Zoology 100 Notes 5

ZOOLOGY 100 NOTES (5) Cell Division Cell division increases the number of somatic (body) cells, and consists of: 1. Karyokinesis (division of nucleus) 2. Cytokinesis (division of cytoplasm) Apoptosis (cell death) decreases the number of cells. Both cell division and apoptosis occur during normal development and growth. The Cell Cycle It is an orderly sequence of events that occurs from the time when a cell is first formed until it divides into two new cells. 2 major stages: 1.interphase is the stage where most of the cell cycle is spent 2. mitotic stage or cell division proper The cell cycle showing the stages of interphase

Cells synthesize proteins needed for cell division 1. The Mitotic Stage Includes: 1. Mitosis 2. cytokinesis. During this stage, sister chromatids of each chromosome separate, and become the nuclei of the two daughter cells. ends in cytokinesis (cleaving of the cytoplasm is complete) Three checkpoints where the cell cycle is controlled 1. During G1 prior to the S stage 2. During G2 prior to the M stage 3. During the M stage prior to the end of mitosis DNA damage can also stop the cell cycle at the G1 checkpoint.

Biological Relationship of Mitosis and Meiosis

Cell Cycle

Apoptosis Is programmed cell death. Occurs because of two sets of capsases (enzymes). 1. “initiators” (the 1st set) receive a signal to activate the second set, the “executioners”. 2.”executioners” (the 2nd set) activate enzymes that tear apart the cell and its DNA. Maintaining the Chromosome Number When a eukaryotic cell is not dividing, the DNA and associated proteins is a tangled mass of thin threads called chromatin. At the time of cell division, the chromatin condenses to form highly compacted structures called chromosomes. Each species has a characteristic number of chromosomes.

Overview of Mitosis The diploid number of chromosomes is found in the somatic (non-sex) cells. The diploid (2n) number of chromosomes contains two chromosomes of each kind. The haploid (n) number of chromosomes contains one chromosome of each kind. In the life cycle of many animals, only sperm and eggs have the haploid number of chromosomes. The nuclei of somatic cells undergo mitosis, a nuclear division in which the number of chromosomes stays constant. Before nuclear division occurs, DNA replication takes place, duplicating the chromosomes. A duplicated chromosome is made of two sister chromatids held together in a region called the centromere. Sister chromatids are genetically identical. At the end of mitosis, each chromosome consists of a single chromatid. During mitosis, the centromeres divide and then the sister chromatids separate, becoming daughter chromosomes.

Following mitosis, a 2n parental cell gives rise to two 2n daughter cells, or 2n → 2n. Mitosis occurs when tissues grow or when repair occurs. Following fertilization, the zygote divides mitotically, and mitosis continues throughout the lifespan of the organism. Mitosis in Detail the spindle distributes the chromosomes to each daughter cell. spindle contains fibers made of microtubules that disassemble and assemble. Chromosomes attach to the spindle fibers through their centrosomes. four phases: 1. Prophase 2.Metaphase 3. Anaphase 4.Telophase

Late Interphase

Early Prophase

Late Prophase

Anaphase

Metaphase

Telophase

Cytokinesis Other term: cytoplasmic cleavage accompanies mitosis. Cleavage of the cytoplasm begins in anaphase, but is not completed until just before the next interphase. Newly-formed cells receive a share of cytoplasmic organelles duplicated during the previous interphase. cleavage furrow begins at the end of anaphase. contractile ring or band of actin and myosin filaments slowly forms a constriction between the two daughter cells. A narrow bridge between the two cells is apparent during telophase, then the contractile ring completes the division.

Cytokinesis

OTHER INFOS AFTER CELL DIVISION: 1. cell division in prokaryotes 2. Meiosis Cell Division in Prokaryotes binary fission is the process of asexual reproduction 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. MEIOSIS: Reducing the Chromosome Number It reduces the chromosome number such that each daughter cell has only one of each kind of chromosome. It ensures that the next generation will have: 1. the diploid number of chromosomes 2. a combination of traits that differs from that of either parent.

