The only place that meiosis
occurs is within the testes
or the ovaries
The only cells that have 23
chromosomes in the human body
are the sperm and egg cells
Therefore, when sperm and egg
unite, a single cell (zygote) with
the correct amount of DNA is
produced.
The zygote then begins to divide rapidly
through mitosis
Similar, yet strangely different…
Homologous chromosomes pair up
during prophase I.
The process is called “synapsis”.
Crossing Over:
During synapsis,
neighboring
chromatids may wrap
around each other,
break off, and then
reattach to a
different chromatid.
=Genetic Recombination
Homologous pairs line up randomly along the
equator. This also increases genetic variability.
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The random migration of chromosomes to different
poles (during Anaphase I) is called independent
assortment.
***Notice that when one chromosome from
each pair moves to the opposite cell, the
chromosome number is cut in half…
And again,
What are
those four
cells at the
end of
meiosis??
Gametes!!
(sperm or eggs…)
FYI…
Although four sperm are produced at the end
of meiosis in a male, only one of the final four
cells becomes an egg in females.
Why Not Asexual Reproduction??
Sexual Reproduction increases genetic variability through
several methods
•independent assortment of homologous chromosomes
•crossing over of DNA from one homologue to another
•random fertilization
http://www.naturephotography.net/images/dancingcranes.jpg
Why is Genetic Diversity an Advantage?
Provides advantages in a fluctuating environment
As the environment changes (sometimes due to natural
disasters, sometimes due to a species becoming nomadic), new food
sources become available, old food sources disappear, new infectious
diseases arise, climates change, new predators arise, etc. Genetic
variability increases the odds that some offspring will be born who
can survive conditions that may have been unfavorable to previous
generations.