Biology 30
Module 3
Lesson 10
Reproduction: Cells and Organisms
Copyright: Ministry of Education, Saskatchewan
May be reproduced for educational purposes
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Lesson 10
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Lesson 10
Lesson 10 Reproduction: Cells and
Organisms
Directions for completing the lesson:
Text References for suggested reading:

Read BSCS: An Ecological Approach 8th edition
Pages 125-133, 170-174
OR
Nelson Biology
Pages 618-625, 634-639

Study the instructional portion of the lesson.

Review the vocabulary list.

Do Assignment 10.
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Lesson 10
Vocabulary
adenine
anticodon
asexual reproduction
budding
centriole
centromere
chromatid
cloning
codon
cogenesis
conjugation
crossing over
cytosine
diploid
DNA (deoxyribonucleic acid)
fission
fragmentation
genetic code
genetic continuity
guanine
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haploid
hermaphroditism
homologous
meiosis
mitosis
mRNA (messenger RNA)
nucleotide
parthenogenesis
regeneration
RNA (ribonucleic acid
sexual reproduction
spermatogenesis
spindle fibre
thymine
transcription
translation
tRNA (transfer RNA)
uracil
zygote
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Lesson 10
Lesson 10 – Reproduction: Cells and
Organisms
Introduction
The idea of remaining at a certain age or in a certain condition is one which occupies
a good deal of our attention, whether we realize it or not. This often translates into
what we do with (or to) our bodies and how we feel about them. Following certain
diets or exercise programs, grooming and applying makeup, undergoing facelifts or
hair transplants and taking certain medicines or health treatments, are some actions
which could be carried out with the intentions of looking and feeling good.
Appearance and physical conditions do not remain constant, however. When we rise
each morning, we leave thousands of dead skin cells on our bedsheets or bed
clothing. Washing our hands and faces takes off many more, and if we run combs
through our hair, we will likely find more evidence of cells or tissues which have
reached the ends of their life spans. Our overall body conditions at particular times
depend upon cells reproducing and the rates at which they do so. For a body to grow
or for body functioning to be maintained, there must be a continual production of
new cells.
Our individual survivals are based on cells reproducing. New cells add to or replace
those which are continually dying from normal "wear-and-tear", injuries or disease.
Just as with cells, all organisms have limited time spans. Points are reached where
rates of cell reproduction cannot be maintained or where important cells are not
replaced. Decline inevitably leads to death and the end of a particular organism.
This does not necessarily mean that all life ceases at that point. Some cells or parts
of an individual's body could have been involved in the formation and development of
new individuals. Successful reproduction on this larger level, resulting in the
production of new individuals, ensures the continued survival of a species. With
reproduction so important for the survival of individuals and species, this module will
focus on that action as it applies to cells and organisms.
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Lesson 10
After completing this lesson you should be able to:
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explain the importance of reproduction to individual
organisms and to species.
•
explain the general role of DNA and RNA in cell actions.
•
describe the general structures of DNA and RNA.
•
summarize the actions involved in the synthesis of
proteins under the direction of DNA.
•
discuss the important relationships between DNA and
cellular reproduction.
•
describe the function of mRNA, tRNA, amino acids and
ribosomes in protein synthesis.
•
describe the actions of mitosis and meiosis.
•
distinguish between asexual and sexual reproduction.
•
explain the importance of meiosis to sexual reproduction.
•
define parthenogenesis.
•
describe some asexual and sexual reproductive methods
of invertebrates.
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Lesson 10
Deoxyribonucleic Acid (DNA) and Its Role
Early microscopic cell studies and experiments quickly established the importance of
a nucleus to the continued functioning and survival of a eukaryotic cell. Cell
differentiation, metabolic actions and movements of substances into and out of a cell
are ultimately controlled by the nucleus. This was confirmed early by the rapid
deaths of cells following the removal of their nuclei.
Protein formation is a critical action in metabolic-related processes within a cell.
Proteins are important structurally in that they make up a large part of a cell, such
as its membranes, organelles and plasma matter both inside the nucleus and in the
general cytoplasm.
Other important substances composed in large part of proteins are enzymes.
Enzymes have significant regulatory functions. These compounds take part in
building up or synthesizing and breaking down substances (including themselves),
controlling the rates of these actions and also affecting the permeabilities of
membranes. By doing so, enzymes are directing the kinds of actions or responses a
cell is making to stimuli and, on a larger level, how an entire organism is reacting.
Therefore, the production of proteins is important to the characteristic structures and
actions of, not only individual cells, but entire organisms. Directions for the
production of proteins come from the nucleic acids in a nucleus. It is these nucleic
acids and DNA in particular, which are the real control centers around which cell
and body actions revolve. What activates or inhibits the DNA will be discussed in the
section describing DNA actions in protein synthesis.
Deoxyribonucleic Acid Structure
Although the name and scale model of a DNA molecule seems intimidating, the basic
structure is not that complex. References commonly compare the double strand
molecule to a rope ladder, with metal rungs, which is twisted about itself. This gives
it a double helix shape, which was first described in l953 by two English scientists
Watson and Crick.
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Lesson 10
DNA double helix and structure
The DNA molecule is made up of two chains of nucleotides wound around each other.
The “handrails” of the molecule are made up of alternating sugar and phosphate groups,
with the phosphate groups serving as bridges between nucleotides. The nitrogenous
bases protrude at regular intervals into the interior of the molecule.
There are three components of a DNA strand:
1. phosphate group
2. 5 carbon sugar unit - deoxyribose sugar
3. nitrogen base
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Lesson 10
The phosphate group and the sugar are joined together in an alternating fashion to
form the ‘handrails’ of the DNA molecule. Joined to each sugar group is the nitrogen
base. The nitrogen bases form the ‘rungs’ of the DNA molecule.
There are five types of nitrogen bases:
Two of these are purines: adenine (A) and guanine (G). Three are pyrimidines:
thymine (T) and cytosine (C), and uracil (U). Uracil replaces thymine in a related
nucleic acid, RNA. Therefore uracil is found in RNA and thymine is found only in
DNA.
