Normal cell division

Every second of your life, cells in your body divide to make new cells. They do this because we all need to grow and develop. We all need to heal. And we all need to replace cells lost naturally. For example, we use up to 40,000 dead skin cells every minute. That’s 50 million skin cells alone we need to replace each day. Our cells need to divide in order to meet this need. You may have heard the terms mitosis and meiosis before. These are two specific stages in cell division. During the creation of new body, or somatic, cells, they undergo mitosis whereas as meiosis is unique to the formation of gametes, the sperm and egg cells.

There are three stages of the cell cycle. First, interphase, the stage of normal cell growth and DNA replication. Secondly, mitosis, where duplicated genetic material in the cell nucleus separates. And finally, cytokinesis, the stage of division into two separate cells. During the interphase stage, the cell is engaged in normal cell growth metabolic activities. It is during this phase that the chromosomes replicate themselves to produce two identical copies of each chromosome. Mitosis follows interphase and occurs in all body cells other than those of the sperm and eggs. Mitosis involves a number of precise steps, resulting in two identical daughter cells. Each daughter cell will have 46 chromosomes, the same as the parent cell.

At the beginning of mitosis, the chromosomes condense and become visible, as they are made shorter and thicker. At this point, the chromosome consists of two identical parts joined at the centromere. These identical parts are called sister chromatids. Cells spindle fibres, then form and move to opposite ends of the cell to produce a web-like structure across the cell. This web-like structure pulls the chromosomes, so that they are all lined up in pairs at the centre of the cell spindle. The sister chromatids are then pulled apart to opposite ends of the cell.

A nuclear membrane forms around each set of chromosomes and the chromosomes uncoil and become less visible. During cytokinesis, the final stage of cell division, the cytoplasm then divides, and two genetically identical daughter cells are formed. Cell division in sperm and egg cells is different, to reduce the number of chromosomes to 23 rather than 46. This process is known as meiosis. This allows, when the sperm and egg come together, that the fertilised cell has 46 chromosomes. The 23 chromosomes in the sperm and egg cells are referred to as the haploid number whereas, the 46 chromosomes in all the other cells are referred to as the diploid number. Meiosis is a two-step process, referred to as Meiosis 1 and Meiosis 2.

The reduction to 23 chromosomes occurs during Meiosis 1. As with mitosis, meiosis is preceded by interphase, where the chromosomes replicate. The homologous chromosome pair align along the spindle and swap genetic material, a process called recombination, an essential to producing offspring with different traits to their parents. Following recombination, the pairs of chromosomes are pulled to opposite ends of the cell by the spindle, and daughter cells are formed. In Meiosis 2, the sister chromatids are separated. Therefore, from the single progenitor, four daughter cells are formed, each with 23 chromosomes.