Chapter 2. Mitosis and Meiosis

Chromosome Theory of Heredity What structures within cells correspond to genes? The development of genetics took a major step forward by accepting the notion that the genes are parts of specific cellular structures, the chromosomes.

Chromosomes contain a very long DNA molecule with thousands of genes. Individual chromosomes are only visible during cell division. They are packaged as chromatin. Before a cell starts dividing, the chromosomes are duplicated. This process produces sister chromatids. Sister chromatids Centromere

Each duplicated chromosome consists of two sister chromatids which contain identical copies of the chromosome s DNA. As they condense, the region where the strands connect shrinks to a narrow area, is the centromere. Later, the sister chromatids are pulled apart and repackaged into two new nuclei at opposite ends of the parent cell.

Binary Fission of a Prokaryotic Cell Prokaryotic chromosome Duplication of chromosome and separation of copies Plasma membrane Cell wall Prokaryotic chromosomes Continued growth of the cell and movement of copies Division into two cells

Budding Yeast

Eukaryotic Cell Cycle and Mitosis A eukaryotic cell has many more genes than a prokaryotic cell. The genes are grouped into multiple chromosomes, found in the nucleus.

The ability of organisms to reproduce their kind is one characteristic that best distinguishes living things from nonliving matter. The continuity of life from one cell to another is based on the reproduction of cells via cell division. This division process occurs as part of the cell cycle, the life of a cell from its origin in the division of a parent cell until its own division into two. The division of a unicellular organism reproduces an entire organism, increasing the population.

Cell division on a larger scale can produce progeny for some multicellular organisms. Cell division is also central to the development of a multicellular organism that begins as a fertilized egg or zygote. Multicellular organisms also use cell division to repair and renew cells that die from normal wear and tear or accidents. Cell division requires the distribution of identical genetic material (DNA) to two daughter cells. A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and then splits into two daughter cells.

Cell Division Distributes Identical Sets of Chromosomes to Daughter Cells A cell s genetic information, packaged as DNA, is called its genome. In prokaryotes, the genome is often a single long DNA molecule. In eukaryotes, the genome consists of several DNA molecules. A human cell must duplicate about 2m of DNA and separate the two copies such that each daughter cell ends up with a complete genome.

DNA molecules are packaged into chromosomes. Every eukaryotic species has a characteristic number of chromosomes in the nucleus. Human somatic cells (body cells) have 46 chromosomes. Human gametes (sperm or eggs) have 23 chromosomes, half the number in a somatic cell.

Each eukaryotic chromosome consists of a long, linear DNA molecule. Each chromosome has hundreds or thousands of genes, the units that specify an organism s inherited traits. Associated with DNA are proteins that maintain its structure and help control gene activity. This DNA-protein complex, chromatin, is organized into a long thin fiber. After the DNA duplication, chromatin condenses, coiling and folding to make a smaller package.

The process of the formation of the two daughter nuclei, mitosis, is usually followed by division of the cytoplasm, cytokinesis. These processes take one cell and produce two cells that are the genetic equivalent of the parent. Each of us inherited 23 chromosomes from each parent: one set in an egg and one set in sperm. The fertilized egg or zygote underwent trillions of cycles of mitosis and cytokinesis to produce a fully developed multicellular human.

These processes continue every day to replace dead and damaged cell. Essentially, these processes produce clones - cells with the same genetic information. In contrast, gametes (eggs or sperm) are produced only in gonads (ovaries or testes). In the gonads, cells undergo a variation of cell division, meiosis, which yields four daughter cells, each with half the chromosomes of the parent. Fertilization fuses two gametes together and doubles the number of chromosomes to 46 again.

Cell Cycle Mitosis: nuclear division associated with the somatic cell division Meiosis: nuclear division associated with the gametic cell division

The Mitotic Phase Alternates with Interphase in the Cell Cycle The mitotic (M) phase of the cell cycle alternates with the much longer interphase. The M phase includes mitosis and cytokinesis. Interphase accounts for 90% of the cell cycle.

During interphase the cell grows by producing proteins and cytoplasmic organelles, copies its chromosomes, and prepares for cell division. Interphase has three subphases: the G 1 phase ( first gap ) centered on growth, the S phase ( synthesis ) when the chromosomes are copied, the G 2 phase ( second gap ) where the cell completes preparations for cell division, and divides (M). The daughter cells may then repeat the cycle.

Mitosis is a continuum of changes. For description, mitosis is usually broken into five subphases: Prophase Prometaphase Metaphase Anaphase Telophase

By late interphase, the chromosomes have been duplicated but are loosely packed. The centrosomes have been duplicated and begin to organize microtubules into an aster ( star ).

In prophase, the chromosomes are tightly coiled, with sister chromatids joined together. The nucleoli disappear. The mitotic spindle begins to form and appears to push the centrosomes away from each other toward opposite ends (poles) of the cell.

During prometaphase, the nuclear envelope fragments and microtubules from the spindle interact with the chromosomes. Microtubules from one pole attach to one of two kinetochores, special regions of the centromere, while microtubules from the other pole attach to the other kinetochore.

The spindle fibers push the sister chromatids until they are all arranged at the metaphase plate, an imaginary plane equidistant between the poles, defining metaphase.

At anaphase, the centromeres divide, separating the sister chromatids. Each is now pulled toward the pole to which it is attached by spindle fibers. By the end, the two By the end, the two poles have equivalent collections of chromosomes.

