1. Chapter 13 – Meiosis and Sexual Life Cycles

2. Heredity
Heredity: the transmission of traits from one generation to the next
Variation: offspring differ somewhat in appearance from parents and siblings
Genes: segments of DNA that encode for proteins, the hereditary unit
Locus: a genes specific location on a chromosome
Asexual reproduction:
produces clones
example budding in hydra
Sexual reproduction: results in variation

3. Human life cycle 23 pairs of homologous chromosomes (46)
1 pair of sex and 22 pairs of autosomes = a set
Somatic cells – body cells
Gametes – sex cells
Sex chromosomes – X and Y
Autosomes – non-sex chromosomes
Gametes are haploid while all other cells are diploid
Haploid (1n) = 1 set of chromosomes
Diploid (2n) = 2 sets of chromosomes
Fertilization (syngamy) results in a zygote

4. Karyotype
Micrograph of chromosomes, usually arranged by size

5. Alternative life cycles
Fungi/some algae - meiosis produces 1n cells that divide by mitosis to produce 1n adults (gametes by mitosis)
Plants/some algae - Alternation of generations: 2n sporophyte, by meiosis, produces 1n spores; spore divides by mitosis to generate a 1n gametophyte; gametes then made by mitosis which then fertilize into 2n sporophyte

6. Meiosis Overview Preceded by chromosome replication (interphase)
Followed by 2 successive cell divisions (Meiosis I & Meiosis II)
Reduces the chromosome number
Produces 4 haploid daughter cells
Variation

7. Meiosis 1 overview

8. Meiosis 2 overview

9. Prophase I Occupies 90% of the time to complete meiosis
Chromosomes condense
Homologous chromosomes pair up, aligning gene to gene
Synapsis – tetrads (4 homologous chromatids) are grouped together and held in place by a protein structure called the synaptonemal complex
Each tetrad has one or more chiasmata – criss-crossed regions where crossing over occurs; holds tetrad together until Anaphase I
Crossing over – non-sister chromatids exchange DNA
Everything from mitosis prophase and prometaphase also occurs except the kinetochore microtubules only attach to each homologous pair, not each chromatid

10. Prophase I

11. Crossing Over

12. Metaphase I Tetrads line up on metaphase plate
One chromosome from each homologous pair facing a pole

13. Metaphase I

14. Anaphase I Pairs of chromosomes move towards the poles
Original sister chromatids remain attached

15. Anaphase I

16. Telophase I and Cytokinesis
At end of telophase, each new cell has a complete set of chomosomes, but each is still attached as sister chromatids
Cytokinesis occurs simultaneously with telophase I same as mitosis
The cells do not enter interphase but enter a stage of interkinesis (a pause between meiosis I and meiosis II)

17. Telophase I and Cytokinesis

18. Meiosis II Same as mitosis
End product is 4 haploid gametes with genetic variation

19. Meiosis II

20. Meiosis vs. Mitosis Synapsis/tetrad/ chiasmata (prop I)
Homologous vs. indiv. chromosomes (meta I)
Sister chromatids do not separate (ana I)
Meiosis I separates homologous pairs of chromosomes, not sister chromatids of individual chromosomes.

21. Origins of Genetic Variation
Independent assortment: homologous pair of chromosomes position and orient randomly (metaphase I) and nonidentical sister chromatids during meiosis II n

22. Origins of Genetic Variation
Crossing over (prophase I): the reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I (recombinant chromosomes)
Random fertilization: 1 sperm (1 of 8 million possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 64 trillion diploid combinations!