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.

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.

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

• Results in 4 daughter cells with only 23 chromosomes in each

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

http://post.queensu.ca/

Animation

http://www.johnkyrk.com/meiosis.html

Animation

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

What is a zygote and how is it formed?

A zygote is a cell that is formed when a sperm fertilizes and egg.

Compare and contrast sexual and asexual reproduction.

Sexual reproduction – offspring is produced when sex cells (egg & sperm) combine. Offspring are genetically different from each other and the parents.

Asexual reproduction – One parent produces genetically identical offspring