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What is cytokinesis?

Cytoplasmic division following nuclear division, resulting in two new daughter cells.


What is interphase?

Phase of cell cycle where the cell is not dividing; it is subdivided into growth and synthesis phases.


What is mitosis?

Type of nuclear division that produces daughter cells genetically identical to each other and to the parent cell


What could early researchers see when observing cell division under the microscope?

They could easily see the behaviour of chromosomes during mitosis which is nuclear division followed by cytokinesis or cytoplasmic division, resulting in two daughter cells.


What could early scientists not see in cell division?

Nuclear and cytoplasmic division, called the M-phase, occupy only a small part of the cell cycle. Between each M-phase is an interphase. Interphase when studied under the microscope, appears to be uneventful. However, more sophisticated techniques have enabled scientists to learn that during interphase there are elaborate preparations being made for cell division, in a carefully ordered and controlled sequence, with checkpoints.


What are the two man checkpoints?

The G1/S checkpoint, also called the restriction point, and the G2/M checkpoint.


Name another checkpoint? (not the two main checkpoints) State its purpose.

There is one halfway through mitosis and one in early G1.
The purpose of the checkpoint is;
To prevent uncontrolled division that would lead to tumours (cancer).
To detect and repair damage to DNA (for example, damage caused by UV light).


Molecular events that control the cell cycle happen in a specific sequence, what does this ensure?

The cell cycle cannot be reversed.
The DNA is only duplicated once during each cell cycle.


Cells should usually undergo a certain number of divisions or cycles, what is this known as and how many is it?

The number is about 50 and known as the Hayflick constant.


What happens if the cell division becomes uncontrolled?

If cell division becomes uncontrolled, then a tumour can form which may become malignant or cancerous.


What helps regulate cell division?

There are proto-oncogenes that help regulate cell division by coding for proteins that help regulate cell growth and differentiation.


What happens if the proto-oncogenes fail?

If these proto-oncogenes mutate, then they may become oncogenes and can cause cells to fail to undergo apoptosis and instead to keep on dividing, leading to a tumour.


Describe the divisions of a tumour.

Tumours form because cells divide uncontrollably. The division phase, mitosis, of each cancerous cell takes the same length of time as in a non-cancerous cell. Because the cell division is uncontrolled, there is a greater proportion of cells dividing within a tumour than within normal tissue.


Why is the gene p53 important?

It triggers the two main checkpoints in the regulation of the cell cycle.


What is the gene p53 known as?

A tumour suppressor gene.


What other regulatory chemicals are there apart from p53?

Other regulatory chemicals are proteins called cyclins and CDK (cyclin-dependant kinases). Cyclins are synthesised in response to cell-signalling molecules such as growth factors.


Describe the prokaryotic cell cycle.

This occurs by a process called binary fission, the cell grows to its limit of size and then splits into two. Before the cell divides, its DNA is replicated. The new loops of DNA are pulled to opposite ends of the cell and a wall forms which begins to separate the bacterial cell. Each new cell also contains replicated plasmids and synthesised ribosome's.


Apart from bacterial cells what other cells split by binary fission?

Mitochondria and chloroplasts within eukaryotic cells also divide by binary fission.


In the M - phase what does the chemical trigger?

A checkpoint chemical triggers condensation of chromatin.


What phase of the cell cycle if the M-phase in?

Halfway through the cell cycle, the metaphase checkpoint ensures the cell is ready to complete mitosis.


What events within the cell occur in the M-phase?

Cell growth stops.
Nuclear division (mitosis) consisting of stages : prophase, metaphase, anaphase and telophase.
Cytokinesis (cytoplasmic division)


What cells do not have a G 0 (gap 0) phase?

Epithelial cells lining the gut do not have this phase.


When and how is the G 0 (gap 0) chemical triggered?

A resting phase triggered during early G 1 at the restriction point (see below), by a checkpoint chemical.


What events happen in G 0?

In this phase, cells may undergo apoptosis (programmed cell death) differentiation or senescence.
Some types of cell (e.g. neurones) remain in this phase for a very long time or indefinitely..


At what phase of the cell cycle and which checkpoints are involved in G1 (gap 1) phase - also called the growth phase.

A G1 checkpoint control mechanism ensures that the cell is ready to enter the S phase and begin DNA synthesis.


State all (5) of the events in G1 phase.

Cells grow and increase in size.
Transcription of genes to make RNA occurs.
Organelles duplicate.
Biosynthesis, e.g. protien synthesis, including making the enzymes needed for DNA replication in the S phase.
The p53 (tumour suppressor) gene helps control this phase.


Describe the sequence of the S phase of interphase.

Because the chromosomes are unwound and DNA is diffuse, every molecule of DNA is replicated. There is a specific sequence to the replication of genes - those which are active in all types of cells, are duplicated first. Genes that are normally inactive in specific types of cells are replicated last.


State all (4) of the events in S phase.

Once the cell has entered this phase, it is committed to completing the cell cycle.
DNA replicates.
When all chromosomes have been duplicated, each one consists of a pair of identical chromatids.
This phase is rapid, and because the exposed DNA base pairs are more susceptible to mutagenic agents, this reduces the chances of spontaneous mutations happening.


What is the purpose of G 2 (gap 2) phase of interphase.

Special chemicals ensure that the cell is ready for mitosis by stimulating proteins that will be involved in making chromosomes condense and in formation of the spindle.


State all of the events in G 2 (gap 2) phase of interphase.

Cells grow.