Human Biology (BIOL 104)
Talk Eleven:
Cell Reproduction
Chapter 18
A Quick look at DNA
• DNA is organized into structures called chromosomes in
the nucleus of a cell.
• When a cell is not dividing, its threadlike chromosomes are
dispersed in the nucleus.
• As a cell prepares to divide, however, each chromosome is
copied and each copy is coiled and packed tightly.
•
Each copy is condensed as it coils back on itself again and
again
• The DNA loops around some proteins (called histones),
forming beadlike structures. The “beads” then coil up into
a long fiber.
Types of cells
• In biology, reproduction is when a “parent” cell produces a
new generation of cells, or when parents produce a new
individual.
• Reproduction is part of a life cycle, a recurring series of
events in which individuals grow, develop, maintain
themselves, and reproduce a new generation.
• The instructions for the human life cycle are encoded in our
DNA, which we inherit from our parents.
• Two types of cells:
– Somatic Cells
– Gametes
Somatic Cells
• Non-sex Cells.
• Diploid
• Contain a full compliment
of chromosomes
• Means double number.
• Characteristic to their
species.
• Designated 2n
• Referred to as the
diploid number of
chromosomes.
• All somatic cells in an
organism have the 2n or
diploid number of
chromosomes.
Somatic Cells
Human somatic cells have 46
chromosomes.
A The presence of pairs of
chromosomes (two of each type) tells
you that they came from a diploid
cell.
One member of each pair contains
genetic instructions inherited from
the father.
The other member contains
instructions from the mother.
B Close-up of a pair of homologous
chromosomes from an animal cell.
Gametes
• Sex Cells
• Haploid
• Means single number.
• Contain one half the
compliment of chromosomes
characteristic to their
•
species.
• Referred to as the haploid
number of chromosomes.
Designated n
• All gametes formed by an
organism have the n or
haploid number of
• chromosomes.
The Human cell cycle
• G0 phase
•
The term "post-mitotic" is sometimes
used to refer to both quiescent and
senescent cells. Nonproliferative cells
in multicellular eukaryotes generally
enter the quiescent G0 state from G1
and may remain dormant for long
periods of time, possibly indefinitely.
•
This is very common for cells that are
fully differentiated.
•
Cellular senescence occurs in response
to DNA damage or degradation that
would make a cell's progeny nonviable
The Human cell cycle
•
•
G1 phase
It is also called the growth phase.
•
During this phase the biosynthetic
activities of the cell, which had
been considerably slowed down
during M phase, resume at a high
rate.
•
This phase is marked by the use of
20 amino acids to form millions of
proteins and later on enzymes that
are required in S phase, mainly
those needed for DNA replication.
•
Duration of G1 is highly variable,
even among different cells of the
same species.
The Human cell cycle
•
•
S phase
Starts when DNA replication
commences; when it is complete, all
of the chromosomes have been
replicated, i.e., each chromosome
has two (sister) chromatids.
•
Thus, during this phase, the amount
of DNA in the cell has effectively
doubled, though the ploidy of the
cell remains the same.
•
During this phase, synthesis is
completed as quickly as possible
due to the exposed base pairs
being sensitive to external factors
–
ie - U.V, Heat, drugs
The Human cell cycle
• G2 phase
• During the gap between
DNA synthesis and
mitosis, the cell will
continue to grow.
• The G2 checkpoint
control mechanism
ensures that everything
is ready to enter the M
(mitosis/ Mieosis) phase
and divides
Mitosis or Meiosis
• For much of the cell division process, each chromosome and its copy
stay together.
• During this time they are called sister chromatids.
– A sister chromatid has at least one “pinched in” region called a centromere.
• It will provide attachment sites for microtubules that move
chromosomes when a cell nucleus is dividing.
Mitosis or Meiosis
• Mitosis maintains the
chromosome number.
• After mitosis is complete,
the parent cell’s cytoplasm
divides.
• Mitosis occurs in cells that
are dividing as the body
grows or repairs itself.
• It assures that each new
“daughter” cell is diploid,
with a full complement of
46 parental chromosomes.
Mitosis or Meiosis
• Meiosis occurs in germ cells in the ovaries and testes
• It halves the number of parental chromosomes
• Two rounds of division, called meiosis I and meiosis II produces gametes
that are haploid cells
• half the diploid number of chromosomes.
Stages of Division- Mitosis
• Prophase--nuclear envelope
breakdown, chromosome
condensation, spindle formation.
• Metaphase--chromosomes are
lined up precisely on the
metaphase plate, or middle of the
cell.
• Anaphase--spindle pulls sister
chromatids apart.
• Telophase--chromatids begin to
decondense and become
chromatin. Spindle disappears.
• Cytokinesis--divide cell and
organelles. Actin ring, or cleavage
furrow splits cell.
Mitosis Begins with Prophase
• During interphase each
chromosome was duplicated,
forming two sister chromatids
joined at the centromere.
• The sister chromatids of each
chromosome twist and fold into
a more compact form.
• By the end of prophase, all
chromosomes will be condensed
into thick rod shapes.
• Chromosomes move into new
positions with help of a spindle.
• This structure consists of
microtubules that extend from
centrioles
Moving from Prophase to Metaphase
• A lot happens between prophase
and metaphase—so much that this
transitional period has its own
name, “prometaphase.”
• The nuclear envelope breaks
apart, allowing the chromosomes in
the nucleus to interact with
microtubules extending toward
them from the poles of the
forming spindle.
• When all the duplicated
chromosomes are lined up midway
between the poles of a spindle, we
say the cell is in metaphase
(meta-means “midway between”).
