Topic 6

• Phases of the cell cycle

  Interphase, mitotic phase, cytokinesis
• Phases of interphase


  G1, S, G2
• Interphase


  Long period of growth and working that separate cell divisions
• Why is interphase not a resting phase?

  It isn't really resting
• Stuff the cell does in interphase


  DNA is replicated and checked for errors, protein synthesis, mitochondria and chloroplasts grow and divide, normal metabolic processes
• G1


  Proteins that are used to make organelles are produced, organelles replicate, cell increases in size
• S


  DNA is replicated in the nucleus
• G2


  Cell increases in size, energy stores increase, duplicated DNA checked for errors
• Parts of the mitotic phase


  Mitosis, cytokinesis
• What happens in mitosis?

  Nucleus divides
• What happens in cytokinesis?

  Cytoplasm divides, two new cells produced
• G0


  Phase when the cell leaves the cycle
• Reasons for entering G0

  Differentiation, damaged DNA, ageing
• Reasons for having checkpoint proteins

  To ensure the production of two identical daughter cells from the parent cell
• Checkpoints in the cell cycle

  G1 checkpoint, G2 checkpoint, spindle assembly checkpoint
• G1 checkpoint


  At the end of the G1 phase, cell size, nutrients, growth factors and DNA damage are checked.
• G2 checkpoint


  At the end of the G2 phase, cell size, DNA replication and DNA damage are checked.
• Spindle assembly checkpoint


  It is checked whether all chromosomes are attached to spindles and have aligned.
• Things mitosis is used for

  Growth of organism, repair and replacement of tissues, asexual reproduction
• Asexual reproduction


  The production of genetically identical offspring from one parent
• What must chromosomes do before mitosis?

  DNA molecules converted into chromatids, joined at the centromere
• Why are centromeres necessary?

  To keep the chromosomes together so they can be moved and separated equally
• Phases of mitosis


  Prophase, metaphase, anaphase, telophase
• Prophase


  Chromatin fibres coil up and condense to form chromosomes, nucleolus disappears, nuclear membrane breaks down, protein microtubules form spindle-shaped structures to link the poles of the cell, centrioles move to opposite poles of the cell, spindle fibres move chromosomes towards the right place
• Metaphase


  Chromosome line up to form the metaphase plate
• Anaphase


  Chromosomes are pulled apart by the division of centromeres and the shortening of spindle fibres, chromatids pulled centromeres first
• Telophase


  Chromatids reach piles, nuclear envelope reforms around sets of chromosomes, chromosomes uncoil, nucleolus forms
• Why is meiosis important?

  Production of haploid cells, introduction of genetic variation by independent assortment or crossing over
• Homologous chromosomes


  A pair of chromosomes with one maternal and one paternal which has the same genes at the same loci but may have different alleles
• Stages of meiosis


  Interphase prophase 1 metaphase 1 anaphase 1 telophase 1 prophase 2 metaphase 2 anaphase 2 telophase 2
• Prophase 1


  Chromosomes condense, nuclear envelope disintegrates, nucleolus disappears, spindle formation begins, homologous chromosomes pair up to form bivalents, crossing over
• How does crossing over happen?

  As chromosomes move through the cytoplasm, they may entangle with other chromatids
• Metaphase 1


  Homologous pairs of chromosomes line up along the metaphase plate, orientation of each pair is random in independent assortment
• Independent assortment of chromosomes

  Maternal or paternal chromosomes can face either pole, regardless of any other homologous pair
• Anaphase 1


  Homologous chromosomes being pulled to opposite poles, crossing over can occur
• Crossing over


  Section of chromatids become entangled at chiasmata, forms recombinant chromatids, exchanged genes can be different alleles, sister chromatids no longer identical
• Telophase 1


  Chromosomes assemble at each pole, nuclear membrane reforms, chromosomes uncoil, cytokinesis
• Prophase 2


  Chromosomes condense, nuclear envelope breaks down, spindle formation begins
• Metaphase 2


  Individual chromosomes assemble on metaphase plate, independent assortment of chromatids
• Anaphase 2


  Chromatids of individual chromosomes are pulled to opposite poles centromeres first