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Asexual reproduction yields two identical
organisms (with exception of mutations)
Sexual reproduction merges the genes of the
two parents
Result: two versions of each chromosome
◦ Called homologous chromosomes
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Organism is diploid (2n=46)
All somatic cells are diploid
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All can be lined up based on size, creates a
karyotype
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First 22 are autosomes
Chromosome 23 is the sex chromosome
Haploid (n=23) sex cells merge to create a
zygote in sexual reproduction
Meiosis creates these sex cells (gametes)
The overall process is the sexual life cycle
Interphase I of
Meiosis
Homologous chromosomes
Chromosomes replicate
Meiosis I
Meiosis II
Homologous chromosomes separate
Sister chromatids separate
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Similar to mitosis, chromatin replicates
Centrioles replicate, proteins form around to
make the centrosomes
◦ Also called the microtubule organizing centre
◦ Collectively, with microtubules, is called mitotic
spindle
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Longest part of meiosis
Chromatin condenses, chromosomes become
visible
◦ Homologous chromosomes come pair up, forming a
tetrad – called synapsis
◦ The chromatids can overlap, called a chiasmata
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All other features are similar to mitosis
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Almost identical to metaphase in mitosis,
with the tetrad exception
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Sister chromatids remain attached, tetrads
separate (cells are now haploid)
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Homologous chromosomes are at opposite
poles of cell
◦ Each end is now haploid
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Cytokinesis takes place simultaneously
Some cells proceed to interphase II, others
directly to prophase II
No cells replicate chromatin again
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Spindle forms again, chromosomes move
towards equator
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Closely resembling mitosis, metaphase plate
is formed
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Sister chromatids finally separate
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Mitotic spindle breaks down, nucleus reforms
around two daughter nuclei
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Some of the processes can go awry
If anaphase I or II does not proceed correctly,
the chromosomes may not be shared evenly
This is called nondisjunction
More of a problem if it occurs during meiosis
I
Can take place in mitosis, but not as much of
a concern
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An abnormal chromosome number is called
aneuploidy
If faulty gamete succeeds in fertilization, the
offspring with have health consequences
If a gamate with an extra chromosome
fertilizes, the zygote is trisomic (2n+1)
If the gamete has a chromosome missing, the
zygote is monosomic (2n-1)
Ex. Extra chromosome 21 – Down syndrome
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If the extra chromosome is an X in a male
(XXY), called Klinefelter syndrome
The individual will have male sex organs, but
the testes will be abnormally small, and he
will be sterile
As well, feminine characteristics such as
breast enlargement is common
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Final example is Turner syndrome
Example of monosomy X
Individuals are classified X0, and are female
However, sex organs do not mature at
adolescence and secondary sex
characteristics fail to develop
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Also possible for anaphase I or II to
completely fail and all chromosomes move to
one cell
Referred to as polyploidy
Could have three chromosome sets (triploidy,
3n) or four (tetraploidy, 4n)