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Meiosis
Chromosome Number

The number of cells per cell vary between
organisms. Humans have 46 chromosomes,
fruit flies have 8.

Humans have 23 different pairs of
chromosomes, fruitflies have 4.
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The number of different pairs is given as n,
which is the haploid number of chromosomes
Haploid #
Diploid #
Humans
n = 23
2n = 46
Fruitflies
n=4
2n = 8
Homologous Chromosomes

Each partner in pair of chromosomes
has the same basic shape and
structure. They also contain the same
exact genes, but may carry different
alleles.
Human Karyotype
Creating a Karyotype
Mitosis - Refresher

Mitosis produces daughter cells having the
same number of chromosomes as the parent
cell 2n  2n

Produces genetically identical cells

One cell to two cells

Process used to replace old cells or dying
cells
Meiosis

Process of nuclear division that reduces
the number of chromosomes by half.

Used in sexual reproduction to make
gametes (i.e. – sperm and egg cells)
2n  1n (diploid cells become haploid)
 Two nuclear divisions  1 cell  4 cells

Meiosis
Phases
Prophase I

DNA strands thicken and coil

Spindle fibres appear

Nuclear membrane & nucleolus disappear

SYNAPSIS occurs (process by which each chromosome
finds its homologue)

TETRAD formation (process by which homologous
chromosomes line up with each other)

CROSSING-OVER – process whereby homologous
chromosomes or sister chromatids twist around each other
and exchange genes.
Crossing-Over
Crossing-Over
Crossing-Over
Metaphase I

The tetrads move
to the equator of
the cell

Homologous pairs
(tetrads) remain
together
Anaphase I

The homologous pairs of
chromosomes separate

One chromosome is
pulled to each pole.

Each chomosome is still
composed of two
chromatids joined by a
centromere.
Telophase I

The cytoplasm
divides, forming two
daughter cells, with
half the number of
double-stranded
chromosomes.

2nn
Male
Female
Meiosis II

Occurs in each cell formed during
meiosis I and is NOT preceded by DNA
replicaton
Prophase II

Chromosomes are coiled and new
spindle fibres form
Metaphase II

Chromosomes move to the cell equator.

Each chromosome is made of two sister
chromatids that are joined by a
centromere that is attached to a spindle
fiber.
Anaphase II

The centromeres
joining the
chromatids divide,
freeing each sister
chromatid from the
other.

The sister
chromatids move
toward an opposite
pole.
Telophase II

Spindle
dissolves

Nuclear
membrane
reforms

Cytokinesis
occurs
Male
Female
Meiosis – Result

The two nuclear divisions of meiosis
result in four daughter cells from a
single parent cell, each with half the
chromosome number of each parent

46 double stranded chromosomes

23 single stranded chromosomes
Law of Independent Assortment
Meiotic Accidents &
Genetic Disorders
Nondisjunction during Meiosis

During this type of accident homologous
chromosomes fail to separate during
anaphase I or a double stranded
chromosome fails to separate during
anaphase II

Most nondisjunctional accidents are fatal and
cause the embryo to spontaneously abort
Trisomy 21 – aka Down Syndrome

Most common autosomal nondisjunction
accident

Contains a third chromosome 21

The 21st pair of chromosomes fails to
separate during meiosis in the mother

Is not a fatal accident and therefore
mother’s carry their baby to term
Characteristic Syptoms Include:
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•
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Notably round face
Skin fold at the inner corner of the eye
Flattened nose bridge
Small, irregular teeth
Short stature
Heart defects
Hightened susceptibility to respiratory infections
Leukemia
Alzheimer’s disease
Various degrees of mental retardation
Usually sterile, but some exceptions
Half the egg cells will have two 21 chromosomes, so
there is a 50 % chance of transmission to child
Incidence of Down Syndrome
Compared to Maternal Age
Klinefelter Syndrome
(XXY)

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Reduced testes
Sterile
Breast development
Normal intelligence
Also found in other individuals with
more than one additional sex
chromosome – XXYY, XXXY,
XXXXY (more chance of mental
retardation)
Turner Syndrome
(XO)
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Short stature
Sterility caused by
underdeveloped ovaries
Small breasts
Webbed necks
Normal intelligence
Only syndrome where
having only 45
chromosomes is not fatal
Chromosomal Mutations
Cri-du-Chat Syndrome
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Deletion mutation in chromosome 5
Diagnosed usually by a characteristic cry in children
that sounds like a distressed cat mewing
Mental retardation
Widely-spaced eyes
Epicanthic folds
The degree of disability is directly related
to the amount of genetic material missing
Chronic Myelogenous Leukemia