🪺Biology Unit 3 Reproduction

• The cell cycle is a regulated process in which cells divide into two identical daughter cells, and that this process consists of three main stages: interphase, mitosis and cytokinesis

  
• Mitosis contributes to growth, repair and asexual reproduction
• Meiosis


  1. Results in haploid gametes

  2. Results in genetic variation through recombination of alleles, including independent assortment and crossing over

  
• Chromosome mutations


  • Translocations
  • Non-disjunction leading to polysomy (Down's syndrome) and monosomy (Turner's syndrome)

  
• Pollen grain formation in anther

  1. Embryo sac formation in ovule

  2. Male nuclei reach embryo sac via tube nucleus, pollen tube and enzymes and fertilise the female gamete and 2 polar nuclei

  3. Double fertilisation to form triploid endosperm nucleus and zygote
• In eukaryotes, DNA is cut into shorter lengths and each length is tightly wrapped up with histone proteins to form chromatin
• Before cell division, DNA is replicated and more histone proteins are synthesised, so there is temporarily twice the normal amount of chromatin
• Following replication, chromatin coils up even tighter to form short fat bundles called chromosomes
• Chromatids


  The two arms of an X-shaped chromosome, joined at the centromere

  
• Karyotype


  • Displays the individual chromosomes of a cell, showing their characteristic size, shape and banding pattern
  • Allows identification and numbering of chromosomes
  • Shows homologous pairs of chromosomes
  • Shows sex chromosomes (non-homologous in one sex)

  
• Homologous chromosomes


  Two chromosomes of the same size and shape, one originating from each parent, containing the same genes but different alleles
• Cell cycle

  • Interphase (G1, S, G2)
  • Mitosis
  • Cytokinesis

  
• Checkpoints

  Points in the cell cycle where the cell is checked to see if it is safe to proceed to the next phase
• Mitosis

  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase

  5. Cytokinesis
• The purpose of mitosis is to make two genetically-identical cells
• Prophase 1

  1. Nuclear envelope disappears

  2. Chromosomes condense

  3. Homologous chromosomes join together to form bivalents
• Metaphase 1


  Bivalents line up on the equator
• Anaphase 1


  1. Bivalents separate and homologous chromosomes move to opposite poles

  2. Chromatids do not separate
• Telophase and Cytokinesis 1


  1. Two nuclei with replicated chromosomes

  2. Chromosomes don't uncoil

  3. Cells divide, but remain joined
• Prophase II

  1. Centrioles move to form new poles at 90° to original poles

  2. Chromosomes already coiled
• Metaphase II


  Chromosomes line up on new equator
• Anaphase II


  1. Centromeres finally split, separating chromosomes

  2. Chromatids move to opposite poles
• Telophase II and Cytokinesis II

  1. Nuclear envelopes form around each set of chromosomes

  2. Cells divide, but remain joined to form a tetrad
• Meiosis


  • Halves the chromosome number from diploid (2n) to haploid (n)
  • Rearranges chromosomes to form new combinations of alleles
• Crossing over


  Sections of sister chromatids are swapped (crossed over) within bivalents during prophase I
• Chiasmata


  Points at which the chromosomes actually cross over
• Independent assortment


  Maternal and paternal versions of different chromosomes can be mixed up in the final gametes during metaphase I
• Every gamete produced by an individual is genetically unique due to crossing over and independent assortment
• Nondisjunction


  Chromosome mutation where chromatids or chromosomes remain stuck together and move together to one pole of the cell during anaphase

  
• Polysomy


  • Trisomy (e.g. Down syndrome)
  • Monosomy (e.g. Turner syndrome)
• Polyploidy

  Nondisjunction causes all the chromosomes to remain together, forming a gamete with two copies of every chromosome
• Translocation


  Chromosome mutation where one part of a chromosome is swapped with a part of a completely different, non-homologous, chromosome
• Male gametes

  Small cells that can move, either motile (e.g. animal sperm) or non-motile (e.g. plant pollen)
• Sexual reproduction


  1. Meiosis

  2. Fertilisation
• Gametes


  Haploid sex cells that will fuse together to form a new diploid individual
• Types of gametes


  • Male
  • Female
• Male gametes

  • Many
  • Mobile
  • Minute
• Female gametes

  • Few
  • Fixed
  • Food
• Those individuals that produce small mobile gametes are the males, and those that produce the larger gametes are the females
• In some species the same individual organisms can produce both male and female gametes, so they do not have distinct sexes and are called hermaphrodites