Advanced Higher Biology Unit 1- Cell Cycle
Cells are reproducing constantly in the body. Mitosis is the process which provides us with enough new cells to continue living. As a cell grows its volume increases, but this does not continue forever. Eventually the cell surface cannot provide sufficient resources for the survival of the cell contents. The cell divides, producing two 'daughter cells' which can then begin to grow and divide again. This process is called the cell cycle.
The cell cycle has two vital processes-
the genetic material must be duplicated to ensure that each new cell has a full set of instructions
the two copies of the genetic material must be equally distributed to the two daughter cells
Mitosis takes up only a small part of the cell cycle. The rest of the process is known as interphase (G1, S and G2). The term interphase suggests that not much is happening, which is far from the truth. The cell is extremely busy preparing for mitosis which requires a wide variety of molecules to be synthesised.
G1 (or first gap)- cell mass increases
This stage is where the cell grows and increases in mass. A newly formed cell will not divide until it has nearly doubled in mass. Extra organelles need to be synthesised and more cell membrane is made. There is also chemical preparation for DNA replication as deoxyribose sugar, phosphates and bases need to be manufactured. In plant cells, more cell wall is laid down.
Outside conditions can slow G1, but once the cell has passed the RESTRICTION POINT, the cycle is completed. There is a switching on process here. Cells that are not switched on at the restriction point go into a non-diving state called G0.
S (synthesis)- chromosome synthesis by DNA replication
Each double helix unwinds and unzips at many INITIATION POINTS to make sure the strands are fully seperated. The single strands are used as templates for many complementary strands. Two identical double helices are formed that are known as chromatids.
The two chromatids are joined together by a centromere to form the chromosome seen in mitosis.
The centriole pairs also duplicate. These are a part of the centrosome (or microtubule organising centre (MTOC)) which lies close to the nucleus. Normally the MTOC controls the cytoskeleton of the cell. During mitosis the cytoskeleton breaks down and the sub-units are reassembled to form the spindle which grows out from the two MTOCs.
G2 (second gap)- energy production
After replicating the cell enters a second growth phase. During this period the cell builds up an energy reserve to last it through the process of mitosis. Sufficient ATP reserves are built up in this stage to last the cell through the rest of the cycle.
Mitosis is a continous sequence of events with a set order. Because there is so much going on, biologists have split the process into four steps: prophase, metaphase, anaphase, and telophase.
This is where DNA and associated histone proteins fold into really really folded loops to give the condensed chromosome structure that is visible under microscopes. In this condensed form they can be separated and moved without tangling, a bit like a bundle of string.
DNA has an overall negative charge, which should really prevent it from packing tightly, as the DNA should repel other parts of itself. The histone proteins have a positive charge which neutralises the negative charge on the DNA. As the DNA-histone complex as no overall charge it can coil tightly. Once the DNA is condensed it can no longer be involved in transcription and so protein synthesis stops.
This is a prophase chromosome. The two supercoiled chromatids are joined by a centromere. This is a more interesting structure than it first appears. Each chromatid has a region within the centromere called a kinetochore made of motor proteins.
At the start of prophase, the cytoskeleton disintegrates to provide a large pool of sub-units which is used to make the microtubules of the spindle. When the cytoskeleton disassembles the cell is free to take up its natural sppherical shape. The spindle begins to form from microtubules between the two centrosomes. The nuclear membrane, Golgi body and ER disintegrate and are distributed around the cell as vesicles.
Once the MTOCs reach the poles of the cell they act as a fixed point from which the spindle contnues to grow using the sub-units from the cytoskeleton.
There are 3 different types of spindle microtubules:
Polar microtubules- elongate when sub-units are added and push the MTOCs to the poles of the cell, eventually stretching from both poles to the equato
Astral microtubules- radiate out from the MTOCs towards the plasma membrane at the poles
Kinetochore microtubules- the part of the spindle that separate the chromatids and draw them to the poles
After the nuclear membrane disintegrates, the kinetochore microtubule tips can grow in amongst the chromosomes which now can move about the whole cell. The kinetochores grab onto the end of one of the kinetochore microtubules. The crhomatids can use their kinetochore protein motors to "walk" up their microtubules and so they line up across the equator of the cell. The equator can also now be called the metaphase plate.
The phase begins when the centromeres all divide together and the chromatids are drawn apart. The sister chromatids are dragged towards opposite poles by the kinetochore motor proteins "walking" up their microtubules, which are being dismantled behind them. Whilst they are being draw to the poles the chromatids begin the uncoiling process. Eventually the chromatids reach the region close to the poles of the cell.
Whilst this is happening, the polar microtubules lengthen making the distance between the poles greater. The cell becomes much longer in readiness for division. This is caused by the addition of sub-units to the microtubules in both the MTOCs. Motor proteins also form bridges between the tips of the polar microtubules at the equator and use the energy from ATP to "walk" the polar microtubules in opposite directions towards the poles. This cell elongating process continues into telophase.
In this stage, a lot of the changes in the cell that occured during prophase are reversed.
The chromatids have reached the poles of the cell. The nuclear membrane reforms to re-enclose the two identical groups of chromosomes to give two daughter nuclei. The decondensing of the chromosomes produces long strands of DNA again. THey are now in a form where transcription can resume and so protein synthesis begins again.
The Golgi apparatus reassembles. The nucleolus also reappears in the two nuclei. As its function is over the spindle disassembles and reassembles as the cytoskeleton of the two new cells.
The first signs of cleavage start in anaphase and continue throughout telophase. A ring of contractile microfilaments made of actin and myosin encircles the cell, attached to the inner surface of the plasma membrane. The circle is at the spindle equator.
Actin and myosin can contract powerfully. As the filament ring begins to contract a cleavage furrow forms all the way around the cell at the equator. The plasma membrane is pulled in towards the centre until the circle of contracting filament pinches the cell in two.
Cytokinesis not only divides the two nuclei, but also divides the many cell organelles and molecules. Cells contain many mitochondria, ribosomes etc., and so both cells would be likely to receive some of every organelle. The element of chance is reduced as many organelles attach themselves to the microtubules of the spindle.
The first checkpoint is at the end of the G1 phase. The cell monitors its size and the surroundings and decides whether to move on to S phase.
During G1 the cell grows and manufactures enough cell organelles and molecules for both daughter cells for the rest of interphase and then mitosis. Enzymes that are not normally needed in the cell are activated for this period of frantic metabolic activity. At the end of this preparation the cell reaches the RESTRICTION POINT. The cycle is pushed past this point by a signal from outwith the cell, a protein complex known as the mitosis promoting factor (MPF).
The second checkpoint is after DNA replication. Any cell with damaged or unreplicated DNA is held in the G2 phase until it can be repaired. If the damage is too much the cell is induced to commit suicide (apoptosis)
The third checkpoint is at the end of metaphase. If the spindle has not been assembled properly, or all the kinetochores have not been attached, the cycle may not move on to anaphase.
