The Cell Cycle

Types of Cell Division Mitosis Meiosis provides cells for growth, cell replacement, and asexual reproduction maintains chromosome number Meiosis produces cells for sexual reproduction reduces chromosome number by half

Chromosome Replication All the DNA is duplicated. Duplicated chromosomes separate as cell divides.

Asexual Reproduction in Bacteria Bacteria reproduce asexually by binary fission. A bacterium possesses a single, circular chromosome which is replicated; and then the replicates (chromosomes) are distributed to two new cells formed by division of the original cell.

The Eukaryotic Cell Cycle Interphase G1 growth phase; synthesis of organelles S synthesis of DNA (replication) G2 growth; synthesis of proteins essential to cell division Mitosis prophase metaphase anaphase telophase

Important Terms chromatin - the form taken by chromosomes when a eukaryotic cell is not dividing monad chromosome - unreplicated chromosome (single chromosome) dyad chromosome - replicated chromosome (double chromosome) chromatid - half of a dyad chromosome centromere - structure by which chromatids of a dyad chromosome are held together

Mitosis* 1. Prophase: dyad (replicated) chromosomes resemble long threads (chromatin); replicated centrioles move apart toward opposite poles; (animal cell); nucleolus disappears; dyad chromosomes condense (coil tightly); nuclear membrane disintegrates; spindle fibers appear between centrioles; dyad chromosomes migrate toward the equator of the cell

Mitosis* 2. Metaphase: dyad chromosomes line up along the equator of the cell

Mitosis* 3. Anaphase: dyad chromosomes separate forming monad chromosomes; monad chromosomes move toward opposite poles

Mitosis* 4. Telophase: monad chromosomes cluster at poles; cytokinesis occurs gradually; nuclear membrane forms; nucleolus reappears; chromosomes uncoil; cytokinesis is completed; two daughters cells are formed

Daughter Cells of Mitosis have the same number of chromosomes as each other and as the original cell from which they were formed e.g. Mitosis in a cell which possesses 4 chromosomes will result in the formation of two daughter cells each having 4 chromosomes.

Can you identify the stages (phases) of mitosis in both plant cells and animal cells?

Can you cite differences between plant cell mitosis and animal cell mitosis?

Types of Cell Division Mitosis Meiosis provides cells for growth, cell replacement, and asexual reproduction maintains chromosome number Meiosis produces cells for sexual reproduction reduces chromosome number by half

Meiosis preceded by interphase which includes chromosome replication two meiotic divisions four daughter cells produced daughter cells contain half the number of chromosomes as the original cell

Important terms in the study of meiosis monad chromosome - unreplicated chromosome dyad chromosome - chromosome composed of two chromatids (replicates) homologous - similar in size and shape synapsis - pairing of homologous chromosomes crossover - exchange of segments between chromatids of homologous chromosomes

Chromosomes are replicated during S-phase of interphase Chromosomes are replicated during S-phase of interphase. As a result, monad chromosomes become dyad chromosomes.

Meiosis I (Figure 6.6, p. 106) Prophase I: homologous chromosomes of diploid cell synapse; homologous pairs migrate toward equator of cell; crossover may occur during synapsis Metaphase I: dyad pairs of homologous chromosomes align at the equator of the cell Anaphase I: homologous chromosomes move apart toward opposite poles Telophase 1: chromosomes cluster at poles of cell; cytokinesis occurs forming two haploid daughter cells each containing one member of each homologous pair thus reducing chromosome by half

Meiosis II (resembles mitosis) (Figure 6.6, p. 106) Prophase II: dyad chromosomes move toward the equator of each cell (2) Metaphase II: dyad chromosomes align at equator of cell Anaphase II: dyad chromosomes separate forming monad chromosomes (chromatids separate); monads migrate toward opposite poles of each cell Telophase II: cytokinesis occurs at right angles to the first plane of division in each cell (2); four daughter cells are formed each having half the number of chromosomes as the original cell

Gamete formation - meiosis meiosis in males - spermatogenesis meiosis in females - oogenesis

Spermatogenesis 4 mature sperm cells produced from meiosis in a single spermatocyte

Oogenesis one mature ovum (egg cell) and 3 polar bodies formed from meiosis in a single oocyte human female embryo - oocytes formed by mitosis and then complete prophase I of meiosis by time of birth puberty - one oocyte in alternating ovaries resumes meiosis under the influence of hormones; at ovulation the oocyte is at metaphase II oogenesis will be completed in the event of fertilization

Comparison of Mitosis and Meiosis (Table 6.2; Figure 6.8) synapsis does not occur one division occurs two daughter cells are formed chromosome number is maintained Meiosis synapsis occurs during Prophase I two divisions occur four daughter cells are formed chromosome number is reduced by half

Nondisjunction (Essay 6.1, p. 109) failure of chromosomes to separate during anaphase results: daughter cells contain abnormal number of chromosomes

Chromosome anomalies Down syndrome Turner syndrome Klinefelter syndrome

Down syndrome Cause: trisomy 21 Effects: mental retardation, weakened cardiovascular system, shortened limbs, widely-spaced eyes, receding forehead and chin, protruding tongue

Turner syndrome Cause: one X chromosome in female (XO) Effects: underdeveloped ovaries; failure of secondary sex characteristics to appear; sterility; webbing of neck; weakened pancreas, liver, and heart in 40%; short stature

Klinefelter syndrome Cause: extra X chromosome in males (XXY) Effects: underdeveloped testes, sterility, failure of secondary sex characteristics to appear, taller than average

Detection of chromosome anomalies in a fetus: amniocentesis chorionic villi sampling

Questions for next class meeting: During which weeks of a pregnancy, may the following procedures be conducted: amniocentesis chorionic villi sampling