Biology 101
Section 5 Notes
Cellular Reproduction
These notes will correspond to Chapter 8 in the textbook.
What is cellular division? One cell growing in size and dividing to produce two new cells
This is an important question. Since the first scientist observed a cell dividing, people have
wondered why cells do this and how. Early biologists observed that cells reproduce, or make
more of them, by dividing in half. Simply one cell splitting into two. However, there is a lot
more to it than that. First a cell must increase its size before dividing. If I cut a block of wood in
half, I get two blocks of wood, but each only half the size of the original. I get nowhere that
way. So a cell grows rapidly to nearly twice normal size and divides. This growth entails an
increase in cytoplasm, DNA replication, an increase of membrane and organelles; a basic
doubling up of all cellular components.
One of the most important events to occur before a cell can divide is the DNA must be copied,
faithfully, error free and totally. This is called DNA Replication. This must happen before a cell
can divide. It guarantees that each new cell receives a copy of all the genetic instructions.
Another thing to note: all terms when dealing with cells are in feminine terms. The parent cell is
called the mother cell, the news cells are called daughter cells, and the two daughter cells are
called sister cells to one another.
Also called "simple cellular division”
Functions for:
1) Growth and development
Cell division functions for all growth of all parts of the body: skin, hair, nails and any
other part that grows or increases. Also any part of the body that develops or changes
must develop through cell division.
2) Repair and reproduction
Anytime the body is damaged or hurt, it is repaired by cells dividing. You cut your
hand; the cut seals itself and repairs itself by cell division. All forms of reproduction,
from the cellular level to larger organisms, are through cell division. If you are a
single-celled organism like protists, then you only reproduce by splitting in half. But
even larger, multi-celled, organisms reproduce by dividing cells. Humans reproduce
by using reproductive cells called sperm and eggs. A sperm or egg is just one cell.
They are produced in the body by cell division in the gonads. They fuse in the
process of fertilization to produce a single cell called a zygote. This zygote then
divides to grow and form a complete human. All reproduction on Earth involves
cellular division.
Reproduction = single or multiple celled organisms producing a new generation
2 Types:
1) Asexual: production of offspring by a single parent, no sexes or genders
This is the most common form of reproduction. Almost all living things can carry out
some form of asexual reproduction. This trait is only lost as we get to higher life
forms such as mammals. This is the preferred method for most organisms: it is faster,
cheaper (requires fewer resources), less space, and is not reliant on finding a mate.
2) Sexual: production of offspring by two or more parents resulting in genetic
recombination
For the majority of organisms, this method is not used. Even those capable of sexual
reproduction will typically reproduce asexually if it is an option. Sexual reproduction
requires more resources (water, food, shelter), more time, larger territories to supply
the resources, and depends on finding a member of the opposite sex of the same
species; which for some species such as Pandas, is actually quite difficult. The only
positive this form of reproduction has in its favor is Genetic Recombination!! For
some species, such as higher animal forms, this benefit is large enough to out weigh
the costs. So animals, like humans, only reproduce sexually. In reality, Genetic
Recombination is the main point of sexual reproduction, its purpose for existence.
Here a distinction is made between sex (intercourse or copulation) and sexual
reproduction. The purpose of sexual reproduction is to shuffle and recombine genetic
material (DNA) from two individuals into a new, third person. Not for enjoyment or
pleasure. For many animals, this is achieved without any form of copulation or even
production of gametes (sex cells).
Purpose: to pass on the genome (An organism’s genome is the total collection of all its genes.
Reproduction is an attempt to pass on as many of those genes as possible to the next generation.)
In Prokaryotes: No nucleus!
Remember, prokaryotes have no organelles! This includes the nucleus. However, they
still contain DNA, and pass it on to the new cells.
Use process called Binary Fission, the DNA is replicated (copied) and divided between two new
cells. This is the most basic form of cell division, simple singular division of one cell into two
cloned copies.
In Eukaryotes: Nucleus!!
Eukaryotes have a nucleus! In essence, most of the events and information about cell division in
eukaryotes revolve around the events associated with the nucleus, what is done with it, and how
the DNA molecule is handled. Because of this, the following notes, especially dealing with
mitosis, are explicitly in reference to the nucleus, not the entire cell division process. What
happens with the nucleus is called Nuclear Division. The division of the rest of the contents of
the cell is called Cytoplasmic Division.
→ Simple Cell Division: has 2 parts
1) Nuclear Division: division of nuclear contents (I.e. the DNA, chromosomes)
2) Cytoplasmic Division: division of the cytoplasm (Organelles and everything else)
Nuclear Division (only eukaryotes) can take one of two forms:
MITOSIS
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This is often referred to as simple cell division
Basic, general division of all cells
Used for growth of the organism and tissues and repair of damage
Used for asexual reproduction in organisms that reproduce asexually
One parent cell gives rise to two daughter cells, direct split from one to two
Each daughter cell exactly identical genetically to parent, clones
Occurs in somatic cells, ordinary cells that make up the body (skin, muscle hair, etc.)
