Download MITOSIS-MEIOSIS SUMMARY

MITOSIS
● Mitotic Phase- a cell will undergo Mitosis to
form 2 new nuclei
● Mitosis- the process of dividing the duplicated
DNA of a cell into two new nuclei; split into
distinct stages.
Prophase
- First stage in mitosis
- DNA condenses, organizes, and the classic
chromosome structure appears.
- We first see the classic chromosome
structure
which
occurs through a
condensation process.
- At the same time, protein strands called
“Microtubules”
appear
from
the
centrosomes in animals.
- Finally, the structure found within the
nucleus, the “Nucleolus” disappears.
Prometaphase
- microtubules attach to the chromosomes
- begins when the nuclear membrane is
broken down
- At the same time, microtubules strands or
spindle fibers are growing from the
centrosomes
- Spindle fibers attach to a protein structure
called the kinetochore
- one kinetochore is attached to the
centromere of each Sister chromatid
Metaphase
- chromosomes align
- the sister chromatids align along the center
of the cell so that both chromatids face
toward opposite poles of the cell
- the sister chromatids are then ready to be
separated
Anaphase
- chromosomes/ sister chromatids separate
- separation through a shortening of the
microtubules/ spindle fibers attached to the
kinetochores
- the poles of the cell move farther apart,
causing increased separation of sister
chromatids
- At the end of anaphase, the sister
chromatids have moved to the two ends of
the cell.
Telophase
- nuclear membranes reappear around the
two sets of chromosomes
- the final stage of mitosis
- the components of the new cells begin to
appear
-
spindle fibers are broken up
a new nuclear membrane surrounds the
chromosomes at the end of each cell
chromosomes uncoil and return to an
unconditioned state
Mitosis is then complete, the formation of
two cells is all that remains.
●
After mitosis, 2 new cells are formed by a
process called "cytokinesis".
- First, the cell is compressed by a contractile
ring that divides the cell in nearly equal
halves.
- By now the organelles in the cell have been
replicated and are now divided between the
two halves of the cell. This includes
mitochondria, Golgi bodies, and the rough
ER. plant cells also have chloroplasts.
- Once split, the two new cells are now fully in
the G one stage of interface and ready again
to begin their growth.
●
Mitosis is only one part of what is called the
cell cycle.
For many eukaryotic cells, a cell is duplicated
every 24 hours.
●
Interphase- in which most of the life of a cell
is spent in; consists of 3 stages, called: G1, S
and G2.
G1 or Gap 1
- the first growth stage of Interphase
- cell grows to nearly it’s full size, and
performs many of its specific biochemical
functions that aid the organism
S or Synthesis Phase
- an important stage, because it is when the
DNA in the nucleus is replicated
G2 or Gap 2
- cell finishes growing
●
●
●
Once the cell has duplicated DNA in the
nucleus and 2 centrosomes have appeared in
the cytoplasm, Mitosis can begin.
For a typical eukaryotic cell, Mitosis will last
about 80 minutes.
Mitosis begins with prophase ( DNA condensing
into chromosomes), then prometaphase
(microtubules appear and the nuclear
membrane
breaks
down),
metaphase
(chromosomes are aligned at the center of the
cell), anaphase (chromosomes are moving
apart), and lastly telophase (appearance of new
nuclear membranes). This is the end of mitosis.
Finally, the splitting of the cell occurs during
cytokinesis. The two new cells are now ready to
grow and perform their specialized functions.
MEIOSIS
Many organisms pass their genes to their
offspring through sexual reproduction.
● Sexual reproduction begins when two
gametes unite to form an embryo that is
genetically unique from the parent organisms.
The embryo then grows into an adult who in
turn passes their genetic information onto
their own offspring.
● Gametes are formed through a process called
meiosis.
●
Germline cells
- cells that undergo Meiosis to produce the
gametes
- In diploid organisms, they have two copies
of each chromosome.