The combination of a phosphate group, sugar and nitrogen base makes up the basic
unit or nucleotide of one strand. Three DNA nucleotides make up a DNA codon,
which usually codes or identifies one specific amino acid.
IMAGES THAT CORRESPOND TO THE FOLLOWING HEADING BELOW WOULD BE
USEFUL
DNA nucleotide structure
Joining of nucleotides to form
one side of the DNA double helix
As mentioned, nucleotides are long chains of a phosphate unit, 5 carbon sugar and a
nitrogen base. To form the ‘ladder shape’ or helix structure of DNA, two strands or
chains of nucleotides are wound around each other with connections made at the
nitrogen bases. Hydrogen bonds hold the nitrogen bases together. Certain enzymes
can easily break these hydrogen bonds, permitting the strands of a DNA molecule to
separate.
When joining two nitrogen bases to form a ‘rung’, they follow a set pattern:
adenine only pairs with thymine (A – T or T – A pairing)
guanine only pairs with cytosine (G- C or C – G pairing)
Though the pairings are always the same, the sequences vary along the strands.
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Lesson 10
For example, if the sequence along part of one strand (or half of the ladder) was ACCTGA using the A - T and G – C pairing, the corresponding part of the other strand
(the other half of the ladder) would be TGG-ACT.
Strand 1 A C C - T G A
Strand 2 T G G
A C T
Considering that there are over an estimated 8 billion nucleotide pairings in the DNA
of the 46 human chromosomes, there could be vast numbers of sequence
combinations despite only four nitrogen bases. It is these sequences which supply
the directions for all cellular activities.
DNA and Protein Synthesis
DNA is responsible for protein synthesis. Recall from an earlier lesson that proteins
are organic compounds made up of one or more polypeptides, and that polypeptides
are long chains of amino acids. There are 20 different amino acids that can be used
to form polypeptide chains or proteins. Like the letters of an alphabet, amino acids
can form tremendous numbers of different proteins ("words"). All organisms contain
and make proteins. Proteins are vital structural components found in hair,
fingernails and blood. Proteins also make up enzymes, the substances which control
a bodies chemical reactions.
Directions for the formations of these proteins come from the DNA in the nucleus.
Since the type of protein is dependent on the amino acids joined together, the DNA
can be thought of as a code that specifies how to join the amino acids in a
certain order to form the desired protein. This genetic code is in the form of the
nucleotides. Since the DNA remains in the nucleus, while proteins are synthesized
around the ribosomes in the cytoplasm, there has to be some sort of messenger
system between the DNA and the ribosomes. This messenger is another nucleic acid
called ribonucleic acid or RNA.
RNA – the messenger system for communication between DNA
and ribosomes for protein synthesis.
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Lesson 10
RNA differs from DNA in a number of ways:



RNA is a single, generally shorter
strand nucleic acid
RNA contains the sugar base, ribose
uracil is used in place of thymine in the
nitrogen base
Structure of ribose (left) and deoxyribose (right) sugars
The process of the transfer of genetic information is sometimes termed gene
expression. It can be summarized as follows:
Transcription -the genetic information is transcribed from DNA to RNA
Translation - the genetic information is translated from the RNA to the protein
The DNA within the chromosomes of a nucleus appears to be sensitive to the kinds or
amounts of substances which are present in the cell. These substances could serve as
activators or inhibitors of DNA actions. If a cell is low in a certain needed substance, a
particular strand, or part of a strand, of DNA may be activated. This part then directs
the formation of a structural protein or an enzyme which will be used to build up the
needed material. When the concentration of the substance exceeds a certain level, the
DNA is inhibited so that no more protein or enzyme is produced. Sometimes,
substances which come from outside a body can serve as activators or inhibitors. The
antibiotic penicillin inhibits the DNA in certain bacteria from producing enzymes
responsible for cell wall formation. In this way, bacterial growth is stopped.
Transcription:
Transcription is the term given to the formation, or synthesis of RNA. This process
occurs in the nucleus. The goal of transcripton is to make a copy of a small piece of
the DNA. When synthesis occurs, there are a series of steps which occur:



the nitrogen bases disconnect so that there is a complete or partial separation
between strands.
Nucleotide units, which are present in the plasma inside and outside the nucleus,
line up along the freed nitrogen bases so that guanine and cytosine always pair
up while adenine and uracil do the same.
Gradually, a single (complementary) strand ribonucleic acid is built up along the
DNA. This strand is the RNA.
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Lesson 10
As in DNA, the nucleotides in RNA are also grouped in series of three to serve as
messengers in protein formation. These are called RNA codons or codon triplets.
Transcription, of RNA stops at a certain point and the single strand ribonucleic acid
separates from the DNA and leaves the nucleus through its membrane. The RNA is
ready to transport the information for protein synthesis to the ribosomes. At this
point it is termed messenger RNA (mRNA). Once outside the cell, the mRNA finds a
ribosome and attaches to its outer surface.
Translation
The process in which codons and anticodons form proteins is called translation
Once the mRNA reaches the ribosomes, the genetic information is ready to be
deciphered, or translated.
Transfer RNA (tRNA) is another kind of RNA found in the cell's cytoplasm. The
nucleotides in transfer RNA are not identical to those in the messenger RNA, but
complement them according to the A-T (A-U) and C-G pairings. For this reason, the
term anticodons is sometimes applied to transfer RNA.
The short strands of tRNA, or anticodons, form attachments with specific amino
acids. Transfer RNA finds spots to line up along the messenger RNA so that the
adenine-uracil, guanine-cytosine, pairings are consistent. As this happens, the
amino acids join to (eventually) form a protein. After an amino acid is joined, a
transfer RNA disconnects from both the amino acid and messenger RNA to return to
the cytoplasm for another amino acid. In this manner, a protein is built up
according to original directions from the DNA in the nucleus.
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Lesson 10
Follow the process for a single codon through DNA codon – (messenger) RNA codon –
(transfer) RNA anticodon for the example below.
