Y10 Reproduction

Cell Division Learning objectives  Know that mitosis is the division of a diploid cell that produces two genetically identical cells.  Understand that meiosis is the division of a diploid cell to produce four haploid cells that show genetic variation. Learning outcomes  Understand that division of a diploid cell by mitosis produces two cells that contain identical sets of chromosomes.  Understand that mitosis occurs during growth, repair, cloning and asexual reproduction.  Understand that division of a cell by meiosis produces four cells, each with half the number of chromosomes, and that this results in the formation of genetically different haploid gametes.

Mitosis Mitosis is the division of body cells for growth and repair. It creates two cells which are genetically identical. This process creates all of the cells in an organisms body, apart from sex cells.

 There are 23 pairs of chromosomes in every human body cell.  In every pair, the same genes are present, though they will be slightly different from one another (alleles).  The pairs are called homologous.  In a sex cell (gamete), you have one chromosome from each pair.

Terminology

Start of mitosis The microtubules are also called spindle fibres, they attach to the spindles.

End of mitosis

Reasons for Mitosis  Every body (somatic) cell are formed by mitosis from the zygote (single cell formed at fertilization).  Each daughter cell is an exact copy of the parent cell, they all contain identical copies of all the chromosomes and therefor genes of that zygote.  Mitosis occurs for growth and repair, in some places mitosis occurs more regularly than others:  Skin – looses thousands of cells every time we touch something;  Cells are scraped of our gut lining as food moves past;  Our spleen destroys about 100,000,000,000 worn out red blood cells per day;  Cancer cells divide by mitosis, this copies the parent cells exactly, including the mutation that makes cells divide uncontrollably.

Meiosis Meiosis is similar to mitosis, in that chromosomes are doubled. It is very different in how the doubled chromosomes separate initially and then divide again to create a haploid cell. A haploid cell is one which contains half the usual amount of chromosomes. They are gametes, or sex cells. Meiosis creates genetic variation in several ways.

 Meiosis 1 starts with the duplication of chromosomes.  These line up along the equator due to the spindles and spindle fibres.  The difference here compared to mitosis is that the duplicated pair of chromatids separates from it’s opposing number, instead of the pair splitting in half.  This creates two new cells, each with the original number of chromosomes, but no difference between the pairs, as they are duplicates.

 In the second stage of meiosis the chromosomes line up at the equator, again they move due to the spindles and spindle fibres.  In a process similar to mitosis, the chromosomes split in half, with one chromatid moving to one cell, and one chromatid moving to the other.  At the end of mitosis there are 4 daughter cells. Each with on chromosome of each type, but all genetically different.

Genetic variation through meiosis Cross over – when the pairs of chromosomes line up during meiosis 1, they get entangled with one another, this can lead to sections of DNA being transferred from one chromosome to another.

Genetic variation through meiosis Random alignment / Independent Assortment – during metaphase 1, the homologous pairs line up along the equator before they divide apart. The way in which they line up, and therefore the side to which they are divided and gamete the end up in, is random and causes variation. The number of possible combinations = 2 n n = pairs of chromosomes 2 pairs = 2 2 = 4 4 pairs = 2 4 = pairs (humans) = 2 23 = 8,388,608 combinations

Genetic variation through meiosis Random fertilisation – The gametes, sperm and egg, all show variation from each other. Which sperm out of the 8.3 millions different possibilities, fertilises which particular egg, again out of 8.3 million possibilities gives another cause of variation. Even more variation exists as the sperm and egg come from different organisms