- undergo meiosis to produce haploid
gametes, which only have 1 copy of each
chromosome. These haploid gametes fuse
to form a diploid embryo that grows into the
adult.
- Undergoes meiosis, which is just one step in
the life cycle of a germline cell
- Similar to Mitosis, the cells also pass
through the interphase, G1, S, and G2
stages, before they enter meiosis.
- The DNA inside a germline cell is
duplicated before Meiosis begins during the
S phase.
Sister chromatids
- duplicated germline chromosomes
- remain attached to each other until the
second cell division event in Meiosis
Meiosis
- has 2 cell division events
- first division: Meiosis I, results in 2 unique
daughter cells that have half the amount of
DNA as the parent germline cells
- second division: Meiosis II, results in 4
unique haploid cells that only have 1 copy of
each chromosome.
- These haploid cells that are produced are
the gametes that could go on to produce an
offspring through sexual reproduction.
Meiosis I
Prophase 1
- wherein Meiosis begins
- DNA condenses to form chromosomes
- The duplicated sister chromatids join
together at the centromere and stay fused
at the centromere throughout Meiosis I
- Next,
each
pair
of
homologous
chromosomes undergoes synapsis to form
-
-
a complex involving two pairs of sister
chromatids.
Recombination/
crossing
overChromosomal material is exchanged
between the two pairs of sister chromatids.
After crossing over, the sister chromatids
for each chromosome are no longer
identical to one another (why no two
siblings aside from twins are genetically
identical)
nuclear membrane begins to break down
the two centrosomes migrate to opposite
ends of the cell and microtubules appear
microtubules
then
attach
to
the
chromosomes
Metaphase 1
- the synapsed chromosomes align at the
equator of the cell.
- chromosomes align randomly which results
in different combinations each time meiosis
occurs
Anaphase 1
- homologous chromosomes separate and
migrate to the two poles of the cell
- the sister chromatids remain attached at
their centromeres
Telophase 1 and Cytokinesis
- final steps of Meiosis I
- Here, the cell divides into 2 daughter cells,
each of these 2 cells now undergo Meiosis
II.
Meiosis II
- similar to mitosis
Prophase II
- The first stage of Meiosis II
- Again, chromosomes condense, the nuclear
envelope breaks down, and the spindle
apparatus forms.
- The major difference between Prophase II &
Prophase I, Is the fact that the daughter
cells only have 1 copy of each homologous
chromosome.
- In Prophase II, there is no synapsis of
homologous chromosomes or crossing
over.
Metaphase II
- the chromosomes align at the equator of
the cell
- the alignment is random.
- Since the sister chromatids are no longer
identical, there will be many different
possible ways for these chromosomes to
align.
Anaphase II
- the sister chromatids are pulled apart as the
microtubules shorten
- the ends of the cell are pushed further apart
as microtubules elongate
Telophase II
- the nuclear membrane reforms and the
cytoplasm is divided into the two haploid
daughter cells (division process is called
cytokinesis)
●
●
●
Since Meiosis II began with 2 cells and each
of those cells were split into 2 cells, we now
have 4 unique haploid cells at the end of
meiosis. These cells are gametes.
Two gametes; one from a father, and one from
a mother may fuse to produce a diploid
embryo.
The resulting embryo then grows through
many cycles of Mitosis.
DNA, chromatin, chromosomes, chromatid
Chromatin
- is composed of DNA and histones that are
packaged into thin, stringy fibers
- undergoes further condensation to form the
chromosome
- is a lower order of DNA organization, while
chromosomes are the higher order of DNA
organization.
Chromosomes
- are single-stranded groupings of condensed
chromatin
- During the cell division processes of mitosis
and meiosis, chromosomes replicate to
ensure that each new daughter cell receives
the correct number of chromosomes.
- A
duplicated
chromosome
is
double-stranded and has the familiar X
shape.
- The two strands are identical and connected
at a central region called the centromere.
