Chromosomes, Chromatids and
Centromeres
Meiosis
A packaged
chromosome
Gamete Production
Chromosome
arm
Biology 12
A. Allen
Centromere
Chromatid
Identical
chromatid
Two identical
chromosomes
Replication
Anaphase
Chromosome
arm
Gametes Are Haploid
In The Beginning
• Humans and many other complex multi-celled
organisms incorporate genetic recombination in
their reproduction
• Reproduction in which there is a re-mixing of the
genetic material is called sexual reproduction
• Two cells, a sperm and an egg, unite to form a
zygote, the single cell from which the organism
develops
• Meiosis is the process of producing sperm and
eggs (gametes)
• Gametes must have half the genetic material of a normal
cell
• If the genetic material in the gametes was not halved,
when they combined the zygote would have more genetic
material than the parents
• Meiosis is specialized cell division resulting in cells with
half the genetic material of the parents
• Gametes have exactly one set of chromosomes, this state
is called haploid (1n)
• ‘Regular’ (somatic) cells have two sets of chromosomes,
this state is called diploid (2n)
Stages Of Meiosis
Meiosis
• Meiosis resembles mitosis except that it is actually
two divisions not one
• These divisions are called Meiosis I and Meiosis II
• Meiosis I results in haploid cells with
chromosomes made up of two chromotids
• Meiosis II is essentially mitosis on haploid cells
• Stages of meiosis resemble mitosis with two
critical differences: the first in prophase I and the
second in Metaphase I
Stages of Meiosis
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Stages of Meiosis
Stages of Meiosis
Stages Of Meiosis - Meiosis I
Prophase I - The beginning phase • DNA which was unraveled and
spread all over the nucleus is
condensed and packaged
• Homologous chromosomes
(each made of two identical
chromatids) come together
(synapsis) and form tetrads
(4 chromatids)
• Crossing over, in which
chromatids within tetrads
exchange genetic material,
occurs (IMPORTANT)
Metaphase I - Middle stage –
• Tetrads line up along the
equator of the cell
Can you spot the differences from mitosis in
these two phases?
Stages Of Meiosis - Meiosis I
Anaphase I
• One copy of each chromosome
still composed of two
chromatids moves to each pole
of the cell
Telophase I
• End stage - New nuclear
membranes are formed around
the chromosomes and
cytokinesis (cytoplasm division)
occurs resulting in two haploid
daughter cells. Each haploid cell
still contains 2 copies of each
chromosome
Stages Of Meiosis - Meiosis II
Stages Of Meiosis - Meiosis II
Meiosis II is Separation is sister chromatids by mitosis
•
Prophase II
• Cells do not typically go into
interphase between meiosis I and
II. Chromosomes are already
condensed
Metaphase II
• Chromosomes line up at the
equator of the two haploid cells
produced in meiosis I
•
Stages of Meiosis
Mother cell
Anaphase II - Chromosomes
made up of two chromatids split to
make chromosomes with one
chromatid which migrate to the
poles of the cells
Telophase II - Cytokinesis and
reformation of the nuclear
membrane in haploid cells each
with one set of chromosomes made
of one chromatid
Stages of Meiosis
Stages Of Meiosis: Meiosis I
Stages Of Meiosis: Meiosis II
Prophase II
The products of meiosis are 4
haploid cells each with a unique set
of chromosomes.
Interphase
Prophase I:
Metaphase II
Condensing
Prophase I:
Chromosomes
Tetrad formation/
The products of mitosis are
2 diploid cells with
identical chromosomes.
Metaphase I
crossing over
Anaphase II
Meiosis II
Anaphase I
Telophase I
Telophase II
Stages of Meiosis
2
Crossing Over
Prophase I:
Tetrad formation/
crossing over
Because of crossing over, every
gamete receives a unique set of
genetic information.
Telophase II
Why Sexual Reproduction?
Metaphase I
Anaphase I
Telophase I
What's the use of Sexual Reproduction?
• Sexual reproduction: a costly process
• Special reproductive cells and structures must be constructed
• Courtship behavior, hormones, colors, scents must be
developed
• Reproductive timing‚ finding and recognizing mates, sperm
and egg available at the same time - must occur
• The developing embryo and young must be cared for, often for
years or decades!
• Why bother, then?
• The genetic diversity that results from sexual reproduction
increases the possibility that the offpsring will survive and
pass its genes on to the next generation, offsetting the
biological "cost".
Unique Features of Meiosis
Meiosis Creates Genetic Variation
• During normal cell growth, mitosis produces
daughter cells identical to parent cell (2n to 2n)
• Meiosis results in genetic variation by shuffling of
maternal and paternal chromosomes and crossing
over..
over
• No daughter cells formed during meiosis are
genetically identical to either mother or father
• During sexual reproduction
reproduction,, fusion of the unique
haploid gametes produces truly unique offspring.
offspring.
Independent Assortment
Ever wonder why siblings don’t
look alike even though they come
from the same DNA?
Wait no more…..
Independent Assortment
Number of combinations: 2n
e.g. 2 chromosomes in haploid
2n = 4; n = 2
2n = 22 = 4 possible combinations
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In Humans
Crossing Over
Chiasma (pl. Chiasmata)– site
of crossing over, occur in
synapsis. Exchange of genetic
material between non-sister
chromatids.
Crossing over produces
recombinant chromosomes.
e.g. 23 chromosomes in haploid
2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations!
Random Fertilization
At least 8 million combinations from Mom, and another 8 million from
Dad …
64 trillion combinations for a diploid zygote!!! Yikes……that’s a lot of
sex…ual reproduction
And that does not even take crossing over into
account!
Important Link
Use the link & watch the animations carefully
Meiosis Animations
Remember, YOU contribute only half the genetic material to your offspring.
So when you are choosing a mate for reproduction remember Indiana Jones
& the Temple of Doom.
-” Choose, but choose wisely.”
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