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Meiosis – A Source of Distinction
Why do you share some but not all characters of each parent?
What are the rules of this sharing game?
At one level, the answers
lie in meiosis.
Meiosis – A Source of Distinction
Meiosis does two things -
1) Meiosis takes a cell with two copies of every chromosome (diploid)
and makes cells with a single copy of every chromosome (haploid).
This is a good idea if you’re going to combine two cells to make a new
organism. This trick is accomplished by halving chromosome number.
In meiosis, one diploid cells produces four haploid cells.
Reduction of Chromosome
Number in the First Division of
Meiosis
Meiosis – A Source of Distinction
The second accomplishment of meiosis:
2) Meiosis scrambles the specific forms of each gene that each sex cell
(egg or sperm) receives.
This makes for a lot of genetic diversity. This trick is accomplished through
independent assortment and crossing-over.
Genetic diversity is important for the evolution of populations and
species.
Keys to Understanding Meiosis
Chromosomes are paired.
Chromosomes carry genes.
The gene forms on a pair of chromosomes may be identical .. or different.
Brown eyes
Brown eyes
Tall
Tall
Brown eyes
Blue eyes
Tall
Short
One Way Meiosis Makes Lots of
Different Sex Cells (Gametes) –
Independent Assortment
Independent assortment produces 2n distinct gametes,
where n = the number of unique chromosomes.
In humans, n = 23 and 223 ≈ 8,000,000.
That’s a lot of diversity by this mechanism alone.
Another Way Meiosis Makes Lots of Different Sex Cells –
Crossing-Over
Crossing-over multiplies the already huge number of different gamete types
produced by independent assortment.
Between Independent Assortment and Crossing-Over, No Two
Gametes Are Identical.
Meiosis Following a Single
Chromosome Pair
Crossing over
The Key Difference Between Mitosis and Meiosis is the Way
Chromosomes Uniquely Pair and Align in Meiosis
Mitosis
The first (and
distinguishing)
division of meiosis
Boy or Girl? The Y Chromosome “Decides”
Y chromosome
X chromosome
Boy or Girl? The Y Chromosome “Decides”
Oogenesis – A Path of Meiosis in
Humans
Woman are less busy in meiosis than
men – meiosis produces only a few
hundred mature eggs over a lifetime.
Spermatogenesis – Another Path
of Meiosis in Humans
Men are always doing meiosis,
producing roughly 250,000,000 sperm
per day.
“Putting It All Together” - Fertilization
What Meiosis is About
Meiosis allows the creation of unique individuals through sexual reproduction.