DNA codon:
ACC
mRNA codon: UGG
tRNA codon:
ACC
(Recall that in messenger and transfer RNA, uracil takes the place of thymine. This
is why messenger RNA in this sequence is UGG, rather than TGG.) Note that the
transfer RNA anticodon is exactly the same as the DNA codon.
The Genetic Code
How is the genetic code written? Recall that there are only 4 nitrogen bases but 20
amino acids. How do we account for twenty amino acids originating from only 4
nitrogen bases? Observations have shown that a single nitrogen base does not
always pair up with just one specific amino acid. It is now thought that the nitrogen
bases arrange themselves in series of threes, called nitrogen base triplets or codons
(or anticodons, in transfer RNA). Each codon identifies with a specific amino acid.
The triplet arrangements or "codes" can give 64 different possible combinations with
the four nitrogen bases. 64 can more than account for all the amino acids. Since
there are 64 combinations and only 20 amino acids, some amino acids have more
than one triplet code. This is comparative to a synonym. For instance, the amino
acid leucine can be coded by the triplets CUA, CUG, CUC and CUU. Just how the
coding works is not known exactly. As well, some triplet codes do not identify
specific amino acids; instead, some of these codons signal the beginnings of protein
chains, others stand for the ends and some do not seem to signify anything. A
sequence of triplet codes in DNA which directs the formation of one complete
protein make ups the gene portion of a chromosome. Proteins themselves are made
up of varying numbers of amino acids, often ranging from 50 to over 300.
Through genetic and biochemical studies, geneticists have identified the specific
amino acids which are coded by particular nitrogen base triplets. By convention, the
genetic code is presented in terms of the mRNA codon, not the original DNA codon.
Be careful not to mix up a nucleotide and a codon. A nucleotide is
a base “unit” of a strand of DNA, made up of a phosphate group, a
sugar and a nitrogen base. Three nucleotides (3 bases) make up a
codon in DNA and RNA (includes mRNA and anticodon in tRNA).
Remember:
One codon identifies an amino acid.
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Lesson 10
The following table shows the genetic code which relates messenger RNA codons to
their amino acids.
First Base
(or letter)
in the Triplet
Second Base (or letter) in the Triplet
Third Base
(or letter)
in the
Triplet
U
C
A
G
U
Phenylalanine
Phenylalanine
Leucine
Leucine
Serine
Serine
Serine
Serine
Tyrosine
Tyrosine
Stop chain
Stop chain
Cysteine
Cysteine
Stop chain
Tryptophan
U
C
A
G
C
Leucine
Leucine
Leucine
Leucine
Proline
Proline
Proline
Proline
Histidine
Histidine
Glutamine
Glutamine
Arginine
Arginine
Arginine
Arginine
U
C
A
G
A
Isoleucine
Isoleucine
Isoleucine
Methionine
(start)
Threonine
Threonine
Threonine
Threonine
Asparagine
Asparagine
Lysine
Lysine
Serine
Serine
Arginine
Arginine
U
C
A
G
G
Valine
Valine
Valine
Valine
Alanine
Alanine
Alanine
Alanine
Aspartic acid
Aspartic acid
Glutamic
acid
Glutamic
acid
Glycine
Glycine
Glycine
Glycine
U
C
A
G
Table 10.1 Genetic Code Table
How to use the table: Suppose you identified a DNA codon as TGC and you wish to
determine the amino acid it relates to. First, determine the RNA codon which would
be formed. In this instance, it would be ACG. Go to the table and locate A in the
column with the heading First Base in the Triplet.
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Lesson 10
First Base in
the Triplet
U
Second Base in the Triplet
C
A
G
U
C
A
G
Third Base in
the Triplet
U
C
A
G
U
C
A
G
U
C
A
G
U
C
A
G
C is the second letter in ACG. Locate this in the row Second Base in the Triplet.
First Base in
the Triplet
U
Second Base in the Triplet
C
A
U
C
A
G
G
Third Base in
the Triplet
U
C
A
G
U
C
A
G
U
C
A
G
U
C
A
G
The last letter in the codon is G. Locate this in the third column Third Base in the
Triplet so that it is in the same row as the first letter A. Run your finger across from
G towards A until it is under C, the second letter of the table. You can now identify
Threonine as the amino acid coded for by the DNA nucleotide triplet TGC.
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Lesson 10
First Base in
the Triplet
U
Second Base in the Triplet
C
A
U
C
A
Threonine
G
G
Third Base in
the Triplet
U
C
A
G
U
C
A
G
U
C
A
G
U
C
A
G
Tables could also be used to determine the nucleotide triplets, knowing what the
amino acids are. For example, given the amino acid Valine, consulting the table, the
corresponding nucleotide triplets are GUU, GUA, GUC and GUG.
The table may take on the appearance of a ‘Codon Wheel’ as shown below. When
naming a codon, choose the corresponding first nitrogen base from the centre of the
wheel. Move to the next circle to the corresponding second nitrogen base. Again,
move to the next circle to the corresponding third nitrogen base. Directly outside this
base is the codon sequence followed by a shortened version of the amino acid name.
The abbreviated form is translated below the wheel.
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Lesson 10
Codon Wheel
Some nitrogen base triplets, such as (messenger RNA) UAA, do not stand for any
particular amino acid. Rather, these codons seem to signal the end of an amino acid
sequence or that a protein chain has been completed. Another codon, AUG, appears
to signal the beginnings of various proteins.
Although there are different forms in which the genetic code can be presented, if
interpreted correctly, the same amino acids would be selected from an original DNA
codon triplet. Some tables may present the code in terms of the DNA codon or the
tRNA anitcodon.
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Lesson 10
Example:
Given the DNA codon sequence: ACA – TGG – CGT determine the sequence for the
mRNA, tRNA and name each of the 3 amino acids formed.
