1. 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

2. 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

3. 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

4. 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?

5. 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

6. 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)

7. 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

8. - 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. CHROMOSOMAL BASIS FOR EQUAL SEGREGATION OF ALLELES INTO GAMETES

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

17. 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

18. 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)

19. 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