Upload
helena-nichols
View
215
Download
0
Tags:
Embed Size (px)
Citation preview
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.
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.
Crossing Over
• Where a section of one chromosome switches places with the same section from the other chromosome of the pair.
• Recombination
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.