reproduction

• Sexual reproduction


  The process involving the fusion of the nuclei of two gametes to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other
• Gamete


  A sex cell (in animals: sperm and ovum; in plants: pollen nucleus and ovum)
• Gametes


  • They contain half the number of chromosomes found in other body cells - they have a haploid nucleus
  • They 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 chromosomes
• Zygote


  The fertilised egg cell that contains the full 46 chromosomes, half of which came from the father and half from the mother - it has a diploid nucleus
• Advantages and disadvantages of sexual reproduction


  • Advantages
  • Disadvantages
• advantages of sexual reproduction: increase gentic variation
• advantages of sexual reproduction: the species can adapt to new enviroments due to variation, giving them a survival advantage. disease is less likely to affect population
• disadvanatges: takes time to find mates. diffucult for isolated members of the species to reproduce
• what cell divsion takes place in sexual reproduction: meiosis, once the gametes are formed, they undergo meiosis to produce haploid gametes
• diploid = 46 chromosomes
• haploid= half the original number of chromosomes
• an offspring will look similar to his parents because it inherited gemes from its parents
• asexual reproduction is when one parent produces offspring with the same genes as itself
• • Bacteria produce exact genetic copies of themselves in a type of asexual reproduction called binary fission
• Sexual reproduction


  Reproduction involving two parent organisms
• Asexual reproduction


  Reproduction involving one parent organism
• Key differences between sexual and asexual reproduction


  • Number of parent organisms
  • How offspring are produced (type of cell division)
  • Level of genetic similarity between offspring
  • Possible sources of genetic variation in offspring
  • Number of offspring produced
  • Time taken to produce offspring
• Asexual reproduction


  • 1 parent organism
  • Mitosis
  • Genetically identical offspring
  • Mutation
  • Usually large numbers
  • Usually fast
• Sexual reproduction


  • 2 parent organisms
  • Meiosis
  • Genetically unique and different from parents
  • Contribution of 50% DNA from each parent, random assortment of chromosomes
  • Usually limited numbers
  • Usually slow
• mitosis vs meiosis: mitosis is a type of cell division that produces two genetically identical daughter cells, while meiosis is a type of cell division that produces four genetically different daughter cells
• asexual reproduction takes place in plants, fungi, bacteria and some animals like star fish
• mutation is a change in the DNA sequence of a gene that can lead to a change in the amino acid sequence of a protein
• a karyotype is a diagram of the chromosomes in a cell
• • Fertilisation is defined as the fusion of gamete nuclei, and as each gamete comes from a different parent, there is variation in the offspring
• fertilization in plants: the ovule is what eventually develops into a seed, the haploid female nucleus is located in the middle of the ovary, the ovary wall will eventually develop into a fruit. the pollen tube is the male haploid nucleus, the male and female nuclei fuse in fertilization.
• Flowers


  Reproductive organs of plants
• 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)
• Pollination


  Pollen is transferred from the anther (male part of the flower) to the stigma (female part of the flower)
• Mechanisms of pollination


  • Insects (or sometimes other animals like birds or bats)
  • Wind
• Insect-pollinated flowers


  • Adapted for their specific function
• Wind-pollinated flowers


  • Adapted for their specific function
• Insect-pollinated flowers


  Flowers of many plant species where the pollinating agents are insects (e.g. bees)
• Insect-pollinated flowers


  • Insects visit these flowers to collect nectar (a sugary substance produced by insect-pollinated flowers and the base of their petals, which provides the insects with energy)
  • As an insect enters the flowers in search of nectar, it often brushes against the anthers, which deposit sticky pollen onto the insect's body
  • When the insect visits another flower, it may brush against the stigma of this second flower and in the process, may deposit some of the pollen from the first flower, resulting in pollination
• The structures of an insect-pollinated flower ensure that the flower is well-adapted for pollination by insects
• pollination occurs when pollen lands on the stigma of a flower
• the ovary, the style, and the stigma make up the female reproductive system in plants called the carpel. the filament and anther make up the male reproductive system called the stamen.
• the insect-pollination plant is adapted with specialized features. such as large, brightly colored petals to attract insects, a nectary which releases scent and provides a sugary liquid, pollen grains are produced in large quantities of sticky pollen to attach to insects. there are enclosed and stiff anthers help up high in the flower by the filament, they are stiff so they can brush against insects to transfer pollen. the stigma that is inside the flower is also sticky so that the pollen can stick to it.