1. Sexual Reproduction
Chapter 12 AP

2. Discovery Edouard Joseph Louis Marie Van Beneden
Observed cells in roundworms and noticed the gametes had half the number of chromosomes as the somatic cells
Fusion of gametes = fertilization/syngamy; forms zygote
Even before discovery scientists knew there was some mechanism for halving the number of chromosomes
Otherwise, the next generations would have double the chromosomes
Meiosis is the process of reduction division

3. Sexual Life Cycle Diploid cells have two sets of each chromosome
Found in somatic cells of adults
Haploid cells have only one of each chromosome
Found in sex cells (egg & sperm)
Sexual reproduction
Process of producing offspring by the fusion of haploid cells produced by meiosis from diploid cells

4. Sexual Life Cycle Somatic tissues Germ-line tissues
After fertilization, the single diploid cell divides by mitosis
Single cell gives rise to all cells
These cells called somatic meaning body (2n)
Divide by mitosis
Germ-line tissues
Cells predestined to be gametes are set aside early in development
Divide by meiosis

5. Features of Meiosis Synapsis Homologous recombination
Pairing of homologues early in first nuclear division
Homologous recombination
Crossing over
Exchange of genetic information between homologues
Reduction division
Chromosomes do not duplicate between the two nuclear divisions of meiosis, thus each resulting cell has half the original number of chromosomes

6. Sequencing Meiosis The first division (Meiosis I) Prophase I Leptotene
Chromosomes condense tight (4 chromatids for each type of chromosome)
Zygotene
Protein lattice laid down between homologues (synapsis)
Called synaptonemal complex
Pachytene
Synaptonemal complex holds homologues together gene to gene
DNA unwinds at certain sites allowing bases to pair with complimentary bases from the other chromosome (crossing over)
Recombination nodules assist in recombination
Crossing over evidence = chiasmata (seen through microscope)

7. Sequencing Meiosis (cont)
Diplotene
Synaptonemal complex breaks down
Chromosomes decondense/transcription occurs
Diakinesis
Transcription stops/chromosomes condense

8. Sequencing Meiosis (cont)
Metaphase I
Chiasmata move down the chromosomes until it reaches the end holding the homologues together
Forces one side of centromere outward
Spindle is then only able to attach to one centromere per chromosome
Homologues then line up on metaphase plate
Orientation of pair is random (mom’s doesn’t have to stay with mom’s and visa versa) = independent assortment

9. Sequencing Meiosis (cont)
Anaphase I
Spindles shorten and break chiasmata pulling centromeres to opposite poles
One chromosome from the homologous pair is now on opposite sides
Telophase I
Nuclear envelope reforms
Because of crossing over, sister chromatids no longer identical
Cytokinesis may occur

10. Sequencing Meiosis (cont)
The second division (Meiosis II)
Brief interphase (no DNA synthesis)
Resembles normal mitotic division
Prophase II
Nuclear envelope breaks down/spindle reforms
Metaphase II
Spindle fibers bind to both sides of centromere
Anaphase II
Fibers contract spiltting centromeres and moving sister chromatids to opposite poles
Telophase II
Nuclear envelope reforms

11. Result of Meiosis Four haploid cells
No two cells are alike because of crossing over and independent assortment
These haploid cells can
Directly become gametes (animals)
Divide mitotically producing more haploid cells making even more gametes (fungi, plants, protists)

13. Origin of Sex Unknown Asexual reproduction works Parthenogenesis
Bacteria, protists, plants, some animals (budding)
Parthenogenesis
Development of adult without fertilization
Arthropods (females diploid, males haploid)
some amphibians and lizards (gamete goes through mitosis, but doesn’t divide forming a diploid cell)

14. Origins of Sex (cont)
Some species benefit from variability generated from meiosis
*However, the more specifically adapted an organism is, the more likely its offspring will not benefit from genetic variability that comes from meiosis; the more likely that offspring will suffer and be less likely to survive.
Then why do we have sexual reproduction?

15. Sex and Evolution
Read
DNA Repair Hypothesis (pg 236)
The Contagion Hypothesis (pg 236)
The Red Queen Hypothesis (pg 237)
Miller’s Ratchet (pg 237)
Sexual reproduction has a huge impact on evolution
***Test soon to follow***