Cell Cycle
1. Phases of the cycle
a. G1: The preparation for mitosis. It’s where the cell produces lipids, proteins,
❗ ❗
energy and the mRNA necessary for the daughter cell.
Important G0: Without the presence of mitogens or the correct
proteins, the cell stops on G1. If it’s long enough in that phase, it’s now called
G0. Most cells can come back from that stage, but muscle and nerve cells
can’t.
b. S: When the genetic material is duplicated.
c. G2: When specific proteins necessary for mitosis are produced.
d. M: When the cell divides.
2. Cell Cycle Control System:
a. Concept: It’s a system of proteins that work as molecular stops to verify if the
necessary events occurred for the division or if the environment is favorable.
They stop the phase’s transition at determined checkpoints.
b. Checkpoints:
i. G1 → S: When the cell verifies its DNA and the environment. If the
environment is favorable and there are extramolecular signs for
division, the cell enters S.
ii. G2 → M: Where the cell verifies if everything went correct on G1, S,
and G2. It’s important to verify if the DNA’s replication went correctly
and if there are any molecular damages to the DNA. If everything is
fine, the cell continues to the M phase.
iii. M: It’s verified if the chromosomes are correctly bound to the mitotic
spindle.
3. Molecular Machinery:
a. General: There are two proteins: CDK and cyclins. The CDK is always
present in the cytoplasm and cyclins are produced when they are needed.
b. Activation: The complex cyclin-CDK by itself is not functional. Two inhibitory
phosphates have to be removed from the CDk and one activating phosphate
❗ ❗
has to be there.
Important When activated, the complex phosphorylate proteins related
to the progression of that phase.
c. Types of Complexes:
i. G2 → M: It’s controlled by the M cyclin that forms the M-CDK complex.
ii. G1 → S: It’s controlled by the S-cyclins and G1/S-cyclins.
d. Inhibition:
i. Destruction: If the cyclin is marked with ubiquitin, it will be destroyed by
the proteasome.
1. Phases of the cycle
a. G1: The preparation for mitosis. It’s where the cell produces lipids, proteins,
❗ ❗
energy and the mRNA necessary for the daughter cell.
Important G0: Without the presence of mitogens or the correct
proteins, the cell stops on G1. If it’s long enough in that phase, it’s now called
G0. Most cells can come back from that stage, but muscle and nerve cells
can’t.
b. S: When the genetic material is duplicated.
c. G2: When specific proteins necessary for mitosis are produced.
d. M: When the cell divides.
2. Cell Cycle Control System:
a. Concept: It’s a system of proteins that work as molecular stops to verify if the
necessary events occurred for the division or if the environment is favorable.
They stop the phase’s transition at determined checkpoints.
b. Checkpoints:
i. G1 → S: When the cell verifies its DNA and the environment. If the
environment is favorable and there are extramolecular signs for
division, the cell enters S.
ii. G2 → M: Where the cell verifies if everything went correct on G1, S,
and G2. It’s important to verify if the DNA’s replication went correctly
and if there are any molecular damages to the DNA. If everything is
fine, the cell continues to the M phase.
iii. M: It’s verified if the chromosomes are correctly bound to the mitotic
spindle.
3. Molecular Machinery:
a. General: There are two proteins: CDK and cyclins. The CDK is always
present in the cytoplasm and cyclins are produced when they are needed.
b. Activation: The complex cyclin-CDK by itself is not functional. Two inhibitory
phosphates have to be removed from the CDk and one activating phosphate
❗ ❗
has to be there.
Important When activated, the complex phosphorylate proteins related
to the progression of that phase.
c. Types of Complexes:
i. G2 → M: It’s controlled by the M cyclin that forms the M-CDK complex.
ii. G1 → S: It’s controlled by the S-cyclins and G1/S-cyclins.
d. Inhibition:
i. Destruction: If the cyclin is marked with ubiquitin, it will be destroyed by
the proteasome.
