Mitosis and the Cell Cycle
I. Reproduction occurs in cells as well as in multicellular organisms.
A. All organisms begin life as a single cell.
1. When single-celled organisms divide, they
form new organisms.
2. Cell division is part of the growth process
in multicellular organisms, as well as the agent of
tissue renewal and repair.
B. New cells come only from preexisting cells, as
the cell theory states.
C. Cells divide to remain small, since a smaller
cell has a more effective plasma membrane-to-volume ratio and all materials
that enter or leave the cell must pass through the plasma membrane.
D. In multicellular
organisms, cell division allows the production of specialized cells for various
functions by passing on copies of the chromosomes that contain the information
for all parts of the organism.
II. Prokaryotic chromosomes and cell division
A. Prokaryotic chromosomes are present in a
single copy per bacterial cell.
1. There is no nucleus, but the chromosome is
compacted into an electron-dense, irregularly shaped region called the nucleoid that is not bounded by a membrane.
2. The bacterial chromosome is a circular loop
about 500 times the length of the cell, attached to the inside of the plasma
membrane.
3. The bacterial chromosome lacks associated
proteins and consists of just DNA.
B. Cell division in prokaryotes is by binary
fission, producing 2 daughter cells identical to the original parent cell.
1. Before division, the cell replicates its DNA
to make 2 chromosomes that are attached to the inside of the plasma membrane.
2. The chromosomes separate by elongation of the
cell which pushes the chromosomes apart.
3. When the cell is twice its original length,
the plasma membrane grows inward and a cell wall forms, dividing the cell in
half and forming 2 daughter cells.
4. A prokaryotic cell may have several nucleoids since the replication process is occurring at
several stages at once, so that a third chromosome starts to develop before the
first 2 are separated from each other.
III. Eukaryotic chromosomes and cell division
A. Eukaryotic chromosomes have a true nucleus
bounded by a double membrane, the nuclear envelope, that
contains pores that allow substances to pass between the nucleus and cytoplasm.
1. Chromatin is the tangled mass of DNA and
associated protein within the nucleus.
2. Chromatin condenses into chromosomes at the
time of division, as DNA coils tightly.
3. The nucleus contains at least one nucleolus
formed by special regions of particular chromosomes and involved in the
production of ribosomes.
4. It is possible to photograph and count
chromosomes during cell division.
a) There are 46 chromosomes in humans.
b) This is the full diploid (2N) number for
humans, in nearly all cells of the body, containing 2 copies of each kind of
chromosome.
c) Half the diploid number is the haploid (N)
number, with only one copy of each kind of chromosome, found in sperm and egg
cells in humans and most other animals.
B. Cell division in eukaryotes involves nuclear
division and cytokinesis (division of the cytoplasm).
1. There are 2 types of nuclear division,
mitosis and meiosis.
2. Mitosis is a nuclear division in which the
number of chromosomes remains constant.
a) If a parent cell is 2N, the daughter cells
are also 2N.
b) Mitosis is involved in growth and repair of
the body.
3. Meiosis is nuclear division where the
chromosome number is halved.
a) A 2N parent cell produces daughter cells that
are N.
b) Meiosis produces gametes, the egg cells and
sperm.
c) Fertilization joins the egg and sperm and
restores the 2N number of chromosomes.
4. DNA replicates before nuclear division
occurs, so that each chromosome is duplicated and has 2 identical parts called
sister chromatids.
a) Sister chromatids are identical, with the
same genes.
b) They are constricted and held together at a
region called the centromere.
5. There are 4 stages of nuclear division.
a) In one stage, the centromeres
divide and each duplicated chromosome gives rise to 2 daughter chromosomes, one
from each chromatid.
b) Each daughter cell produced by cell division
gets one copy of the genes from the parent cell, a complete copy of the genetic
material of the organism.
IV. The cell cycle has 4 phases for growth and
division.
A. The period of the cell cycle between cell
divisions was not at first recognized as anything more than a resting stage
between divisions, called interphase.
1. It was discovered in the 1950s that DNA
replicated during this period, and the concept of the cell cycle emerged.
2. The phase of cell division, including mitosis
and cytokinesis, is called the M phase.
3. The period of DNA synthesis during interphase is called the S phase of the cell cycle, and
proteins associated with DNA are also made at this time.
4. Following S phase and before M phase is G1
phase.
5. Following M phase and before the next S phase
is G2 phase.
6. When first named, not much was known about G1
and G2 phases, and they were thought of as gap phases.
7. During G1 the cell grows in size and
synthesizes organelles, while during G2 enzymes and other proteins for mitosis
are produced.
8. Interphase consists
of G1, S, and G2 phases.
