Chromosomes
Thread like structure made up of a long molecule of DNA and histone proteins, chromosomes pair up (homologus pairs) and the 23rd pair are sex chromosomes. All others are autosomes.
Chromosomes in division
When a cell is not dividing, chromosomes are not visible. When they are about to divide, chromosomes condense and become invisible. They appear as two threads and joined by a centromere
Life cycle of a body cell
Interphase, mitosis and cytokenisis
Interphase
DNA of each chromosome is replicated (copies (called sister chromatids) held together at the centromere) Organelles and centrioles replicated and proteins are made and cell carries out its functions
Mitosis
Prophase, metaphase, anaphase and telophase
Prophase
Chromosomes condense and become visible, centrioles move to opposite sides of cell in animal cells, spindle forms from microtubules and nuclear envelope disintergrates
Metaphase
Centromeres of chromosomes attach to spindle and line up at the equator
Anaphase
Spindle fibres shorten, centromere seperates and individual chromatids are pulled to the poles centromere first
Telophase
Spindle breaks down, chromosomes uncoil and nuclear envelope forms
Cytokinesis
Cytokinesis is the division of the cytoplasm to create the two new genetically identical cells
Animal cells in cytokinesis
Splitting happens in the centre of the cell (cytoplasm splits) due to contraction of ring made of two proteins to make a cleavage furrow
Mitosis in plants
Only takes place in root and shoot tips, these cells are called meristem cells. There are no centrioles in plant cells and during cytokinesis, a cell plate (droplets of cell wall material) develop from the centre out
Tumour suppressor genes
Prevent rapid cell division in cell tissue
Oncogenes
Genes mutate or if the cell is infected by a virus