At telophase, the cell continues to elongate as free spindle fibers from each centrosome push off each other. Two nuclei begin to form, surrounded by the fragments of the parent s nuclear envelope. Chromatin becomes less tightly coiled. Cytokinesis, division of the cytoplasm, begins.

Meiosis Reduces the Number of Chromosome Sets from Diploid to Haploid n is the number of chromosomes in a set. Cells with only one set (n) of chromosomes are called haploid. Cells with two sets (2n) of chromosomes are called diploid.

Like mitosis, meiosis is preceded by the replication of chromosomes. Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II. The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis. Each daughter cell has only half as many chromosomes as the parent cell.

The Stages of Meiosis In the first cell division (meiosis I), homologous chromosomes separate. Meiosis I results in two haploid daughter cells with replicated chromosomes; it is called the reductional division. In the second cell division (meiosis II), sister chromatids separate. Meiosis II results in four haploid daughter cells with unreplicated chromosomes; it is called the equational division.

Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I Meiosis II 1 Homologous chromosomes separate Haploid cells with replicated chromosomes 2 Sister chromatids separate Haploid cells with unreplicated chromosomes

Meiosis I Division in meiosis I occurs in four phases: Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis

Prophase I Prophase I typically occupies more than 90% of the time required for meiosis. Chromosomes begin to condense. In synapsis, homologous chromosomes loosely pair up, aligned gene by gene. In crossing over, nonsister chromatids exchange DNA segments. Each pair of chromosomes forms a tetrad, a group of four chromatids. Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred.

Leptotene stage: The chromatin begins to condense and the chromosomes become visible. Homology search begins during this stage. Zygotene stage: The chromosomes continue to shorten and thicken during this stage. Homologous chromosomes undergo initial alignment with one another. Pachytene stage: The chromosomes continue to coil and shorten during this stage. Diplotene stage: During this stage, each tetrad consists of two pairs of sister chromatids. The result of crossing over is visible only when the duplicated chromosomes begin to separate. Diakinesis: The final stage of prophase I. The nucleolus and nuclear envelope break down, and the two centromeres of each tetrad attach to the spindle fibers.

Metaphase I In metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole. Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad. Microtubules from the other pole are attached to the kinetochore of the other chromosome.

Prophase I Metaphase I Sister chromatids Centrosome (with centriole pair) Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore

Anaphase I In anaphase I, pairs of homologous chromosomes separate. One chromosome moves toward each pole, guided by the spindle apparatus. Sister chromatids remain attached at the centromere and move as one unit toward the pole.

Telophase I and Cytokinesis In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids. Cytokinesis usually occurs simultaneously, forming two haploid daughter cells.

Anaphase I Telophase I and Cytokinesis Sister chromatids remain attached Homologous chromosomes separate Cleavage furrow

Meiosis I Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Sister chromatids Centrosome (with centriole pair) Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Sister chromatids remain attached Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore Homologous chromosomes separate Cleavage furrow

Division in meiosis II also occurs in four phases: Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Meiosis II is very similar to mitosis.

Prophase II In prophase II, a spindle apparatus forms. In late prophase II, chromosomes (each still composed of In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate.

Metaphase II In metaphase II, the sister chromatids are arranged at the metaphase plate. Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical. The kinetochores of sister chromatids attach to microtubules extending from opposite poles.

Anaphase II In anaphase II, the sister chromatids separate. The sister chromatids of each chromosome now move as The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles.

Telophase II and Cytokinesis In telophase II, the chromosomes arrive at opposite poles. Nuclei form, and the chromosomes begin decondensing. At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes. Each daughter cell is genetically distinct from the others and from the parent cell.

Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Sister chromatids separate Haploid daughter cells forming

Accidents during meiosis can alter chromosome number Nondisjunction is the failure of chromosomes or chromatids to separate during meiosis During Meiosis I: Both members of a homologous pair go to one pole During Meiosis II: Both sister chromatids go to one pole Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes

Down Syndrome

Nondisjunction in Meiosis I Nondisjunction in meiosis I Normal meiosis II Gametes n + 1 n + 1 n 1 n 1 Number of chromosomes

Nondisjunction in Meiosis II Normal meiosis I Nondisjunction in meiosis II Gametes n + 1 n 1 n n Number of chromosomes

Abnormalities of Sex Chromosome Number in Humans

A Comparison of Mitosis and Meiosis Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell. Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell. The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis.

MITOSIS MEIOSIS Parent cell Chiasma MEIOSIS I Prophase Chromosome replication Chromosome replication Prophase I Replicated chromosome 2n = 6 Homologous chromosome pair Metaphase Metaphase I Anaphase Telophase Daughter cells of meiosis I Anaphase I Telophase I Haploid n = 3 2n 2n MEIOSIS II Daughter cells of mitosis n n n Daughter cells of meiosis II n

Three events are unique to meiosis, and all three occur in meiosis l: Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information. At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes. At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate.

Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4

Prophase I of meiosis Pair of homologs Nonsister chromatids held together during synapsis Chiasma Centromere Anaphase I Anaphase II Daughter cells Recombinant chromosomes

P Gene loci a B Dominant allele Genotype: P PP Homozygous for the dominant allele a aa Homozygous for the recessive allele b Bb Heterozygous Recessive allele