Download Communication - Mrs Jones A

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MEIOSIS
MITOSIS
Forms body cells
 Forms sex cells/gametes
Growth and repair
 For sexual reproduction
Cells produced are
 Cells produced are
genetically identical
genetically different
One division
 Two divisions
Homologous pairs do not
 Homologous pairs meet
meet
 Crossing over
No crossing over
 Produces 4 cells from
Produces 2 cells from each
each division
division
 Haploid number of
Diploid number of
chromosomes
chromosomes
In both the chromosomes replicate before division begins
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Gametes
Haploid
Meiosis I
Meiosis II
Prophase
Metaphase
Anaphase
Telophase
DNA replication
Sister chromatids
Centromere
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Chromatin
Homologous pairs
Bivalent
Loci
Maternal/paternal
Chiasmata
Crossing over
Spindle
Random assortment
Fertilisation
Mutation

Describe, with the aid of diagrams and
photographs,
 the behaviour of chromosomes during
meiosis,
 the associated behaviour of the nuclear
envelope, cell membrane and centrioles.
 (Names of the main stages are expected, but
not the subdivisions of prophase)
 Pages 118-9
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All living organisms can reproduce, this can be:
Asexual reproduction
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Single organism divides by mitosis
New organism is genetically identical to the parent
Genetic variation can only be by mutation
Mitosis (in eukaryotes) or binary fission (in
prokaryotes).
Sexual reproduction
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Meiosis produces haploid gametes
Which fuse at fertilisation to form a diploid zygote
This produces genetic variation amongst offspring
Diploid
Zygote
46
fertilisation
Mitosis
Haploid
Sperm
23
Haploid
Egg
23
Meiosis
Adult
46
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Chromosomes in a nondividing cell (unduplicated)
exist as single-armed structures
(each is equivalent to one of the
chromatids in a metaphase
chromosome prepared for cell
division)
It would not be visible as a
coiled structure but would be
unwound
Metaphase (duplicated)
chromosome has 2 sister
chromatids. Each chromatid
contains an identical copy of
the genetic material. (DNA
molecule)
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Chromatid vs. Chromosome: Remember that when two
DNA molecules are joined together, each molecule is called
a chromatid. When a DNA molecule (and proteins) is not
attached to another one then that single molecule of DNA is
not a chromatid but an unduplicated chromosome
Chromatin: During certain times of the cell's life cycle the
chromosomes are not visible. This is because the
chromosomes are stretched out very thin to allow surfaces
for the various chemical reactions that involve
chromosomes to take place. When the nucleus is stained
and examined, it appears uniformly coloured and the
chromosomes collectively are termed chromatin.
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Read the worksheet and complete the
definitions
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Sexually reproducing
organisms in nearly all cases
have paired sets of
chromosomes, one set coming
from each parent
(maternal/paternal)
The equivalent chromosomes
that form the pair are called
Homologous
pairs/homologues
Each homologue carries an
identical assortment of genes,
but the version of the gene, the
allele, from each parent may
differ
Some basic points… showing that different versions (alleles) occupy
the same position (locus) on a chromosome.
A pair of homologous chromosomes
centromere
Paternal
chromosome,
T
originating from
the sperm
Locus of gene for height (dominant allele T = tall)
Maternal
chromosome,
t
originating from
the egg
Locus of gene for height (recessive allele t = short)
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Meiosis is a reduction division
Resulting daughter cells have half the original
number of chromosomes
 Daughter cells are haploid
 Can be used for sexual reproduction
 Source of genetic variation
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Meiosis has two divisions
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Meiosis I and Meiosis II
Each division has 4 stages
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Prophase, metaphase, anaphase, telophase
(PMAT x2)
An animation of Meiosis at
http://www.cellsalive.com/meiosis.htm
 Just watch first
 Then again….Let’s go!!!!
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During interphase the DNA replicates forming two sister
chromatids joined at the centromere
PROPHASE 1:
The chromatin condenses, supercoils and become
visible.
The homologues pair up in a process called synapsis to
form bivalents
One of the chromosomes is maternal and one is
paternal.
The chromosomes in a bivalent may cross over at points
called chiasmata where they may swap sections of
DNA/alleles.
The nucleolus disappears and the nuclear envelope
breaks down.
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This swapping of genetic
information is called
crossing over.
The centriole divides
(animal cells only) and
starts migrating to opposite
poles of the cell.
A spindle forms (protein
microtubules) produced by
centrioles lining up at the
equator.
Prophase 1 can last days,
months, years!!
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METAPHASE 1:
The bivalents randomly line up on the equator.
(random assortment)
The spindle fibres attach to them at their
centromeres.
Each member of the homologous pair facing
opposite poles
This allows for independent assortment
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ANAPHASE 1:
The homologous chromosomes of each
bivalent are pulled apart by the spindle fibres
contracting, towards the poles. (independent
assortment)
The chiasmata separate as each homologous
pair moves towards it separate pole.
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TELOPHASE 1:
In animal cells the chromosomes will de
condense a little and the nuclear envelope will
reform.
Cytokinesis will occur and the cell will split.
However most plant cells progress straight to
Meiosis II
Prophase I
Chromatin
condenses
Homologous pairs form
a bivalent. Crossing over
occurs
Nucleolus disappears
Spindle forms
Anaphase I
Homologous
chromosome in each
bivalent are pulled to
opposite poles by
spindle contracting
Metaphase I
Bivalents
line up on
equator of cell
Attached by
centromere
Random
Telophase I
Two
new nuclear
envelopes form
Cell divides by
cytokinesis
Each cell has???
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PROPHASE 2:
Everything from now on happens in two cells.
The chromosomes condense.
The centrioles replicate and the spindle starts
to develop perpendicular to the previous
division.
The nuclear envelope disintegrates.
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METAPHASE 2:
The duplicated chromosomes randomly line
up on the equator.
They are attached to the spindle fibres at their
centromeres.
ANAPHASE 2:
The centromeres divide to allow the
chromosomes to split.
The chromatids (now called chromosomes,
just to confuse things) are pulled to the poles
by the contracting spindle fibres
The chromatids randomly segregate
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TELOPHASE 2:
The chromosomes reach the poles and de
condense.
The nuclear envelope reforms, forming four
separate nuclei.
Cytokinesis may now occur and the cells
divide.
Each cell has half the number of chromosomes
as the original parent cell, the cells are haploid.
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Metaphase II
Nucleolus disappears
Chromosomes condense
Spindle forms
chromosomes
arrange themselves on
equator
Attach by centromere to
spindle fibres
Random arrangement of
chromatids
Anaphase II
Telophase II
Prophase II
Centromeres divide
Chromatids pulled apart
to opposite poles by
contracting spindle
Duplicated
Nuclear
envelopes reform
around haploid nuclei
Cell divides by cytokinesis
4 daughter cells formed
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1.
2.
https://www.youtube.com/watch?v=16enC385R
0w
Learning outcome states diagrams and
pictures: SORT in pairs! (Homework!)
Recap on some definitions
 Homologous pairs
 Bivalent
 Loci
 Maternal chromosomes
 Paternal chromosomes
Mitosis
No. of Divisions One
Products Two genetically
identical daughter
cells
Chromosome number Maintained
Meiosis
Two
Four cells, genetically
different from each
other & parent cells
Halved
Bivalents formed? No
Yes
Crossing over? No
Yes