How long does the S phase normally take?
How long does the M phase normally take?
G1, S, G2
At what phase is the cell at its lowest level of activity?
G1. It starts with a slightly smaller daughter cell.
Why is biosynthetic rate so high in G1?
The cell is assembling the machinery necessary for DNA replication that will happen in S phase. It is also trying to get the smaller daughter cell up to speed.
Why is the biosynthetic rate decreased during S phase?
There is a fixed amount of energy a cell can use. Biosynthetic rates of proteins decreases so histone synthesis rates can increase.
What do you have at the end of S phase.
Two identical sister chromatids joined by a centromere.
Why does biosynthesis increase again in G2 phase?
The cell synthesizes microtubules and centrosomes for the bipolar spindle apparatus.
6 sub phases of M phase
Prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis
What happens during prophase?
Sister chromatids begin condensing. Mitotic spindle apparatus begin aligning at polar sides of the cell.
What pulls the centrosomes to different ends of the cell?
Motor proteins (dyneins and kinesins)
What happens during pro metaphase?
Sudden breakdown of the nuclear envelope by phosphorylation of nuclear laminas. Kinetochore is established and microtubules attach to sister chromatids on both sides.
What stabilizes microtubule attachment to the sister chromatids?
Opposite poles must establish equal tension on the centromere
What happens during metaphase?
Tug-of-war of chromatids to center of cell by microtubules.
What happens during anaphase?
Microtubules depolymerize and become shorter and shorter pulling chromatids apart. Centromeres polarize to opposite ends of the cell.
What happens during telophase?
Chromosomes decondense, new envelop forms around each pole and contractile ring is formed.
What composes the contractile ring?
Ring of motor proteins (actin) that will allow the cell to contract during cytokinesis
What happens during cytokinesis?
The contractile ring pulls the cell membrane from within like a draw-string. The cells then fill in the mid body with plasma membrane and the cells separate.
G1/S Checkpoint checks what?
Looking for appropriate cell size and undamaged DNA. No external inhibitory signals, only positive signals.
G2/M Checkpoint checks what?
Make sure all DNA is replicated, environment is favorable, is the cell size okay and makes sure there are no external inhibitory cell signals
Metaphase-Anaphase transition mitotic checkpoint checks what?
Are all the chromosomes attached and oriented in the correct fashion?
Cell cycle regulators
Cyclin-dependent kinases, cyclins, cyclin dependent inhibitory proteins (CKIs)
What determines CDK activity in cell cycle regulation?
They must be bound to a cyclin to be active. When active they phosphorylate targets to trigger cell cycle events.
How do cyclins control cell cycle?
They are not always present. They undergo synthesis and rapid degradation. When absent, CDKs are inactive and cell cycle events do not happen. They control movement from one phase to another.
What is the role of CDKIs?
They fine-tune CDK activity by forming complexes with cyclin and CDK to block activation of CDKs.
How is cyclin activity limited at the end of mitosis?
Ubiquitination tags the cyclins for degradation. Then cyclins can no longer activate CDKs.
Phosphorylates nuclear lamins to initiate nuclear envelope breakdown. Phosphorylates motor proteins on microtubules for chromatid separation.
Phosphorylates origin recognition complex to initiate DNA replication. Phosphorylates CDC6 which leads to replication termination.
Anaphase promoting complex (APC)
Signals for S and M cyclin degradation and CDK inactivation. Phosphorylates securin, protein that allows sister chromatids to separate in anaphase.
What inhibitory signals regulate the cell cycle checkpoints?
p21, p27, p16 CDKIs. Activated by p53 after DNA damage or other similar events