1. Meiosis Ch. 9

2. Gametes
Gametes are reproductive cells (eggs and sperm) that contain half the complement of chromosomes found in somatic cells
the gametes fuse to form a new cell called a zygote, which contains two complete copies of each chromosome
the fusion of gametes is called fertilization, or syngamy
egg + sperm = Zygote

3. Haploid Cell vs. Diploid Cell
Typically, each cell of your body has 46 chromosomes..23 from each parent
So, you have what we call a Diploid value of 46
Or, referred to as 2N = 46
Your gametes, however, have 1N values
1N = 23….This is a Haploid condition
All your normal body cells are diploid, only your gametes are haploid
THIS IS TRUE FOR MOST ORGANISMS!!!!!!
Sex Cells

4. Chromosome Number
Every organisms inherits chromosomes from their parents.
Asexually reproducing organisms pass on their full set of chromosome creating offspring that are essentially clones of themselves
Sexually reproducing organisms pass on half of their chromosomes through their sex cells. The offspring created inherits half of its chromosomes from each parent.

5. Human Life Cycle Gamete- egg and sperm
Zygote- cell produced when egg and sperm fuse
Fertilization/ syngamy- fusion of gametes

6. Traits
Inheritable traits are carried on factors called genes and are passed down from generation to generation.
Genes are located on DNA within the nucleus of the cell.
There are two alleles for every gene. One allele is inherited from each parent
Alleles are alternate versions of a gene
The process by which chromosomes are separated in the production of sex cells is known as Meiosis.

7. Homologous Chromosomes
Homologous Chromosomes- chromosomes that carry the alleles for the same gene.
Diploid Cells- (2N) “Two sets”; have both sets of homologous chromosomes (All body cells are diploid)
Haploid Cells- (1N) “one set”; has a single set of genes. (All sex cells are haploid)

8. Meiosis
Meiosis- is the process by which the homologous chromosomes are separated in a diploid cell to produce a haploid cell
Meiosis involves two divisions, meiosis I and meiosis II
meiosis I separates the homologues in a homologous pair
meiosis II separates the replicate sister chromatids

9. Meiosis I
Meiosis I is traditionally divided into four sequential stages
Prophase I
Homologous chromsomes pair up and exchange segments
Metaphase I
The paired homologous chromosomes align on a central plane
Anaphase I
Homologues are pulled apart and move to opposite poles of the cell
Telophase I
Individual replicated chromosomes gather at each of the two poles

10. Prophase I
Prior to the start of Meiosis, chromosomes are replicated during Interphase.
During Prophase I homologous chromosomes pair up to form a tetrad (there are 4 chromatids in a tetrad)
Crossing over may occur when the tetrads exchange a portion of the chromatids
During crossing-over chromatids break and may be reattached to a different homologous chromosome.
Human female eggs remain in Meiosis I until puberty… years

11. Meiosis I: Metaphase I
Each homologous set of chromosomes line up along cell center
Orientation on the metaphase plate is random…with either parental homologue on a side. This means that there is a chance for the daughter cells to get either the mother's or father's homologue for each chromosome.

12. Metaphase I
During metaphase I, the orientation of the homologous chromosome pairs is a matter of chance
each possible orientation of which homologue in a homologous pair faces which pole results in gametes with different combinations of parental chromosomes
this process is called independent assortment

13. Meiosis I: Anaphase I Homologs are pulled apart to opposite poles
Chromosomes, each with two chromatids, move to separate poles.
Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids.

14. Meiosis I: Telophase I Homologous chromosomes have fully separated
Results in a Haploid (1N) set of chromosomes at each pole
Nuclear envelopes may reform, or the cell may quickly start meiosis 2.

15. Overview: Meiosis I

16. Meiois II Meiosis II is also divided into four stages Prophase II
new spindle forms to attach to chromosome clusters
Metaphase II
spindle fibers bind to both sides of the centromere and individual chromosomes align along a central plane
Anaphase II
sister chromatids move to opposite poles
Telophase II
the nuclear envelope is reformed around each of the four sets of daughter chromosomes

17. Meiosis II
What does Meiosis II look like????

18. Meiosis Overview

19. Meiosis vs. Mitosis

20. Why Sex??
Sexual reproduction has an enormous impact on how species evolve because it rapidly generates new genetic combinations
Three mechanisms help produce this variety
Independent assortment
Crossing over
Random fertilization

21. Gametogenesis
Gametogenesis the process of forming gametes (haploid, 1n) from diploid cells (somatic cells) of the germ line.
Spermatogenesis is the process of forming sperm cells by meiosis. In spermatogenesis all 4 meiotic products develop into gametes and human males produce 200,000,000 sperm per day
Oogenesis is the process of forming an ovum (egg) by meiosis in specialized gonads known as ovaries. Human females female produce one egg (usually) each menstrual cycle

22. Gamete Formation