Chromatid
- is either of the two strands of a replicated
chromosome.
- connected by a centromere, are called sister
chromatids.
- At the end of cell division, sister chromatids
separate
and
become
daughter
chromosomes in the newly formed daughter
cells.
Mitosis
Meiosis
What is the purpose of this process?
In a unicellular organism, the purpose of
mitosis is to proliferate asa species. In a
multicellular organism, the purpose can
be to grow during development, or to
repair or regenerate
adamaged tissue, for example.
To create gametes with only one copy of the
organism’s genetic information, in preparation
for sexual reproduction. Various steps in
meiosis create opportunity for genetic
diversity in the daughter cells. This is the raw
substrate for evolution.
What is the outcome of this process?
Two diploid cells with identical genetic
information.
Four haploid cells with different genetic
information.
Which organisms perform this process?
Mitosis is performed by unicellular and
multicellular eukaryotes.Bacteria have
their own version of mitosis called
“binary fission”.This is distinct from
meiosis as bacteria typically have one
circular chromosome,which is not
contained within a nucleus, like
eukaryotic chromosomes.
Only organisms which perform sexual
reproduction. Archaeaand bacteria do not do
this, so it might be tempting to think that
unicellularorganisms do not sexually
reproduce. However, there are exceptions;
buddingyeast will form haploid spores under
nutritional deprivation.
How long does this process take?
Mitosis is usually shorter than meiosis.
Meiosis has various timescales in different
The process can take over 10 hours for
organisms, which can be affected by several
mammalian cells in culture [2], budding factors including temperature and environment
yeast can take ~80 minutes to complete
of the organism, and the amount of nuclear
a cell cycle [3], whilst bacteria can divide
DNA. The process lasts 6 hours in yeast but
every 20 minutes.
can last more than 40 years in human females,
due to a developmental hold at prophase I,
until ovulation. Other examples are 1-2 days in
male fruit flies and ~ 24 days in human males.
[1]
What is an example of a disease caused by an error in this process?
Uncontrolled mitosis occurs in cancer,
Errors in meiosis can lead to the wrong
where either genes that stop cell division number of chromosomes ending up in germ
(tumour suppressors) are switched off, cells, this is called aneuploidy. This can trigger
or genes that encourage cell division
miscarriage, but is occasionally tolerated. One
(oncogenes) are overactive.
example is Down’s syndrome, caused by
trisomy 21. Another example is Klinefelter
syndrome, where XY males have an additional
X chromosome.
Etymology?
Mitosis is the Greek word for thread, after Meiosis means a “lessening” in Greek. This
the thread-like chromosomes that can be refers to the outcome of meiosis, where the
seen under the microscope in dye-stained genetic information in each new cell is halved.
cells during cell division.
First described by?
Walther Flemming in his 1882 work “Cell Oskar Hertwig described the fusion of egg and
substance, nucleus and cell division.” [5]
sperm in the transparent sea urchin egg in
1876. [4]
Summary
Meiosis and mitosis both have a prophase, metaphase, anaphase, telophase and
cytokinesis.
In meiosis, prophase, metaphase, anaphase
and telophase occur twice. The first round of
division is special, but the second round is
more like mitosis.
In mitosis, prophase, metaphase,
anaphase and telophase occur once.
Prophase
Chromosomes condense and the centrosomes begin to form an early spindle.
●
●
Meiotic prophase I is much longer that
mitotic prophase.
During prophase I homologous
chromosomes make contacts with each
other called chiasmata and “crossing
over” occurs. This is where
chromosomes exchange sections of
DNA. This is important for generating
genetic diversity but is also crucial
mechanically to hold homologous
chromosomes together.
●
●
Mitotic prophase is much shorter
that meiotic prophase I.
There is no crossing over in
mitosis.
Metaphase
In metaphase II of meiosis, and metaphase of mitosis, chromosomes line up along the
metaphase plate due to the action of microtubule spindle fibres emanating from the
centrosomes located at opposite cell poles. These fibres are attached to the
chromosomes by kinetochores at the centromeres of the chromosomes.
