• Have your compare & contrast mitosis & meiosis ws out on the table

The Big Questions…

• Why do you look a little like your mother a little like your father?

• Why do you have traits that your grandparents had but your parents didn’t?

The beginning of THE ANSWERS

IS…Meiosis!I will post these notes on the hw web so pay

attention & ask questions

Review “Acting our Mitosis”…

I.) In chromosomes exist in pairs. One from mother and one from its father.

A.) In humans, we have a total of 46 chromosomes. 23 from mom and 23 from dad.

1.) Therefore, we have 23 pairs of chromosomes.

2.) These pairs of chromosomes are called HOMOLOGOUS pairs.

a.) Same in SIZE and FUNCTION.

b.) BUT Slightly different versions of the same genes, called alleles

c.) The genes determine the TRAITS

WHAT’S a Trait???

Diploidy

• For most genes you have 2 alleles (i.e. brown eye color proteins and green eye color proteins).

• Sometimes you express only one of them and sometimes both

A.) Any cell with 2 sets of chromosomes (1 set from mom and 1 from dad) is called a DIPLOID cell. (46 chromosomes)

a.) ALL the cells in your body are diploid EXCEPT the GAMETES (sex cells).

b) Gametes have 1 set of chromosomes (a mix of mom & dad) SO we call them HAPLOID. (23 chromosomes)

c. )WHY? If the chromosome # wasn’t halved then each generation the chromosome number would double.

Remember

Sperm/Egg• But you only pass

one of your homologous chromosomes on to each sperm or egg cell.

• Each gamete randomly receives 1 of the 2 versions of each chromosome

Purpose of meiosis• To allow

individuals to pass on half of their chromosomes to offspring (sexual reproduction)

• A germ cell in the ovary or testis divide 2x to create egg or sperm

Meiosis I-division of homologous pairs• Instead of the replicated chromosomes being

separated – the homologous chromosomes get pulled apart

Meiosis II- similar to Mitosis…• EXCEPT the cell begins with only half the

chromosomes

Meiosis• Each parent cell produces 4 haploid daughter

cells

• Each daughter cell has 1 set of chromosomes (instead of the usual 2) OR (half of the parent’s chromosomes)

Why Sexual Reproduction?

• DIVERSITY!

• Meiosis produces an incredible amount of diversity

• Diversity is good!

• B/C diversity increases chances that our species will survive

How does diversity happen?

• #1 Crossing over

• #2 Independent Assortment

• #3 Recombining genes of 2 people (aka sexual reproduction

Crossing OverA.) During Prophase I

1.) Unlike mitosis, each pair of HOMOLOGOUS chromosomes “find” each other and pair up.

a.) the 2 “huggers” are paired so tightly that they can exchange pieces of themselves (exchange GENES) in a process called CROSSING OVER.

b.) Now ALL of the chromatids are unique with new combinations of GENES

Independent Assortment: the different ways the chromosomes could align with the maternal and paternal chromosomes switching sides. This alignment is random.

Which side of the cell the chromosome lines up on = 223 + crossing over!!!

You can get either 1 of your mother’s pair of chromosomes OR all 23!

Recombination of genes• You get genes from your mother and your father-

new combinations can be beneficial• There are 223 possible chromosome combinations

to make a gamete• =8,388,608 possibilities• So you are 1 out of 8,388,6082 possibilities• You are quite literally 1 in a trillion (1 of

70,368,744,200,000 possible combinations of your parents)

• AWE!