BIG QUESTION:

Explain why I look like my parents.

Is the DNA equal from each parent?

What role does reproduction play into this process? Why would that make me look like my mother and father?

Do you get equal amount of genes from each parent?

Why dont my siblings look alike?

Cell Division

Meiosis

Definition • Cell division by which eggs and sperm

are produced. • The production of sex cells, which are

not genetically identical, through a series of cell divisions.

● Homologous chromosomes: one of the matching pair of chromosomes, one from each parent.

● Haploid cell (n): a single set of chromosomes

● Gamete: a reproductive cell that contains the single (haploid) set of chromosomes

● Diploid cell (2n): 2 sets of homologous chromosomes

● Almost all human cells are diploid.● The total number is 46;

○ 23 from your mother○ 23 from your father

● Ensures that humans have the same number of chromosomes in each generation.

● It is a two-step process that reduces the chromosome number by half—from 46 to 23—to form sperm and egg cells.

● This is a reduction in genetic material.

Why do the gametes only have 23

chromosomes???????

● When the sperm and egg cells join at fertilization, each contributes 23 chromosomes

● So the resulting embryo will have the usual 46.

Meiosis also allows genetic variation through a process of

DNA shuffling while the cells are dividing.

Meiosis• TWO divisions• ONE DNA replication

Meiosis• Interphase: Before meiosis begins, genetic material

is duplicated. • First division of meiosis

– Prophase 1: Duplicated chromatin condenses. Each chromosome consists of two, closely associated sister chromatids. Crossing over can occur during the latter part of this stage.

– Metaphase 1: Homologous chromosomes align at the equatorial plate.

– Anaphase 1: Homologous pairs separate with sister chromatids remaining together.

– Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

Meiosis• Second division of meiosis: Gamete formation

– Prophase 2: DNA does not replicate.

– Metaphase 2: Chromosomes align at the equatorial plate.

– Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole.

– Telophase 2: Cell division is complete. Four haploid

daughter cells are obtained.

• One parent cell produces four daughter cells. Daughter cells have half the number of chromosomes found in the original parent cell and with crossing over, are genetically different.

http://www.palaeos.com/

Crossing Over• Where a section of one chromosome

switches places with the same section from the other chromosome of the pair.

• Recombination

http://www.accessexcellence.org

Animation

http://www.phschool.com/science/biology_place/biocoach/meiosis/process.html

Animation

http://www.csuchico.edu/~jbell/Biol207/animations/meiosis.html