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Skip to 0 minutes and 13 secondsEvery 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 lose 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-- undergo mitosis, whilst meiosis is unique to the formation of gametes, the sperm and egg cells. There are three stages of the cell cycle.

Skip to 0 minutes and 54 secondsFirst, the interface, 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 interface stage, the cell is engaged in normal cell growth and metabolic activities. It is during this phase that the chromosomes replicate themselves to produce two identical copies of each chromosome. Mitosis follows interface and occurs in all body cells other than the 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.

Skip to 1 minute and 40 secondsAt 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. Cell 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.

Skip to 2 minutes and 17 secondsA 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 allowss-- when the sperm and egg come together, 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."

Skip to 3 minutes and 11 secondsThe reduction to 23 chromosomes occurs during meiosis 1. As with mitosis, meiosis is preceded by interface, where the chromosomes replicate. The homologous chromosome pairs align along the spindle and swap genetic material-- a process called recombination, and 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.

Normal cell division

In this step, we learn about the processes of normal cell division - mitosis and meiosis.

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This video is from the free online course:

The Genomics Era: the Future of Genetics in Medicine

St George's, University of London

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