Meiosis in humans. Have 23 pairs of chromosomes, or 46 chromosomes total. Before meiosis I, DNA replication occurs. During meiosis, there will be two nuclear divisions, and the result will be four haploid nuclei. No replication of DNA occurs between meiosis I and meiosis II. Meiosis I separates homologous pairs of chromosomes. Daughter cells are haploid, but chromosomes are still in duplicated condition. Synapsis occurs during meiosis I Meiosis II separates sister chromatids. The completely haploid daughter cells mature into gametes. Fertilization restores the diploid number of chromosomes during sexual reproduction. Meiosis in Detail four phases: 1. prophase 2. metaphase 3. anaphase 4. telophase occur during both meiosis I and meiosis II. interkinesis is the period of time between meiosis I and meiosis II - no replication of DNA occurs here because the DNA is already duplicated.

5 substages of prophase I 1. Leptonema 2. Zygonema 3. Pachynema 4. Diplonema 5. Diakinesis 1. Leptonema • chromosomes begin to condense, but are still diffuse. • chromosomes (DNA) have already replicated (during S phase), 'chromosome' consists of two sibling chromatids. • each chromosome begins to search the nucleus for its homologue. 2. Zygonema • chromosomes continue to condense,

each

•

homologous chromosomes find each other and begin to align to each other in a process known as "rough pairing".

3. Pachynema • synapsis happen; aligned homologous chromosomes become closely associated Tetrad is the synapsed homologous pair of chromosomes - consists of four chromatids. - undergo crossing over - continue to condense. 4. Diplonema • homologous chromosomes in each tetrad begin - remain connected at points of crossing over - chiasma is the point of crossing over (plural: • nuclear envelope begins to break down.

to separate chiasmata)

5.Diakinesis • homologous chromosomes separate further • chiasmata terminalize • the nuclear envelope has completely disintegrated • The centromeres of the chromosomes become attached to spindle fibers.

Meiosis II (similar to mitosis)

Genetic Recombination 2 sources of genetic recombination in meiosis 1. Crossing-over. Non-sister chromatids of a chromosome pair exchange genetic material. 2. Independent assortment. Homologous chromosomes are distributed to daughter cells randomly. Both events assure new genetic combinations in the offspring. Synapsis and crossing-over

Independent assortment

The sources of genetic recombination 1. Independent alignment/ assortment of paired chromosomes along the metaphase I plat 2. Synapsis 3. Crossing-over during prophase I 4. Combining of chromosomes of genetically different gametes 5. Fertilization between haploid gametes results in a third source of genetic recombination Comparison of Meiosis I to Mitosis Meiosis I: Prophase I - pairing of homologous chromosomes Metaphase I – homologous pairs line up at metaphase plate Anaphase I – homologous chromosomes separate Telophase I – daughter cells are haploid Mitosis: Prophase has no pairing Metaphase – chromosomes align at metaphase plate Anaphase – sister chromatids separate Telophase – diploid cells Meiosis I - Two cycles of division completes the cycle - Chromosomes have genetic variability - Chromosomes randomly have synapsis,indepen-dent assortment, and segregation Mitosis - One cycle of division completes the cycle - Chromosomes have identical genetic information - Chromosomes do not exhibit synapsis,indepen-dent assortment, and segregation

Meiosis

The human life cycle requires both mitosis and meiosis. In males, meiosis occurs as spermatogenesis and produces sperm. In females, meiosis occurs as oogenesis and produces egg cells. Mitosis is involved in the growth of a child and repair of tissues during life.

Spermatogenesis in human males produces four viable haploid sperm. - Diploid primary spermatocytes undergo meiosis I to produce haploid secondary spermatocytes. Secondary spermatocytes divide by meiosis II to produce haploid spermatids. Spermatids mature into sperm with 23 chromosomes.

During oogenesis, a diploid primary oocyte undergoes meiosis I to produce one haploid secondary oocyte and one haploid polar body. The secondary oocyte begins meiosis II but stops at metaphase II and is released at this stage from the ovary. Meiosis II will be completed only if sperm are present. After meiosis II, there is one haploid egg cell with 23 chromosomes and three polar bodies. Polar bodies serve as a dumping ground for extra chromosomes.

Animal Gametogenesis

Summary Cell division increases the number of body cells; Apoptosis decreases cell number. Cells goes through a cell cycle.

Each species has a characteristic number of chromosomes. Mitosis 1. produces daughter cells that are identical to the parental cell. 2. has four phases designed to maintain the chromosome number. 3. is used for growth and repair of tissues. Meiosis 1. reduces the chromosome number. 2. includes two nuclear divisions. 3. results in non-identical haploid gametes. The human life cycle includes both mitosis and meiosis. The process of meiosis and fertilization in humans and other sexually reproducing organisms result in offspring with new genetic combination.