- Cell Division
- The cell cycle consists of interphase, mitosis and cytokenisis. Mitosis occupies only about 5% of the time while the rest is used for the duplication and checking of DNA.
- In interphase DNA replicates, so that each chromosome is made up of two identical chromotids joined at the centromere.
- During mitosis, the nuclear membrane breaks down, spindle fibres form, attach themselves to the condensed chromosomes and move them to the equator of the cell. The centromeres then break & the spindle fibres pull the separated chromatids to opposite ends of the cell. New nuclear membranes form around each group of chromatids.
- The phases of mitosis are Prophase, Metaphase, Anaphase and Telophase.
- During cytokenisis, the cytoplasm slpits and two new cells are formed.
- Mitosis produces 2 daughter cells that are genetically identical to each other & to the parent cell. Mitosis is used for growth, repair and asexual reproduction.
- Multicellular organisms usually contain many types of cells, which have become diferentiated to perform different functions. They are often grouped into tissues, containing cells that have the same function e.g. squamous and ciliated epithelium in animals, xylem and phloem in plants.
- Tissues are grouped into organs and organs are grouped into organ systems.
- When animal cells become diferentiated it is normally not able to become any other type of cell. However stem cells have the ability to divide and diferentiate. Stem cells in a young embryo are able to diferentiate into any kind of cell, in an adult stem cells appear to have a limited range of specialised cells they can form.
- For example stem cells in bone marrow
- Cells may divide by meiosis, which produces genetically different cells with half the number of chromosomes of the parent cell. Body cells are diploid, meaning that they have 2 complete sets of chromosomes. Matching or homologous, chromosomes pair up in meiosis and are then shared out into the daughter cells. These are haploid meaning that they have only 1 set of chromosomes.
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