Chapter 5: Cell Division

Cell Increase and Decrease Cell division increases the number of somatic (body) cells, and consists of: Mitosis (division of nucleus) Cytokinesis (division of cytoplasm) Apoptosis (cell death) decreases the number of cells. Cell division occurs throughout life. Cell division replaces worn out or damaged cells, and heals wounds. Apoptosis prevents a tumor from occurring. Cell division and apoptosis are two opposing processes that keep the number of healthy cells in balance.

The cell cycle Most of the cell cycle is spent in interphase. Fig. 5.1 Most of the cell cycle is spent in interphase. Following interphase, the mitotic stage of cell division occurs. Cells go through a cycle that consists of four stages: G1, S, G2, and M. During G1, growth occurs as organelles double. During the S stage, DNA replication occurs as chromosomes duplicate. During the G2 stage, growth occurs as the cell prepares to divide. During the M stage, mitosis and cytokinesis occur.

The stages of interphase G1 stage – cell growth, # of organelles doubles S stage – DNA synthesis and replication occurs G2 stage – protein synthesis for cell division The amount of time a cell spends in interphase varies widely. Interphase lasts about 20 hours in mammalian cells, but embryonic cells complete the cell cycle in a few hours.

The Cell Cycle: G1  S  G2  M Fig. 5.1 Following interphase is the M stage, including mitosis and cytokinesis. The cell cycle ends when cytokinesis, the cleaving of the cytoplasm, is complete. Cells go through a cycle that consists of four stages: G1, S, G2, and M. During G1, growth occurs as organelles double. During the S stage, DNA replication occurs as chromosomes duplicate. During the G2 stage, growth occurs as the cell prepares to divide. During the M stage, mitosis and cytokinesis occur. Interphase Mitosis and cytokinesis The Cell Cycle: G1  S  G2  M

The cell cycle is controlled at three checkpoints. Fig. 5.1 G1 checkpoint G2 checkpoint M checkpoint The cell cycle is controlled at three checkpoints. The barriers in this figure represent three checkpoints when the cell cycle either stops or continues on, depending on the internal signal it receives. Researchers have identified a signal called cyclin that increases and decreases as the cell cycle continues. Cyclin has to be present for the cell to proceed from the G2 stage to the M stage, and for the cell to proceed from the G1 stage to the S stage. At the G1 checkpoint, apoptosis can occur if DNA is damaged. At the G2 checkpoint, mitosis will not occur if DNA is damaged or not replicated. At the M checkpoint, mitosis stops if chromosomes are not properly aligned. In mammalian cells, the p53 protein stops the cycle at the G1 checkpoint when DNA is damaged. It attempts repair, but will initiate apoptosis when repair is not possible. Many kinds of tumors lack an active p53 gene. DNA damage can stop the cell cycle at the G1 and G2 checkpoint. If chromosomes are not properly aligned, cell cycle stops at the M stage

Apoptosis Apoptosis - programmed cell death. - occurs because of two sets of enzymes called capsases. “initiators” - receive a signal to activate the “executioners”. Executioners activate enzymes that tear apart the cell and its DNA.

Maintaining the Chromosome Number During interphase, the DNA and associated proteins is are called chromatin. During Mitosis , the chromatin condenses to form highly compacted structures called chromosomes.

Overview of Mitosis The haploid (n) number of chromosomes = the number of kinds of chromosome. The diploid (2n) number of chromosomes = two chromosomes of each kind. Humans have 23 types (haploid) of chromosomes, so we have a total pf 46 chromosomes (diploid).

DNA replication takes place before nuclear division occurs. Page 85 A duplicated chromosome is made of two sister chromatids held together at the centromere. During mitosis, the centromeres divide and the sister chromatids become daughter chromosomes. genetically identical.

Mitosis overview Diploid Fig. 5.3 Following DNA replication during interphase, each chromosome in the parental nucleus is duplicated and consists of two sister chromatids. During mitosis, the centromeres divide and the sister chromatids separate, becoming daughter chromosomes that move into the daughter nuclei. Therefore, daughter cells have the same number and kinds of chromosomes as the parental cell. In this figure, the blue chromosomes were inherited from one parent, and the red chromosomes were inherited from the other parent.

Following mitosis, a diploid parental cell gives rise to two diploid daughter cells, or 2n → 2n. Mitosis occurs when tissues grow (throughout the lifespan of the organism) or when repair occurs.