– This alignment sets the stage
for anaphase
Anaphase, then Telophase
• During anaphase, the sister
chromatids of each chromosome
separate from each other and move
to opposite spindle poles.
• Once each chromatid is separated
from its sister, it is an independent
chromosome.
• Telophase begins as soon the two
clusters of chromosomes arrive at a
spindle pole.
• New nuclear envelope forms around
each cluster, separating it from the
cytoplasm.
• Once two nuclei form, telophase is
over—and so is mitosis
How the Cytoplasm Divides
• Cytokinesis, usually begins toward the end of anaphase.
• By this time, the two sister chromatids of each chromosome have been
separated and are independent chromosomes. About midway between
the cell’s two poles, a patch of plasma membrane sinks inward, forming
a cleavage furrow.
• Now there are two new cells, each with a nucleus, cytoplasm, and a
plasma membrane.
What is the Function of Meiosis?
• Meiosis is the first step in the
formation of gametes—sperm
and eggs—for sexual
reproduction.
• Meiosis reduces the parental
chromosome number by half, to
the haploid number.
• In males, meiosis and gamete
formation are called
spermatogenesis.
• In females, these two
processes are called oogenesis.
Meiosis – a two stage process
Pieces of Chromosomes
may be Exchanged
• No one ever looks, or has a
body that operates, exactly
like his or her parents.
• Most of the trait variations we
take for granted result from
changes to chromosomes.
• These occur during meiosis, when
germ cells were forming sperm
in a father’s testes or eggs in a
mother’s ovaries.
Pieces of Chromosomes
may be Exchanged
• Some genetic variations come about
during prophase I of meiosis.
• This is a time when parts of the
duplicated chromosomes—the
sister chromatids—are rearranged.
• These homologues therefore can be
called “maternal” and “paternal”
chromosomes and they carry genes
for the same traits.
• During meiosis I, the two pairs of
sister chromatids (one maternal,
the other paternal) line up very
closely.
Pieces of Chromosomes
may be Exchanged
• Crossing Over (Synapsis):
• Leads to variation in the details of
inherited traits because a gene may
have several chemical forms.
• Often, particular forms of genes on
one chromosome differ from
corresponding ones on its
homologue partner.
• When a crossover occurs between
chromosomes in a germ cell, both
then have a slightly different
version of their genes than they
had before.
Meiosis I
• The first stage of meiosis I is prophase I. During this
phase, the chromosomes condense, align, and exchange
segments. The centrosome is duplicated along with its two
centrioles.
– Crossing over (SYNAPSIS) occurs here!!!!!!
• Pairs of centrioles are positioned at the opposite sides of
the nucleus as the nuclear envelope breaks up (step 1).
• Microtubules of the spindle link the chromosomes to the
spindle poles so that each chromosome is attached to one
spindle pole, and its homologous partner is attached to the
other.
• At metaphase I, all the chromosomes are aligned midway
between the spindle poles (step 2).
Meiosis I
• In anaphase I, the spindle microtubules separate the
homologous chromosomes and pull them toward opposite
spindle poles (step 3).
• During telophase I, the chromosomes reach the spindle
poles (step 4). New nuclear envelopes form around the two
clusters of chromosomes.
• Each of the two haploid nuclei that form contains one set of
duplicated chromosomes. The cytoplasm divides at this point
(by cytokinesis), forming two haploid cells, but the cells’
DNA is not copied before meiosis II begins.
First stage of Meiosis
Things to remember!!!!
• The first meiotic division effectively ends when the
chromosomes arrive at the poles.
• Each daughter cell now has half the number of chromosomes
but each chromosome consists of a pair of chromatids, .
• Remember, Cytokinesis completes the creation of two
daughter cells.
– Sister chromatids remain attached during telophase I
• So, as there is no replication of DNA before prophase II,
the separation of chromosomes/chromatids result in four
haploid, genetically different cells.
Meiosis II
• In prophase II No more crossing over, and no duplication
of chromosomes (step 5)
• In metaphase II, the same events occur in each new nucleus
as in metaphase I (step 6).
• In anaphase II, the spindle microtubules pull the sister
chromatids apart (step 7).
– Each chromosome now consists of one DNA molecule.
• During telophase II, the chromosomes cluster around each
spindle pole. There are four clusters in all.
• Each one is a set of unduplicated chromosomes.
• New nuclear envelopes form around the four clusters,
forming four haploid nuclei—two in each cell (step 8).
– Cytokinesis then divides the cytoplasm of each cell, producing
four haploid cells.
Second stage of Meiosis
What are the Key Events of
Meiosis?
• In meiosis the nucleus of a diploid germ cell divides twice.
Ultimately four haploid nuclei form, each with 23
chromosomes—one of each type.
• Meiosis takes place in two steps, called meiosis I and
meiosis II.
• Similar to mitosis, each step occurs in stages called
prophase, metaphase, anaphase and telophase.
• Cytokinesis occurs at the close of both meiosis I and
meiosis II. The result is four haploid cells.
Sexual life cycles
• Haploid Gametes join to form a zygote
• Somatic cells divide by Mitosis to produce adult organism
• Meiosis produces gametes in sex cells
Human variation
• A grand total of 223, or
8,388,608, combinations of
maternal and paternal
chromosomes are possible
every time meiosis in a germ
cell produces a gamete!
• This is why striking mixes of
traits can show up even in
the same family
Summary
• Somatic Cells - Non-sex Cells. Contain a
full compliment of chromosomes.
Characteristic to their species. Referred to
as the diploid number of chromosomes.
• Gametes - Sex Cells. Cell which carry
genetic information for sexual reproduction.
Contain one half the compliment of
chromosomes characteristic to their species
.Referred to as the haploid number of
chromosomes.
The End.
Any Questions?