3B

• locus


  location of a specific gene on a chromosome
• diploid


  a cell with a nucleus containing 2 full sets of chromosomes (2n)
• haploid


  a cell with a nucleus containing 1 full set of chromosomes (n)
• zygote


  cell formed when 2 haploid gametes fuse together
• fertilisation


  fusing of haploid nuclei from 2 gametes to form a diploid zygote in sexual reproduction
• polyploidy

  cell or organism with more than 2 sets of chromosomes
• homologous pairs


  matching pairs of chromosomes in an individual which carry the same genes, but they can have different alleles
• crossing over


  process by which large multi-enzyme complexes cut and re-join parts of the maternal and paternal chromatids in prophase 1, to introduce genetic variation
• chiasmata


  points where chromatids break during crossing over
• independent assortment


  process by which the maternal and paternal chromosomes are distributed into gametes randomly
• Meiosis - first division:

  • Prophase 1 - chromosomes are condensed with 2 chromatids, homologous pairs pair up, crossing over occurs
  • Metaphase 1 - pairs of chromosomes line up on the metaphase plate, spindle fibres form
  • Anaphase 1 - 1 chromosome from each pair moves to the end of the cell, so each cell has half the number of chromosomes
  • Telophase 1 - nuclear membrane reforms
• Meiosis - second division:

  • Prophase 2 - new spindles are formed
  • Metaphase 2 - chromosomes line up on metaphase plate
  • Anaphase 2 - centromeres divide and chromatids move to the end of the cell
  • Telophase 2 - nuclear membrane reforms, chromosomes return to interphase, cytokinesis occurs
  • 4 genetically different haploid daughter cells
• sperm specialisation:

  • acrosome - break down layers surrounding ovum
  • condensed haploid chromosomes - less energy to transport
  • lots of mitochondria - ATP for movement
  • microtubules - whip-like movement in tail
  • tail - propels
  • 50-60 micrometers
• acrosome


  membrane-bound storage site found in the sperm head for enzymes that digest the layers surrounding the ovum and allow the head to penetrate
• ovum specialisation:

  • zona pellucida - protective coat
  • food rich cytoplasm - provides nutrients for zygote
  • 100 micrometers (human)
• zona pellucida

  layer of protective jelly around the unfertilised ovum
• mitosis functions:

  • repair
  • growth
  • asexual reproduction
• Phases of cell cycle:

  1. G1 - cell takes in material, grows, and develops between the previous round of mitotic division and the start of chromosome replication
  2. S - chromosomes replicate to become chromatids
  3. G2 - organelles and other materials are synthesised in preparation for cell division
  4. Mitosis - nucleus is actively dividing
  5. Cytokinesis - new cells separate during final stage of cell division
• mitosis


  cell divides to produce 2 genetically identical daughter cells
• asexual reproduction


  production of genetically identical offspring from a single parent
• sexual reproduction


  production of genetically different offspring from a parent by the fusing of 2 gametes
• meiosis 


  cell division where the chromosome of the original cell is halved, forming gametes
• histones

  positively charged proteins involved in the coiling of DNA to form dense chromosomes during cell division
• nucleosomes 


  dense clusters of DNA wound around histones
• karyotype


  displaying an image of the chromosomes of a cell to show the homologous pairs
• cell cycle


  regulated process of 3 stages in which cells divide into 2 genetically identical daughter cells
• interphase

  period between active cell division when cells increase in size and mass, replicate their DNA and carry out metabolic activities
• cytokinesis

  final stage of cell division with the division of cytoplasm at the end of mitosis
• chromatid 


  one strand of the replicated chromosome pair that is joined to the other strand at the centromere
• cyclins


  small proteins that build up during interphase to control the cell cycle
• cyclin-dependent kinases (CDKs)

  enzymes involved in the control of the cell cycle
• stages of mitosis:

  1. prophase
  2. metaphase
  3. anaphase
  4. telophase
• Prophase:

  • chromosomes condense
  • nucleolus breaks down
  • centrioles start to pull apart to form spindle fibres
• Metaphase:

  • nuclear membrane breaks down
  • centrioles moved to opposite poles of cell, spindle fibres fully formed
  • chromatids line up along cell equator on the metaphase plate
  • spindle fibres attach to centromeres
• Anaphase:

  • contractile fibres in spindle fibres contract
  • centromeres split, so chromatids become separate
  • chromatids are pulled to opposite poles of the cell
• Telophase:

  • spindle fibres break down
  • nuclear membrane forms around each set of chromosomes
  • nucleolus and centrioles reform
  • chromosomes begin to unravel
• Cytokinesis:

  • cytoplasm divides
  • ring of contractile fibres tighten around centre of cell
  • 2 genetically identical daughter cells are formed
• Cytokinesis in plant cells:

  1. some spindle fibres remain after telophase, and guide Golgi vesicles to the centre of the cell
  2. vesicles enlarge and fuse together, forming a cell plate
  3. basic structure of the cell walls forms within each vesicle, and the vesicles fuse to join the cell wall together
  4. small gaps are left between the vesicles which form plasmodesmata
• Asexual reproduction is useful as it does not require finding another mate and it produces offspring rapidly. However, all of the offspring are clones of the parent, so they are less able to change with a changing environment.
• The mitotic index is a measure of how actively the cells in a tissue are dividing.