Chromosomes and Chromosome
Number
 Human body cells have 46
chromosomes
 Each parent contributes 23
chromosomes
 Homologous chromosomes -one of
two paired chromosomes, one from
each parent
Homologous Chromosomes
 Same length
 Same centromere position
 Carry genes that control the same
inherited traits
 One came from Dad, &
one came from Mom!
Why Meiosis?
 An organism produces gametes to
maintain the same number of
chromosomes from generation to
generation, and also introduce and
maintain variation within the population.
 We will discuss later, what happens when
there is an error in meiosis. Mutations and
Evolutionary events
Compare diploid and haploid number
Diploid: cell that contains
two of each kind of
chromosome (2N)
 Body cells(somatic cells)
are diploid

Haploid: cell with only
one kind of chromosome
(N)
 Gametes (sex cells) are
haploid

Meiosis I
 The sexual life cycle in animals involves
meiosis.
 Meiosis produces
gametes.
 When gametes combine in fertilization,
the number of chromosomes is restored.
This is often called alternation of generations, the cycling
between haploid and diploid organisms (cells)
Meiosis I
 Prophase I- each chromosome pairs with its
corresponding homologous chromosome to
form a tetrad(4 chromatids)
 The nuclear envelope breaks
down.
 Spindles form.
*Crossing-over- chromosomes will switch some
genes. This gives us genetic variation.
Crossing-over is more common the farther it is
located from the centromere. Remember map units in
Sordaria
Crossing Over
Metaphase I
 Homologous pairs of chromosomes,
still in their tetrad, line up together
at the middle of the cell
Chromosome centromeres attach to
spindle fibers.
Anaphase I
 homologous chromosomes separate (the
tetrad seperates) & move to opposite
ends. (centromeres do not split)
Telophase I & Cytokinesis
spindle disappears and the cell divides
Meiosis I results in 2 daughter cells, each
with the same number of chromosomes as the
original cell however sister chromatids are
still attached to each other
Meiosis II
During the second meiotic division,
nothing is replicated, everything is going
to be just divide
Prophase II
spindle apparatus
reforms and the
chromosomes
condense, while the
nuclear membrane
dissolves again
Metaphase II
 chromosomes still attached at the
centromere line up at the equator
(middle)
Anaphase II
 sister chromatids are pulled apart
at the centromere & move to opposite
poles
Telophase II (includes cytokinesis)
 when the 2 daughter cells divide into 4
new daughter cells, each new daughter
cell has 23 chromosomes. This is half
the number of the original parent cell
Difference in
meiosis






In females: oogenesis
1 mature egg, 3 polar
bodies which break down
Women born with all
eggs they will have,
meiosis I occurs before
birth, meiosis II occurs
once a month
Much larger
Have all X chromosomes
Has no method of
movement






In males: spermatogenesis
4 mature sperm
Males begin to produce
sperm after puberty,
produced constantly until
death; meiosis II occurs
immediately after meiosis
I
Much smaller than egg
May have X or Y
chromosomes
Have flagella to move
Meiosis/gamete production
Females have XX as their 23rd pair of
chromosomes
 Males have XY which are nonhomologous
 Since all eggs are X, father determines the
sex of the child since the sperm may be X
or Y

Meiosis
There are 2 divisions of meiosis- Meiosis I and
Meiosis II. It must be this way in order to end
up with the sex cells (sperm & egg) only having 23
chromosome.
Meiosis occurs in the testes of the male and is
called spermatogenesis (4 viable sperm)
And in the ovaries of the female it is called
oogenesis. ( produces 1 egg and 3 polar bodies)
23
+
23 =
46 chromosomes
Mitosis/Meiosis
Mitosis
 Cell division producing
identical somatic cells
 Results in 2 daughter
cells
 One division
 Daughter cells have
same number of
chromosomes (2N)
Meiosis
 Cell division producing
unique gametic cells
 Results in 4 daughter
cells
 Two divisions
 Daughter cells have
half the number of
chromosomes (N)
Cell division
cell type
# of daughter
# of
cells
chromosomes
Mitosis
somatic(body)
2
46(diploid)
Meiosis
gametes(sex)
4
23(haploid)
www.pbs.org
How cells
divide:Mitosis
vs. meiosis