B13 - Reproduction

• Asexual reproduction


  Reproduction involving only one parent, with no fusion of gametes and no mixing of genetic information, leading to genetically identical offspring (clones)
• Sexual reproduction


  Reproduction involving the fusion of male and female gametes formed by meiosis, leading to the mixing of genetic information and variation in the offspring
• Gamete


  A specialized sex cell, such as an egg cell or sperm cell, formed by meiosis and containing half the normal number of chromosomes
• Variation


  Differences between organisms of the same species, resulting from the mixing of genetic information during sexual reproduction
• In asexual reproduction, the offspring are genetically identical to their parent, but in sexual reproduction the offspring have a mixture of characteristics from both parents
• Organisms that reproduce asexually are more affected by changes in the environment than those that reproduce sexually, as asexual reproduction does not introduce variation
• Meiosis


  1. Genetic material is copied, then the cell divides twice to form four gametes, each with a single set of chromosomes

  2. Gametes are genetically different from each other
• Meiosis takes place in the reproductive organs (ovaries and testes) to produce gametes
• Meiosis


  The type of cell division that produces gametes with half the normal number of chromosomes
• Fertilisation restores the full number of chromosomes by combining the genetic material from the male and female gametes
• Asexual reproduction involves mitosis, while sexual reproduction involves meiosis and fertilisation
• Advantages of asexual reproduction

  • Only one parent needed
  • Time and energy efficient
  • Rapid production of large numbers of identical offspring
• Disadvantages of asexual reproduction

  • No variation, so if environment changes all offspring may be unable to survive
  • Clones are vulnerable to diseases and pests
• Advantages of sexual reproduction

  • Produces variation in offspring, giving a survival advantage if environment changes
  • Natural selection can act on the variation
• Disadvantages of sexual reproduction

  • Requires two parents or gametes
  • Time and energy needed to find a mate and spread gametes
  • Slower than asexual reproduction
• Fungi commonly reproduce asexually using spores, but can also reproduce sexually to introduce variation when conditions are unfavourable
• Plants reproduce sexually through flowers, with pollen and egg cells produced by meiosis, but many can also reproduce asexually through runners, bulbs, etc.
• Malaria parasite life cycle

  1. Asexual reproduction in human liver and blood cells

  2. Sexual reproduction triggered in mosquito, with fusion of gametes and meiosis to produce new asexual parasites
• Asexual reproduction


  Not an alternative only if conditions are bad
• Malarial parasites reproduce asexually in human liver and blood cells
• Malaria parasite reproduction in mosquito

  1. Temperature drop triggers sexual reproduction

  2. Sexual forms develop, burst out of blood cells, and meet to form zygotes

  3. Zygotes undergo meiosis to produce new asexual parasites
• Malaria parasites show a lot of variation
• Advantages and disadvantages of using both asexual and sexual reproduction in the life cycle, with examples from different organisms
• Mitosis occurs in malaria parasites in humans
• Figure 2 shows the fast changing variation in malaria parasites results from a life cycle that includes mitosis alone in humans and a combination of meiosis and mitosis in the body of a female mosquito
• Sexual reproduction


  • Produces variation that helps survival through natural selection if the environment changes
  • Natural selection is sped up by humans in selective breeding
• Asexual reproduction


  • Needs only one parent
  • Time and energy efficient
  • Often faster than sexual reproduction
  • Many identical offspring are produced when conditions are favourable
• Some organisms depend on both asexual and sexual reproduction depending on the circumstances
• Malaria parasites reproduce sexually in mosquitoes and asexually in their human host
• Many fungi reproduce asexually by spores but can also reproduce sexually to give variation
• Many plants produce seeds sexually but also reproduce asexually, for example by runners or bulb division
• Synthesis


  The process of combining parts or elements to form a whole
• Mutation


  A change in the DNA sequence of a gene
• Gene expression


  When a gene codes for a protein that is synthesised in the cell
• Most of the DNA does not actually code for proteins - there are only 21000 genes but 3 billion pairs of bases</b>
• Scientists are still discovering what the non-coding part of the DNA does
• Non-coding DNA


  Involved in switching genes, or parts of genes, on and off
• Variations in the non-coding areas of our DNA may affect how our genes are expressed
• Changes in the non-coding DNA can have a big effect on the phenotype of the organism
• Mutation


  A change in the sequence of bases in a strand of DNA