Meiosis

Reproduction of sex cells

I. History

In 1882, a scientist observed that the sex cells of roundworms contained different numbers of chromosomes in comparison to the body (somatic) cells of the individuals.

These observations led scientists to believe that a different type of cell division occurred that reduced the number of chromosomes by half

o In human and other eukaryotic organisms there are two types of cells

1. Somatic cells or basic BODY cells that have 46 chromosomes or 23 PAIRS

-Diploid cells contain a FULL SET of

chromosomes

o In human and other eukaryotic organisms there are two types of cells

2. Gametes or SEX cells that have 23 chromosomes

-Haploid cells have HALF the number of chromosomes

Sperm

Egg

This was important because when 2 sex cells fused together during FERTILIZATION they produce a new DIPLOID cell called a ZYGOTE

II. Human Chromosome Types:Within a diploid cell there are two different types of

chromosomes

1. _AUTOSOMES (non sex chromosomes): 44 chromosomes (22 sets)

-most are HOMOLOGOUS CHROMOSOMES or pairs of chromosomes from mom and dad that are alike and carry the same genetic traits

• Each pair have genes in the SAME location

• _MALE sex chromosomes are NOT homologous

Are the following homologous or nonhomologous?

The other type of chromosomes are…

2. SEX chromosomes: these determine the gender

a. _XX – female (these are homologous)

b. _XY – male (these are non homologous)

Chromosomes can be seen during different stages of the cell cycle. A KARYOTYPE is a photograph that shows these chromosomes.

III. The Sexual Life Cycle

A. MEIOSIS: DIVISION of a diploid cell to produce SEX CELLS with 23 chromosomes

KEY POINTS:

1. _TOGETHER meiosis and fertilization represent SEXUAL reproduction

2. _GAMETE cells produces are haploid sperm and egg cells

3. Results in 4 genetically NON-IDENTICAL daughter cells

4. Ensures GENETIC VARIABILITY by mixing of genes

RESULT:1. Through the process of sexual reproduction

PARENTS will pass genetic information onto the next generation

2. The offspring will INHERIT a great variety of gene combinations

IV. A Closer Look at Meiosis

• Two rounds of cell division called MEIOSIS I and MEIOSIS II

DNA replication only occurs 1 time between the nuclear divisions to reduce the original chromosome number by half

Draw this flow map across your paper…then add details under each square

Ovary/Teste Interphase Meiosis I Meiosis II

Production of Gametes

• Site of Meiosis

Ovary/Teste

• DNA copies• (just like in Mitosis)

Interphase

• Nuclear division• Crossing over of

homologous chromosomes• Independent assortment of

chromosomes

Meiosis I

PMAT

• No interphase, DNA does NOT copy

• Otherwise, looks like mitosis

• Nuclear division• Chromosomes separate

Meiosis II

PMAT

• 4 Haploid cells• Sperm/egg

Production ofGametes

V. Phases of Meiosis I

• To understand why haploid cells are genetically different, Meiosis I needs to be explained

A. Interphase: chromosomes copy (s-phase)(in humans, 46 chromosomes copy, making

92)

B. During Prophase:o _HOMOLOGOUS CHROMOSOMES PAIR

and CROSSOVERo Crossing over: ensures genetic variability in the

offspring because of RECOMBINATION or MIXING of genetic material

Color code your chromosomes…

D. During Anaphase:o _INDEPENDENT ASSORTMENT occurso After metaphase, pairs of chromosomes SEPARATE

and SORT randomly during cell division to produce gametes containing one chromosome of each type

E. Telophase and Cytokinesis:• Pairs are separated into two new cells

VI. Phases of Meiosis II:• _NO INTERPHASE occurs between two

divisions, the new cells produces will have a REDUCTION in chromosome number and only have half the number of chromosomes

• PMAT, cytokinesis occurs: looks like mitosis

• _4 NEW cells will RESULT and will be non identical due to crossing over and random separation of chromosomes

MEIOSIS IPMAT

MEIOSIS IIPMAT