Meiosis and

Sexual Reproduction

Homologous Chromosomes• Chromosomes of each pair are similar in

length and centromere position• Both carry genes controlling the same

inherited characteristics• We inherit one chromosome of each homologous pair from our mother and the other from our father…(Therefore, these are NOT identical!)

Meiosis• A type of cell division that…

– Occurs only in the sex organs• Sperm cells in men• Eggs in women

– Produces 4 genetically unique cells each with HALF the number of chromosomes as the parent cell

– Keeps the number of chromosomes in a species the same generation to generation

Karyotype

• Body cells are called somatic cells.• Germ cells develop into sex cells.

– Gametes are sex cells: egg and sperm.– Gametes have DNA that can be passed to

offspring

Two Types of Cells

Haploid vs. Diploid

• Fertilization between egg and sperm occurs in sexual reproduction.

• Diploid (2n) cells have two copies of every chromosome.– Body cells are diploid.– Half the chromosomes come from each parent

Haploid (n) cells have one copy of every chromosome.

– Gametes are haploid.– Gametes have 22 autosomes and 1 sex

chromosome

Meiosis makes haploid cells from diploid cells

Some details of meiosis…• As with mitosis, meiosis begins after the

chromosomes have been duplicated during Interphase

• Meiosis consists of two distinct parts– Meiosis I – Meiosis II

Meiosis I

Homologous chromosomes exchange genetic material (cross over) and separate during anaphase

• Crossing over results in new combinations of genes

Meiosis IISister chromatids are separated much as

they are in mitosis except the resulting cells are haploid and not diploid

Key Differences between Mitosis and Meiosis

• Mitosis:– One cell division– No pairing of homologous

chromosomes– Produces two offspring cells

each with the same number of chromosomes as the parent cell (46)

– Produces genetically identical copies of the parent cell

• Meiosis:– Two cell divisions– Homologous

chromosomes pair– Produces four offspring

cells each with one set of chromosomes—half the number as the parent cell (23)

– Creates genetically unique cells different from the parent cell

• This is because homologous chromosomes exchange genetic information

Which process is this?

• Provides for growth, repair, and asexual reproduction

• Produces TWO daughter cells that are genetically identical to the parent cell

• Involves one division of the DNA• Diploid parent cells produce diploid daughter

cells• Happens in most cells of the body

Which process is this?

• Yields haploid daughter cells with only ONE member of the homologous chromosome pair

• Produces FOUR daughter cells that are genetically different from the parent cell

• Involves TWO divisions of the DNA• Exchange of genetic material occurs between

homologous chromosomes

Gametogenesis

• All sperm and eggs produced during gametogenesis are genetically unique.

• What would happen if humans created gametes that were all genetically identical to each other? (Think about your siblings)

• Twins—identical vs. fraternal (how does this occur?)

Trisomy 21—Down’s Syndrome

• Three copies of chromosome 21

Maternal Age

Incidence at Birth

20 1 in 1500

30 1 in 900

35 1 in 400

40 1 in100

45 1 in 30

Klinefelter’s Syndrome• Male—XXY • 1 in 1000 males• Sterile• Tall, thin, and slightly lower

IQ

Other Disorders

• Turner’s Syndrome—XO– 1 in 500 female births (most

are aborted before birth)– Very short, infertile, broad

chest and may have a webbed neck

• Edward’s Syndrome—Trisomy 18– 1 in 3000– Most babies die within the

first month to the first year