Cell division - mitosis Flashcards
Chromosome structure
DNA forms a complex with RNA/proteins to form chromatin
chromatin packed into beads called nucleosomes consisting of two loops of the DNA helix wrapped around 8 histones
supercoiling and condensation of chromatin forms chromosomes
What is chromatin?
A complex of DNA, histones and nonhistone proteins from which chromosomes are made
Is chromatin basophilic or acidophilic? In other words how will it stain?
basophilic
What are the 2 types of chromatin?
Heterochromatin
Euchromatin
What is heterochromatin?
a highly condensed form of interphase chromatin which is transcriptionally inactive
Stains very intensely!
What is euchromatin?
the least condensed extended state of chromatin
known as active chromatin - transcriptionally active
Stains very lightly!
In order for a eukaryotic cell to divide into two, the two into four, etc. two processes must alternate…
What are these two processes?
doubling: doubling of DNA in S phase of the cell cycle
halving: of that genome during mitosis (M phase)
What happens during M phase of mitosis?
Mitosis
and division of parent cell into daughter cells
What is the G0 phase ?
Term applied to cells which divide infrequently or not at all
It is the period when cells carry out their differentiated function
What happens during G1 phase of the cell cycle?
also called the gap phase, it’s the interval between completion of mitosis and the beginning of S phase
What occurs during S phase
Replication of DNA/RNA/proteins
duplicated chromosomes called sister chromatids
What happens during G2 phase of the cell cycle?
It’s the interval between the end of S phase and the beginning of mitosis
protein synthesis occurs
What are the G1, S and G2 phases (together) termed?
interphase
What is the purpose of interphase (G1, S and G2)?
to allow the cell additional time to grow and duplicate its chromosomes
What are the 6 stages of Mitosis?
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis
Summarize the events of Prophase
1st Stage of Mitosis
Chromatin condense to form visible chromosomes
chromosomes not yet attached to the mitotic spindle
chromosomes consist of 2 sister chromatids joined at centromere
centrioles duplicate and move to opp poles acts as microtubule organising centres
microtubules form the spindle apparatus between the centrioles
Summarize the events of Prometaphase
Starts with the disassembly of the nuclear envelope
nucleoli regress
chromosomes move to equator
Microtubules from the mitotic spindle bind to specialized protein complexes (kinetochores) on each pair of sister chromatids
Summarize the events of Metaphase
The chromosomes align along the equatorial plate of the spindle
spindle consists of microtubules running from pole to pole
Summarize the events of Anaphase
connections between sister chromatids are cut by proteolytic mechanisms
Daughter chromosomes moved to opposite poles of the spindle by action of the microtubules
Summarize the events of Telophase
the condensed chromosomes decondense into their interphase state
nucleoli reappear
Nuclear envelope re-assembles around each group of chromosomes to form two daughter nuclei
spindle disappears
Summarize the events of cytokinesis
Process by which the cytoplasm is cleaved in two
Begins in anaphase
involves a transient structure made of actin filaments (contractile ring)
contractile ring attaches to the membrane-associated proteins on the cytoplasmic face of the membrane
Describe the main cell cycle regulators
Early Response Transcription Factors like c-fos and c-jun promote cell division
Cyclins (G1, S-phase, and M-phase cyclins) - promote the events that happen in each phase, conce rise and fall as each phase occurs
Cyclin-dependant kinases - CDKs (G1 CDKs, S-phase CDKs, M-phase CDKs) - levels remain fairly stable
What can happen if cells progress to the next phase of the cell cycle before the previous phase is properly completed?
Catastrophic genetic damagge can occur
What is the purpose of a cell cycle checkpoint?
Control mechanisms at such checkpoints make sure the cell progresses through the cell cycle - completing each step in order (otherwise castastrophic genetic damage can occur)
What do mitogens do in the cell cycle?
They stimulate cells to enter G1
What was the term “tumor” formerly used to denote?
any localized swelling in the body caused by inflammation or abnormal cell proliferation
Now we use it as a synonym for neoplasm
What are neoplasms?
abnormal mass of tissue formed by uncoordinated cell proliferations
Can be benign or malignant depending on their growth rate and invasiveness
How is the cell cycle regulated
how can rate of mitosis be increased/decreased ?
Repressor genes reduce mitosis
some hormones and growth factors trigger cell division
usually rate of cell production = rate of cell death
most cancers caused by mutation of repressor genes
How do CDKs work?
CDKs become activated upon binding with cyclin
activated CDKs cause phosphorylation to activate or inactivate target proteins
these proteins allow the cells entry into the next phase of the cell cycle
what are the centrosomes ?
structures the microtubules attach to, forming the spindle
organelle located near the nucleus in the cytoplasm that divides to form two centrioles that migrate to opposite poles of the cell during mitosis
