MEIOSISVariety is the Spice of Life!

SEXUAL REPRODUCTION

The production of offspring from the fusion of two sex cells

Usually from two different parents

The genetic makeup of the offspring is different from that of either parent

TERMS TO KNOW SOMATIC CELLS: cells that perform

mitosisEXAMPLE: skin cells

GAMETE CELLS: cells that perform meiosis

EXAMPLE: sex cells FERTILIZATION: when two gamete cells

combine to form a single zygote cell ZYGOTE: the first cell of a new individual

HOMOLOGOUS CHROMOSOMES

Pairs of chromosomes that are similar in structure and carry similar genetic information

You get one chromosome from your mother and one chromosome from your father

CHROMOSOMES

The number of chromosomes in somatic cells

There are 46 chromosomes in a typical human somatic cell

The number of chromosome in gamete cells

The number of chromosomes in gamete cells is half the number of somatic cells

There are 23 chromosomes in a typical human gamete cell

DIPLOID NUMBER HAPLOID NUMBER

FERTILIZATION

MEIOSIS A form of cell division in which the

resulting daughter cells have half the number of chromosomes as the parent cell

There are two stages: Meiosis I and Meiosis II

Results in the formation of four gamete cells

It takes place only in reproductive tissue of sexually reproducing organisms

MITOSIS VS. MEIOSIS

MITOSIS MEIOSIS

STAGES OF MEIOSIS

Prophase I Metaphase I Anaphase I Telophase I

Prophase II Metaphase II Anaphase II Telophase II

MEISOSIS I MEIOSIS II

PROPHASE I Similar events

occur in prophase I of meiosis as in prophase of mitosis

Chromosomes come together in homologous pairs

The homologous pairing forms a TETRAD

GENETIC VARIATION During PROPHASE I there are two events

that occur to ensure that you have genetic variability:

RANDOM ASSORTMENT: chromosomes that come from the father will not stay together nor will chromosomes that come from the mother

RECOMBINATION: the exchanging of genetic information called CROSSING OVER

METAPHASE I

The tetrads migrate towards the centre of the cell and align across the middle of the cell

ANAPHASE I The homologous

chromosomes move to opposite poles of the cell

Only ONE chromosome from each homologous pair will be found in each new daughter cell

TELOPHASE I The cells begin

to divide Each daughter

cell has only one member of each original pair

These cells are now HAPLOID

MEIOSIS II Begins immediately after Telophase I There is NO duplication of DNA

between Meiosis I and Meiosis II The chromosomes still consist of sister

chromatids, however, the sister chromatids are no longer identical

Meiosis II continues on like Mitosis PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II

GAMETOGENESIS

The production of sperm cells

There is even division the of the cytoplasm at each stage of meiosis

The final product is four SPERM cells

The production of egg cells

The cytoplasm doesn’t divide equally at each stage of meiosis

Final product is a single OVUM

SPERMATOGENESIS OOGENESIS

NON-DISJUNCTION The separation of chromosomes

occurs incorrectly It can happen during Meiosis I or

Meiosis II It results in a cell that can contain too

many or too few chromosomes If that cell is involved in fertilization,

the zygote will have an abnormal number of chromosomes

NON-DISJUNCTION

A chromosomal abnormality in which there are three homologous chromosomes

A chromosomal abnormality in which there is a single chromosome

TRISOMY MONOSOMY

DOWN SYNDROME An individual that

has three copies of chromosome number 21

These individuals experience physical and mental challenges

TURNER SYNDROME An individual that

has only one X chromosome

These individuals are female in appearance but do not mature sexually and are sterile

KLINEFELTER SYNDROME

An individual with two X chromosomes and one Y chromosome

These individuals are male in appearance with feminine body characteristics and are usually sterile