Meiosis
and
Sexual Reproduction
Homologous Chromosomes
• Chromosomes of each pair are similar in
length and centromere position
• Both carry genes controlling the same
inherited characteristics
• We inherit one
chromosome of each
homologous pair from
our mother and the
other from our
father…(Therefore,
these are NOT
identical!)
Meiosis
• A type of cell division that…
– Occurs only in the sex organs
• Sperm cells in men
• Eggs in women
– Produces 4 genetically unique cells each
with HALF the number of chromosomes as
the parent cell
– Keeps the number of chromosomes in a
species the same generation to generation
Karyotype
Two Types of Cells
• Body cells are called somatic cells.
• Germ cells develop into sex cells.
– Gametes are sex cells: egg and sperm.
– Gametes have DNA that can be passed to
offspring
Haploid vs. Diploid
• Fertilization between egg and sperm occurs in
sexual reproduction.
• Diploid (2n) cells have two copies of every
chromosome.
– Body cells are diploid.
– Half the chromosomes come from each parent
Haploid (n) cells have one copy of
every chromosome.
– Gametes are haploid.
– Gametes have 22 autosomes and 1 sex
chromosome
Meiosis makes haploid cells from
diploid cells
Some details of meiosis…
• As with mitosis, meiosis begins after the
chromosomes have been duplicated during
Interphase
• Meiosis consists of two distinct parts
– Meiosis I
– Meiosis II
Meiosis I
Homologous chromosomes exchange genetic
material (cross over) and separate during
anaphase
• Crossing over results in new combinations of
genes
Meiosis II
Sister chromatids are separated much as
they are in mitosis except the resulting
cells are haploid and not diploid
Key Differences between Mitosis
and Meiosis
• Mitosis:
– One cell division
– No pairing of homologous
chromosomes
– Produces two offspring cells
each with the same number
of chromosomes as the
parent cell (46)
– Produces genetically
identical copies of the
parent cell
• Meiosis:
– Two cell divisions
– Homologous
chromosomes pair
– Produces four offspring
cells each with one set of
chromosomes—half the
number as the parent cell
(23)
– Creates genetically unique
cells different from the
parent cell
• This is because homologous
chromosomes exchange
genetic information
Which process is this?
• Provides for growth, repair, and asexual
reproduction
• Produces TWO daughter cells that are
genetically identical to the parent cell
• Involves one division of the DNA
• Diploid parent cells produce diploid daughter
cells
• Happens in most cells of the body
Which process is this?
• Yields haploid daughter cells with only ONE
member of the homologous chromosome pair
• Produces FOUR daughter cells that are
genetically different from the parent cell
• Involves TWO divisions of the DNA
• Exchange of genetic material occurs between
homologous chromosomes
Gametogenesis
• All sperm and eggs produced during gametogenesis are genetically
unique.
• What would happen if humans created gametes that were all
genetically identical to each other? (Think about your siblings)
• Twins—identical vs. fraternal (how does this occur?)
Trisomy 21—Down’s Syndrome
• Three copies of
chromosome 21
Maternal
Age
20
Incidence at
Birth
1 in 1500
30
1 in 900
35
1 in 400
40
1 in100
45
1 in 30
Klinefelter’s Syndrome
•
•
•
•
Male—XXY
1 in 1000 males
Sterile
Tall, thin, and slightly lower
IQ
Other Disorders
• Turner’s Syndrome—XO
– 1 in 500 female births (most
are aborted before birth)
– Very short, infertile, broad
chest and may have a webbed
neck
• Edward’s Syndrome—
Trisomy 18
– 1 in 3000
– Most babies die within the
first month to the first year