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.h
tml
Animation
http://www.csuchico.edu/~jbell/Bio
l207/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