Most common form of division in all living things, 99.99999% of all divisions
Ex. protistans, fungi, plants and animals (basically everything)
Or
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MEIOSIS
Also called Reduction Division
One purpose only, and that is sexual reproduction
Only occurs in those species that are capable of sexual reproduction (so not everything)
One parent cell will give rise to four daughter cells
Each daughter cell is nonidentical genetically to parent or each other
Occurs only in germ cells, cells that make up the gonads and divide special to form the
reproductive cells called the sperm and egg, which are collectively called the gametes
Reduces number of genes in gametes to 1/2 of parents, so called reduction
Chromosomes and Duplication
Chromosome Numbers and Sets
The DNA molecule in most things is quite large. Too large really. So it is broken up into
smaller fragments, each consisting of several thousand genes, called chromosomes (because they
are easy to color with stains). The number of chromosomes for each species is specific. Humans
have 46 chromosomes. However, they are not all different. Technically, we have duplicates of
all chromosomes, one inherited from each parent. We get half our DNA from each parent, but
not half our genes in a strict sense. The full complement, all the instructions for “human” will fit
onto 23 chromosomes. We just get a complete set of instructions from each parent, and chose to
follow Chapter one from one set and Chapters 3, 4, and 5 from the other set. Because of this,
scientists say humans have two sets of chromosomes, each set consisting of 23 chromosomes.
These are also setup in pairs, so we have 23 pairs of chromosomes, one from the pair from one
parent and one from the pair from the other parent. For example, let’s use m = mother and f =
father, we get:
Chromosomes
Im
If
IIm
IIf
IIIm
IIIf
IVm
IVf
Vm
Vf
VIm
VIf
VIIm
VIIf
.
.
.
XXIIIm
.
XXIIIf
So on and so on for 23 pairs. There are two chromosome number VII, one from each parent.
They will contain the same genes on them. So recap: humans have 46 chromosomes, divided
into 2 sets, each with 23 chromosomes. These can be paired based on similarity of genes, so we
have 23 pair.
Duplication: each daughter cell gets a full copy of genes
When a cell gets ready to divide, it carries out the most important thing it will ever do, and that is
copy all its chromosomes. We have many words for this: copy, duplicate, replicate.
Technically, it is called DNA Replication. This is to guarantee that each daughter cell gets a full
copy of the genetic information. It is done before the cell even begins dividing, sort of like
making a full backup of a computers hard-drive.
Duplicated Chromosome Structure: centromeres and sister chromatids
When the chromosomes are replicated, they still remain attached to one another at a small region
of unreplicated DNA called the centromere. These two pieces, exact copies of each other, are
together called a duplicated chromosome. Both pieces together will continue to be referred to as
one chromosome while they are attached at the centromere. Eventually, when the cell is
dividing, the last, central region of uncopied DNA will be replicated and the two copies will
separate. When this happens, both pieces will now be referred to as chromosomes, sort of like a
promotion. While attached and called the duplicated chromosome together, the two pieces of
DNA (original and copy) are called sister chromatids. For the original chromosome, if you think
about it, appears to be demoted. These duplicated chromosomes are what give the classic X
shape to chromosomes so common in photographs shown in textbooks.
The Cell Life Cycle
The cell life cycle covers from one division event to the next. Since the parent cell doesn’t die,
but doesn’t continue on, the cycle is shown as a circular diagram. You need to study a figure of
the cell life cycle, either in your book or find one from the internet!
Total cycle: Has 2 major parts or phases.
For more information on the cell life cycle and mitosis, refer to the addendum notes in this
section on the internet.
1. INTERPHASE
Has 3 Subphases
1) G (gap) one (G-1)
2) S (synthesis) phase (S-phase)
3) G (gap) two (G-2)
2. MITOTIC PHASE (mitosis + cytokinesis)
Has 4 Subphases (4 phases of mitosis)
1) Prophase
2) Metaphase
3) Anaphase
4) Telophase
- Then cytokinesis
Mitosis:
Prophase (early):
 Chromosomes coil up and become visible
 Centrioles start to move towards opposite ends of cell
 Mitotic spindle begins to form
Prophase (late):
 Centrioles finally at opposite ends of cell
 Nucleolus degenerates and disappears
 Nuclear envelope breaks down and disappears
 Rest of spindle forms
Metaphase:
 Chromosomes line up in a straight single row
 Centromeres of each chromosome attach to mitotic spindles from both sides
Anaphase:
 Centromeres finish duplicating and break apart, separating the sister chromatids of the
chromosome
 Spindles shorten, pulling sister chromatids toward opposite ends of the cell
Telophase:
 Chromosomes (previously sister chromatids) finish migrating to ends of cell
 New nuclear envelope forms around each batch of chromosomes
 Nucleoli reform inside each nucleus
 Eventually cytokinesis occurs
Differences between animal and plant mitosis:
1. Cytokinesis: animal cells develop a cleavage furrow or split around the equator of the cell
while plant cells stretch and form a new cell wall down the middle of the long cell
2. Plants lack centrioles, but still have a mitotic spindle which anchors to the cell wall
Homologues
Chromosomes in the body come in pairs, called homologous pairs or homologues
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One homologue from each pair comes from one parent while the other homologue
comes from the other parent (we discussed this earlier)
Homologues are similar in that they have the same genes at the same loci (location on
a chromosome), same overall shape, size and banding pattern. But they may contain
different alleles, or variants, for the genes.
