Biology Topic 3B

• Cells divide in a sequence of events known as the cell cycle. The 3 stages are interphase (containing G1, S and G2), mitosis and cytokinesis.
• Mitosis is the division of the nucleus that results in two nuclei, each with an identical set of chromosomes.
• Cytokinesis is the division of the cytoplasm and whole cell.
• The stage G1 of interphase is when the cell takes in material, grows and develops.
• The stage S of interphase is when chromosomes replicate and become double stranded chromatids (which remain attached until they divide in mitosis).
• The stage G2 of interphase is when organelles and other materials are made before a cell can divide as it needs 2 copies of everything.
• The cell cycle is controlled by chemical signals (cyclins) at different points called checkpoints in the cell cycle.
• Mitosis occurs in single celled organisms that divide by asexual reproduction following mitosis, growth (in animals and plants) and after fertilisation of gametes (fetal growth).
• During prophase in mitosis, the nuclear envelope breaks down and chromatin condenses into chromosomes.
• During metaphase in mitosis, spindle forms from the centrioles and attaches to the centromeres of chromosomes, and the chromosomes line up along the equator of the cell.
• During anaphase in mitosis, spindle shortens and this seperates the chromosomes back into chromatids, before arriving at the opposite poles of the cell, where they are chromosomes again (now with only one chromatid).
• During telophase in mitosis, the nuclear envelope reforms around each group of chromosomes.
• The purpose of mitosis is to produce 2 genetically identical daughter cells.
• The purpose of cytokinesis in animal cells is to form the cleavage furrow in order to separate the cells.
• The purpose of cytokinesis in plant cells is to form the cell plate in order to aid the cell wall formation.
• During prophase I in Meiosis I, the nuclear envelope breaks down, the homologous chromosomes pair up (to form bivalents) and crossing over occurs between non-sister chromatids at the chaismata.
• During metaphase I in Meiosis I, pairs of chromosomes line up in a random order along the equator of the cell and spindle forms which attaches centrioles to centromeres.
• During anaphase I in Meiosis I, pairs of chromosomes get pulled apart as spindle shrinks and they move to opposite poles of the cell and chromosomes contain 2 chromatids each.
• During telophase I in Meiosis I, the nuclear envelope reforms around each group of chromosomes.
• During prophase II in Meiosis II, the nuclear envelope breaks down and new spindle forms perpendicular to the original spindle in Meoisis I.
• During metaphase II in Meiosis II, the chromosomes line up along the equator of the cell.
• During anaphase II in Meiosis II, spindle shortens, therefore chromosomes are seperated and the now chromatids move to the opposite poles of the cell.
• During telophase II in Meiosis II, the nuclear envelope reforms around groups of chromosomes and each chromosome contains 1 chromatid. 4 haploid daughter cells are formed.
• The purpose of meiosis is the production of gametes that are genetically different and therefore producing variation in offspring.