Download Meiosis and independent assortment

Meiosis
Gamete formation
Revision
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The human chromosome complement is 46 chromosomes
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This means humans have 23 pairs of chromosomes
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There are 22 matching pairs of autosomes
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There is one pair of matching sex chromosomes
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Males have an X and a Y chromosome, whereas females have two X
chromosomes
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Matching pairs of chromosomes are referred to has homologous
pairs, therefore males have 22 homologous pairs whereas females
have 23 pairs
Male karyogram showing 22
homologous autosomes and one
pair of sex chromosomes (XY)
female karyogram showing 22
homologous autosomes and one pair of
homologous sex chromosomes (XX)
Haploid and Diploid Summary
Normal body
cell/zygote
Gametes (sex cells)
Contains 46 chromosomes or 23
pairs = diploid (2n)
Contains 23 chromosomes, half of
normal body cells = haploid (n)
Meiosis
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Meiosis is a cell division
in which the number of
chromosomes in a
gamete mother cell
(cell which produces
gametes) is reduced from
two sets (diploid) to a
single set (haploid)
http://www.cellsalive.com/meiosis.htm
First Meiotic Division
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Chromosomes exist as single threads
of information
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Chromosomes undergo DNA
replication prior to cell division
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To increase the variation in genetic
information in each cell, some sections
of genetic information are
swapped over between
homologous chromosomes
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This point of crossing over is
called chiasmata
http://www.johnkyrk.com/meiosis.html
Second Meiotic Division
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The chromosomes align at the
equator of the cell once again
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Chromosomes are pulled to
opposite poles
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Cell and nuclear division occurs
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The end product is four daughter
cells, each containing the haploid
number of chromosomes
Significance of the haploid Number
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Gametes produced at the end of
meiosis must have the haploid number
of chromosomes
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If they do not, the normal chromosome
complement will not be achieved at
fertilisation
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This can result in various genetic
abnormalities or failure to produce
viable offspring
Crossing-over
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Homologous pairs of
chromosomes are in very close
contact with each other
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This results in segments of
genetic information being
swapped with its homologous
partner
The point at which this
crossing over occurs is
called chiasmata
Crossing-over
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Exchange of chromosomal
information in this way produces
further variation in gametes
Independent assortment and
crossing over increase variation
in gametes
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Gametes have a unique genotype,
therefore a zygote is also unique as
its genotype is very different to
other zygotes
Independent Assortment
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The random alignment of homologous
pairs of chromosomes is called
independent assortment
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In this example, blue represents genetic
information from an ovum and yellow
represents genetic information from a
sperm
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As a result of the random nature in
which chromosomes pair up with their
homologous partner, four different
gametes are produced
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Independent assortment ensures
variation
First
Meiotic
division
Second
Meiotic
division
Possible
Gametes
Task 1
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Meiosis animation of all stages
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Collect a cut out sheet to help you summarise the stages
of meiosis I and II.
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Use pages 118-119 from your textbook to help you write
descriptions of each stage.
Title: Significance of meiosis
Learning question: Why is meiosis important?
Starter List three difference between meiosis and mitosis
Meiosis
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Is the special form of cell division used to produce
gametes
It has two functions:
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To form haploid cells with half the chromosome number
To re-arrange the chromosomes with a novel combination
of genes
Meiosis
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Meiosis comprises of two successive divisions
without DNA replication in between
The stages of these divisions are on your
worksheet…
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Either draw or cut and stick the corresponding picture in
the boxes
Meiosis
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Rearrange the following steps of meiosis
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Separation of chromatids
Formation of bivalents
4
Pairing by homologous chromosomes
2
Production of haploid cells
1
Chiasma formation and exchange between chromatids
5
3
Genetic Variation
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Meiosis and sexual reproduction allows for genetic
variation
This allows species to adapt to their environments
through evolution
There are three sources of genetic variation in
sexual reproduction
Can you name them?
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Independent Assortment
Crossing Over
Random Fertilisation
Independent Assortment
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Happens in metaphase I, when the bivalent line up
on the equator
Each bivalent is made up of two chromosomes
The chromosomes can line up in any order on the
equator resulting in different possible combinations
Independent Assortment
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In the sample below, n=2 and there are 4 possible
gametes
In humans n=23 so there are over 8 million possible
different gametes!
Crossing Over
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This occurs in prophase I
While the two homologous chromosomes are joined
in a bivalent, bits of one chromosome are swapped
(crossed over) with the corresponding bits of the
other chromosome
Crossing Over
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There are always equal amounts crossed over so
chromosomes stay the same length
Crossing over means that the maternal and paternal
alleles can be mixed
Random Fertilisation
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This takes place when two gametes fuse to form a
zygote
Each gamete has a unique combination of genes
and any of the numerous male gametes can fertilise
the egg
Gene Mutation
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DNA is very stable but bases can change during
DNA replication
Most mutation have no phenotypic effect, these are
called silent mutations
Variation
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Below is an outline of causes of variation
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As well as genetic causes, individuals are
influenced by the environment
Genotype + environment = phenotype
Discontinuous Variation
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Are distinct, separate peaks/bars
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Are distinct categories to place individuals
Tends to have no overlap between categories
Are controlled by a small number of genes
Are largely unaffected by the environment
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Continuous Variation
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Is a continuous range of values so get a smooth
curve
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Are no distinct categories to place individuals
Tends to have overlap between categories
Are controlled by a large number of genes
Are significantly affected by the environment
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Sex Determination
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Females produce eggs which carry the X
chromosome.
Sperm carry either X or Y
Males therefore determine the sex of the child
Females = XX
Males = XY
Sex Determination
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It is though that Y chromosomes dislike acid conditions
within the female reproductive systems
Y chromosomes are smaller and will therefore float in a
sample of semen if centrifuged
Clinics can use this information to select sex