Solution:
Use the A - T and G – C pairings, remembering that U replaces T in RNA:
DNA:
mRNA:
tRNA:
ACA – TGG – CGT
UGU – ACC – GCA
ACA – UGG – CGU
Using mRNA codons to name amino acids gives:
UGU –
ACC –
GCA –
Cysteine
Threonine
Alanine
DNA and Reproduction
Cell reproduction is a necessary process if an individual organism or a species is to
survive. The division of a mother cell into two daughter cells ensures that life is at
least temporarily maintained. For the daughter cells to survive and carry on the
same processes as the original parent cell, it is important that they each receive the
same "blueprints" that the parent had. For this reason, there must be particular
actions the "blueprints" or DNA must go through before ending up in the new cells.
If cells are to be duplicated, where one cell gives rise to two,
there must also be a duplication or replication of the DNA before
it is divided among the new cells. The manner in which DNA
does this has not been difficult to follow. Just before cell
division, enzymes break the hydrogen bonds between the
nitrogen bases of DNA. This allows the two halves of the double
DNA strands to separate. Almost immediately, the separated
strands begin replicating the sequence of bases which were
present in the other strand. This is done with other nucleotide
units present in the plasma. At the end of the process and
when the two original DNA strands have separated, each will
have replaced its partner with a new, but matching one. When
this has been fully completed, the DNA will have doubled with
two identical sets. In eukaryotic cells, some DNA is also found
in some organelles such as chloroplasts and mitochondria. Similar replications
occur within these organelles just before they reproduce.
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Lesson 10
The manner in which cells divide and what happens to the DNA could follow one of
two basic processes: mitosis and meiosis. Before examining these processes, it
should be noted that continuing descriptions will mainly use the term chromosome,
of which a major part is DNA. The directing influence of a cell has so far been
examined from the point of view of deoxyribonucleic acid or DNA. Most of this DNA
really makes up the chromosomes inside a nucleus. In addition to the DNA, there
are also some ribonucleic acids, proteins, calcium and fats to fill out the structure of
a chromosome.
Mitosis
Organisms are continually experiencing cell divisions. Among unicellular and
multicellular life forms, divisions of cells become a necessary part of cell or organism
life cycles. As cells increase in size, their internal volumes increase much more in
proportion to the increases in the surface areas of their external membranes. This
characteristic of cell growth makes it increasingly difficult for enough nutrients to
enter and pass into all parts of the cytoplasm or for all wastes to pass out. Division
of a larger cell into two smaller ones helps by increasing the ratio of surface area
relative to internal volume. In a multicellular organism, cell divisions are also
necessary for body growth and for normal replacement of cells which have reached
the end of their life spans or have been lost due to diseases or injuries. In addition to
growth and replacement, cell division takes place in the production of entirely new or
different individuals in asexual reproduction.
Normally, cell division includes two related but different actions:
1. nuclear or genetic duplication
2. equal division into two nuclei.
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Lesson 10
This is important in ensuring that the daughter cells will show genetic continuity,
that is they have the same genetic material that the parent cell had. Having the
same chromosomes and genes will permit new cells to differentiate and function in
the same way as the original cells. The second process, which often begins before
nuclear duplication and division is complete, divides the cytoplasm of the original cell
into two parts. This cytoplasmic cleavage or cytokinesis finally separates the two
new cells. The close relationship between the two actions frequently results in the
term mitosis being applied to both. However, mitosis really refers to the duplication
and division of the nucleus.
Mitosis-cytokinesis was originally described in lesson 2 of this course. Some of
the following sections will really be a review of the cell cycle.
The Actions of Mitosis and Cytokinesis
Microscopic examination of a cell in a normally functioning state shows the nucleus
as no more than a dense body within the cytoplasm. Larger magnifications may pick
out the presence of a smaller body, the nucleolus, within the nucleus. In an animal
cell there is a rounded body or centrosome (consisting of a centriole pair) just outside
the nucleus. In the cell cycle, interphase occurs before mitosis begins. During
interphase, DNA duplication occurs in the S phase of interphase. Two complete
identical sets of the cell’s DNA result. The cell now has twice its normal DNA. In
animal cells, the centrioles duplicate and prepare to form the spindle fibres. As well
a substance is released that causes mitosis to proceed.
INSERT DIAGRAM HERE
Interphase
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Lesson 10
In the prophase stage, the nucleolus disappears and the nuclear membrane begins
to disintegrate as well so that at the end of this stage the nucleus is not visible. The
chromatin or chromosome material thickens and shortens so that the chromosome
threads become visible.
INSERT DIAGRAM HERE
Early prophase
As prophase progresses, chromosomes shorten and thicken even more. A high
magnification may show each chromosome to be made up of two parallel threads
joined at a centromere. Each of the threads is called a chromatid and is the result of
the DNA doubling which took place at the end of interphase. The centrioles move to
opposite ends of the cell just as the chromosomes move towards the middle.
Thread-like spindle fibers appear and extend across the cell from the locations of the
centrioles or poles of the cell.
INSERT DIAGRAM HERE
Late prophase
In the metaphase stage, the chromosomes will have arranged themselves in the
middle or "equatorial" plane of a cell. Each "sister" chromatid of a chromosome will
have its centromere attach to a spindle fiber from a pole, with "sisters" attached to
opposite poles.
INSERT DIAGRAM HERE
Metaphase
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Lesson 10
The anaphase stage begins when the centromeres of the sister chromatids separate
and the chromatids or chromosomes begin to be pulled to opposite poles.
INSERT IMAGES FOR EARLY AND LATE ANAPHASE HERE
The end of the anaphase stage has the chromosomes pulled to the poles and still
distinctly visible.
In telophase, changes opposite to those in late interphase and during prophase take
place. Chromosomes lengthen and become so thin as to begin to disappear. At the
same time, the nuclear membrane re-forms and the nucleoli reappear.
INSERT IMAGE OF TELOPHASE IN ANIMAL AND PLANT CELLS
HERE THAT SHOW CLEAVAGE FURROW VS CELL PLATE
At the end of the telophase stage of mitosis, the nuclei of the daughter cells will
contain the same kinds and numbers of chromosomes which the mother cell
originally had.
In describing the different stages of mitosis, it should be emphasized that it is a
continuous process and that the stages are not always distinct. Certain points
during the process will fall in between identifiable stages.
