Mitosis

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Mitosis
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What is a genome?
A genome is all the genetic information of a cell
in the form of DNA, which is packaged into
chromosomes
Eukaryotic organisms have a characteristic number
of chromosomes, all of which are linear
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All these chromosomes are found in the nucleus of
the cell
When the cell is not dividing, the DNA and its
associated proteins exist in the form of chromatin
Chromatin begins to condense into chromosomes
when the cell is ready to divide
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What is Mitosis?
Technically, mitosis is the division of the
nucleus of a somatic cell
A somatic cell is a body cell, as opposed to a
gamete, which is a sex cell
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The Mitotic Cell Cycle
The mitotic phase alternates with interphase in
the cell cycle
Mitosis is just one part of the cell cycleThe
mitotic (M) phase, which includes both mitosis
and cytokinesis, is usually the shortest part of
the cell cycle
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Mitotic cell division alternates with a much
longer interphase, which often accounts for about
90 of the cell cycle
It is during interphase that the cell grows and
copies its chromosomes in preparation for cell
division
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Interphase can be divided into sub-phases
These sub-phases are
G1 Phase (First Gap)
S Phase (Synthesis)
G2 Phase (Second Gap)
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During all three sub-phases, the cell grows by
producing proteins and cytoplasmic organelles
Chromosomes are duplicated only during the S phase
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Why must the chromosomes duplicate?
Each daughter cell must have a complete
complement of chromosomes (2n, diploid) to
survive (46 in humans)
Only the gametes have reduced chromosome number
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Some Background
What is the difference between homologous
chromosomes and sister chromatids?
The two chromosomes of a homologous pair are
individual chromosomes that were inherited from
different parents
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These are sister chromatids
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Set of 46 homologous chromosomes of the human male
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Mitosis is conventionally broken down into five
different sub-phases
Prophase Prometaphase Metaphase Anaphase Telophase
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Cytokinesis in animal and plant cells
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Mitosis in an onion root
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Cell Cycle Checkpoints
A checkpoint in the cell cycle is a critical
control point where stop and go-ahead signals can
regulate the cycle
There are three major checkpoints found in G1,
G2, and M phases
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G1 checkpoint is the restriction point in
mammal cells
If a cell gets a go-ahead signal at the G1
checkpoint, it will usually complete the cycle
and divide
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If a cell does not receive the go-ahead signal at
the G1 checkpoint it will exit the cycle,
switching into a non-dividing state called G0
phase
Most cells of the human body are in the G0 phase,
such as nerve and muscle cells that never divide
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There is a molecular (chemical) control system
driving the cell cycle. These regulatory
molecules are proteins of two main types
Protein kinases (Cdks) and the cyclins to which
they are attached
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To be active, the kinase must be complexed with
its cyclin partner, and the cyclin concentrations
fluctuate cyclically in the cell
The activity of a Cdk (always present in a
constant concentration) rises and falls with
changes in cyclin concentration
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One of these cyclin-Cdk complexes is called MPF,
and is responsible for triggering the cells
passage past the G2 checkpoint into M-phase
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When the cyclins that accumulate during G2
associate with Cdk molecules, the resulting MPF
complex initiates mitosis, apparently by
phosphorylating a variety of proteins
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This diagram shows the fluctuation of MPF
activity and cyclin during the cell cycle
3. One indirect effect of MPF is the breakdown of
its own cyclin the Cdk component of MPF is
recycled
1. Accumulated cyclin molecules combine with Cdk
molecules to produce many molecules of MPF by the
G2 checkpoint
2. MPF promotes mitosis by phosphorylating
various proteins, including other enzymes
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For example, MPF causes the nuclear envelope to
fragment by phosphorylating, and stimulating
other kinases to phosphorylate, proteins of the
nuclear lamina, the lining of the nuclear envelope
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Later in the M-phase, MPF helps switch itself off
by initiating a process that leads to the
destruction of its cyclin by a special protein
breakdown mechanismwhich also is involved in
driving the cell cycle past the M-phase checkpoint
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Meiosis
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What is Meiosis?
