MEIOSIS & Genetic Variation
Goal: Divide up chromosomes & cytoplasm to
sex cells so that when they unite the zygote has
the correct # of chromosomes.
• Only in eukaryotic cells
• Only is gametes
2 Cell Divisions (Meiosis I & II)
1 Duplication of chromosomes
•
If Meiosis did not occur the chromosome # in each
new generation would double…. The offspring would
die.
Organisms that reproduce sexually are
made up of two different types of cells.
1. Somatic Cells
–
–
–
–
“body” cells
“Normal” # of chromosomes (46 for humans)
Diploid # (the symbol is 2n).
Humans have 23 pairs of chromosomes for a total of 46
chromosomes. Diploid #: 46
2. Gametes
– “sex” cells (sperm & eggs)
– ½ the normal number of chromosomes
– Haploid # (the symbol is n)
– Humans: n = 23 (23 pairs of chromosomes)
Making Gametes
• Meiosis in males = spermatogenesis = make
sperm in testes
• Meiosis in females = oogenesis = make ova in
ovaries
Ovum+ Sperm  Zygote (fertilized egg)
n=23
egg
Sperm
n = 23
2n=46
zygote
Spermatogenesis
Secondary Spermatocyte
n=23
human
sex cell
2n=46
sperm
n=23
Primary Spermatocyte
n=23
Secondary Spermatocyte
haploid (n)
n=23
diploid (2n)
n=23
4 sperm cells are
produced from each
primary spermatocyte.
meiosis I
n=23
meiosis II
Oogenesis
*** The polar bodies die… only one ovum
(egg) is produced from each primary oocyte.
Homologous Chromosomes
2n = 46
n = 23 … pairs of chromosomes
1 set from mom; 1 set from dad
Homologous Chromosomes =
Matching pairs of chromosomes
Locus: position of a gene
Paternal
eye color
locus
hair color
locus
Maternal
eye color
locus
hair color
locus
Humans have 23 Sets of Homologous Chromosomes
Homologue
Autosomes
Chromosomes
# 1 – 22; code
for most traits
Homologue
Sex
Chromosomes
(Pair 23)
XY (male)
XX (female)
23
Homologous Chromosomes
Homologous Chromosomes
Tetrad = homologous pairs when they line up and touch
consists of 4 chromatids
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
Stages of Meiosis
2 cell divisions with only 1 duplication of chromosomes.
Interphase I
• Similar to mitosis
interphase.
• Chromosomes replicate
(S phase).
• Each duplicated
chromosome consist of
two identical sister
chromatids attached at
their centromeres.
• Centriole pairs also
replicate.
nucleus
nuclear
membrane
nucleolus
Meiosis I
• Cell division that reduces the chromosome
number by one-half (2n  n)
• Stages:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I
Prophase I
•
•
•
•
Longest; most complex phase.
90% of meiosis
Chromosomes condense.
Synapsis occurs: homologous
chromosomes come together to
form a tetrad.
• Tetrad is two duplicated
chromosomes (4 chromatids)
• CROSSING OVER occurs!
Also: Nuclear membrane
disappears, spindle fibers
form, etc.
Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad
sister chromatids
Prophase I : “Crossing Over”
• Segments of nonsister
chromatids break and
reattach to the adjacent
chromatid.
• RESULTS IN GENETIC
RECOMBINATION!
(New mixture of genetic
material; Creates
diversity in gene pool)
Crossing Over
Humans: 1 – 3 crossover events per chromosome pair in meiosis
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
variation
Prophase I
spindle fiber
aster
fibers
centrioles
Metaphase I
• Shortest phase
• Tetrads align in middle of
cell.
• INDEPENDENT
ASSORTMENT OCCURS:
1. Orientation of homologous
pair to poles is random.
2. Results in Variation
Formula for genetic combinations: 2n
Example: 2n = 4
then
n = 2;
thus
22 = 4 combinations
Metaphase I
OR
metaphase plate
metaphase plate
Anaphase I
Sister chromatids
attached
• Homologous
chromosomes separate
and move towards the
poles.
• SEGREGATION of
chromosomes.
• Sister chromatids remain
attached at their
centromeres.
Homologous
chromosomes
separate
Telophase I
• Each pole now has
haploid set of
chromosomes.
• Cytokinesis occurs
and two haploid
daughter cells are
formed.