9. Cells that temporarily or permanently leave
the cell cycle are said to be in G0 phase.
a) This can arise through cell specialization.
b) Liver cells specialize but can reenter the
cell cycle to repair injury.
c) Muscle and nerve cells of animals and
sugar-transport cells of plants permanently lose the ability to divide.
B. Control of the cell cycle
1. One thing that affects whether a cell
continues to divide or not is outside influences.
a) Cells contain surface receptors for growth
factors.
b) Normal cells of multicellular
animals divide only if they are adhering to a solid surface, since the
cytoskeleton cannot function properly without a base.
c) When normal cells contact neighboring cells,
they stop dividing; cancer cells can continue to divide even with such contact.
2. Internal factors within the cytoplasm can
also cause the cell to divide.
a) There is direct control, since the first
events such as protein synthesis must be completed before division can take
place.
b) Indirect control seems to be exerted at 2
critical “checkpoints” during interphase.
(1) When G1 becomes S and synthesis of DNA occurs,
apparently due to a substance in the cytoplasm at that time.
(2) When G2 becomes M and mitosis begins, which
requires a maturation promoting factor (MPF) in fertilized frog eggs.
(3) cdc2 protein and cyclin protein are in MPF and are involved in the actions
leading to mitotic activity.
(4) The cdc2 gene is remarkably similar in all
eukaryotes, from yeasts to humans.
V. Mitosis is division of the eukaryotic nucleus
so that daughter cell nuclei have the same number and kinds of chromosomes as
in the parent cell nucleus.
A. At the beginning of mitosis, each chromosome
has 2 identical chromatids, while at the end there are twice as many
chromosomes and each has only one chromatid.
1. The mitotic spindle is a microtubular
structure that forms early in mitosis and produces the orderly distribution of
chromosomes to daughter cells.
2. Prokaryotic cells do not use a spindle during
binary fission.
3. Mitosis is also called karyokinesis.
4. There are 5 stages of mitosis: prophase, prometaphase,
metaphase, anaphase, and telophase.
B. Mitosis in animal cells
1. Prophase is the stage when the chromatin
condenses and chromosomes become visible structures that contain 2 chromatids
each.
a) DNA has already replicated, producing the 2
chromatids.
b) The nucleolus disappears.
c) The nuclear envelope fragments.
d) The spindle begins to assemble as pairs of
centrioles move to opposite poles.
e) Microtubule organizing centers (MTOC) contain
centrioles in animal cells (but not in plant cells), and these serve as the
center to organize the spindle.
f) Short rays of microtubules called asters
form around the centrioles.
2. Metaphase is the second stage of mitosis.
a) Fragmentation of the nuclear envelope and
extension of polar microtubules are continuing.
b) Kinetochores
develop on either side of each chromosome’s centromere, and microtubules
(kinetochore fibers) extend toward the pole from each kinetochore.
c) Chromosomes line up at the equator of the
cell as a result of interaction of kinetochore fibers and spindle microtubules
attached to the poles.
3. Anaphase begins with the division of the centromeres that hold sister chromatids together.
a) The separated chromatids are now known as
daughter chromosomes, each of which has only one chromatid.
b) The mechanism of centromere division is not
known but may just be a region where DNA has not yet replicated, and the
replication is completed at this stage.
c) Apparently 2 processes account for the
movement of daughter chromosomes toward opposite poles.
(1) Polar spindle fibers lengthen and push the
chromosomes away from the equator.
(2) Kinetochore fibers shorten and pull the chromosomes
toward the poles.
d) As the chromosomes reach the poles, cytokinesis begins.
4. Telophase rebuilds the nuclear envelopes
around the daughter chromosomes.
a) Chromosomes become diffusely distributed into
chromatin again.
b) The spindle apparatus breaks down.
c) The nucleolus reforms in each daughter
nucleus.
d) Cytokinesis
continues and cell division is completed.
5. Cytokinesis (cytoplasmic cleavage) usually accompanies mitosis in animal
cells.
a) A cleavage furrow, an indentation of the
plasma membrane, forms between the daughter nuclei, beginning near the end of
anaphase.
b) The furrow deepens as actin
filaments constrict in a band called the contractile ring, which soon separates
the cytoplasm into 2 daughter cells.
C. Mitosis in plant cells has exactly the same
stages but has no centrioles or asters.
1. Meristem tissue,
found in the root and shoot tissues and stems of plants, retains the ability to
divide throughout the life of the plant.
2. Cytokinesis in
plant cells occurs by a different process than that in animals.
a) Instead of furrowing, vesicles derived from
the Golgi apparatus fuse at the equator to form a
cell plate.
b) The cell plate forms the plasma membranes of
both daughter cells and releases substances that form the plant cell walls.
