cell cycle, chromosomes and karyotypes Flashcards
What happens in G1?
the cell grows and develops
What happens in S?
the DNA is replicated
What happens in G2?
In G2, replication of DNA has been completed and the cell contains double amount of nuclear DNA than in G1; and gets ready for division
What is G0?
stable non-dividing period of variable length
What happens at G1/S checkpoint?
The G1/S checkpoint determines the size and condition of the DNA, if the DNA is damaged or the cell is below a certain size, the process is halted. The DNA must be fully repaired before entering the synthesis phase.
What happens at the G2/M checkpoint?
At the G2/M checkpoint, the condition of the DNA is checked again, if replication is not complete or the DNA is damaged, the cell cycle is arrested.
What happens at the M checkpoint?
the correct configuration of the metaphase plate is analyzed: the spindle fibres must have formed between the two poles of the cell and each chromosome must have engaged through the kinetochore associated with the centromere to the spindle fibres. (The kinetochore is the structure responsible for the movement of the chromosome as the spindle fibres retract towards each pole).
What happens to centrioles during mitosis?
The centrioles will duplicate, migrate to opposite poles and form spindle fibres that will extend from one to the other pole.
What happens to chromosomes during mitosis?
Chromosomes will condense and place themselves at the middle of the spindle fibres—usually referred to as the equatorial plane or the metaphase plate.
What is the first level of packing of DNA around histones?
The first level of packing is the nucleosome, which is essentially a fragment of DNA of 147 bp length coiled around an octamer of histones. This octamer contains 8 histones molecules of four different types, in other words, two copies of each of the histones H2A, H2B, H3 and H4. The nucleosomes are visible in electron microscopy micrographs- referred to in text books as “beads on a string”, which are visible only when the DNA is extracted from the cell at high salt concentrations.
What is the second level of packing of DNA around histones?
Nucleosomes then organize into solenoids-several nucleosomes coiled closely together. The structure of the solenoid is unclear but it is estimated to represent 6-fold compaction of DNA- and visible by electron microscopy. During mitosis the solenoids will coil further into a 300 nm fibres which will then finally condense into a 700nm chromosome.
Do all cells that enter G0 come back to the cell cycle at some point?
No, most differentiated cells in multicellular organisms remain in G0 phase indefinitely. Some, such as cardiomyocytes or motoneurons, once they are terminally differentiated in the adult never re-enter the cell cycle.
What do you think is the point of having a G0 in pluricellular organisms?
It allows for terminal specialization of cells or for some type of cells re-entering the cell cycle under special circumstances, for example when regeneration is required, as is the case with satellite cells and skeletal muscle.
What would be the consequence of a defective M checkpoint?
The most likely outcome will be unequal separation of chromosomes, with a daughter cell receiving more chromosomes than the other.
What would be the consequence of a defective G1/S checkpoint?
It may allow DNA replication to proceed even if DNA is damaged, leading to an accumulation of mutations in the daughter cells. If these mutations affect the very genes that control the regulation of the cell cycle, cells may divide uncontrollably. This is the underlying cause of many forms of cancer.
