AQA AS Level Biology Unit 2 - The Cell Cycle

The cells that make up organisms are always derived from existing cells by the process of division. Cell division occurs in two main stages:

• Nuclear division - is the process by which the nucleus divides. There are 2 types of nuclear division, mitosis and meiosis
• Cell division - follows nuclear division and is the process by which the whole cell divides

Before a nucleus divides its DNA must be replicated. This is to ensure that all the daughter cells have the genetic information to produce the enzymes and other proteins that they need.

The process of DNA replication is clearly very precise because all the new cells are more or less identical to the original one. This is a remarkable achievement when one considers the complexity of the DNA molecules.

For semi conservative replication to take place there are four requirements:

• The four types of nucleotide with their bases of adenine, guanin, cytosine and thymine must be preseent
• Both strands of the DNA molecule must act as a template for the attachment of these nucleotides
• The enzyme DNA polymerase is needed to catalyse the reaction
• A source of chemical enerfy is required to drive the process

The process takes place as follows:

• The enzyme DNA helicase breaks the hydrogen bonds linking the base pairs of DNA
• As a result the double helic separates into its two strands and unwinds
• Each exposed polynucleotide strand and acts as a template to which complementary nucleotides are attracted
• Energy is used to activate these nucleotides
• They activated nucleotides are joined together by the enzyme DNA polymerase to form the 'missing' polynucleotide strand on each of the two original polynucleotide strands of DNA
• Each of the new DNA molcules contains on of the riginal DNA strands i.e half te original DNA has been saved and built into each of the new DNA molecules. The process is therefore termed 'semi-conservative replication'.

• 1. A representatice portion of DNA which is about to undergo replication
• 2. An enzyme DNA helicase causes the two strands of the DNA to separate
• 3. DNA helicase completes the splitting of the strand. Meanwhile free nucleotides that have been activated are attraccted to their complementary bases
• 4. Once the activated nucleotides are lined up, they are joined to gether by DNA polymerase (bottomthree nucleotides). The remaining unpaired bases continue to attract their complementary nucleotides
• 5. Finally all the nucleotides are joined to form a complete polynucleotide chain using DNA polymerase. In this way, two identical strands of DNA are formed. As each strand retains half of the original DNA materical, this method of replication is called the semi-conservative method.

There are two hypotheses:

• The conservative model  suggested that the parental DNA remained intact and that a separate daughter DNA copy was built up from new molecules of deoxyribose, phosphate and organic bases. Of the two molecules present, one would be made of entirely new material while the other would be entirely original material
• The semi-conservative model proposed that the DNA molecule split into two separate strands, each o which then replicated its mirror image. Each of the two new molecules would tehrefore have one strand of new material and one strand of original material

They based their work on three facts:

• All the bases in DNA contain nitrogen
• Nitrogen has two forms: lighter nitrogen (14N) and the isotops (15N) which is heavier.

Mitosis produces two daughter nuclei that have the same number of chromosomes as the parent cell and each other

Mitosis is the division of the nucleus of a cell that results in each of the daughter cells having an exact copy of the DNA of the paarent cell. Except in the rare event of a mutation, the genetic make-up of the 2 daughter nuclei is also identical to that of the parent nucleus. Mitosis is always preceded by a period during which the cell is not dividing. This perid is called interphase. It is a period of considerable cellular activity that includes a very important event, the replication of DNA. Although mitosis is a continuous process it can be divided into 4 stages for conveinience:

• 1. Prophase - in which the chromosomes become visible and the nuclear envelope disappears
• 2. Metaphase - in which the chromosomes arrange themselves at the centre (equator) of the cell
• 3. Anaphase - in which each of the two threads of a chromosome (chromatid) migrates to an opposite pole
• 4. Telophase - in which the nuclear envelope reforms

Mitosis is important in organisms because it produces daughter cells that are genetically identical to the parent cells. There are three main reasons:

• Growth - When two haploid cells (e.g spearm and ovum) fuse together to form a diploid cell, this diploid cell has all the genetic information needed to form the new organism. If the new organism is to resemble its parents all the cells that grow from this original cell must possess the same set of genertic information. Mitosis ensures that this happend. The cell firstly divides to give a group of identical cells
• Differentiation - these cells change or differentiate to give groups of specialised cells e.f. epithelium in animals of xylem in plants. These different cell types each divide by mitosis to give tissues made up of identical cells which perform a particular function. This is essential as the tissue can only function efficiently if all its cells haev the same structure and perform the same function
• Repair - If cells are damaged or die it is important that the new cells produced have an identical struction and funtion to the ones that have beem lost. If they were not exact copies the tissue would not function as effectively as before, Mitosis is therefore a means by which new cells replace damaged or dead ones

Mitosis is a continuous process. When mitosis is viewed under a microscope the observer only gets a snapshot of the process at one moment in time. In this snapshot, the number of cells at each stage of mitosis is proportional to the number of cells at each stage of mitosis is proportional to time each cell spends undergoing that stage

Cells do not divide continuously, but undergo a regular cycle of division separated by periods of cell growth. This is known as the cell cycle and has three stages.

1. Interphase, which occupies most of the cell cycle and is sometimes known as the resting phase because no division takes place. It is divided into three parts:

(a) First Growth (G1) phase, when the proteins from which th cell organelles are synthesised are produced

(b) Synthesis (S) phase, when DNA is replicated

(c) Second Growth (G2) phase, when organelles grow and divide and energy stores are increased

2.Nuclear division - when the nucleus divides wither into two (mitosis) or four (meiosis)

3. Cell Division - which followsnuclear division either into two (mitosis) or four (meiosis)

The length of a complete cycle varies greatly amongst organisms (mammal = 24h) 90% is interphase

Cancer is a group of diseases(around 200 in total) caused by a growth disorder of cells. It is the result of damage to the genes that regulate mitosis and the cell cycle. This leads to uncontrolled growth of cells. 

As a consequence, a group of abnormal cells, called a tumour, developes and constantly expands  in size.

Cancers can develop in any organ of the body but are most commonly found in the lungs, pancreas, oesophagus, stomach, large intestine, breast and ovaries(women) and prostate gland(male)

The treatment of cancer often involves blocking some part of the cell cycle. In this way the cell cycle is disrupted and cell division and hence cancer growth ceases.

Drugs used to treat cancer (chemotherapy) disrupt the cell cycle by:

• preventing DNA from replicating e.g cisplatin
• inhibiting the metaphase stage of mitosis by interfering with spindle formation e.g vinca alkaloids

The problem with such drugs is that they also disrupt the cell cycle of normal cells. However, the drugs are more effective against rapidly dividing cells. As cancer cells ave a particulary fast rate of division, they are damages to a greater degree than normal cells. Those normal body cells, such as hair-producing cells that divide rapidly are also vulnerable to damage. This explains the hair loss in patients undergoing cancer treatment.