VI. Importance of binary fission and mitosis
A. These processes assure that daughter cells
receive equal amounts of genetic material and cytoplasm.
B. Binary fission does not utilize a spindle for
chromosome separation, while mitosis does.
C. Evolution of the spindle
1. Primitive eukaryotes such as dinoflagellates and fungi suggest possible steps in the
development of mitosis.
2. Division of chromosomes may have been similar
to fission but with the chromosomes attached to the nuclear envelope rather
than to the plasma membrane.
3. Microtubules may have gotten involved to
support the nuclear membrane and later took over the role of chromosome
attachment completely, since the nuclear membrane has no role in mitosis now.
D. Asexual versus sexual reproduction
1. Reproduction of the organism by binary
fission or mitosis is referred to as asexual reproduction.
2. While some multicellular
forms reproduce asexually, in most mitosis is the process of growth and repair
of tissues, and sexual reproduction of the organism occurs through meiosis and
fertilization.
I. Sexual reproduction involves gamete
formation and fusion in fertilization to form a zygote.
A. Gametes must not contain the same number of
chromosomes as other body cells, or joining together 2 of them would double the
chromosome number.
B. Meiosis reduces the chromosome number from
diploid (2N) to haploid (N).
C. Meiosis occurs at different points during the
life cycle of various kinds of organisms.
1. In animals, meiosis occurs during gamete
production.
2. In plants, meiosis produces spores that
divide mitotically to form haploid plants that
produce gametes.
3. In fungi and some algae, meiosis occurs
directly after zygote formation, and the adult is always haploid, producing
gametes by mitosis.
D. All 3 different kinds of life cycles have a
diploid stage and a haploid stage.
1. In animals, the adult is diploid.
2. In plants, there is a diploid adult and a
haploid adult.
3. In fungi and some algae, the adult is
haploid.
II. Overview of Meiosis
A. In a diploid cell, chromosomes come in pairs
called homologous chromosomes, or homologues.
1. Homologues look alike and carry genes for the
same traits.
2. One member of each pair of homologues was
contributed by the male gamete and one by the female gamete that joined to
produce the diploid organism.
B. Meiosis requires 2 nuclear divisions and
produces 4 haploid daughter cells.
1. Each daughter cell receives one of each kind
of chromosome.
2. Each daughter cell is thus haploid, while the
parent cell was diploid.
3. The 2 nuclear divisions are meiosis I and
meiosis II.
4. Prior to meiosis, DNA replication has
occurred and the chromosomes have 2 chromatids each, so meiosis I begins with 2 copies of DNA for each chromosome.
5. In meiosis I, the number of chromosomes is
reduced, and in meiosis II the chromatids separate to give the daughter
chromosomes.
6. In meiosis I, the homologues come together
and pair, forming a bivalent or tetrad, with 2 chromosomes and 4 chromatids.
a) Crossing-over of genetic material may occur
between nonsister chromatids of the tetrad.
b) Homologues exchange genetic material, making
them no longer identical.
c) Homologues undergoing crossing-over interact
with the spindle as if they were a single chromosome and separate from the
tetrad in meiosis I to allow only one copy of each kind of gene to be present
in each daughter cell.
d) There is no restriction on where each
homologue goes, since each homologous pair separates independently of all other
pairs.
7. DNA replication does not occur between
meiosis I and meiosis II.
8. During meiosis II, the centromeres
divide and a haploid set of chromosomes moves toward each pole.
9. At the completion of meiosis II, there are 4
daughter cells with a haploid number of chromosomes.
C. Meiosis I has the same 4 stages as mitosis, but
with a difference in prophase I.
1. Prophase I contains
chromosomes with 2 chromatids, since DNA replication has already occurred.
a) The homologous chromosomes pair up during synapsis.
b) A nucleoprotein lattice (synaptonemal complex) forms between the homologues,
holding the bivalent members so that their DNA is aligned. At this point, the two homologous chromosomes
(4 chromatids) form the bivalent,
also called a tetrad.
c) Crossing-over occurs, exchanging genetic
material between nonsister chromatids of opposite
homologues. Prophase I, and later
Anaphase II, result in the most genetic variation in the daughter cells of
meiosis.
d) The lattice then starts to break down,
allowing homologues to move away from each other slightly, with chiasmata still
holding them together at the sites of crossing-over.
e) Spindle fibers form as the centriole pairs move apart, the nuclear envelope fragments,
and the nucleolus disappears, as the chromatin condenses into chromosomes.
f) Prophase I is
the longest stage of meiosis, taking up most of the time required for the
process.
2. Metaphase I has the
chromosome bivalents arranged along the equator.
a) Kinetochore
fibers extend from only one side of the centromere that holds the sister
chromatids together.
b) The bivalent is held together by chiasmata.
c) The bivalents are arranged randomly with
respect to each other and to the pole they face, so that chromosomes assort
independently of one another.
