Cells: Cancer, Mitosis And The Cell Cycle Flashcards
Mitosis
The cycle starts and ends here. Cytokinesis occurs here (when the membrane is formed between the two cells and one cell becomes two)
The Cell Cycle overview
The time between a new cell forming and a cell dividing to produce two identical cells is the cell cycle. The cell cycle contains a period of cell growth and DNA replication called Interphase and a period of cell division called mitosis:
- Interphase is divided into three separate growth stages - G1, S, G2
- Mitosis: Cell division, nucleus divides into two (consisting of PMAT)
- Cytokinesis: Cytoplasm divides to produce two cells (cell membrane is forming)
In mammals, the cell cycle usually takes about 24 hours, of which 90% is Interphase (including G1, S, G2)
Interphase
During the interphase, the cell carries out normal functions, but also prepares to divide. The cells DNA is unravelled and replicated, to double its genetic material. The organelles are also replicated and its ATP content is increased (energy needed for cell division)
The structure of chromosomes in mitosis
As mitosis begins, the chromosomes are made of two strands joined in the middle by a centromere. The seperate strands are called chromatids while two strands of the same chromosome are called sister chromatids. The two strands are identical copies formed during the interphase
What happens in gap phase 2?
Cell keeps growing and proteins needed for cell division are made
What happens in gap phase 1?
Cell grows and new organelles and proteins are made
Cytokinesis process
An important final step in the cell cycle because during this stage the cytoplasm divides forming two genetically identical cells. In animal cells, a ‘cleavage furrow’ forms and separates the daughter cells. In plants, a ‘cell plate’ (the precursor to a new cell wall) forms at the site of the metaphase plate. Once the cell plate reaches the cell walls of the parent cell, new cell walls are produced, separating the new daughter cells
Anaphase
The centromeres divide, separating each pair of sister chromatids. The spindles contract, pulling chromatids to opposite poles of the spindle, centromere first. This makes the chromatids appear v-shaped. This stage requires energy in the form of ATP which is provided by respiration in the mitochondria
Prophase
The chromosomes condense and became visible. In animal cells the centrioles seperate and move to opposite poles of the cell. The centrioles are responsible for creating spindle fibres which are released from both poles to create a spindle apparatus - these will attach to the centromere and chromatids on the chromosome in later stages. Plants have a spindle apparatus, but lack the centrioles. The nuclear envelope breaks down and the chromosomes lie free in the cytoplasm
What happens in the ‘S’ synthesis stage?
Cell replicates its DNA ready to divide by mitosis
Telophase
The chromatids reach the opposite poles on the spindle: they uncoil and become thin again (chromosomes.) The spindle fibres disintergrate. A nuclear envelope forms around each of the chromosomes, so there are two nuclei. The cytoplasm divides and there are two daughter cells that are genetically identical to the original cell and the parent cell
Metaphase
The chromosomes (each with two chromatids) line up along the middle of the cell (along the equator) and become attached to the spindle by their centromere
G2 checkpoint
Controlled by DNA replication completion, DNA mutations, cell size: this stage ensures that the cell has completed DNA replication and that the replicated DNA is undamaged before entering mitosis
What occurs at the M-spindle checkpoint (metaphase)?
Check spindle fibre (microtubules) attachment to chromosomes at kinetochores (anchor sites)
What causes cancer?
- Mutated genes that cause cancer are called oncogenes
- It is thought that several mutations need to occur to give rise to cancer
- Cells that are old or not functioning properly normally self-destruct and are replaced by new cells
- Cancerous cells do not self-destruct and continue to divide rapidly producing millions of new cancerous cells
Targets of cancer treatments within the cell cycle
G1 (cell growth and protein production):
- Chemical drug prevents the synthesis of enzymes needed for DNA replication
- Cells cannot enter the synthesis phase, disrupting the cell cycle and forcing the cell to kill itself (apoptsis)
S (DNA replication):
- Radiation and chemical drugs damage DNA
- At several points in the cell cycle, the DNA is checked for damage
- If severe DNA damage is detected, the cell will kill itself - preventing further tumour growth
What characterises benign tumours?
Do not spread from their site of origin, are more compact, but can compress surrounding cells e.g brain tumour, warts. They tend to grow slowly and are less likely to be life threatening
What happens at the G1 checkpoint?
The most important checkpoint:
Controlled by cell size, growth factors, environment
‘Go’ —> completes whole cell cycle
‘Stop’ —> cell enters no dividing state (G0 phase)
Nerve, muscle cells stay at G0: liver cells called back from G0
Cancer treatment
Often works by killing dividing cells by blocking part of the cell cycle.
Procsses in the cell cycle a drug (chemotherapy) targets to prevent cell division include:
1. prevent DNA replication
2. inhibit metaphase - interfering with spindle fibre formation
These drugs cannot distinguish between normal cells and cancerous ones so rapidly dividing cells such as hair producing cells and cancerous ones so rapidly dividing cells such as hair producing cells can also be destroyed on these kind of therapies
Cancer is caused by…
…a mutation in a gene that controls the rate of cell division. This results in uncontrolled cell division producing a mass of abnormal cells - a tumour
- The division of normal cells is precisely controlled. New cells are only formed for growt or for the replacement of dead ones
- Cancerous cells divide repeatedly out of control even though they are not needed, they crowd out other normal cells and function abnormally. They can also destroy the correct functioning of major organs
Why are secondary tumours so bad?
The tumours can tire the body out as they both need a huge amount of nutrients to sustain the rapid growth and division of the cells
What characterises malignant tumours?
Grow rapidly, are less compact and more likely to be life-threatening. They can spread from the original site and cause secondary tumours. They interfere with neighbouring cells, block blood vessels, the gut, glands, lungs etc…
