Growth, asexual reproduction, tissue repair and maintenance are processes that require the creation of new cells In eukaryotic cells, division of the nucleus to form 2 genetically identical nuclei is called MITOSIS The life of the cell can be considered an ordered sequence of events called the cell cycle M - Mitosis and cytokinesis G1 - Growth phase (cell grows and organelle number increase) S - Synthesis phase (chromosomes are replicated in preparation for the next cell division) G2 - Another growth phase (leading to M phase again) Newly produced cells may:  grow and reproduce again  differentiate to become specialised cells  perform a particular function, age then die R – Point – The R-Point is known as the RESTRICTION POINT. It is the most important of the control points of the cell cycle. It is here that the cell determines whether it will undergo mitosis or not. If it has a mutation or is cancerous it may be terminated or remain dormant.

Apoptosis is the programmed death of a cell Some cells live for a lifetime (brain cells) others different times Eg. Skin cells (few weeks), stomach lining (few days) and RBCs (few months) Cancer cells avoid apoptosis and continue growing unchecked During Apoptosis, the programmed cell:  signals that it is going to die  shrinks  digestive enzymes are released breaking the cell up  some parts are engulfed by macrophages whilst other organelles can be reused by neighbouring cells Cell death is important as:  it protects from ill health by destroying infected cells  help divide cells in an ordered manner  it helps shape organs and tissues Necrosis is a form of cell death that results from irrepairable injury to the cell

VIDEO: CRASHCOURSE MITOSIS VIDEO: THE EUKARYOTIC CELL CYCLE

 Meiosis is a type of cell division that produces gametes (sex cells)  Cells produced are haploid (contain HALF the original chromosome number)  Process accounts for great genetic variation due to  RANDOM ASSORTMENT  CROSSING OVER  2 cell divisions: 1 st : PROPHASE I METAPHASE I ANAPHASE I TELOPHASE I 2 nd : PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II

Random Assortment and Crossing Over play an important role in increasing genetic variation within a species. They do this by increasing the number of different combinations that can be formed in the gametes.

During Anaphase I, if the homologous pair fails to separate then one cell will receive BOTH chromosomes In the 2 nd division the paired chromatids separate but the gametes will receive 2 copies of the chromosome Can also result from FAILURE OF PAIRED CHROMOSOMES TO SEPARATE at anaphase II. One gamete receives both chromosomes and the other none

VIDEO: CRASHCOURSE MEIOSIS VIDEO: NON-DISJUNCTION VIDEO: CROSSING OVER AND GENETIC VARIABILITY