3. Anaphase I begins
when the chiasmata disappear as crossing-over is
completed, so the homologues separate from each other as the bivalent is
disrupted.
a) The chromosome number is reduced from the
diploid to the haploid number.
b) At the end of anaphase I, each chromosome
still contains 2 chromatids.
c) Movement of chromosomes is due to a push
provided by spindle elongation and a pull due to kinetochore disassembly.
4. Telophase I may not
be present in some species.
a) If it is present, the nuclear envelope
reforms and nucleoli reappear, possibly with cytokinesis
and daughter cell formation.
b) In some species, this phase is omitted.
D. Interkinesis
is similar to interphase between mitotic divisions,
except that DNA replication does not
occur, since the chromosomes are already duplicated.
E. Meiosis II contains prophase II, metaphase II,
anaphase II, and telophase II.
1. It varies considerably between species.
2. Meiosis II is essentially similar to
mitosis, since the chromatids separate at anaphase and move to opposite
poles at telophase.
3. At the end of telophase II, following cytokinesis, there are haploid cells.
4. The daughter cells are not genetically
identical to the parent cell because they are haploid and they have different
genetic compositions because of crossing-over.
5. Genetic recombination has occurred.
6. Haploid cells become gametes in animals, form
spores that mature into haploid adults in plants, and in fungi and some algae
they form the main part of the life cycle.
F. Meiosis in humans is separated into the germ
line, the cells that produce the gametes.
1. Oogenesis is the process of producing egg cells
in the ovaries of the female.
a) Oogenesis produces
2 cells in the first meiotic division, one of which is considerably smaller
than the other.
b) The smaller cell is called a nonfunctional polar
body and remains attached to the larger cell, the oocyte.
c) The oocyte matures
into the egg.
d) The second meiotic division does not occur
unless the egg is fertilized.
e) If the second division does occur, one
haploid egg cell and at least 2 nonfunctional polar bodies are formed.
f) The egg is very large and contributes most
of the cytoplasm and nutrients for the zygote, which results from
fertilization.
2. Spermatogenesis
is the process of producing sperm cells in the testes of the male.
a) Four viable sperm cells can result
from each original cell that enters meiosis.
b) The sperm is a tiny flagellated cell that
swims toward the egg.
c) When fertilization occurs, the resultant
zygote develops into the new individual.
III. Importance of meiosis
A. It provides a way to keep the chromosome
numbers constant from one generation to the next.
1. Each daughter cell receives one copy of each
kind of chromosome.
2. This ensures that each daughter cell gets one
copy of each gene.
B. It helps ensure that genetic recombination
occurs with each generation.
1. There is independent assortment of
chromosomes, as they are distributed to daughter cells in various combinations.
2. Variations are also introduced by crossing-over between
homologues.
3. Fertilization assures that chromosomes from
different parents are combined to form a new combination in the zygote.
4. The possible combinations in a human are
staggering.
C. Direct testing of diploids by the environment is
not possible, but haploid adults, as in plants and fungi, are tested for any
variability by the environment.
D. Sexually reproducing populations have a
storehouse of genetic recombinations that may be
advantageous in evolution, especially when the environment is changing.
E. Asexually reproducing populations like
prokaryotes must depend mainly on mutation to generate variation,
but this is sufficient since they reproduce so frequently.
F. Mutation also provides the material of
variation in sexually reproducing organisms, but sexual reproduction allows
shuffling of the genetic material that allows more rapid adaptation to a
changing environment.
IV. Comparison of meiosis and mitosis
A. Occurrence
1. Meiosis occurs only at certain times in the
life cycle.
2. Mitosis is more common because it allows
growth and repair of body tissues.
B. Process
1. DNA is replicated once before mitosis and
once before meiosis, but there are 2 nuclear divisions in meiosis and only one
in mitosis.
2. Homologous chromosomes pair and undergo
crossing-over in prophase I of meiosis but not in mitosis.
3. Paired homologous chromosomes (bivalents)
align at the equator in meiosis, while individual (duplicated) chromosomes
align at the equator in mitosis.
4. Homologous chromosomes (with centromeres intact) move to opposite poles at anaphase I in
meiosis, while centromeres divide and daughter
chromosomes move to opposite poles in anaphase of mitosis.
C. Daughter nuclei and cells
1. Four daughter cells are produced from a
single cell in meiosis, while 2 daughter cells are produced in mitosis.
2. Daughter cells are haploid in meiosis and
diploid in mitosis.
3. Daughter cells in meiosis are not genetically
identical to each other or to the parent cell, while daughter cells from
mitosis are genetically identical to each other and to the parent cell.