Cells We all started life as one cell! So how do we go from one to billions of cells? 25 July 2019

Cell Division Multicellular organisms begin life as a single fertilised egg cell called a ZYGOTE. It then divides over and over again by MITOSIS Zygote Embryo 25 July 2019

Cell Division Mitosis is nuclear division giving rise to genetically identical cells 25 July 2019

Mitosis This cell will then divide many times by mitosis to form the new organism. Egg Zygote Embryo Sperm 25 July 2019

Cell division For growth , repair, replacement of cells and asexual reproduction cells need to divide this happens using mitosis. This is when a cell with 46 chromosomes copies itself to become two identical cells with 46 chromosomes. 25 July 2019

MITOSIS Here is a cell with 4 chromosomes The chromosomes then all line up down the middle of the cell The cell then splits down the middle. Before you copy a cell you need to copy all the DNA. Therefore all the chromosomes double. Normally a cell would have 46 but for the purposes of the diagram we cannot fit them all in. Then the two copies of the chromosomes split apart to opposite ends of the cell. There are now two identical copies of the original cell. 25 July 2019

Stages of Mitosis Before mitosis an exact copy of each chromosome is made As the cell divides these chromosomes are separated to give two identical daughter cells Note that STEM CELLS are undifferentiated cells. After division these cells become specialized 25 July 2019

Mitosis This all happens by mitosis! Mitosis doesn't’t stop when you finish growing. It is occurring all the time around our bodies in order to replace lost cells. We lose about 40,000 skin cells a minute which need to be replaced! We make red blood cells at the rate of 144,000,000 a minute to replace old cells. This all happens by mitosis! 25 July 2019

Meiosis Meiosis is nuclear division giving rise to cells that are genetically different A* definition Meiosis is reduction division in which the chromosome number is halved from diploid to haploid resulting in genetically different cells 25 July 2019

Cell division for reproduction Some simple organisms can just reproduce by splitting into two by mitosis, like bacteria, but humans are more complicated. We combine sex cells (gametes) in fertilisation to form a zygote. Sperm Egg Cell 25 July 2019

Cell division for reproduction Because you are adding two cells together, they need to have half the normal number of chromosomes so that when they combine they have 46 in total. Therefore gametes have 23 chromosomes and are called haploid cells. The type of cell division to make gametes is called MEIOSIS. 23 23 46 25 July 2019

Here is a cell with 4 chromosomes Meiosis Here is a cell with 4 chromosomes So 1 diploid cell makes 4 haploid cells in Meiosis Before you copy a cell you need to copy all the DNA. Therefore all the chromosomes double. However this time, you end up with 4 cells each with 2 chromosomes in, which is half the original number. 25 July 2019

Meiosis In sexual reproduction, the sex cells gametes are made using meiosis. This produces haploid cells with only 23 chromosomes we call this reduction division Once gametes combine they make a zygote with 46 chromosomes the new cell can then continue to divide using mitosis. In meiosis four cells non-identical are made from the original cell. These cells are called haploid cells as they each have half the number of chromosomes. 25 July 2019

Meiosis Chromosomes move in to each cell with some randomness, this gives wider variation. Chromosomes may also swap information, we call this crossing over 25 July 2019

Comparing Mitosis With Meiosis Growth, repair, cell placement, asexual reproduction Sexual reproduction 2 identical cells 4 non-identical cells No crossing over, no random assortment Crossing over and random assortment Diploid cells Haploid cells 25 July 2019

Comparing Mitgosis With Meiosis Mitosis Meiosis Growth, repair, cell placement, asexual reproduction Sexual reproduction 2 identical cells 4 non-identical cells No crossing over, no random assortment Crossing over and random assortment Diploid cells Haploid cells 25 July 2019