Meiosis and Gametes 1

Chapter 11 Sexual Reproduction and Meiosis

Rev Question

Meiosis and Gametes 2

When and where does meiosis occur in humans?

Why is meiosis necessary?

“reductive division”

Meiosis and Gametes 3

What are the stages of Meiosis?

Meiosis I vs Meiosis II

Prophase I

Metaphase I

Anaphase I

Telophase I

McGraw-Hill Meiosis

Meiosis unique features

Meiosis and Gametes 4

How does Prophase I differ from Prophase of Mitosis?

1) Bivalents form -- “synapsis”

-- homologous chromosome pairs

2) Crossing over occurs-- exchange of DNA segments-- “chiasmata”

During Metaphase 1,bivalents are aligned at equatornot individual chromosomes

Question

Meiosis and Gametes 5

During Meiosis I parental homologous chromosomes are independently sorted to daughter cells

Chromosome reduction occurs during Meiosis I

Interkinesis

What happens during Meiosis II?

Chromosome reduction ---- sister chromatids separate

Gametes produced

McGraw-Hill sorting

Question

Meiosis and Gametes 6

Comparison of Meiosis and Mitosis

Metaphase looks differentfor each process

2N=4Question

Question

Meiosis and Gametes 7

Homework assignment:10 points: due next class

For a cell where 2N=6, draw a series of diagrams showing the chromosomes moving through the stages of meiosis.

In one of the diagram clearly label:-- a pair of homologous chromosomes-- a bivalent

Label all stages

Make diagrams large, clear and neat

Also answer the following questions:For an organism with a chromosome number of 2n = 56:

A. How many pairs of homologous chromosomes does each normal body cell possess? ___B. How many chromosomes does each cell possess after mitosis? ____C. How many pairs of sister chromatids are present at the beginning of meiosis? ___D. How many bivalents will form during prophase-I? ____E. How many chromosomes does each cell possess after meiosis? ___

Drawings must be neatly done and chromosomes drawn with

adequate detail. Chromosomes, bivalents and spindle fibers

must be recognizable as these structures.

Meiosis and Gametes 8

Why does meiosis in females yieldonly 1 viable gamete?

Why does spermatogenesis in males assure 50:50 ratio of males to females?

Sex of the sperm determines sex of the offspring

Meiosis and Gametes 9

How can errors during meiosis causeCongenital birth disorders?

Non-disjunction of chromosomes

e.g., Down’s syndrome

What is karyotyping?

“trisomy 21”-- an example of “aneuploidy”

Many effects-- reduced intellectual development-- heart and kidney defects

-- often fatal-- early onset Alzheimer's

Meiosis and Gametes 10

How does meiosis create genetic diversity?

1) crossing over

2) random assortment of chromosomes

and later …

3) random pairing of gametes