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.

 

Meiosis—The formation of haploid gametes (sex cells)

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.