Reproduction

• Asexual Reproduction

  • Asexual reproduction does not involve sex cells or fertilisation
  • Only one parent is required so there is no fusion of gametes and no mixing of genetic information
  • As a result, the offspring are genetically identical to the parent and to each other (clones)
  • Asexual reproduction is defined as a process resulting in the production of genetically identical offspring from one parent
• Bacteria produce exact genetic copies of themselves in a type of asexual reproduction called binary fission:

  
• Plants can reproduce asexually using bulbs and tubers; these are food storage organs from which budding can occur, producing new plants which are genetically identical to the parent plant:

  
• Some plants grow side shoots called runners that contain tiny plantlets on them (a good example of this are strawberry plants. These will grow roots and develop into separate plants, again being genetically identical to the parent plant:

  
• Advantages of asexual reproduction

  • Population can be increased rapidly when conditions are right
  • Can exploit suitable environments quickly
  • More time and energy efficient
  • Reproduction is completed much faster than sexual reproduction
• Disadvantages of asexual reproduction:

  • Limited genetic variation n population - offspring are genetically identical to their parents
  • Population is vulnerable to changes in conditions and may only be suited for one habitat
  • Disease is likely to affect the whole population as there is no genetic variation
• • Specifically in crop plants, asexual reproduction can be advantageous as it means that a plant that has good characteristics (high yield, disease-resistant, hardy) can be made to reproduce asexually and the entire crop will show the same characteristics
• Sexual Reproduction

  • Sexual reproduction is a process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other
  • Fertilisation is defined as the fusion of the nuclei of gametes, and as each gamete comes from a different parent, there is variation in the offspring
• Gametes


  Sperm and ovum in animals; pollen nucleus and ovum in plants
• Gametes


  • Contain half the number of chromosomes found in other body cells
  • Have a haploid nucleus
  • Only contain one copy of each chromosome, rather than the two copies found in other body cells
• In human beings, a normal body cell contains 46 chromosomes but each gamete contains 23
• Fertilisation


  1. Male and female gametes fuse to become a zygote (fertilised egg cell)

  2. Zygote contains the full 46 chromosomes, half of which came from the father and half from the mother

  3. Zygote has a diploid nucleus
• • The nuclei of gametes are haploid
  • They contain half the number of chromosomes of a normal body cell
  • In humans, this is 23 chromosomes
  • The nucleus of a zygote is diploid
  • It contains the same number of chromosomes as a normal body cell
  • In humans, this is 23 pairs of chromosomes
  • The zygote continues to stay diploid as it grows into a fetus and embryo during pregnancy

  
• Advantages of sexual reproduction:

  • increases genetic variation
  • the species can adapt to new environments due to variation, giving them a survival advantage
  • disease is less likely to affect population(due to variation)
• Disadvantages of sexual reproduction:

  • Takes time and energy to find mates
  • Difficult for isolated members of the species to reproduce
• • Most crop plants reproduce sexually and this is an advantage as it means variation is increased and a genetic variant may be produced which is better able to cope with weather changes, or produces significantly higher yield
  • The disadvantage is that the variation may lead to offspring that are less successful than the parent plant at growing well or producing a good harvest
• Flowers


  The reproductive organ of the plant
• Flowers


  • Usually contain both male and female reproductive parts
• Pollen


  Contains a nucleus inside that is the male gamete
• Pollen is not capable of locomotion (moving from one place to another), unlike the male gamete in humans (sperm)
• Pollination


  Transfer of pollen from the anther to the stigma
• Mechanisms of pollination


  • Transferred by insects (or other animals like birds)
  • Transferred by wind
• Insect and wind-pollinated flowers


  • Have slightly different structures as each is adapted for their specific function
• Parts of a flower:

  
• General flower structure

  A) Petal

  B) Stamen

  C) Anther

  D) Filament

  E) Sepal

  F) Stem

  G) Receptacle

  H) Carpel

  I) Stigma

  J) Style

  K) Ovary

  
• Features of an insect-pollinated flower

  A) Ovary

  B) Nectary

  C) Ovule

  D) Sepal

  
• Features of a wind-pollinated flower

  A) Stamen

  B) Filament

  C) Stigma

  
• • The pollen produced by insect and wind-pollinated flowers is also different:
  • Insect pollinated flowers produce smaller amounts of larger, heavier pollen grains that often contain spikes or hooks on the outside so they are better able to stick to insects
  • Wind pollinated flowers produce large amounts of small, lightweight pollen grains that are usually smooth 
• Pollination

  • Pollination is discussed above as the ways in which pollen grains can be transferred from an anther to a stigma
• Fertilisation


  Pollen nucleus fuses with ovum nucleus in the ovule
• Pollen


  • Has no 'tail' to swim to the ovary of a plant
  • Grows a pollen tube to reach the 'female' nucleus in the ovary
• Pollen grain lands on the right kind of stigma


  Pollen tube grows down the style towards the ovary
• Ovary


  Contains one or more ovules, each containing an ovum with a female nucleus that a male pollen nucleus can fuse with
• Fertilisation


  1. Nuclei join together

  2. Zygote formed

  3. Zygote starts to divide

  4. Seed forms within the ovule
• Different plants have different numbers of ovules


  Explains why different fruits have different numbers of seeds
• Think of pollination as the plant’s equivalent to human sexual intercourse – after sex, the male sex cells (sperm) have been deposited into the female. But, for fertilisation to occur, the nucleus from a male sperm cell has to fuse with the nucleus of a female sex cell (egg) and the sperm has to travel to find the egg before this happens. It’s exactly the same in plants!
• Factors Affecting Germination of Seeds

  • Germination is the start of growth in the seed
  • Three factors are required for successful germination:
  • Water - allows the seed to swell up and the enzymes in the embryo to start working so that growth can occur
  • Oxygen - so that energy can be released for germination
  • Warmth - germination improves as temperature rises (up to a maximum) as the reactions which take place are controlled by enzymes
  • As carbon dioxide is not necessary for germination but also does not inhibit it, it makes no difference whether it is present or not
• Investigating Germination

  • Set up 4 boiling tubes each containing 10 cress seeds on cotton wool
  • Set each test tube as shown in diagram below
  • Leave tubes in set environment for a period of time: A, B and C incubated at 20°C; D placed in a fridge at 4°C
  • Compare results and see which tube has the greatest number of germinated seeds

  
• Cross-pollination


  Pollen grains from the anther of a flower to the stigma of a flower on a different plant of the same species
• Self-pollination


  Pollen grains from the anther of a flower transferred to the stigma of the same flower or a different flower on the same plant