In multicellular organisms, a division of cytoplasm and organelles begins before the
telophase stage of mitosis is complete. Unlike mitosis, cytokinesis does not
necessarily result in exact and equal divisions. One daughter cell could end up
with more cytoplasm than the other. Organelles in the cytoplasm tend to remain
undivided as they end up in one cell or the other and, again, one cell may receive
more of the different organelles. This inequality of divisions does not present any
great problem as new cells soon form new or more organelles as needed.
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Lesson 10
Cytokinesis follows a slightly different pattern in plants than in animals. In animals,
a "pinching-in" or infolding begins around the outer edges of the equatorial area.
This continues until the two daughter cells separate completely. In plants, a cell
plate begins in the middle of the equatorial area and works outward towards the
original cell walls. New cell walls are formed on each side of the cell plate by each
daughter cell. Completion of cytokinesis in plants has the daughter cells usually
remaining together. Pectic material deposited in the cell plate area between the two
cells acts somewhat like a bonding agent.
Mitosis and Asexual Reproduction
Mitosis and cytokinesis are the major methods of adding to, or replacing, the body
cells of multicellular plant and animal organisms. The production of additional cells
always originates with single cells.
Asexual reproduction: the form of reproducing, where single cells of both
unicellular and multicellular organisms give direct rise to more cells.
The term vegetative reproduction may be used when referring to the propagation of
new individuals from the vegetative or "body" cells, or organs, of plants. However,
some references may also use the term with respect to animals. Asexual
reproduction is a rapid method of producing more cells, especially when conditions
for growth are good.
Asexual reproduction is also applied more
broadly to situations where new, individual
organisms are formed to go their own separate
ways. For unicellular forms, a very common
method of asexual reproduction is fission (or,
binary fission), where an individual divides into
two, fairly equal-sized halves. Bacteria,
Amoebae and Paramecia are common examples
of this technique.
Image by JWSchimdt
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Lesson 10
Some unicellular forms experience an unequal division into two parts. This is
budding and a very common participant is a yeast cell. An outgrowth or bud appears
in a part of a cell wall. Eventually, one of the nuclei produced in mitosis will enter
this bud. Separation of the bud could occur soon after or the bud could continue to
increase in size. Additional buds could appear on the original cell or even on the
bud(s), before separations take place.
Budding in yeast
Despite the unequal division of the cytoplasm in budding, the nuclei produced have
equal amounts and kinds of chromosomes.
Budding is an asexual technique common to some multicellular organisms also.
Tapeworms have segments which divide and then enlarge before dropping off. In
some multicellular organisms (e.g. Hydra), a number of cells participate in the
production of a bud.
Other examples of asexual reproduction occurs in plants:
 Accelerated cell divisions or outgrowths, eventually producing new individuals, are
common in many plant varieties. Surface stems, such as the stolons of
strawberries, frequently develop new individuals.
 Underground stems or rhizomes are more common sources of new, asexually
produced individuals. Quack (or couch) grass reproduces in this
 Root systems (Canada Thistle, Foxtail...) can either spread or be broken apart to
begin new individual growths. Spreading root systems of willows quickly
distribute these shrubs over wider areas. Some plant varieties can reproduce
asexually if their leaves or if their stems come in contact with soil.
Humans have used their knowledge of natural asexual plant reproduction for
starting particular varieties or more rapidly increasing the numbers of others.
 Tubers, corms and rhizomes are used to begin many plants from stems rather
than seeds.
 Dividing rootstalks is another common technique.
 Bending stems to the ground and covering portions with soil (in a process called
layering) encourages adventitious roots and new individuals in raspberries,
gooseberries and other plants.
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Lesson 10
Humans have also developed techniques, which are not usually naturally
occurring, of asexually producing new plant individuals.



Cuttings are commonly made where leaves, stem portions or root pieces are cut
from plants and "started" in water, peat or some soil mixture. Some of these
pieces will develop root or stem systems, before transplanting to more permanent
media if so desired.
Grafting is used in the propagation of many fruit or ornamental trees and shrubs.
A scion (branch or twig) is inserted into a stock (the rooted portion) of a related
variety so that the cambiums of both are in contact.
Budding can also be carried out by taking a bud from one plant and inserting it
just under the bark of another.
Other natural asexual reproductive methods include fragmentation and
regeneration. Fragmentation is more commonly experienced by less complex or
colonial organisms. The action occurs when an organism or colony is broken apart
into smaller units. Frequently, outside forces are involved. Wind and wave actions
on many Saskatchewan lakes break masses of colonial algae apart.
Worms, such as earthworms, may develop into separate individuals if cut or torn into
pieces. The process of regeneration generally accompanies fragmentation.
Regeneration adds to a colony or body, or it replaces body parts which have been lost
or injured. An earthworm cut in half could have the anterior portion regenerate the
posterior and the posterior segment could redevelop the "head". Regeneration tends
to be more successful from the anterior parts. Crayfish commonly regenerate claws
which have been injured or torn off and it is not unusual to see an individual with
different sized claws. The ability to regenerate decreases as the complexity of
organisms increases. For instance, mammals cannot grow back appendages (limbs)
which are injured or lost. Regeneration in more complex animals is limited mainly to
replacing cells in areas which have experienced wounds or injuries.
Finally, certain life forms produce special reproductive bodies called spores. Some
bacteria, fungi and plants produce spores when growing conditions are unfavorable.
Each spore, containing a nucleus and a small amount of cytoplasm, has an outer
wall capable of providing some protection from adverse conditions. A return to
favorable conditions, or when a spore reaches an ideal area, will cause it to begin new
growth. Some species may even use spore production or sporulation as a means of
dispersal. Many land spores are light and easily spread by wind. Most cereal grain
rusts in our province originate from windblown spores from the United States. Some
aquatic spores have flagella or cilia to assist movements.
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Lesson 10
Cloning
Another technique which has been applied more and more and is receiving greater
attention is that of using tissue cultures. The term cloning is sometimes applied to
this, but any asexual techniques mentioned so far where genetically identical
individuals are produced from a single parent are examples of cloning. Tissue
culturing is based on the principle held by researchers that a single cell really
contains all the genetic material needed to reproduce an entire new individual.
