2. What is mitosis and meiosis
Meiosis – cell division that results in cells that have half the
normal chromosome number(haploid gametes), it is also
called reduction division, specific to create sex cells.
Mitosis – Cell division that results in identical cells, used for
growth and repair of organisms.
3. Cell Cycle : Growth Stage/Interphase
Growth stage also called interphase is where the cell carries
out its regular metabolic function and prepares for its next
division
Interphase is divided into three phases : G1 phase, S phase
and G2 phase.
By the end of interphase, the cell is ready to begins the
process of nuclear division: mitosis.
4. Three phases in Interphase
G1 phase (Growth 1) –The cell goes through rapid growth and
metabolic activities.
S phase (Synthesis) –The middle stage of interphase where the cell’s
DNA is copied exactly.Which means that the chromatin replicates to
create a second identical set of DNA.After the replication, the two
identical chromosomes – sister chromatids – are joined at the
centromere
G2 phase (Growth 2) – During the replication in S phase, the cell
consumed a large amount of energy, therefore this stage allow the cell to
regenerate energy for division.As well, the cell manufactures proteins
and other molecules to make structures required for division of the
nucleus and cell.
6. Cell division : Mitosis
Four phases in the process of mitosis -Prophase, Metaphase,
Anaphase, andTelophase
After the cell has been through all the phases, it enter
cytokinesis - the division of the cytoplasm and the organelles
into two separate cells.
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Mitosis : Prophase
Prophase - During this phase, the chromatin condenses into tightly pack
chromosomes while the nuclear membrane breaks down, releasing
chromosomes into the cytoplasm.
As the nucleolus disppears, pair of cylndrical organelles called centrioles move
their way to the opposte poles of the cell
During the separation of the centrioles, a network fibres called the spindle
apparatus forms between them
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Prophase : Diagrams
Spindle fibre is made of microtubules - hollow
tubes of proten that facilitate movement of
chromosomes within a cell
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Mitosis : Metaphase
During metaphse, the spindle fibres guide the chromosomes
to the centre line of the cell and attach to the centromere of
each chromosome.
The chromosomes are lined up in such way that one sister
chromatid face one pole and the other sister chromatid face
the opposite pole.
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Mitosis : Anaphase
Before anaphase, the chromosomes are line up at the centre line of the cell.
During anaphase, the centromere splits apart and the sister chromatids separate
from one another.
The spindle fibres from each end still attach to the split centromere, it shortens
and pulls the sister chromatid towards its pole.
At the same time, microtubules in the spindle apparatus lengthen and force the
poles of the cell away from one another
By the end of anaphase, one complete diploid set of chromosomes has been
gathered at each pole and the process of lengthening the microtubules result in
a elongated cell
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Mitosis : Telophase
Final phase of mitosis
The chromatids begin to unwind into longer and less visible
strands of chromatin at the opposite pole of the cell
The spindle fibres begin to break down
Nuclear membrane starts to form around each new set of
chromosomes and a nucleolus forms within each new
nucleus
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Cytokinesis
This is the last step of cell division, interphase and mitosis are
the process of nuclear division, Cytokinesis is the division of
the cytoplasm to complete the creation of two new daughter
cells.
In animal cells, an indentation forms in the cell membrane
along the cell equator, the indentation deepens until the cell
is pinched in two.
This result in the equal divison of the cytoplasm and
organelles between the two halves of the cell
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Cytokinesis : Diagrams
Indentation at the
equator
Two daughter cells are
formed
End of cell division, the
daughter cells are now
in G1 phase
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Meiosis
Other form of cell division is called Meiosis, the process that
produces haploid gametes from diploid cells in the ovaries
and testes - sex cells -
Two main parts to Meiosis : Reduction division and
Recombination
During Meiosis, the four phases that occur in mitosis happens
twice, therefore it is called Meiosis I and Meiosis II.
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Interphase before Meiosis
Like somatic cells, germ cells also goes through the growth
and synthesis phases before dividing
Duplicated chromosomes are formed by the end of
interphase
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Meiosis : Prophase I
The homologous chromosomes which contains the same
genes but carry different alleles of these genes are align side
by side in the this phase.
The alignment of the homologous chromosomes are called
synapsis
At synapsis, homologous non-sister chromatids lie side by
side.This is a important step that will be later demonstrated
in genetic recombination
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Prophase I : Diagrams
Crossing over between non-siser
chromatids may occur.This increases
variation in gametes and offspring
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Metaphase I
A spindle fibre attach to the centromere of each chromosome
A spindle fibre from one pole attaches to one pair of sister
chromatids in the tetrad, and a spindle fibre from the
opposite pole attach to the other pair of sister chromatids
The spindle fibres guide each tetrad to the equator of the
cell, however the chromosomes do not line up in single file as
they do in mitosis, instead they line up as homologous pairs.
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Anaphase I
Similar to Mitosis, during anaphase I in Meiosis, the spindle
fibres shorten cause the homologous chromosomes to
separate from one another, making their way to opposite
poles of the cell
Since the sister chromatids are still held together, the
centromeres do not spilt
This result in a single chromosome - made up of two sister
chromatids - from each homologous pair moves toward the
poles.
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Anaphase 1 : Diagrams
Result in a single chromosome - made up of
two sister chromatids
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Telophase I
The homologous chromosomes begin to uncoil and the
spindle fibres disappear
Each new cell contains one set of sister chromatids
and is now haploid
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Meiosis II
By the end of telophase I, new cells contains one set of sister
chromatids are formed and is now haploid
Chromosome replication does not take place before the next
phase of meiosis
The process of Meiosis II are similar to the phases of mitosis
Each cell that enters meiosis II is haploid but consists of
replicated chromosomes.
At the end of meiosis II, the daughter cells are still haploid
but they contain single unreplicated chromosomes.
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Meiosis II : Diagrams
daughter cells are still haploid but
they contain single unreplicated
chromosomes.
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Similarities between Mitosis and Meiosis
Both process go through chromosome replication
Meiosis II is similar to Mitosis
Both Mitosis and Meiosis go through
interphase,prophase,metaphase,anaphase and telophase
Both use spndle fibers to separate chromatids from each
other
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Differences between Mitosis and Meiosis
Mitosis produce identical cells, Meiosis produce 4 daughter cells are
haploid but they contain single unreplicated chromosomes.
Mitosis is used for growth and repair of organisms, Meiosis is used to
reproduce gametes
Four phases in Meiosis I is different from Mitosis, in prophase I synapsis
happen, in metaphase I crhomosomes line up as homologous pairs, in
anaphase I centromeres do not split result in single chromosome from
each homologous pair moves to each pole of the cell, in telophase I
daughter cell contain one set of sister chromatid