Mitosis in Detail Mitosis has four phases: prophase, metaphase, anaphase, and telophase. Each centrosome contains a pair of barrel-shaped organelles called centrioles and an aster, which is an array of short microtubules that radiate from the centrosome. Plant cells lack centrioles; thus centrioles are not required for spindle formation.

Late Interphase Chromatin is condensing into chromosomes. Fig. 5.4 Chromatin is condensing into chromosomes. Centrosomes have duplicated. During late interphase, before the start of mitosis, duplicated chromatin is condensing into chromosomes, and centrosomes have duplicated in preparation for mitosis.

Early Prophase Duplicated chromosomes are visible. Fig. 5.5 Duplicated chromosomes are visible. Nuclear envelope is fragmenting and nucleolus will disappear. Spindle fibers appear between the separating centrosomes. During early prophase, mitosis has begun. Duplicated chromosomes are now visible. Each chromosome is duplicated and composed of sister chromatids held together at a centromere. Centrosomes begin moving apart, the nuclear envelope is fragmenting, and the nucleolus will disappear.

Late Prophase Spindle is in process of forming Fig. 5.5 Spindle is in process of forming Centromeres of chromosomes are attaching to centromeric spindle fibers Chromosomes have no particular orientation During late prophase, the spindle is forming and spindle fibers appear between the separating chromosomes. Centromeres attach to spindle fibers called centromeric (or kinteochore) fibers. The chromosomes have no particular orientation as yet.

Metaphase Spindle is fully formed (poles, asters, and fibers) Fig. 5.5 Spindle is fully formed (poles, asters, and fibers) Chromosomes are at the metaphase plate of the fully formed spindle By the time of metaphase, the fully formed spindle consists of poles, asters, and fibers. The metaphase plate is a plane perpendicular to the axis of the spindle and equidistant from the poles. The chromosomes attached to the centromeric spindle fibers line up at the metaphase plate during metaphase. Polar spindle fibers reach beyond the metaphase plate and overlap. Metaphase plate

Anaphase Centromeres divide Sister chromatids separate Fig. 5.5 Centromeres divide Sister chromatids separate Daughter chromosomes begin to move toward the opposite poles of the spindle During anaphase, daughter chromosomes (each consisting of one chromatid) are moving toward the poles of the spindle. The daughter chromosomes have a centromere and a single chromatid. The movement of the daughter chromosomes occurs because the centromeric spindle fibers disassemble at the region of the centromere, pulling the daughter chromosomes toward the poles, and because the polar spindle fibers push the poles apart as they lengthen and slide past one another.

Telophase Spindle disappears Nuclear envelope reappears Fig. 5.5 Spindle disappears Nuclear envelope reappears Chromosomes become diffuse chromatin again Cleavage furrow visible During telophase, the spindle disappears, and the nuclear envelopes begin to reform around daughter nuclei. Each 2n daughter nucleus contains the same numbers and kinds of chromosomes as the original 2n parental cell. Chromosomes become diffuse chromatin again, and a nucleolus appears in each daughter nucleus. At this point, cytokinesis is also nearly complete.

How Plant Cells Divide Plant cells lack centrioles and asters, but have a centrosome and spindle and the same four stages of mitosis.

Cytokinesis in Plant and Animal Cells Cytokinesis, or cytoplasmic cleavage, accompanies mitosis. Cleavage of the cytoplasm begins in anaphase, but is not completed until just before the next interphase.

Cytokinesis in Plant Cells The rigid cell wall surrounding plant cells does not permit cytokinesis by furrowing. The Golgi apparatus releases vesicles that microtubles move to the cell plate forming between the two new cells. New plant cell walls form and are later strengthened by cellulose fibers.

Cytokinesis in plant cells Fig. 5.7 During cytokinesis in a plant cell, the cell plate forms midway between the two daughter nuclei and extends to the plasma membrane.

Cytokinesis in Animal Cells Fig. 5.8 In animal cells, a cleavage furrow begins at the end of anaphase. A contractile ring (actin and myosin) filaments slowly forms a constriction between the two daughter cells.

Cell Division in Prokaryotes The process of asexual reproduction in prokaryotes is called binary fission. The two daughter cells are identical to the original parent cell, each with a single chromosome. Following DNA replication, the two resulting chromosomes separate as the cell elongates.