Single plant cells or, more commonly, a number of cells are removed from an
individual and placed into artificial, nutrient media where they may divide into more
cells, producing a tissue. Application of proper hormones could then cause cells in
the tissue to begin differentiating into root, stem and other cells of a completely new
individual. This technique is already being commercially used to propagate some
grains and trees. Limitations to more widespread use have largely been due to
difficulties in preparing proper nutrient media and hormone applications for specific
varieties. However, three advantages of tissue culturing will likely continue to
expand its use. Culturing produces new individuals known to have the same genetic
makeup as the parent; the technique is a much faster way of increasing the numbers
of individuals of certain varieties; and, started from one or a few carefully examined
cells, virus or bacteria-free individuals can be produced.
Cloning, by tissue culturing, of more complex animals has not been very successful
so far. The greater degree of specialization among animal cells may be a reason this.
Some success has been experienced in experiments where nuclei of egg cells have
been removed and replaced with nuclei of cells from developing embryos. (This will
be looked at in Lesson 12.) There are also ethical issues that are present in the
cloning of complex animals and humans.
All the actions of asexual reproduction, whether they involve cell divisions,
outgrowths, replacement of parts or spore formation, include mitosis or mitotic
divisions. The end results are always new cells or individuals with the same genetic
characteristics as the parent cell or parent organism.
Mitosis produces cells with the same genetic content of the parent. These cells
are diploid
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Lesson 10
Meiosis
Asexual or vegetative reproduction is a very common method of adding new body
cells or replacing those which are no longer functional. It is also a way in which
many unicellular and multicellular organisms give rise to new individuals. Among
the more complex, multicellular groups, new individuals are frequently formed
through a sexual process.
In sexual reproduction, a union of two cells and their genetic contents occurs.
The resulting union produces a cell called a zygote, which then has an opportunity
to develop into a new individual. The reproductive cells are usually distinct from
body cells. Each is formed by a nuclear process called meiosis, which reduces the
chromosome numbers to one-half of those in the parent cell. The sexual process, or
fertilization, when two reproductive cells fuse, restores the chromosomes to their
normal (diploid) numbers.
Meiosis produces a cell (gamete) with one –half the genetic content of the parent.
These cells are haploid (n).
Different species not only vary in the kinds of chromosomes and genes (DNA) which
they have, but also are distinct with respect to numbers of chromosomes.
Organism
human
horse
pig
cow
goldfish
mosquito
cabbage
barley
wheat (one variety)
Adder’s tongue fern
Biology 30
Chromosome
Number
46
66
40
60
94
6
18
14
42
1260
27
Number of
Chromosome Pairs
23
33
20
30
47
3
9
7
26
630
Lesson 10
Although the kinds and numbers vary, all species have most of their chromosomes as
homologous pairs. That is, each chromosome has a similar partner in length,
perhaps shape and especially in the genes responsible for certain characteristics. For
this reason, biologists just as commonly refer to organisms as having so many pairs
of chromosomes as they do total numbers. For instance, humans have 23 pairs. A
cell which has all the chromosomes or genetic material is said to be diploid, which
can also be represented by the number 2n. For humans, 2n = 46.
Asexual reproduction through mitosis carries out a doubling and then an equal
splitting of genetic material. This ensures that new cells or individuals directly
arising from existing ones continue to maintain a constant chromosome number from
generation to generation. If mitosis continued to be the major process leading up to
the final production of the special sex cells or gametes taking part in sexual
reproduction, a major difficulty would arise. Fusion of sex cells would combine the
genetic contents of the two. If a human egg and a human sperm each had 46
chromosomes, the zygote and new individual would then have 92, instead of the 46 of
each of the parents. Such a doubling generation after generation would not be
possible in the limited space of the nucleus. Difficulties in matching up
chromosomes or their numbers between sex cells would also likely prevent successful
fusion.
Fortunately, such a situation does not usually occur. Mitosis does take place in the
production of cells in special sex organs such as ovaries and testes. However, a
special kind of cell division is responsible in producing the final sex cells. This
process is meiosis and it results in cells having one-half the number of chromosomes
that the parent cell or organism has. Such cells can also be referred to as monoploid
or haploid. Meiosis involves a sequence of phases similar to mitosis: prophase,
metaphase, anaphase and telophase. However, meiosis involves TWO sequences of
these series termed meiosis I and meiosis II. There are two distinct actions which
take place: a reduction in chromosome numbers and then a division of the
chromosomes.
The Stages of Meiosis
As with mitosis, just before meiosis and cell division, there is a doubling of DNA or of
chromosomes. This happens in late interphase. The following graphics show each
chromosome doubling to form two chromosomes or sister chromatids.
INSERT IMAGE OF INTERPHASE HERE
The chromosomes are illustrated to show the doubling. Ordinarily, even though the
action does take place, the chromosomes are not visible during the interphase stage.
Biology 30
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Lesson 10
In prophase I, the nuclear membrane begins to disappear and the chromosomes
shorten, thicken and become more visible. As prophase proceeds, homologous
chromosomes pair up. (This does not happen in mitosis.) During the pairing,
chromosomes often exchange corresponding sections of chromatids in an action
called crossing-over. The significance of this is looked at more closely in another
section.
For the sake of clarity in the illustrations, only 2 chromosomes or 1 homologous pair
will be shown throughout. We will assume that for this organism, 2n = 2.
In metaphase I, all homologous pairs line up in the equatorial area of the cell.
Spindle fibers form and join to the centromeres of the chromosomes.
INSERT IMAGE OF METAPHASE I HERE
The anaphase I stage sees a splitting of the homologous chromosome pairs so that
the chromosomes of each pair move to the opposite sides of the cell. Each
chromosome pair still consists of two chromotids joined by a centromere.
INSERT IMAGE OF ANAPHSE I HERE
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Lesson 10
Telophase I marks the end of the first division of nuclear material during meiosis.
During this stage, cytokinesis will usually be taking place to separate the rest of the
cell into two.
