MITOSIS where in cells is the genetic material? are chromosomes in nucleus genetic material? even...

MITOSISMITOSIS• 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 cellsomatic 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 cellsex 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 synthesisG1 (first GAP) - prepare for DNA replication S - DNA synthesis (chromatid duplication)G2 (second GAP) - prepare for mitosisM - mitosis (no RNA/protein synthesis); short relative to interphase

• 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?

Mitotic Cell Cycle Checkpoints and Transitions

G2/M checkpointM checkpoint

G1/S checkpoint S

G2

MG1

G0

homologous chromosomeshomologous chromosomes - chromosomes that contain the same linear sequence of genes….. they may have different variants (alleles) of the same gene

chromatidschromatids

- the products of a chromosome duplicated at S phase and joined at the centromere; also called sister chromatids …… at anaphase chromatids become separate chromosomes

genomegenome - one complete copy of all genes (found on one set of chromosomes)

diploid (2N)diploid (2N) - a cell or organism with 2 copies of each chromosome(2 genomes)

haploid (N)haploid (N) - a cell or organism with 1 copy of each chromosome(1 genome)

• 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

cytokinesis

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

abc

abc

abc

abc

abc

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

MEIOSISMEIOSIS• 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 divideGametes: gametes have one copy of each chromosome;

one copy of each gene

A

Aa

aBB

bb

A

A

a

a

BBb

b

DD

d

d

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

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

Major differences between mitosis and meiosis.

1. Number of cell divisions and products.

mitosis - one cell division resulting in two daughter cellsmeiosis - 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 pairingmeiosis - pairing at zygotene of prophase I

4. Exchange of genetic material between synapsed homologous chromosomes.

mitosis - does not occurmeiosis - occurs at pachytene of prophase I

(first visible at diplotene)

5. Timing of division of centromeres.

mitosis - occurs at anaphasemeiosis - 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