Lecture 3 - Mitosis & Meiosis Flashcards
what state do most developed multicellular cells exist in?
G0 state
allows cells to enter the cell cycle:
signals from their environment allows cells to initiate the cell cycle
cell division is not carried out correctly, causing daughter cells to not get enough genetic information:
the daughter cells will die
prophase:
replicated chromosomes consisting of two closely associated sister chromatids will condense
outside the nucleus the mitotic spindles assemble between the two centrosomes, which have begun to move apart
prometaphase:
starts abruptly with the nuclear envelope breakdown, chromosomes can now attach to spindle microtubules via their kinetochores and undergo active movement
metaphase:
the chromosomes will be aligned in the equator of the spindle (mid way of the spindle)
the paired kinetochore microtubules on each chromosome attach to opposite poles of the spindle
anaphase:
sister chromatids synchronously separate, and each is pulled slowly toward the spindle pole it is attached to
the kinetochore microtubules (ones attached to the chromosome) get shorter, and the spindle poles also move apart - both contributing to chromosomal separation
telophase:
two sets of chromosomes arrive at the poles of the spindle
new nuclear envelope reassembles around the two new sets of chromosomes - completing the formation of two nuclei and marking the end of mitosis
cytoplasm division begins with the assembly of the contractile ring
cytokinesis:
cytoplasm is divided in two by a contractile ring of actin and myosin filaments which pinches the cell to create two daughters each with their own nucleus
how are chromosomes very tightly packed for mitosis?
condensing complexes bind to chromosomes at the beginning of mitosis and folds them into consecutive cis-loops until highly compacted and condensed
what do SMC complexes use?
SMC complexes use the power of ATP binding and hydrolysis to manipulate DNA
SMC complexes can bring together two different regions of DNA to either form:
a DNA loop or to cohese them together
in eukaryotes, in what part of the cell cycle does SMC complex cohesin coheres sister chromatids together?
from S phase to anaphase
how does the SMC complex condensing fold chromosomes?
by looping into their mitotic condensed form
what inhibits the interaction between the nuclear lamins which coat the inner membrane of the nucleus?
phosphorylation of the nuclear lamins which coat the inner membrane of the nucleus inhibits their interaction with each other
what does phosphorylation of the nuclear lamins which coat the inner membrane of the nucleus cause?
this causes a breakup of the nuclear lamina and attached nuclear membrane while mitotic CDK complexes are active
what happens when mitotic CDK complexes are inactivated at the end of mitosis?
the nuclear envelope reforms
mitotic spindles are designed to:
the mitotic spindle is designed to line up all the duplicated sister-chromatids in the middle of the cell in metaphase, before pulling one copy to opposite ends of the cell before cell division
what attaches chromatids to microtubule s?
The microtubules emanating from the spindle pole become attached to a specialized protein structure on chromosomes called the kinetochore where they can become stably attached
what do microtubules coming from the mitotic spindle do before they bond to the kinetochores?
grow and shrink across the cells until they make constant with the kinetochore
the contact is then stabilised to an end-on contact
chromosome segregation does not take place until:
all kinetochores have two kinetochore –microtubule attachments, one connected to each spindle pole
when all chromosomes are bi-orientated in metaphase the kinetochore microtubules…
are actively trying to pull the chromatids to opposite sides of the cell, the chromosome cohesion provided by cohesin prevents this
___ promotes the destruction of cycling’s and mitotic exit
APC
cytokinesis is different in animals and plants:
plants need to form a new cell wall through the fusion of golgi vesicles during cytokinesis as well as cell membrane
meiosis:
meiosis is a specialised reductional form of cell division, a parental diploid cell divides to produce four haploid cells
Meiosis achieves this by undergoing one S (duplication) phase followed by two divisions of the duplicated chromosomes; meiosis I and meiosis II
meiosis generates genetic diversity via two ways:
genetic recombination during prophase I & independent assortment during metaphase I
producing genetic diversity via crossing over during prophase I:
Double strand breaks are introduced into the chromosomes and these are repaired between homologous chromosomes to produce crossovers, observed as chiasmata (sing. Chiasma).
Resolution of crossover produces recombinant chromosomes composed of parts of both maternal and paternal chromosomes
producing genetic diversity through the mixing of chromosomes in metaphase I:
Chromosome pairs are orientated randomly across the centre of the cell in metaphase I. This leads to the random sorting of homologous paternal and maternal chromosomes with other chromosomes. In humans with 23 pairs of chromosomes this results in 223 = 8388608 possible combinations of paternal and maternal chromosomes.
errors in meiosis:
Non-disjunction errors during meiosis I or II can lead to both chromatids being segregated into the same gamete which leads to trisomy in the fertilised embryo
Too many or too few chromosomes in a cell is known as aneuploidy. Too few and essential enzymes are not expressed. Too many and some processes don’t work properly leading to cell death or a failure in development.
what is the only trisomy that can be handled in the human body?
trisomy of chromosome 21 - which leads to downs-syndrome
how do cells initiate the metaphase to anaphase transition?
cells initiate the metaphase to anaphase transition by triggering the cleavage of cohesion by separase
