Meiosis
Genetic Variation
•Describe how chromosome assortment during meiosis
contributes to genetic variation.
•Explain how crossing over contributes to genetic variation.
•Compare and contrast mitosis and meiosis.
Genetic Variety
• As you already know, offspring that result from sexual
reproduction (2 parents) are genetically different from their
parents and from one another.
• This genetic variety in offspring is the raw material for natural
selection, which will be explored later.
• This section explores how genetic variety arises through
meiosis and fertilization.
Tetrads
• Tetrads form during metaphase I.
• A tetrad (or bivalent) is 2 duplicated,
homologous chromosomes (dyads) joined
together
• The process of dyads pairing up across the
equator of a cell is called SYNAPSIS.
• A tetrad has 4 chromatids.
SYNAPSIS
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Assortments of Chromosomes
Here is one way that meiosis contributes to genetic variety. The example is an organism with
a diploid chromosome number of four (2n = 4). How the chromosomes in each homologous
pair (tetrads) line up and separate at metaphase I is a matter of chance, like the flip of a coin.
So, the assortment of chromosomes that end up in the resulting cells occurs randomly. In this
example, four combinations are possible.
In a diploid cell with four chromosomes (two homologous pairs), there are two equally
possible ways for the chromosomes inherited from the two parents to be arranged
during metaphase I. This variation in the orientation of chromosomes leads to gametes
with four equally possible combinations of chromosomes.
Independent Assortment
• If you know the haploid number for an organism, you
can calculate the number of possible combinations in
the gametes. The possible combinations are equal to
2n, where n is the haploid number.
• For this organism n = 3, so the number of
chromosome combinations is 23, or 8.
Law of Independent Assortment
Homologous
Chromosomes
CLICK for an
excellent
explanation of the
Law of
Independent
Assortment
Homologous
Chromosomes
Independent Assortment of Chromosomes
Increases genetic variability
•For a human, n = 23, so there are 223, or about 8 million,
possible chromosome combinations!
•Simply put, this means that if we only consider the
independent assortment of chromosomes during metaphase
I, it is possible for human males to produce about 8 million
genetically different sperm cells!
Crossing Over
• The number of different chromosome
combinations in gametes is just one
factor that contributes to genetic
variation. A 2nd factor is crossing
over—the exchange of genetic material
between homologous chromosomes.
• This exchange occurs during prophase I
of meiosis.
• When crossing over begins, homologous
chromosomes are closely paired as
tetrads from the process of synapsis.
There is a precise gene-by-gene
alignment between adjacent chromatids
of the two homologous chromosomes.
• Segments of the two chromatids can be
exchanged at one or more sites.
• Early in prophase I, a chromatid from
one chromosome exchanges a segment
with the corresponding segment from
the other chromosome. These altered
chromosomes give rise to what are
known as "recombinant chromosomes"
in the gametes.
Two homologous chromosomes
(dyads) paired up to form a
tetrad.
S
phase
Metaphase I
Independent assortment & crossing over
• So, on top of all the possible
chromosome combinations
resulting from independent
assortment, crossing over
adds another source of
variation.
• Crossing over can produce a
single chromosome that
contains a new combination
of genetic information from
different parents, a result
called genetic
recombination.
• Because chromosomes may
contain hundreds of genes, a
single crossover event can
affect many genes.
genetic variability.
Review: Comparison of Mitosis and Meiosis
You have now learned about 2 versions of cell reproduction in eukaryotic
organisms.
1.
Mitosis, which provides for growth, repair, and asexual
reproduction, produces daughter cells that are genetically identical
to the parent cell.
2.
Meiosis, which takes place in a subset of specialized cells in
sexually reproducing organisms, yields haploid daughter cells with
only one set of homologous chromosomes. This set consists of one
member of each homologous pair.
In both mitosis and meiosis, the chromosomes duplicate only once, in the
preceding interphase. Mitosis involves one division of the genetic
material in the nucleus, and it is usually accompanied by
cytokinesis, producing two diploid cells. Meiosis involves two
nuclear divisions, yielding four haploid cells.
•In prophase I, the duplicated
homologous chromosomes form
tetrads, and crossing over occurs.
Then, during metaphase I, the
tetrads (rather than individual
doubled chromosomes) are
aligned at the center of the cell.
•In anaphase I, sister chromatids
stay together and go to the same
pole when the homologous
chromosomes separate.
•At the end of meiosis I, the
chromosome number in each of
the two daughter cells is haploid,
but each chromosome still
consists of two sister chromatids.
The key events that
distinguish meiosis from
mitosis occur during the
stages of meiosis I.
•Meiosis II is basically identical to
mitosis. The sister chromatids
separate, and each cell divides in
two. Because these cells are
already haploid, the cells they
produce are haploid, too.
•Meiosis II is basically identical to mitosis. The sister
chromatids separate, and each cell divides in two.
•Because these cells are already haploid, the cells they
produce are haploid, too.
Mitosis & Meiosis
• Mitosis and meiosis both make it possible for cells to
inherit genetic information in the form of
chromosome copies.
• Both mitosis and meiosis begin after the
chromosomes have been duplicated during
interphase. Though similar, the results of the two
processes differ in the number of cells produced
and in the number of chromosomes the cells
contain.
9.6 Online Review
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Independent assortment of chromosomes animation
Independent assortment tutorial & quiz
Meiosis overview & crossing over tutorial
Crossing over and genetics tutorial (Advanced)
Meiosis/Mitosis Tutorial
Meiosis Quiz 1, Quiz 2, Quiz 3, Quiz 4, Quiz 5
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