INSERT IMAGE OF TELOPHASE I HERE
In some organisms, the two resulting cells may go into temporary interphase with
chromosomes becoming invisible and nuclei reappearing. However, it is a more
frequent occurrence that even before telophase I is completed, another set of spindle
fibers is appearing to mark the beginning of prophase II, which begins the second
sequence, meiosis II. The second nuclear division is similar to mitosis. The
chromosomes, consisting of sister chromatids, line up at the equator and attach to
the spindle. This time the centromeres split and each chromatid (which may now be
called a chromosome) is pulled to its respective pole or side of the cell.
INSERT IMAGE SHOWING MEIOSIS I AND MEIOSIS II AND THE STAGES
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Lesson 10
The end result after meiosis is concluded is that four cells are produced. Each of
these cells has one-half or the haploid number of chromosomes that the parent cell
had. If we had assumed that 2n was equal to 2 for a particular organism, than its
sex cells would have the characteristic of n = 1. For humans, body cells would be
2n = 46 and eggs or sperm would be n = 23.
Comparison of meiosis and mitosis
The following diagram compares the process of meiosis to the process of mitosis.
Notice that the end product of mitosis is 2 daughter cells, each with 2n number of
chromosomes and the end product of meiosis is 4 daughter cells (although the
diagram shows only two), each containing n number of chromosomes.
Diagram comparing Mitosis to Meiosis
Image by JWSchimdt
.
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Lesson 10
Spermatogenesis – Oogenesis
In most species, the appearance of female and male reproductive cells is different.
Sperm tend to be much smaller than eggs and also have flagella, enabling them to
move by swimming actions through fluids. As well, the production of sperm
(spermatogenesis) and the production of eggs (oogenesis) is slightly different.
Spermatogenesis
Specialized cells which eventually produce sperm and are usually located in
structures called testes are known as spermatogonia. Spermatogonia are diploid (2n)
and experience a number of mitotic divisions to increase their numbers. The final
cells, which will then experience meiosis and sperm production, are primary
spermatocytes. Each diploid primary spermatocyte will go through a reduction stage
to give rise to two haploid cells called secondary spermatocytes. The division stage
further divides these so that four haploid cells, or spermatids, are formed. Some final
developments during an interphase, including the appearance of a "tail" on each,
produce four sperm. The process of sperm formation can occur throughout the year
or may occur only during a certain time of year, or breeding season. In humans,
sperm formation occurs continuously, while in some migratory birds, it occurs only
during the spring and summer months.
Oogenesis
Oogenesis follows a somewhat similar pattern of divisions, although the results are
somewhat different. The initial cells making up the ovaries or female reproductive
organs are oogonia. Mitosis increases the numbers of these diploid cells.
Interestingly, in many vertebrates, the mitotic productions of all the oogonial cells
that female individuals will have throughout their lives will have been completed
before those individuals are even born. In humans, a female fetus normally has all
her oogonia before it is four months old. After birth and during growth, oogonia grow
in size and may enter the first stage of meiosis. Development stops at this point and
the cells are called primary oocytes. Each is surrounded by a cluster of food-bearing
cells which forms a follicle. The next developments take place when a female begins
the reproductive cycle. Regularly, a primary oocyte will continue the first phase of
meiosis in which two cells with haploid chromosome numbers are formed. However,
unlike spermatogenesis, there is an unequal division so that one cell is large and
becomes a secondary oocyte (egg) while the other is small and is known as a polar
body. The second stage involving these two cells may not occur until after the
secondary oocyte is released (ovulated) and is entered by a sperm. When this
happens, another unequal division produces an egg from the secondary oocyte and
another small polar body. The end result has one egg and three smaller polar bodies
or cells. These polar bodies degenerate or disintegrate or are used for purposes other
than zygote formation. In some species, the first polar body degenerates before there
is even a further division of it.
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Lesson 10
Comparing Asexual and Sexual Reproduction
Asexual and sexual reproduction have some points which could be considered as
positive and others as negative. The very fact that many species utilize both forms at
different times or even at the same time seems to argue against one having an overall
superiority over the other.
Speed
In terms of speed, asexual reproduction tends to be quite fast, especially when
environmental conditions are good. One contributing factor for this is that only one
parent cell or organism is needed for asexual reproduction to occur. Sexual
reproduction requires two different cells or parent organisms to come together. One
sexually producing cell or organism can not reproduce by itself. Asexual methods
also tend to be faster than sexual, even if both forms are initiated at the same time.
Genetic Content
Through asexual reproduction, resulting daughter cells or offspring receive the same
genetic content as the original parent. This uniformity in genetic material maintains
structural and functional similarities from generation to generation. Characteristics
which had proven to be useful or important for survival would continue to be passed
along. Sexual reproduction promotes different degrees of variation between offspring
cells or organisms and their parents. Crossing-over and different gene assortments
during meiosis and then different combination possibilities during fertilization
promote the differences. This could mean the appearance of some characteristics
which could be of no benefit and some which could even be harmful to organisms'
chances of survival. At the same time, in terms of species adaptations to changing
Biology 30
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Lesson 10
environmental conditions, sexual reproduction has a definite advantage. Continuous
environmental changes could mean the extinction of a species if some variations do
not appear making that species more suited to new conditions. Asexual reproduction
would be at a disadvantage from an adaptation point of view since it is not a likely
source of new characteristics.
Humans involved in plant and animal propagation see advantages and disadvantages
to both forms of reproduction at different times. Producers wishing to quickly
increase numbers of certain organisms can accomplish this more often through
asexual or vegetative methods. Producers can also be sure of the characteristics or
traits being uniform from one generation to another. Having organisms reproduce
sexually presents possibilities to producers of developing superior individuals in
terms of growth rates, yields, disease resistance and other traits. Sexual
reproduction has been the main foundation for the development of new plant and
animal varieties or breeds.
Parthenogenesis
Parthenogenesis is the development of new individuals from eggs which are not
fertilized. This is a form of reproduction which departs from the usual sexual
method. Meiosis could produce eggs having a haploid number. However, some
stimulation other than fusion with sperm could cause the eggs to begin developing.
