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Meiosis and genetic
variation
Genetics Unit:
Chapter 11 &14
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Asexual Reproduction

Many single-celled organisms reproduce by
splitting, budding, binary fission, etc.
GENETICALLY IDENTICAL OFFSPRING!

Some multicellular organisms can reproduce
asexually, produce clones.
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Sexual reproduction

Fusion of 2 gametes (fertilization) to
produce a single zygote.

Greater genetic variation

With exception of self-fertilizing organisms
(e.g. some plants), zygote is a
combination of gametes from 2
different parents.
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Human Chromosomes

Somatic cells = ALL human body cells
have 23 pairs of homologous
chromosomes


22 pairs of autosomes
1 pair of sex chromosomes
GAMETE cells = reproductive cells
 23 individual chromosomes
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Fig. 11.3, p.171
Review of Chromosome Numbers

Gametes (reproductive cells):





Sperm/ovum cells
Haploid - half the full set of chromosomes
22 INDIVIDUAL autosomes and 1 sex
chromosome
Human haploid number in “n” = 23
After fertilization: results in zygote with 2
haploid sets of chromosomes - now diploid.

Diploid cell; 2n = 46. (n=23 in humans)
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diploid
germ cells
in female
diploid
germ cells
in male
meiosis, gamete
formation in both
female and male:
eggs
sperm
X
×
Y
X
×
X
fertilization:
X
X
X
XX
XX
Y
XY
XY
sex chromosome combinations
possible in the new individual
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Fig. 11.2, p.170
Chromosome
numbers:
All are even numbers –
diploid (2n) sets of
homologous
chromosomes!
Ploidy = number of
copies of each
chromosome.
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Processes of Mitosis and Meiosis
 Mitosis
= creates new
SOMATIC Cells only!
(DIPLOID)
 Meiosis
= creates new GAMETE
cells only! (HAPLOID)
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Meiosis –
key differences from mitosis
Meiosis




Make GAMETES
Chromosome
number is reduced
by half in daughter
cells
All daughter cells are
genetically
DIFFERENT
Two rounds of
divisions (Meiosis I
and Meiosis II)
Mitosis




Makes SOMATIC cells
Same chromosome
number in original
and new cells.
All daughter cells are
genetically
IDENTICAL
One round of divisions
(PMAT)
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Meiosis I involves:

Synapsis – homologous chromosomes pair up
and crossing over of non-sister chromatids.

In Metaphase I, homologous pairs line up at
metaphase plate.

In Anaphase I, sister chromatids do NOT
separate.

Overall, separation of homologous pairs of
chromosomes, rather than sister chromatids
of individual chromosome.
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Homologues

Chromosomes exist in homologous
pairs in diploid cells.
Exception: Sex chromosomes (X, Y).
Other chromosomes are known as autosomes,
they have homologues.
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KEY
DIFFERENCE!
Homologous
pairs
separate first
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CROSSING
OVER!
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Sister
Chromatids separate
at centromeres
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Meiosis 1
First division of meiosis
 Prophase 1: Each chromosome duplicated
and remains closely associated. These are
called sister chromatids. Crossing-over can
occur during the latter part of this stage.
 Metaphase 1: Homologous chromosomes
pair up and align in the middle.
 Anaphase 1: Homologous pairs separate
with sister chromatids remaining together.
 Telophase 1: Two daughter cells are formed
with each daughter containing only one
chromosome of the homologous pair.
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Meiosis II
Second division of meiosis:




(Gamete formation)
Prophase 2: DNA does not replicate.
Metaphase 2: Chromosomes align at the
middle of cell.
Anaphase 2: Centromeres divide and sister
chromatids migrate separately to each pole.
Telophase 2: Cell division is complete. Four
haploid daughter cells, all genetically
different, are formed.
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Mitosis vs. Meiosis
Comparing Mitosis and meiosis
Just meiosis!
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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.
During sexual reproduction, fusion of the
unique haploid gametes produces truly
unique offspring.
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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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Independent assortment
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In humans…
e.g. 23 chromosomes in haploid
2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations!
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Random fertilization
At least 8 million combinations from Mom,
and another 8 million from Dad …
>64 trillion combinations for a diploid
zygote!!!
Mitosis after
fertilization
Fertilization envelope
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Alterations in chromosome number and
individual chromosomes


Many mutations can occur during mitosis or
meiosis that will affect the chromosome numbers
or alter the information on individual
chromosomes.
Mutations can be harmful or beneficial to the
organism
Types of mutations include:




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Nondisjunction
Deletion
Duplication
Inversion
Translocation
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Trisomy 21- Cause of Down Syndrome
Nondisjunction
animation
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
Various animations showing chromosomes alterations
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Review Questions
What happens as homologous
chromosomes pair up during prophase I of
meiosis?
2. How does metaphase of mitosis differ from
metaphase I of meiosis?
3. What is the sole purpose of meiosis?
4. What specific activities, involving DNA,
occur during interphase prior to both mitosis
and meiosis?
1.
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5.
Compare mitosis and meiosis on the
following points:
a. number of daughter cells produced.
b. the amount of DNA in the daughter
cells in contrast to the original cell.
c. mechanism for introducing genetic
variation.
6. What is a zygote and how is it formed?
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SHOW ME MEIOSIS!





Using your phone and your magnetic
chromosomes, show me ALL of meiosis,
beginning with Interphase and ending with
cytokinesis after meiosis 2.
Take a picture of the chromosomes at every
stage.
Use chalk to draw nuclear envelopes, spindles
and centrioles. Use labels in photos for phase
names.
When finished, raise your hand for me to check
your photos for completion.
IF IT’S INCORRECT you need to DELETE photos
and start AGAIN!
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