1. Lecture 17 Gametogenesis at the Chromosomal level: Mitosis and Meiosis
Animal Science 434
John J. Parrish

2. Definitions Chromosome - linear DNA molecule
Centromere - special region of spindle attachment
Homologous Chromosomes
Have the same kind of genes in the same order
1 from father, 1 from mother
Sister chromosomes (X, Y)

3. A chromatid pair has an identical sequence of DNA
Definitions
Homologous Chromosomes
DNA Replication
Chromatids
A chromatid pair has an identical sequence of DNA

4. Cell Cycle
Interphase M Go
MPF
MPF
MPF G1 G2
MPF
MPF
MPF
MPF S

5. M - Phase
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis

6. Grand Scheme Mitosis Meiosis 2N 2N Replicate Division Replicate

7. Homologous chromosomes pair along side of each other
Mitosis G1
M - phase S
Prophase G2
Metaphase
Homologous chromosomes pair along side of each other
Chromatids
Centromere
MPF high
Spindles
(microtubules)

8. Mitosis
Anaphase
MPF decreases
Chromatids pull
apart

9. Mitosis Telophase Chromosomes pull apart further Microtubules
Disappears
Nuclear envelopes
reform

10. Mitosis
Cytokinesis 2N

11. Meiosis 2N 4N Go G1 S G2
Prophase
Primary Spermatocyte
Primary Oocyte

12. The Five Phases of Meiotic Prophase
Leptotene
Chromosomes condense 4N
Zygotene
Homologous chromosomes begin to pair
Bivalents 4N

13. The Five Phases of Meiotic Prophase
Pachytene
Pairing is completed
Crossing over of homologues occur 4N
Diplotene/Dictyatene
Oocytes stop here before puberty
Homologous chromosomes pull apart but remain attached at crossover points (chiasma)
RNA synthesis is possible 4N

14. The Five Phases of Meiotic Prophase
Diakinesis
RNA synthesis stops
The chromatids become visible 4N
Homologous chromosomes linked at chiasma
Chromatids linked at centromere
Crossover (chiasma)
Centromere

15. Homologous chromosomes held together via chiasma
Metaphase I of Meiosis 4N
Homologous chromosomes move toward opposite poles and pair across from one another
Homologous chromosomes held together via chiasma

16. Recombination as a result of crossing over
Anaphase I of Meiosis 4N
Recombination as a result of crossing over

17. Secondary Spermatocyte or Secondary Oocyte
Telophase I
Cytokinesis 2N 4N
Secondary Spermatocyte or Secondary Oocyte

18. Meiosis Metaphase II 2N 2N No Replication Rest Same As Mitosis 2N 2N
Oocytes stop here until fertilized

19. Meiosis This does not exist for oocyte Telophase II Cytokinesis
Anaphase II N N 2N
Spermatid N 2N N

20. Abnormalities Anueploidy Euploidy Non-disjunction
Not an exact multiple
Hyper or hypo
Example: 2N - 1 or 2N + 1
Euploidy
Exact multiple of haploid
Example: 3N, 4N, etc.
Non-disjunction
Causes aneuploidy and occurs in meiosis
Example: N - 1 or N + 1

21. Abnormalities Non-disjunction XXYY XY XXYY XXYY O XX YY XY O XX O Y
XXX XY X
Kleinfelter’s
Syndrome
Turner’s

22. Down’s Syndrome Non-disjunction Extra copy of chromosome 21
Probability increases with age
Age 35, 2 per 1000 births
Age 40, 6 per 1000 births
Age 45, 16 per 1000 births