This is the process from which one cell divides in two daughter cells, genome information is duplicated and segregated.
There are different stages in mitosis:
- chromosome condensation in profase, nuclear envelope breakdown which brings you into pro-metaphase at which point the chromosomes are ...
this is the process from which one cell divides in two daughter cells
genome information is duplicated and segregated there are different stages in mitosis chromosome condensation in profase
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Concepts in Molecular Biology
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15 LC Concepts in Molecular Biology 31.10.2022
MITOSIS
This is the process from which one cell divides in two daughter cells, genome information is duplicated and
segregated.
There are different stages in mitosis:
- chromosome condensation in profase, nuclear
envelope breakdown which brings you into pro-
metaphase at which point the chromosomes are
starting to align on the mitotic spindle that is
alreading forming on the outside of the nucleous.
- In metaphase there is an align state; once the cell
has conformed that indeed al the chromosomes
are properly attached to microtubules emanating
from the two spindle poles, anaphase is on set,
chromosomes segregate.
- In telophase the chromosomes start to
decondense beginning to assemble the cleavage
fur that will physically separate the two cells.
in 1980s people did know that mitosis was an important process and noticed that in cancer cells this was
going wrong, with mitotic mitotic figures.
Theodor Boveri came up with what is now know as the aneuploidy hypothesis → the origin of cancer is
one cell with an incorrectly combined karyotype and this aneuploid state is the source of this abnormal
growth tendency that constitutes cancer.
➢ Anatomy of the mitotic spindle
- Microtubules base spindle that has at its poles the replicated
centrosomes; you have condensed chromosomes and site of
attachment of those chromosomes to the spindle in the form
of kinetochores.
Mitotic chromosome segregation is a very complicated process; yet
there are three main elements: condensed chromosomes, the
centrosomes and kinetochores; they can give an overview of mitosis.
o Chromosome condensation
This involves taking the decontained interphase dna and further compacting it 20x fold and
then also resolving those individual chromosomes into individual units that can then be
align on the metaphase plate.
The proteins that are essential for chromosome organization protein interphase and in
mitosis are those stabilio maintence of chromosomes SMC proteins → the cohesins and the
condensisns.
▪ Cohesin
SMC proteins are involved in loop
extrusion, and thereby in the organization
of the interphase chromatine. Through this
process you are developing the
topologically associated domain TADs, seen
in the map.
,15 LC Concepts in Molecular Biology 31.10.2022
▪ Condensin
Depetion of cohesion release factor called wapl
would induce a hyper compaction of the interphase
dna the made a nucleus turn into something called
Vermicello.
→ depletion of cohesion release factor results in DNA hyper-compaction in
interphase (vermicelli chromosomes).
What condensin does is very similar, forms dna loops in an atp dependent manner.
This was shown using fna curtain assay (you attach dna, add recombinant complex,
add atp, apply flow and you can see dna looping occurring in vitro).
From experiments like this was concluded that condensing
does indeed loop dna on one side only off this ring around
the dna.
There is condensin I and II that share the same SMC
subunits. Condensin 2 is involved in chromosome
compaction and prophase, and Condensin 1 is involved on
further looping on top of the loops that you got with the
condensing 2.
These condensins will inset in the chromosome axis and this
is why in the early experiments it was seen that
chromosomes scuffles of proteins that seemed to remain in
the shape of a x shaped mitotic chromosome and those
proteins were those condensins as well as one other
proteins which is topoisomerase II.
Topoisomerase II decatenate dna, it set
off one dsDNA through the other.
As dna is replicating in s phase, you’re entangling this
two stands but once you try to segregate them in
mitosis you need to be ab le to have this nice them
apart with a nice X shape where only at the centromere
the sister chromatids are held together.
➢ Keeping mitotic chromosomes apart – the role of Ki-67
After we nicely condenced chromosomes, we need to make sure they stay apart.
One protein involved in this is Ki-67 that sits in the nucleolus in the interphase and then re-localizes to the
chromosome periphery.
If this protein is depleted the consequences are dramatic:
- In the image is found on the left the wt and on the right the protein repleted rnai; you have the
condensed chromosomes in red and the nuclear envelope marker. It can be seen that dna will have
a hard time being properly segregated in anaphase.
The model that is proposed is that Ki67 coats the mitotic chromosomes and creats a charged enviroments
that will repel other chromosomes.
, 15 LC Concepts in Molecular Biology 31.10.2022
➢ A role for histone modifications in mitotic chromosome condensation
An other thing that is important for chromosome condensation is histone modification, in particular de-
acetylation.
It was known that interphase chromosomes are hyper-acetylated and as you go into mitosis this
acetylation is lost.
If you perturb this mitotic de-acetylation using TSA, something goes wrong with chromosomes
condensation, everything looked a little bit fuzzy. The consequencies don’t seem very dramatinc because
you will have the condensins; but what’s really quite striking is what happens if you get rid of both
condensins and de-acetilation → no real process of condensing mitosis is completed.
This tells us that we need both histone de-acetilation and the action of condensing to condense mitotic
chromosome.
➢ What is de-acetylation good for?
This is where liquid-liquid phase separation comes into play; it was found that if you take nucleosomes you
can, under physiologically salt conditions, see liquid-liquid phase separation behavior in vitro. This behavior
was actually dependent on histone-tail; if removed (histone tails) you are left with no condensation. The
same could be seen if you work to acetylate these histones.
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