MITOSIS where in cells is the genetic material? are chromosomes in nucleus genetic material? even though they divide all cells have the same number of chromosomes - how is this achieved? mitosis proceeds by a highly conservative division of genetic material into two cells…. critical for development and everyday life

somatic cell a cell that is not destined to become a gamete; a cell whose genes will not be passed on to future generations ….. divide by mitosis sex or germ cell a cell that is destined to become a gamete; a cell whose genes can be passed on to future generations ….. divide by mitosis and meiosis

The Mitotic Cell Cycle Interphase - (G0, G1, S, and G2) RNA and protein synthesis G1 (first GAP) - prepare for DNA replication S - DNA synthesis (chromatid duplication) G2 (second GAP) - prepare for mitosis M - mitosis (no RNA/protein synthesis); short relative to interphase

Mitotic Cell Cycle Checkpoints and Transitions G1/S checkpoint G0 G2/M checkpoint M checkpoint G2 G1 M G1/S checkpoint: cell size; is DNA ready to replicate? (i.e. not damaged) G2/M checkpoint: is DNA replication complete? M checkpoint: are chromosomes ready to separate into daughter cells?

homologous chromosomes - chromosomes that contain the same linear sequence of genes….. they may have different variants (alleles) of the same gene chromatids - the products of a chromosome duplicated at S phase and joined at the centromere; also called sister chromatids …… at anaphase chromatids become separate chromosomes

genome - one complete copy of all genes (found on one set of chromosomes) diploid (2N) - a cell or organism with 2 copies of each chromosome (2 genomes) haploid (N) - a cell or organism with 1 copy of each chromosome (1 genome)

centromeres divide at anaphase cytokinesis centromeres divide at anaphase at anaphase chromatids become chromosomes; disjunction occurs; karyokinesis daughter cells are diploid - two copies of each gene, two copies of each chromosome

- genes (a, b, c) arranged in linear order along chromosome chromosome (G1) - genes (a, b, c) arranged in linear order along chromosome DNA replication (S) chromatids (G2/Metaphase) (held together at centromere prior to anaphase) daughter chromosomes (one per daughter cell) Mitotic division of one member of a pair of homologous chromosomes a b c a b c a b c a b c

Important concepts: once replication (S phase) is complete each chromosome consists of two chromatids held together by a common centromere….this is true during prophase and metaphase common centromere divides at anaphase as chromatids separate to move to opposite poles……at this point chromatids become chromosomes mitosis is a conservative process designed to make two exact copies of an existing cell…..one diploid (2N) cell divides to form two diploid (2N) daughter cells

MEIOSIS occurs in germ or sex cells only diploid cell divides twice leading to four haploid products fusion of egg and sperm (fertilization) required to reconstitute diploid organism

Leptotene: chromosomes condense; homology searching Zygotene: synapsis, the close pairing of homologous chromosomes, begins - paired chromosomes called bivalents (tetrads) Pachytene: crossing-over occurs, the exchange of corresponding chromosome parts between homologs by breakage and reunion - not visible until diplotene Diplotene: chiasma visible; the physical sites of cross-overs Diakinesis: chiasma resolved

Anaphase I: reductional division - number of chromosomes per cell is halved Anaphase II: equational division; chromatids divide Gametes: gametes have one copy of each chromosome; one copy of each gene

Novel combinations of alleles are created by recombination without recombination: DaB, Dab, dAB, dAb with recombination: DaB, Dab, dAB, dAb, DAB, DAb, daB, dab sister chromatids A B a B A b a b

recombination - a process that generates new gene or chromosomal combinations not found previously in the cell - during meiosis this leads to haploid products with genotypes different from either of the two haploid genotypes in the diploid progenitor cell ….crossing over leading to recombination can only involve non-sister chromatids

CHROMOSOMAL BASIS FOR EQUAL SEGREGATION OF ALLELES INTO GAMETES

CHROMOSOMAL BASIS FOR INDEPENDENT ASSORTMENT Anaphase I Anaphase II Gamete CHROMOSOMAL BASIS FOR INDEPENDENT ASSORTMENT

Major differences between mitosis and meiosis. 1. Number of cell divisions and products. mitosis - one cell division resulting in two daughter cells meiosis - two cell divisions resulting in four products 2. Ploidy (# copies of each chromosome) mitosis - cells are always diploid (2 copies each chromosome) meiosis - cells become haploid at anaphase I 3. Synapsis of homologous chromosomes. mitosis - no pairing meiosis - pairing at zygotene of prophase I

4. Exchange of genetic material between synapsed homologous chromosomes. mitosis - does not occur meiosis - occurs at pachytene of prophase I (first visible at diplotene) 5. Timing of division of centromeres. mitosis - occurs at anaphase meiosis - occurs at anaphase II but not at anaphase I 6. Genetic variation. mitosis - conservative process; does not lead to genetic variation meiosis - leads to increased genetic variation following recombination (crossing-over)

Role of Meiosis in Generating Genetic Diversity humans have 23 pairs of chromosomes; over 8 million unique gametes can be formed by unique combinations of maternal and paternal chromosomes for two individuals there are 7 x 1013 possible unique combinations of fused gametes (offspring) with no recombination recombination greatly increases the potential number of genetically different offspring possible