Reducing the Chromosome Number Meiosis reduces the chromosome number such that each daughter cell has only one of each kind of chromosome (Reduction Division). Meiosis ensures that the next generation will have: the diploid number of chromosomes A single copy of each type of chromosome from each parent.

Overview of meiosis Fig. 5.9 Following DNA replication, each chromosome is duplicated. During meiosis I, the homologous chromosomes pair during synapsis and then separate. During meiosis II, the centromeres divide and the sister chromatids separate, becoming daughter chromosomes that move into the daughter nuclei.

Meiosis in Detail The same four phases seen in mitosis – prophase, metaphase, anaphase, and telophase – occur during meiosis I and meiosis II. Interkinesis - period between meiosis I and meiosis II. No replication of DNA occurs during interkinesis because the DNA is already duplicated.

Meiosis I in an animal cell From Fig. 5.12 During prophase I, the spindle appears while the nuclear envelope fragments and the nucleolus disappears. Because DNA replicated during interphase prior to meiosis I, the homologous chromosomes each have two sister chromatids. During synapsis, crossing-over can occur. If so, the sister chromatids of a duplicated chromosome are no longer identical. During metaphase I, homologous pairs are aligned at the metaphase plate. Crossing over can occur here Homologous chromosomes line up

Haploid – but chromosomes are duplicated From Fig. 5.12 During anaphase I, homologous chromosomes separate and move to opposite poles of the spindle. Each chromosome still consists of two chromatids. At telophase I, nuclear envelopes may reform and nucleoli may reappear. Cytokinesis may or may not occur at this point. Interkinesis is a stage between the two meiotic divisions when chromosomes still consist of two chromatids. Haploid – but chromosomes are duplicated

Meiosis II From Fig. 5.13 No need for duplication – just separation Haploid Metaphase II At the beginning of prophase II, a spindle appears while the nuclear envelope disassembles and the nucleolus disappears. During anaphase II, sister chromatids separate, becoming daughter chromosomes that move into the daughter nuclei. Anaphase II Prophase II Telophase II

Comparison of Meiosis with Mitosis In both mitosis and meiosis, DNA replication occurs only once during interphase. Mitosis requires one division while meiosis requires two divisions. Two diploid daughter cells result from mitosis; four haploid daughter cells result from meiosis.

From Fig. 5.14

Genetic Recombination There are two sources of genetic recombination during meiosis: crossing-over of nonsister chromatids and independent assortment of homologous chromosomes. Both events assure new genetic combinations in the offspring. Genetic recombination is the process by which the combination of genes in an organism's offspring becomes different from the combination of genes in that organism

Synapsis and Crossing-over Occurs During Prophase I Fig. 5.10 During meiosis I, from left to right, duplicated homologous chromosomes undergo synapsis; nonsister chromatids break and then rejoin, so that two of the resulting daughter chromosomes have a different combination of genes.

Independent assortment Fig. 5.11 Organism with 3 types of chromosomes. - 3 types of chromosomes, 2 copies of each. - one copy from each parent. Large = chromosome 1 Medium = chromosome 2 Small = chromosome 3 1 F 2 F 3 M 3 F 1 M 2 M 1 M 2 M 3 M 3 F 1 F 2 F Four possible orientations of homologous pairs at the metaphase plate are shown. Each of these will result in daughter nuclei with a different combination of parental chromosomes. When a cell has three pairs of homologous chromosomes, there are 23 possible combinations of parental chromosomes in the daughter nuclei. 1 F 2 M 3 F 3 M 1 M 2 F 1 M 2 F 3 F 3 M 1 F 2 M

Life cycle of humans The human life cycle requires both mitosis and meiosis. oogenesis spermatogenesis Meiosis in human males is a part of sperm production, and meiosis in human females is a part of egg production. When a haploid sperm fertilizes a haploid egg, the zygote is diploid, The zygote undergoes mitosis as it develops into a newborn child. Mitosis continues after birth until the individual reaches maturity; then the life cycle begins again.

Spermatogenesis Spermatogenesis produces four viable haploid sperm cells.

Oogenesis Oogenesis produces one egg and two or three polar bodies. The extra cytoplasm of the single egg cell serves as a source of nutrients for the developing embryo.

Sources of Genetic Recombination in Humans Independent assortment of chromosomes during metaphase I Crossing-over during prophase I Upon fertilization, recombination of chromosomes occurs.