Cell Divison

Each of your body cells has 46 chromosomes, or 23 pairs.

Each pair is made of HOMOLOGOUS CHROMOSOMES

Homo = same

These chromosomes contain the same genes in the same order

Half of your chromosomes (23) came from your mom

Half of your chromosomes (23) came from your dad

Each parent gave you one chromosome from a homologous pair.

gene

From Mom

From Dad

From Mom

From Dad

When your BODY cells (autosomal) divide, they make exact copies

Results = two cells with the same type and number of chromosomes as the parent cell.

Many animals reproduce ASEXUALLY – through mitosisex: flatworms, sponges, jellyfish

If humans were created through mitosis:1st generation

46 from mom + 46 from dad = 92 chromosomes for baby

then2nd generation

92 from mom + 92 from dad = 184 chromosomes for baby

And so on, and so on, and so on…

A second type of cell division that occurs in the SEX CELLS (gametes: sperm and egg)

Division creates four cells with half the number of chromosomes as the parent cell.

Body cells (autosomal cells) are DIPLOID DI = 2; Each cell has 2 copies of each

chromosome These are considered “2n”

◦ “n” is the chromosome number

• Sex cells are HAPLOID

• Each sex cell has HALF the normal number of chromosomes

•These are considered “n”

Sperm are the sex cells in males Eggs are the sex cells in femalesBoth are produced through MEIOSIS

Any reproduction that involves SPERM and EGGS is considered SEXUAL REPRODUCTION.

Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II

First Division

Second Division

**Meiosis contains TWO divisions whereas Mitosis only has one!

** Similar to Mitosis…Meiosis does experience interphase.

**Chromosomes are replicated during “S” phase.

Prophase I Chromatin coil and condense,

chromosomes are visible. Spindle form Homologous chromosomes line up gene by

gene to form a tetrad.◦ Tetrad: two homologous chromosomes made of

two sister chromatids paired tightly◦ Crossing over occurs.

Occurs between two non-sister chromatids. Can happen many times or not at all Average: 2 to 3 crosses per each pair of

homologues. Results in genetic variation

◦ Explains why siblings look alike but not identical.

Interphase: All 23 pairs of chromosomes duplicate.

Prophase begins.

Metaphase I◦ Spindle fibers attach to centromere of each

chromosome.◦ Tetrads pulled to the center◦ Line up beside each other

In Mitosis they line up end to end…not paired.

Meiosis: Chromosome line up beside their homologous pairs.

Mitosis: Chromosomes line up end to end

Anaphase I Homologous chromosomes separate

◦ Head to opposite sides of cell Centromeres do not split

◦ Unlike mitosis Ensures that each new cell will receive one

chromosome from each homologous pair

Meiosis: Sister Chromatids stay together

Mitosis: Sister Chromatids separate

Telophase I Prophase Reversed Cytokenisis Occurs Still has one chromosome from each

homologous pair Start the division process over again!

Meiosis: Still have one chromosome from each homologous pair

**need to separate them in order for each sex cell to have ½ of the genetic material.

Prophase II Chromosomes Condense Spindle Form and attach to centromere

◦ NOTICE….did not undergo inerphase therefore the chromosomes did not replicate again!

Metaphase II◦ Chromosomes line up at the equator

End to end this time….like mitosisAnaphase II

◦ Sister chromatids split apart and move to opposite poles

Telophase II◦ Nuclei reform◦ Spindle Breaks down

Cytokenisis

Meiosis I

Meiosis II

Four haploid cells Contains 1 chromosome of each kind

2n2n

nn

nn

nn

nn

http://www.youtube.com/watch?v=D1_-mQS_FZ0