Meiosis is the process by which gametes, or sex
cells, are created
During mitosis, the chromosome number is
maintained During meiosis, the chromosome number
is halved
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Whereas mitosis occurs in all parts of the body,
involving all types of somatic cells, meiosis
takes place only in the gonads Ovaries Testes
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A normal complement of chromosomes in humans is
referred to as 2n
This is because we have two copies of each
somatic chromosome, one from each parent (besides
X and Y)
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After meiosis, the normal complement of
chromosomes is cut in half We end up with
(½)2n n
This is the haploid state. At fertilization
(fusion of gametes) the 2n diploid number is
restored
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Meiosis, like mitosis, is preceded by the
replication of chromosomes
Unlike mitosis, during meiosis, this replication
is followed by two consecutive cell divisions
instead of just one
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These consecutive cell divisions are
called Meiosis I and Meiosis II
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Metaphase during mitosis
Homologous pair at metaphase plate
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2n
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Metaphase during meiosis I
Homologous pair at metaphase plate
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We are now haploid
n
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Metaphase Plate in Meiosis II
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Four haploid daughter cells
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Mendels pea plants
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To create hybrid plants, Mendel snipped off the
male stamens from the reproductive organs to
prevent self-fertilization. He then used a
paintbrush to transfer pollen from another plant
for fertilization.
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Mendel also did reciprocal crosses A female of
phenotype A is crossed with a male of phenotype B
and vice versa
?A ?B
?A ?B
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Purple
White
Parental PP pp
Gametes P only p only
Fusion of Gametes
?
Can we make a Punnett square?
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P
F1
p
All purple flowers
In the F1 generation, we have a single phenotype
and a single genotype
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Now lets allow the F1 generation to
self-fertilize to get an F2 generation. The F1
products of meiosis assort independently
F2 Parental P p P p
Gametes P p P p
Gives us P p P p
Well use these gamete types to make a Punnett
square
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Sperm
F2
P
p
P
Eggs
p
31 Phenotypic ratio
121 Genotypic ratio
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The selfing, or intercross of identical
heterozygous F1 individuals is called a
monohybrid cross It was this type of cross that
provided the 31 progeny ratio that suggested the
principle of equal segregation of chromosomes
(plus the genes residing on them) into gametes
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Mendel then tried to follow the inheritance of
two characters at the same time He wanted to
know if two characters, such as seed shape and
color, are inherited together as a package or if
they assort independently
Will seed color and shape alleles always stay
together?
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Mendels first hypothesis Dependent Assortment
If the two characters segregate dependently (that
is, together), the F1 hybrids can only produce
the same two classes of gametes that they
received from the parents, and after selfing, the
F2 offspring will only show the parental
phenotypes in a 31 ratio
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Lets see how to put a dihybrid cross together
into a Punnett square
YYRR yyrr
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P Generation YYRR yyrr
Gametes YR yr
Y y R r
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This gives us all combinations for our Punnett
square
YR, Yr, yR, yr
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YR
Yr
yR
yr
YR
?
Yr
yR
yr
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YR
Yr
yR
yr
YR
Yr
yR
yr
73
YR
Yr
yR
yr
YR
Yr
yR
yr
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What is a testcross?
We use a testcross to determine the genotype of
an organism that displays an ambiguous phenotype
That is, a purple-flowered pea plant can have
either the PP genotype or the Pp genotype
We can cross the purple-flowered plant with a
homozygous recessive planta white one with a
genotype that is known
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When were done with the testcross, we will
examine the phenotypes of the progeny (F1)
If we get all purple-flowered plants, we know
unambiguously that the purple parent plant is
homozygous dominantPP
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Testcross
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Testcrosses can also be done with the fruit fly,
Drosophila melanogaster
These types of crosses were originally done to
test for linkage by looking for recombinant
phenotypes
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Web Resources
Tetrad Analysis
http//www.usask.ca/biology/rank/316/tetrad/tetrad
.htm