1. Meiosis And Sexual Life Cycle
Chapter 13
Meiosis And Sexual Life Cycle

2. An Introduction to Heredity
Heredity: the transition of traits from one generation to the next
Along w/ similarities, there is also variation- offspring differ somewhat in appearance from parents to offspring
Genetics: the study of heredity and hereditary variation

3. An Introduction to Heredity
Parents pass information to offspring through coded hereditary units called genes.
- 30 to 40 thousand genes in humans
- genes are segments of DNA

4. Concept 13.1 Two types of reproduction:
Asexual reproduction: single parent passes copies of all its genes to offspring; “like begets like.”
Sexual reproduction: two parents give rise to offspring; results in greater variation

5. Concept 13.2
Life cycle: generation-to-generation sequence of stages in the reproductive history of an organism
The human life cycle
- each somatic cell has 46 chromosomes (23 pair); 2 copies of each pair are called homologous chromosomes

6. Concept 13.2 - chromosomes can be displayed through a karyotype
- pairs 1-22 are called autosomes
- pair 23 are called sex chromosomes
- XX = female; XY = male
- we inherit one chromosome of each pair from each parent

7. Concept 13.2

8. Concept 13.2

9. Concept 13.2
- gametes, or sex cells, contain 22 autosomes and 1 sex chromosome; a cell with a single chromosome set is called a haploid cell.
- haploid cells are abbreviated 1n (n = 23)
- the combining of gametes forms a zygote; becomes a diploid cell (2n)

10. Concept 13.2
- The only cells of the human body not produced by mitosis are gametes
- this process is a form of cellular division called meiosis; meiosis reduces the chromosome number in half

11. Concept 13.2 The variety of sexual life cycles Animal cells
- after meiosis, the gametes undergoes no division before fertilization
Fungi
- after meiosis, mitosis occurs and produces a 1n multicellular organism; forms 1n gametes that then fertilize

12. Concept 13.2 Plants (alternation of generations)
- has both diploid (sporophyte) and haploid (gametophyte) multicellular stages;

13. Concept 13.2

14. Concept 13.3
Meiosis, like mitosis, is preceded by the replication of chromosomes; however, the single replication is followed by 2 consecutive divisions (meiosis I and meiosis II)

15. Concept 13.3

16. Concept 13.3
Meiosis I
- during prophase I, chromosomes pair up in synapsis; 4 chromatids form a tetrad
- during metaphase I, homologous pairs line up on equator
- during anaphase I, chromosomes, not chromatids, separate to poles

17. Concept 13.3
Meiosis II
- goes through the same steps as meiosis I, but does not replicate DNA
Meiosis outcome is 4 1n gametes from a single cell

18. Concept 13.4
3 mechanisms contribute to the genetic variation arising from sexual reproduction
Independent assortment of chromosomes
-each homologous pair of chromosomes is positioned independently of the others; variation is 223 or about 8 million

19. Concept 13.4

20. Concept 13.4
Crossing over
- when chromosomes line up along the equator, parts of chromatids can combine genes from parents
Random fertilization
- even w/out considering crossing over, any two parents will produce a zygote with any of 70 trillion combination (223 x 223)

21. Concept 13.4