Meiosis: the creation of sex cells
Meiosis defined
• The production of gametes (sperm and eggs) by
dividing the genetic material in half.
• Meiosis is a process by which the chromosome
number is reduced by half.
– Only occurs in testes and ovaries.
• in cells called “germ” cells.
• Increases genetic diversity of offspring
Think: why is it important for the cell to have half as many
chromosomes in the gametes?
Before Meiosis
• Cells undergo the
phases G1, S, & G2
prior to meiosis.
• Cells that undergo
meiosis start as diploid
(2n) cells.
• Meiosis results in four
haploid (1n) cells being
produced.
Splitting chromosomes
• Body cells are diploid, (2n)
• Chromosomes are in
homologous pairs.
• One “homologue” of a pair
came from mom and the
other from dad.
• These pairs must be split
during the process of
meiosis
Think: “homo”logous mean they
are…
Haploid result
• After meiosis is
complete only
one of each
type of
chromosome is
present in a
sperm or egg.
– i.e. one of each
Autosome and 1
sex chromosome
Think: can you get an X from your mom or dad? What about the Y?
reduction and division
• Chromosomes # is reduced from 2n to 1n.
• Divisions of the cell result in 4 cells being produced.
Creating variation
• The purpose of meiosis is to create sex cells – the
purpose of sex is to create variation.
• Unlike mitosis, which is to create 2 identical cells with
the same DNA, meiosis has steps to create 4 cells
with different DNA.
Achieving variation: crossing over
• At one stage of meiosis
homologous pairs align with
each other.
• Each pair of homologues
chromosomes is called a
tetrad. (4)
• Crossing over occurs during
tetrad positioning of
chromosomes. Portions of
the DNA is swapped from one
chromosome to another
Independent assortment & segregation
• There are 2 cell division
in meiosis.
• The first separates
homologous pairs independently from each
other.
• The second division
separates the chromatids
- segregation
Nondisjunction
• Sometimes when the germ cells
divided the chromosomes are not
distributed evenly
– One gamete can get an extra
chromosome and another can have one
missing.
• Nondisjunction results in
missing or extra chromosomes
Disorders Due to Nondisjunction
• Trisomy means 3
chromosomes instead of
normal pair.
• Monosomy means 1
chromosome instead of
normal pair.
• An extra chromosome #21
causes Down Syndrome
– Most of the time
nondisjunction causes
and embryo to die
Look closely: Each of these karyotypes
shows a nondisjunction – where are the
problems in each?
Contrasting Mitosis & Meiosis
• Mitosis
– Occurs each time a
“body cell” replicates
– Homologues are
randomly distributed
across metaphase plate
– Crossing over never
occurs.
– Sister chromatids
remain identical.
– Sister chromatids
separate in anaphase I.
• Meiosis
– Occurs only in germ
cells of ovaries and
testes.
– Homologous pairs align
(synapsis)
– Crossing over occurs.
– Sister chromatids are
genetically different
– Sister chromatids do
not separate in
anaphase I
– Don’t know what all of
the above means? –
don’t worry it is on the
next lecture.
Summary Review
1.
Cells with a pair of
chromosomes is called
_____.
2. What is the purpose of
meiosis?
3. A) How many cells
result from meiosis?
B) Are the resulting cells
haploid or diploid?
4. What are the 3 ways
variations is achieved?