●
●
In meiotic metaphase I pairs of
homologous chromosomes line up
along the metaphase plate.
The way in which the homologous pairs
are oriented randomly with respect to
the cell poles is referred to as the law
of independent assortment and
ensures a random and independent
distribution of chromosomes to the
daughter cells of meiosis I and
ultimately to the haploid gametes at the
end of meiosis II.
●
●
In mitotic metaphase a single
chromosome/ pair of chromatids
line up along the metaphase
plate.
Sister chromatids are identical
and so the orientation of the
chromosome doesn’t carry any
meaning.
Anaphase
In anaphase, chromosomes are split to opposite poles of the cell.
●
In anaphase of meiosis I cohesin at
the centromeres of the chromosomes
is not cleaved and it therefore
continues to hold sister chromatids
together as the homologous
chromosomes are segregated to
opposite cell poles.
●
In anaphase of mitosis (and
meiosis II), cohesin protein
holding the centromeres of the
sister chromatids together is
cleaved, allowing the sister
chromatids to segregate to
opposite poles of the cell, at
which point they are called
chromosomes.
Telophase
A nuclear membrane reforms around the newly separated chromosomes, which begin to
uncoil, becoming less condense. The spindle microtubules disassociate. Each daughter
cell will inherit one centrosome.
Cytokinesis
The cell plasma membrane pinches, to leave two daughter cells with separate plasma
membranes.
●
In meiosis, cytokinesis must occur
twice: once after telophase I and
again, after telophase II.
●
In mitosis, cytokinesis does not
always occur, some cells divide
and are multinucleate, like
muscle cells.
●
Mitosis
Stage
Chromosomes
Interphase
Are uncondensed but are still organised. The entire genome is replicated
to create two identical semi-conserved copies of each chromosome.
Prophase
Condense. Duplicated chromosomes are called sister chromatids.
Metaphase
Align along the metaphase plate, the midpoint between the two
centrosomes. Sister chromatids are joined at the centromere by proteins
that form a structure called a kinetochore.
Anaphase
Cohesin is cleaved at the centromere of chromosomes, resulting in sister
chromatids being pulled to opposite poles of the cell.
Telophase
Chromosomes begin to uncoil, becoming less condensed.
Cytokinesis
Chromosomes have returned to their interphase structure. This is a topic of
much research, but it seems as though each chromosome occupies its
own territory within the nucleus.
Mitosis
Stage
Centrosomes
Interphase
The centrosome is duplicated.
Prophase
Microtubules begin to form an early mitotic spindle between the duplicated
centrosomes.
Metaphase
The two centrosomes are now located at opposite poles of the cell.
Anaphase
Microtubules emanating from the centrosomes shrink as the tension
holding the chromosomes at the metaphase plate is broken by cohesin
cleavage.
Telophase
The centrosomes remain segregated to opposite sides of the cell. Each
daughter cell will receive one centrosome comprised of two centrioles.
Cytokinesis
Centrosomes signal to the cell that it is okay to proceed with cytokinesis.
Research shows that cells where centrosomes are destroyed with a laser
beam cannot undergo cytokinesis.
Mitosis
Stage
Nuclear Membrane
Interphase
Intact.
Prophase
Intact.
Metaphase
In higher eukaryotes like vertebrates, by the time metaphase occurs the
nuclear envelope has broken down. This is caused by phosphorylation of
nuclear lamin proteins.
Anaphase
Broken down.
Telophase
A nuclear envelope reforms around the chromosomes in each daughter
cell.
Cytokinesis
Intact.
Mitosis
Stage
Plasma Membrane
Interphase
Intact.
Prophase
Intact.
Metaphase
Intact.
Anaphase
Intact.
Telophase
Intact.
Cytokinesis
Pinches to form two separate membranes around the two daughter cells.