These stimulations could come from chemicals, hormones, radiations or some
disturbances to eggs. Individuals produced in this way would be haploid. Male bees
or drones are haploid or n, having developed from unfertilized eggs. In other
instances, the process of meiosis either does not start or is not completed. "Egg"
cells, which are diploid, could again experience stimulations to cause them to develop
into new (diploid) individuals. Many plant varieties actually reproduce in this way.
Pollen from other species could release hormones which trigger egg or cell
development without the sex process of sperm combining with an egg having to take
place. In plants and animals in which parthenogenesis is common, the method is
not used continually. Parthenogenic reproduction seems to alternate fairly regularly
with the usual sexual one. Such alternations may allow some species to benefit from
some of the advantages of either asexual or sexual reproduction at certain times.
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Lesson 10
Invertebrate Reproduction
Invertebrate organisms demonstrate a wide range of reproductive techniques.
Unicellular forms often carry out a relatively fast, asexual fission process where
fairly equal divisions occur. Some multicellular flatworms and annelids can also
divide by pinching in half and then regenerating the missing heads and tails.
INSERT IMAGE OF ASEXUAL FISSION HERE
In budding, a single cell undergoes mitosis and then splits off a smaller cell from a
larger one. In some multicellular species, such as sponges and coelenterates, certain
areas of a main body bulge outward as a number of cells take part in bud formation.
Eventually, this smaller version of the main body separates and begins an
independent existence.
Conjugation, described in the section on spore formation, has two nuclei from
adjacent hyphae fusing to form a zygospore. This primitive sexual process is
common to some other organisms. It is sexual in the sense that there is a fusion of
the contents of one cell with that of another. Some would hesitate to call the process
reproductive, since it seems to form a cell which frequently goes into a dormant or
resting stage. Then, in favorable conditions, this cell undergoes meiosis to produce
two cells which are similar to the ones which existed before the fusion.
I
NSERT IMAGE OF CONJUGATION HERE
In other organisms, conjugation may involve an exchange of nuclear material
between two individuals before they separate and resume reproduction by asexual
methods. This is often seen in Paramecia. Conjugation may be a way of
"rejuvenating" individuals with the exchange of genetic material.
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Lesson 10
As organisms become more complex, there is an appearance of cells or organs which
are clearly involved in the production of special reproductive cells or gametes. The
appearance of ovaries and testes marks a beginning to the process of establishing
individuals of different sexes. Some simple multicellular organisms display
hermaphroditism. A hermaphrodite possesses both female and male sex organs.
Many of the worms show this characteristic, as do some mollusks such as snails.
Even though self-fertilization can occur, many hermaphrodites carry out crossfertilization with other individuals. Aligning themselves parallel to each other,
individuals will exchange sperm with each other. In some mollusks and in
arthropods, the hermaphrodite condition disappears. Individuals develop as either
female or male. Sexual reproduction in these groups includes the transfer of sperm
from male to female. Frequently, sperm is transferred to, and stored in, a special
cavity in a female's body called a seminal receptacle. Females could carry sperm
within these receptacles for months before eggs are laid and fertilized by the sperm in
the process.
Although these last descriptions have focused briefly on the processes in "animal"
reproduction, much more could have been included if life cycles were described in
detail. Some life cycles, especially of parasitic worms, are quite complex with both
asexual and sexual reproduction taking place. The involvement of different hosts by
one species during its complete life cycle further complicates the entire cycle.
Summary
It seems rather ironic that two actions at opposite ends of a life cycle are related. Yet,
there is a link between death and reproduction. Production of new body cells to add
to, or replace, those which are present, is a necessary condition for the continued life
of an organism. Failure to keep up with cell replacement or repair will lead to decline
and eventual death. On a broader level, reproduction of new individual organisms is
necessary for species survival. Failure to keep up in the replacement of individuals
lost to predation, disease, starvation or age could lead to the death or extinction of a
species. Fossil and written records show that this has happened to many species
already and is still continuing.
Activities within cells are controlled through "blueprints" carried on the
deoxyribonucleic acid or DNA of cell chromosomes. By directing the synthesis of
proteins with the help of messenger ribonucleic acid, DNA also directs how a cell is
built and how it functions through its enzymes. To maintain effective functioning
and continuity in new cells, it is important that reproduction should include some
methods of duplicating and then transferring DNA from mother to daughter cells.
These processes are indeed carried out in the successful reproduction of cells and
organisms.
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Lesson 10
Duplication or replication of cellular DNA is generally followed by two possible kinds
of nuclear divisions. In mitosis, the DNA or chromosomes go through a number of
stages. At the end, the chromosomes will have been exactly divided. The resulting
two daughter cells will receive the same genetic information or content as the original
parent cell. Mitosis is very closely related to and part of asexual reproduction. In
this form of reproduction, one cell or one organism gives direct rise to new
individuals. The other kind of nuclear division involves a reduction and then a
division of chromosome content. At the end of meiosis, four daughter cells are
produced from an original parent cell. These new cells have the haploid number, or
one-half the chromosome number of the parent cell. Cells produced through meiosis
are special reproductive cells or gametes. These types of cells enable some species to
carry out sexual reproduction. This form of reproduction sees a fusion of two
different gametes (the sexual process), often from two different organisms. The sex
process restores the normal diploid chromosome number in a zygote, which develops
into a new organism. Meiosis and sexual reproduction permit variations to occur in
offspring and the possible appearance of better adapted individuals.
An increase in complexity of multicellular species is usually accompanied by the
development of organs with clusters of reproductive cells. Some species are
hermaphroditic, in that female and male organs are both found in single individuals.
In many instances, individuals of such species still carry out sperm exchanges or
cross-fertilization. Separation of sexes among higher plants and animals requires
successful transfers of sperm to eggs for reproduction to occur. This introduces
difficulties in the form of finding suitable partners, in the timing of the production of
eggs and sperm and in having sperm deposited close to eggs. Some of these
difficulties are examined in another section.
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Lesson 10