MEIOSIS
3.3 & 10.1
Meiosis:
 A reduction division of a diploid nucleus
to form four haploid nuclei.
 This allows for a sexual life cycle in living
organisms.

Number of
Chromosomes
Description of
condition
Cell Type
46
Diploid (2N)
Typical body
(somatic) cell
23
Haploid (N)
Gamete,
Egg or Sperm cell
Homologous chromosomes: in a
diploid cell, 46 chromosomes are grouped
into 23 pairs of chromosomes.
 Homologous: similar shape and size,
and carry the same genes

Chromosomes
Meiosis
Interphase I –
 All the chromosomes are duplicated and thus
each consists of two identical sister chromatids.

Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Centromere
Figure 13.4
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
In Meiosis I:
◦ Prophase I – Each chromosome pairs with its
corresponding homologous chromosome to
form a bivalent (a.k.a. tetrad)
 Crossing
Over
occurs
during
prophase I,
then the
chromosomes
condense.
Crossing Over
During crossing over there is exchange of
DNA material between non-sister
homologous chromatids.
 This produces new
combinations of
alleles on the
chromosomes of the
haploid cells.
 This leads to genetic
variation.

Nonsister
chromatids
Prophase I
of meiosis
Bivalent
Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Figure 13.11
Recombinant
chromosomes
Chiasmata

A chiasma is an X-shaped knot-like
structure that forms where crossing over
has occurred.
◦ It holds a bivalent together for a while after the
chromosomes condense by supercoiling.

Interphase and meiosis I
MEIOSIS I: Separates homologous chromosomes
INTERPHASE
PROPHASE I
ANAPHASE I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Centrosomes
(with centriole pairs)
Sister
chromatids
Nuclear
envelope
Chromatin
METAPHASE I
Tetrad
Chiasmata
Metaphase
plate
Spindle
Microtubule
attached to
kinetochore
Chromosomes duplicate
Homologous chromosomes
(red and blue) pair and exchange
Figure 13.8
segments; 2n = 6 in this example
Homologous
chromosomes
separate
Bivalents line up
Pairs of homologous
chromosomes split up

After finishing Meiosis I, our results are
two daughter cells with a haploid number
of duplicated chromosomes.
Meiosis II

Telophase I, cytokinesis, and meiosis II
MEIOSIS II: Separates sister chromatids
TELOPHASE I AND
CYTOKINESIS
Figure
PROPHASE II
Cleavage
furrow
Two haploid cells
form; chromosomes
13.8are still double
METAPHASE II
ANAPHASE II
Sister chromatids
separate
TELOPHASE II AND
CYTOKINESIS
Haploid daughter cells
forming
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing single chromosomes

Meiosis I
◦ Homologous chromosomes separate
◦ Reduces the number of chromosomes from diploid to
haploid

Meiosis II
◦ Sister chromatids separate
◦ Produces four haploid daughter cells

Genetic Variation is increased by:
◦ Crossing over (during prophase I)
◦ (Random) Fusion of gametes
◦ Independent assortment
Genetic Variation

Fusion of gametes from different parents
promotes genetic variation.
◦ This allows alleles from two different
individuals to be combined into one new
individual.
◦ The combination of alleles is unlikely ever to
have existed before  genetic variation.
◦ Genetic variation is essential for evolution of a
species.
Sexual Reproduction
Independent Assortment of genes

Organization/ orientation of pairs of homologous
chromosomes during metaphase is random.
Key
Maternal set of
chromosomes
Paternal set of
chromosomes
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Figure 13.10 Combination 1
Combination 2
Combination 3
Combination 4

Non-disjunction: “not coming apart” –
when chromosomes fail to separate
during Meiosis 1 or 2.
Karyotyping
Gametes contain two copies or no copies of a
particular chromosome.
 Offspring have an extra or missing chromosome.

Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n+1
Figure 15.12a, b
n+1
n1
n+1
n –1
n–1
Number of chromosomes
(a) Nondisjunction of homologous
chromosomes in meiosis I
n
n
(b) Nondisjunction of sister
chromatids in meiosis II

Down’s Syndrome – Trisomy 21
◦ The person has 3 (instead of 2)
21st chromosomes

Age of parents vs. Down Syndrome

Do the DBQ on pg. 167 – 168: “Parental
age and non-dsjunction”
Karyotype: a property of a cell – the
number and type of chromosomes present
in the nucleus.
 Karyogram:
picture of
chromosomes
arranged in pairs,
according to their
size and structure
(banding patterns).


Trisomy 18, Trisomy 13

Turner’s Syndrome –
females with only one X

Klinefelter’s Syndrome –
males with XXY
Chromosomal abnormalities

Karyotyping is used for pre-natal (before
birth) diagnosis of chromosome
abnormalities.

Where do we get the cells for doing a
karyotype?
1) amniocentesis
 Extract amniotic fluid,
 Inside are some of
the baby’s cells


Risks:
◦ Miscarriage 1 in 200 to
1 in 400
◦ Accuracy: 99.4%

2) chorionic villus
sampling
Tissue sample from the
placenta’s projections
into the uterus wall

Risks?

◦ Slightly higher chance of
miscarriage than
amniocentesis because it
is done earlier in
pregnancy.
◦ Accuracy: 98%