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Biology 2B Cell division. Cell division by mitosis Needed for: reproduction in unicellular organisms growth in multicellular organisms replacement of.

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Presentation on theme: "Biology 2B Cell division. Cell division by mitosis Needed for: reproduction in unicellular organisms growth in multicellular organisms replacement of."— Presentation transcript:

1 Biology 2B Cell division

2 Cell division by mitosis Needed for: reproduction in unicellular organisms growth in multicellular organisms replacement of damaged or dying cells repair of damaged organs or tissues Involves: replication of DNA controlled division of the nuclear material so new each cell has the same DNA division of cytoplasm and organelles so each new cell has similar contents

3 Cell cycle

4 Stages of mitosis

5 Interphase Phase of growth and normal metabolism Nucleus and nucleolus visible DNA replication occurs just before cell division occurs

6 Prophase First stage of division Nucleus and nucleolus disappear Spindle starts to form DNA condenses and becomes visible as chromosomes Centrioles move to opposite poles (animal cells)

7 Metaphase Chromosomes line up on spindle at the equator of the cell

8 Anaphase Chromatids pull apart at the centromere One chromatid from each chromosome moves to each end of the cell

9 Telophase Chromosomes uncondense and a new nucleus forms around them The cytoplasm splits (cytokinesis) to form two new cells identical to the original except smaller

10 Interphase Phase of growth and normal metabolism Nucleus and nucleolus visible

11 Gametogenesis – cell division by meiosis Sexual reproduction involve the formation of gametes, which are then fertilised to produce a new individual. Gametes are made by a process called gametogenesis. Gametogenesis involves a special type of division called meiosis. In meiosis, the number of chromosomes is halved, so that when gametes unite in fertilisation, they have the correct number of chromosomes. Gametes are called haploid cell as they have only half the number of chromosomes as normal (diploid) cells. Female gametes are made in the ovaries and are called ova. Male gametes are made in the testes and are called sperm. In most species gametes then undergo a period of maturation before they are capable of fertilisation.

12 Events of meiosis

13 Meiosis 1 – the first division prophase I the chromosomes become visible - each chromosome contains two chromatids homologous chromosomes move together & a spindle forms metaphase I homologous pairs of chromosomes line up on the equator, attached by their centromeres anaphase I one of each pair of homologous chromosomes is pulled to either end of the cell telophase I the chromosomes condense cytoplasm is split of and cell divides into two daughter cells, each with 23 chromosomes each chromosome consists of two chromatids

14 Meiosis 2 – the second division prophase II the chromosomes become visible - each chromosome contains two chromatids a new spindle forms, at right angles to the old one metaphase II chromosomes line up on the equator, attached by their centromeres anaphase II one of each chromatid in each chromosome is pulled to either end of the cell telophase II the chromosomes condense cytoplasm is split of and each daughter cell divides into two (ie total of 4 daughter cells), each with 23 chromosomes - each chromosome consists of one chromatid Gametes

15 Comparing mitosis and meiosis MitosisMeiosis Where it occurs Body cellsGonads (reproductive organs) Why it occurs Cell repair, growth, asexual division Sexual reproduction Number of cells produced 24 Number of divisions 12 Number of chromosomes in daughter cells Same as parent (diploid) Half that of parent (haploid) Amount of variation in daughter cells NoneLots

16 Comparing mitosis and meiosis 2 MitosisMeiosis Advantages for reproduction Simple Rapid division Allows variation Disadvantages for reproduction No variationMore complex Slower reproduction


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