Question

short response questions:
describe the events that occur during the s phase of the cell cycle and describe what you think would happen is the events in s phase do not occur, but the rest of the cell cycle takes place anyway.
the dna doesnt get synthesis
explain what you think would happen if a cell had a mutation affecting the proteins that control the g1 causing the cell the grow without stopping?
why are the checkpoints in the cell cycle crucial for maintaining the integrity of the cell cycle and preventing cancer?

Explanation:

First Question (S phase of cell cycle)

During the S (synthesis) phase of the cell cycle, DNA replication occurs, meaning each chromosome is duplicated to form two sister chromatids. If S - phase events (DNA replication) do not occur but the rest of the cell cycle proceeds: when the cell enters mitosis (M phase), the chromosomes will not have been replicated. So, when the cell tries to divide, each daughter cell will receive an incomplete set of chromosomes (or no DNA in some cases). This will lead to genetic abnormalities, and the daughter cells will likely not be viable as they won't have the correct amount of genetic material to function properly. The initial hand - written answer was too brief; a more complete description includes the normal event of S phase (DNA replication) and the consequence of its absence on cell division and daughter cell viability.

The G1 phase of the cell cycle is a gap phase where the cell grows, synthesizes proteins, and prepares for DNA replication. Proteins (like cyclins and cyclin - dependent kinases) control the progression through G1 (including the G1 - S checkpoint). If a mutation affects these proteins such that the cell grows without stopping in G1, the cell will not be properly regulated. It will likely bypass the normal checkpoints that ensure the cell is ready for DNA replication and division. This uncontrolled growth and progression through the cell cycle is characteristic of cancer - causing (oncogenic) behavior. The cell may undergo repeated cell divisions with damaged or unregulated growth, leading to the formation of a tumor as the cells proliferate uncontrollably.

Cell cycle checkpoints (e.g., G1 - S, G2 - M, and spindle - assembly checkpoints) are control points where the cell assesses its internal and external environment. At the G1 - S checkpoint, the cell checks if it has enough nutrients, growth factors, and if its DNA is undamaged before entering S phase. The G2 - M checkpoint ensures that DNA replication in S phase was successful and that the cell is large enough to divide. The spindle - assembly checkpoint in mitosis ensures that chromosomes are properly attached to the mitotic spindle before anaphase. These checkpoints are crucial because they prevent cells with damaged DNA or improper growth from proceeding through the cell cycle. If a cell with damaged DNA (e.g., from mutagenic agents) proceeds without repair, it can lead to mutations being passed on to daughter cells. Accumulation of such mutations can lead to cancer (uncontrolled cell growth). By halting the cell cycle at checkpoints, the cell has time to repair damage (e.g., DNA repair at G1 - S or G2 - M checkpoints) or undergo apoptosis (programmed cell death) if damage is irreparable, thus maintaining the integrity of the cell cycle and preventing cancer.

Answer:

During S phase, DNA replication occurs (each chromosome is duplicated into two sister chromatids). If S - phase events don't occur but the cell cycle continues, daughter cells formed during division will have incomplete/incorrect genetic material (e.g., no replicated DNA, leading to non - viable or genetically abnormal daughter cells).

Second Question (Mutation in G1 - controlling proteins)