• Multiplication of cells takes place by division of pre-existing cells.

• Cell multiplication is equally necessary after the birth of the individual for growth and for replacement of dead cells.

• We also know that Chromosomes within the nuclei of cells carry genetic information that controls the development and functioning of various cells and tissues, and of the body as a whole.

• When a cell divides it is essential that entire genetic information within it be passed on to both the daughter cells resulting from the division, i.e. must have chromosomes identical in number to those in the mother cell.

• This type of cell division is called MITOSIS.MITOSIS.

• A different kind of cell division called MEIOSIS MEIOSIS occurs during the formation of the gametes.

• This consists of 2 successive divisions called 1st and 2nd MEIOTIC DIVISION.

• They differ from other cells of the body because:– The number of Chromosomes is reduced to

half the normal number, and– The genetic information in the various

gametes produced is not identical.

Proteins do the work of the cell: growth, maintenance, response to the environment, reproduction, etc.

Proteins are chains of amino acids. The sequence of amino acids in each protein is coded in the DNA as a specific sequence of A, C, G and T bases.

Each gene codes for a different protein.

Key pointsKey points: All cells within an organism have the same genes. What makes cells different from each other is that

different genes are turned on and turned off in different cells.

The DNA must be copied and then divided exactly so that each cell gets an identical copy.

• Mitosis and Meiosis occur in eukaryotes. Prokaryotes use a different method—”fission” to divide.

• Humans have 46 chromosomes, 23 from each parent.

• Every cell has the same 46 chromosomes• Each species has a characteristic number

of chromosomes: corn has 20, house flies have 10, chimpanzees have 48.

• Some cells divide constantly e.g.: cells in the embryo, skin cells, gut lining cells, etc. These cells are called Labile Cells . Labile Cells .

• Some other cells divide when necessary only to replace themselves. These cells are called Stable or Quiescent Cells. Stable or Quiescent Cells. E.g. Liver cells

• Some other cells almost never divide. These are called Permanent Cells. Permanent Cells. Examples include Neurons, cardiac cells. E.t.c.

• Actively dividing cells go through a cycle of events that results in mitosis.

• Most of the cycle was called “Interphase”

• During interphase the cell increases in size, but the chromosomes are invisible.

• The 3 stages of interphase are called • G1, • S, and • G2.

• The S phase (“Synthesis”) is the time when the DNA is replicated, (when the chromosome goes from having one chromatid to having 2 chromatids

• G1 (“Gap”) is the period between mitosis and S, when each chromosome has 1 chromatid.

• Cells spend most of their time in G1: it is the time when the cell grows and performs its normal function. (Control of cell division occurs in G1: a cell that isn’t destined to divide stays in G1, while a cell that is to divide enters the S phase).

G2 is the period between S and mitosis. (The chromosome has 2 chromatids, and the cell is getting ready to divide).

The M stage stands for "mitosis", and is when nuclear (chromosomes separate) and cytoplasmic (cytokinesis) division occur.

Control of Cell division (and thus tissue growth) is very complex. The following terms are some of the features that are important in regulation.

Cdk (cyclin dependent kinase, adds phosphate to a protein), along with cyclins, are major control switches for the cell cycle.

Causing the cell to move from G1 to S or G2 to M.

MPF (Maturation Promoting Factor) includes the CdK and cyclins that triggers progression through the cell cycle.

• p53 is a protein that functions to block the cell cycle if the DNA is damaged.

• If the damage is severe this protein can cause apoptosis (cell death).

• p53 levels are increased in damaged cells. This allows time to repair DNA by blocking the cell cycle.

• A p53 mutation is the most frequent mutation.

• p27 is a protein that binds to cyclin and CdK blocking entry into S phase.

• Breast cancer prognosis is determined by p27 levels.

• Reduced levels of p27 predict a poor outcome for breast cancer patients

Errors can lead to cancer. Cancer is a disease where regulation of

the cell cycle goes wrong and normal cell growth and behavior is lost.