1. MEIOSIS AND SEXUAL REPRODUCTION

2. Meiosis
Process of nuclear division that reduces the
number of chromosomes in a cell by half
2 Stages
Meiosis I:
Homologous chromosomes are separated
into separate cells
Meiosis II:
Chromatids are segregated into separate cells

3. Complete interphase with S phase (replication of DNA occurs)
BEFORE MEIOSIS
Complete interphase with S phase (replication of DNA occurs)

4. DNA condenses and becomes chromosomes.
MEIOSIS 1
PROPHASE I
DNA condenses and becomes chromosomes.
Spindle forms, nucleolus and nucleus disappears.
3. Homologous chromosomes line up at equator.

5. 4. Synapsis occurs: parallel alignment of sister chromotids
(NOW CALLED A TETRAD)
5. Crossing over occurs
Non sister chromatids
break apart and exchange alleles and re-attach.
Result: Recombination
(NEW COMBOS OF GENES)

6. METAPHASE I
Tetrads move to equator and remain together.
ANAPHASE I
Homologous chromosomes
separate and one of each
pair is pulled to opposite poles.
**chromatids still joined by centromere**

7. Two daughter cells formed.
TELOPHASE I
Cytokinesis occurs.
Two daughter cells formed.
(do NOT have same genetic info due to crossing over)
END RESULT
Two daughter cells with haploid # (N)
** homologous chromosomes have separated, but sister chromatids remain together

8. MEIOSIS II 2 (2N) cells go into meiosis II
MEIOSIS II 2 (2N) cells go into meiosis II. Chromatids are segregated into different cells.
PROPHASE II
1. chromosomes condense.
(brief interphase with
NO DNA replication)
nuclear membrane disappears.
3. chromosomes start to
move to equator.

9. METAPAHSE II
Chromosomes with sister
chromatids joined by
centromere line up at
equator.
ANAPHASE II
Centromeres divide and each sister chromatid moves to opposite poles

10. ** chromatids separate**
TELOPHASE II
1. spindle dissolves & nuclear
membrane reforms around
chromosomes in each
daughter cell
2. cleavage furrow or cell plate
forms and cytokinesis
occurs
END RESULT
Four cells with haploid # (N)
** chromatids separate**
Meiosis animation

11. Why is it important for gametes to have the haploid number?

12. DIFFERENCES: MITOSIS AND MEIOSIS
1 diploid cell makes 2 diploid daughter cells
daughter cells genetically identical to mother cell
occurs in somatic cells
unique features meiosis
Meiosis
1 diploid cell makes 4 haploid cells
resulting cells are genetically different from diploid cells and each other
occurs in gametes
comparison animation

13. HOW MEIOSIS CAUSES GENETIC VARIATION
each of the 23 pairs of homologous chromosomes segregate independently
crossing over adds to recombination
random joining of two gametes with new combinations
Outcomes: x = trillion outcomes

14. GAMETOGENESIS Meiosis is primary event in formation of gametes.
Spermatogenesis Oogenesis
Spermatogenesis:
Production of sperm in testes
- starts with 1 diploid germ cell
- result: 4 mature sperm cells
Oogenesis:
Production of ova (eggs) in ovaries
- result: 1 ovum, 3 polar bodies
- female is born with set number of eggs which are frozen at prophase I
- puberty hormones cause eggs to complete meiosis and then pauses again
- at fertilization: egg undergoes meiosis II

15. TYPES OF ASEXUAL REPRODUCTION
BINARY FISSION FRAGMENTATION BUDDING
bacteria penicillin yeast
PARTHENOGENESIS REGENERATION
rotifer egg starfish
ANIMATIONS

16. Advantages of each type reproduction:
Asexual Sexual
- production of many offspring - quick production of different gene
in short time period combinations
- genetic diversity enhances evolution

17. Eukaryotic Sexual Life Cycles
Haploid: simplest
- Haploid cells occupy major portion of life cycle
- Zygote is only diploid cell, undergoes meiosis as soon as formed, again become haploid
Occurs in protists, fungi, algae

18. Eukaryotic Sexual Life Cycles
Diploid: adults are diploid and inherit chromosomes from parents
- adult individuals are diploid with each individual getting chromosomes from each parent (thru fertilization)
- major portion of life is spent as diploid
- diploid repro. cell undergoes meiosis to form haploid gametes (only haploid cell)
- most animals

19. Eukaryotic Sexual Life Cycles
3. Alternation of generations: life cycle regularly alternates between a haploid phase and diploid phase
- plants
- Sporophytes: diploid phase that produces spores
- Spore: haploid reproductive cell ,undergoes meiosis without fusing with another cell
- Gametophyte: haploid phase produces whereby gametes produced by mitosis fuse and give rise to diploid phase
Ex: moss:
Cycles differ only in which phases become diploid.