Normal cells are diploid, or 2N, they have 2 sets of chromosomes
Sex cells, gametes (sperm and eggs), are haploid, or 1N, they have 1 set of chromosomes
Meiosis
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Used for sexual reproduction
Only one part of process; also requires gametogenesis and fertilization
Gametogenesis
Haploid
Gametes
Fusion
Meiosis
Adult
Mitosis
Fertilization
Diploid
Zygote
Process of Meiosis
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Reduction division (2N→1N) The number of chromosomes in the gametes or sex cells
are reduced to one half. A person cannot pass on all their chromosomes, that would be
too many. You can only pass on half. Meiosis prepares these cells for eventual
fertilization. Also, the chromosomes are not reduced by a random half, but specifically
one full set (chromosomes 1 – 23).
Most steps are similar to mitosis. You need to study mitosis and know it well. The
majority of functions are identical between the two, basic house keeping chores the cell
does before dividing and maintaining itself. If you know mitosis, meiosis will not be
much more difficult.
Consists of 2 separate divisions called
1. Meiosis I
2. Meiosis II
Meiosis itself refers to the whole event, in its entirety. It actually consists of two rapid,
back-to-back divisions, with no intervening space or Interphase. The DNA is NOT
replicated again. There is no pause. This is how we get four daughter cells from it. Even
meiosis follows the basic law that cells always split into two. It is just that in Meiosis we
have two divisions; as soon as one is complete, the second starts without resting between.
Each division has all four phases: PMAT
This means there are Prophase I and Prophase II, Metaphase I and Metaphase II, etc.
Reduction occurs in the first division, Meiosis I
Meiosis I
These notes, more than any other part of the course, must have a textbook accompanying them.
If I discussed it all, this printout would be 20-30 pages. You must read the chapter on mitosis
and meiosis. If there are questions, which there typically are, please ask me. After all, that is
what I get paid for.
Interphase:
 Same as mitosis, DNA replicates, cell prepares to divide
Prophase I:
 Most complex and longest phase of Meiosis
 Chromosomes condense and become visible
 Centrioles migrate towards poles of cell, spindle starts to form
 Nuclear envelope breaks down and disappears
 2 important events occur during prophase I:
1. Synapsis: homologous chromosomes come together as pairs and attach to one
another, resulting in a structure called a tetrad.
2. Crossing-over: non-sister chromatids (pieces) of homologous chromosomes of a
pair exchange segments. Since homologues have same genes, but maybe
different alleles, this creates novel gene combinations and increases genetic
diversity in the species by creating brand new chromosomes that are a
combination of both your parents. This way you pass on only one chromosome,
but may pass on to your children genes from both of your parents.
Metaphase I:
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Homologous chromosome pairs (tetrads) are moved to metaphase plane (equatorial
plane) and aligned in the middle of the cell
Chromosomes now in a double row aligned by pairs
Spindles attach to centromeres, one spindle to one centromere from either side
Anaphase I:
 Tetrads split up. Unlike mitosis, sister chromatids stay together.
 Homologous pairs split up and one entire chromosome migrates to poles of cell
 !! Actual reduction has occurred here!!
Telophase I and cytokinesis:
 Chromosomes arrive at poles
 Now haploid, though still in duplicate form
 Chromosomes uncoil, new nuclear membrane forms, new nucleoli form
 Cytokinesis finally takes place
 No DNA replication between Telophase I and Prophase II
Meiosis II
Essentially the same as mitosis. Meiosis II differs very little from the events that occur in
mitosis with the sole exception that the number of chromosomes are now half what they were to
begin with.
Prophase II
Metaphase II
Anaphase II
Telophase II (and cytokinesis)
Accidents of Meiosis
Non-Disjunction: members of a chromosome pair (tetrad) fail to separate during Anaphase I.
During non-disjunction, the tetrads fail to separate from one another, so both duplicated
chromosomes go to one side while no chromosome goes to the other side. This occurs during the
first meiotic division between homologous chromosomes, not between the two chromatids
making up one of the chromosomes.
Leads to aneuploidy and polyploidy
Aneuploidy = the loss or gain of a chromosome, though typically referring to the loss of one.
Polyploidy = the gaining of one or more extra chromosomes, typically more common.
Some examples of polyploidy and aneuploidy are:
Trisomy 21 = gaining an extra #21 chromosome, leads to Down’s Syndrome
Turner's Syndrome = woman who has inherited only one X chromosome instead of two
Klinefelter's Syndrome = Male with a Y chromosome, but with two X chromosomes
Forms of advanced Asexual Reproduction
These are forms of asexual reproduction on a much larger, multicellular level. Though not
defined here, if you are curious about what these terms are, you can use Google search or visit
www.dictionary.com.
In plants
stolons
grafting
spores
In animals
fission
regeneration
budding
Animal Life Cycle vs. Plant Life Cycle
Please refer to the figure in the textbook or from the internet.
(Note plant gametophyte and sporophyte generations)