Summary

• Viruses


  Non-living structures consisting of nucleic acid (DNA or RNA) enclosed in a protective protein coat called the capsid, sometimes covered with a lipid layer called the envelope
• Viruses can be difficult to treat once an infection has occurred, therefore the focus of disease control should be on preventing the spread
• There are ethical implications to using, or not using, untested drugs for viral outbreaks</b>
• Lysogenic cycle


  1. Viral DNA inserts into the host cell DNA as a provirus, enabling the viral DNA to be replicated via cell division of the host cell

  2. The provirus can stay dormant if the virus produces repressor proteins which inhibit the transcription of the provirus
• Lytic cycle


  1. Viral DNA/RNA inserts into the cytoplasm of the host cell, the viral genome is replicated independently of the host cell genome

  2. Eventually, this leads to lysis of the host cell when a large number of viruses are assembled and ready to infect more cells

  
• When the lysogenic host cells become damaged or the immune system becomes weak


  The dormant viruses can enter the lytic pathway which leads to lysis of the cell and the spread of viral infection
• Cell cycle


  A regulated process in which cells divide to produce two genetically identical daughter cells for growth, repair and asexual reproduction

  
• Stages of the cell cycle


  • Mitosis - prophase, metaphase, anaphase and telophase
  • Cytokinesis - the cytoplasm divides, producing two daughter cells

  
• Mitosis does not give rise to genetic variation as all the cells produced are genetically identical

  
• Lytic pathway of viruses


  1. Viral genome replicates independently of host cell genome

  2. Large number of viruses assemble

  3. Lysis of host cell
• Lysogenic pathway of viruses


  1. Dormant viruses in host cell

  2. Host cell becomes damaged or immune system weakens

  3. Viruses enter lytic pathway

  4. Lysis of host cell

  5. Spread of viral infection
• Cell cycle


  Regulated process in which cells divide to produce two genetically identical daughter cells for growth, repair and asexual reproduction

  
• Mitosis does not give rise to genetic variation

  
• Cell cycle stages


  1. Mitosis

  2. Cytokinesis

  3. Interphase

  
• Meiosis


  • Form of cell division that gives rise to genetic variation
  • Produces haploid gametes and maintains chromosome number

  
• Genetic variation in meiosis

  1. Crossing over

  2. Independent assortment

  
• Chromosome mutations


  • Translocations
  • Duplication
  • Deletion
  • Inversion
  • Non-disjunction
• Polysomy


  More than two chromosomes in a cell (e.g. Down's Syndrome)
• Monosomy


  Less than two chromosomes in a cell (e.g. Turner's Syndrome)
• Spermatogenesis


  1. Primordial germ cells divide to form spermatogonia

  2. Spermatogonia grow to form primary spermatocytes

  3. Primary spermatocytes undergo first meiotic division to form secondary spermatocytes

  4. Secondary spermatocytes undergo second meiotic division to form spermatids

  5. Spermatids differentiate to form mature spermatozoa

  
• Oogenesis


  1. Primordial germ cells divide to form oogonia

  2. One oogonium continues to grow to form a primary oocyte

  3. First meiotic division forms one secondary oocyte and one polar body

  4. Second meiotic division forms one haploid ootid and one polar body
• Ovum


  • Contains zona pellucida
  • Contains haploid nucleus
  • Cortical granules release substances to harden zona pellucida
  • Follicle cells form protective coating
• Spermatozoa


  • Contains many mitochondria for energy
  • Contains acrosomes with digestive enzymes
  • Contains haploid nucleus

  
• Fertilisation


  1. Sperm head contacts zona pellucida, acrosome reaction occurs

  2. Sperm head fuses with egg cell membrane

  3. Cortical reaction hardens zona pellucida

  4. Nuclei fuse to form diploid zygote