Meiosis II:
• Sister chromatids separate
• No interphase II (or very short - no more DNA
replication)
• similar to mitosis
Meiosis
Meiosis I:
Homologous
chromosomes
divide (2n  n)
Meiosis II:
Sister
chromatids
separate
Mitosis vs. Meiosis
Mitosis
Meiosis
1 parent produces 2 DIPLOID
daughter cells (2n  2n)
1 parent produces 4 HAPLOID
daughter cells (2n  n)
Somatic cells
Gametes
Parent & daughter are genetically
identical
Daughter cells are not genetically
identical (They have different
chromosomes)
Prophase: Homologous chromosomes
do not line up
Prophase 1: Homologous
chromosomes SYNAPSE (Tetrads);
Cross-over occurs
Metaphase: Chomosomes line up in
middle of cell
Metaphase 1: Homologous
chromosomes line up together.
Anaphase: Sister chromatids split
Anaphase 1: Homologous
chromosomes split; sister chromatids
still attached.
Sources of Genetic Variation in
Sexual Reproduction.
• 1. Mutation = original source of variation
(in asexual & sexual)
• 2 Random fertilization = offspring are a
combination of paternal + maternal
chromosomes. There different versions of
genes (alleles) for each trait (BB, Bb, bb; B
= brown fur; b = black fur) Different
versions = diff. nucleotide sequences on
chromosomes.
Sources of Genetic Variation in
Sexual Reproduction.
• 3. Independent arrangement of
homologous chromosomes at metaphase
plate in Metaphase I of meiosis. (Do you
get maternal or paternal chromosome?)
– 2n = # combinations n=23 chromosomes
– Over 8 million combinations of chromosomes
4. Crossing over: Exchange of
corresponding segments b/w 2 homologous
chromosomes (Prophase I)
Sources of Genetic Variation:
Crossing Over
• Produces new combinations of traits that
did not exist in maternal or paternal
genome.
New
combinations of
genes = Genetic
Recombination
Genetic Variation: Who Cares?
Variations of traits w/in populations is what
natural selection acts on in the process of
evolution.
What traits are favorable? Unfavorable?
Nondisjunction in Meiosis
• Failure of chromosomes to separate during Meiosis I
or II
• Leads to abnormal # of chromosomes in offspring
(usually fatal)
• Ex: Downs syndrome
Nondisjunction in Meiosis I vs. II
Nondisjunction in meiosis I  all resulting gametes have incorrect
chromosome # (fertilization will result in trisomy or monosomy in zygote)
Nondisjunction in meiosis II Half of resulting gametes have incorrect
chromosome #
Monosomies
Having 45 Chromosomes
• Lethal if autosomal
• Turner Syndrome (XO) - infertile
females, short stature, “webbed” neck,
possible cognitive impairments
http://web.udl.es/usuaris/e4650869/d
ocencia/segoncicle/genclin98/recurso
s_classe_(pdf)/revisionsPDF/chromo
syndromes.pdf
http://en.wikipedia.org/wiki/Monosomy
Trisomies: 47 chromosomes
Autosomes
• Down’s Syndrome (#21) – most common
• Edwards (#18)
• Patau (#13)
Sex Chromsomes (Y determines maleness)
• XXY Klinefelter males (infertile, tall, long limbs, some
secondary female characteristics, possible cognitive
issues)
• XYY normal male
• XXX normal female
http://web.udl.es/usuaris/e4650869/d
ocencia/segoncicle/genclin98/recurso
s_classe_(pdf)/revisionsPDF/chromo
syndromes.pdf
http://en.wikipedia.org/wiki/Monosomy
Nondisjunction in Meiosis
Chromosomal mutations: change in an entire
chromosome; may involve loss or duplication of
multiple genes
Types of Chromosomal Mutations
1. Deletion
2.Duplication
3.Inversion
4.Translocation (balanced &
unbalanced)
Types of Chromosomal Mutations
Partial deletions, duplications, translocations
http://en.wikipedia.org/wiki/Monosomy
Nondisjunction in Meiosis
Links
•
•
http://www.youtube.com/watch?v=Ba9LXKH2ztU&feature=related
http://www.youtube.com/watch?v=sJCWVTnFf5o&feature=related
•
•
http://www.youtube.com/watch?v=QwmpD0OB3AQ&NR=1&feature=endscreen
http://www.youtube.com/watch?v=kVMb4Js99tA&feature=related