Meiosis
How cells create gametes
Fertilization
• When a sperm and an egg
meet, fertilization occurs inside
a sex cell.
• Cell division begins after
fertilization.
• Sex cells are involved in
Meiosis.
Gametes/Zygotes/Embryos
• Gametes are sex cells.
• When a sex cell is fertilized, it is
called a zygote.
• The zygote will eventually
develop into an embryo.
Number of Chromosomes
• Body Cells (Mitosis)
– Humans have 46 chromosomes, 23 pairs
• Sex Cells (Meiosis)
– Humans have 23 chromosomes, not in pairs
Diploid
• Di means 2 (or pairs)
• Diploid cells have pairs of chromosomes
• Body cells are diploid because they contain 23
pairs of chromosomes
• Mitosis produces diploid cells
• Zygotes (2n) are diploid because they contain
pairs of chromosomes from both the sperm
and egg
Diploid Cells Continued…
• Mitosis produces diploid body cells
• If 2 diploid cells combine, offspring would
have TWO times as many chromosomes as the
parent and the animal would not survive
• Body cells are produced by asexual
reproduction
• So how do we get to the correct
number of chromosomes?
Haploid
• Haploid means half the number of
chromosomes as body cells
• Gametes or sex cells are haploid
• That means that sperm are
haploid (n) and eggs are haploid
(n)
• They have 23 chromosomes,
which are half the number of (46)
chromosomes in body cells
Haploid Cells Continued…
• Meiosis produces haploid sex cells through
sexual reproduction
• Meiosis ensures offspring will have the same
diploid number as its parent
Meiosis I
• See pg 128-129 in your textbook for diagrams.
• In Meiosis I, the nucleus divides and produces
2 new cells with 1 nucleus each. This is the
same as the process of mitosis, but with sex
cells.
• An exact copy is made of the original cell.
Meiosis II
• The nuclei divide and the chromatids separate
producing 4 cells with half the number of
chromosomes of the original nucleus.
• From a human cell with 46 chromosomes,
meiosis produces 4 sex cells, each with 23
unpaired (haploid) chromosomes.
Did you know??
• There are more than 8 million combinations of
genes in humans, per parent.
• With 2 parents’ genes, there is a possibility of 70
trillion combinations.
• Meiosis takes an average of 74 hours in humans.
• The fertilized egg, with too few or too many
chromosomes may die, or the growing embryo
may have birth defects. Down Syndrome occurs
in humans when the offspring receives an extra
21st chromosome.
Take a closer look…
• http://www.pbs.org/wgbh/nova/miracle/divi_
flash.html