genetics

• Protein synthesis - translation


  The mRNA strand travels out of the nucleus through small holes in its membrane, called nuclear pores. In the cytoplasm, the mRNA strands attach to ribosomes.

  A ribosome moves along an mRNA strand three bases at a time. Each triplet of bases is called a codon. At each mRNA codon, a molecule of transfer RNA with complementary bases lines up. Each tRNA molecule carries a specific amino acid. As the ribosome moves along, it joins the amino acids from the tRNA molecules together, forming a polypeptide. The polypeptide chain then fold ups to form a protein.
• Meiosis


  The gamete-making cell has two sets of chromosomes. It is diploid.

  The chromosome replicates and the copies stay stuck to one another.

  The cell divides into two and into two again. Each of the four daughter cells has a copy of a chromosome from each pair. They are haploid.
• What are the main steps in the genetic engineering of bacteria to produce insulin?


  1. Use restriction enzymes to make staggered cuts in the DNA and create sticky ends.
  2. Cut plasmids with the same restriction enzyme so they have the same sticky ends.
  3. Mix insulin gene DNA with cut plasmids.
  4. Use ligase to join the DNA ends.
  5. Insert plasmids back into bacteria and grow them in huge tanks.
  6. Extract the produced insulin.
• Variation 


  Causes of variation that influence phenotype include genetic variation and environmental variation.

  Most phenotypic features, eg hair colour, are caused by many genes. Each of these genes may have several alleles, so the phenotype is the result of the combination of different genes. This combination of alleles that an organism inherits is the result of sexual reproduction.
• Mutations


  A mutation or genetic variant is created if the sequence of bases in a gene is changed.

  A mutation in the gene's coding DNA can affect the phenotype of an organism.

  If the amino acid sequence is altered, the activity of the protein produced may also be altered.
• Result of mutations


  The bigger the change to the protein , the larger the effect it has on how the body works.

  A mutation may cause:

  • a large change in the protein produced
  • a small change in the protein
  • no change at all in the protein produced
• Human genome project


  advantages:

  • alerting people that they are at risk of certain diseases. The person may be able to make lifestyle changes to reduce the chances of the disease developing
  • distinguishing between different forms of diseases as some drugs are beneficial to some forms and not others
  • allowing doctors to tailor treatments for some diseases to the individual, where some alleles affect the response to it
• Human genome project


  The human genome project is a collaboration between scientists to decode the human genome and the results are being used to develop new medicines and treatments.

  disadvantages:

  • people who are at risk of certain diseases, eg cancer, may have to pay more for life insurance
  • it may not helpful to tell someone they are at risk for a condition which has no cure.
• DNA extraction


  Steps:

  1. mash organisms - breaks open the cell wall
  2. add detergent - breaks up the cell membrane and the nuclear membrane
  3. add protease - an enzyme that breaks down proteins holding the chromosome together
  4. add ice cold ethanol slowly at an angle - DNA is insoluble in ethanol so it precipitates out
  5. white thin fibres can be seen in ethanol
• Mendel


  He observed many characteristics of pea plants which were not a blend - present or absent. He bred dwarf plants with tall plants.

  First generation$^{ }$

  • only tall pants
  • shows this is the dominant allele

  Second generation

  • 1/4 of the plants were dwarf plants - recessive
• DNA


  A molecule of DNA has 2 strands in a double helix which are joined by 2 bases.

  Each bases is attached to a sugar which is attached to a phosphate group - make up the backbone of DNA.

  The bases have a weak electrical charge - the positively charged attract the negatively charged. This attraction is called hydrogen bonds.

  C and G have 3 hydrogen bonds.

  A and T have 2 hydrogen bonds.
• Sexual reproduction


  Advantages:

  • there is variation and so offspring may be better suited and more likely to survive if conditions change

  Disadvantages:

  • it is slower as they have to find a mate and then lay their eggs/ carry the offspring
• Asexual reproduction


  Advantages:

  • it is quicker as they don't need to find a mate

  Disadvantage:

  • there is no variation so if conditions change it can affect the whole population
• Inheritance


  Punnet squares are used to work out the probabilities of different phenotypes caused by alleles.

  A family pedigree chart shows how genotypes and their resulting phenotypes are inherited in families.
• Alleles (homozygous and heterozygous)

  If the alleles are the same, the organism is homozygous for that gene. If the alleles are different, the organism is heterozygous for that gene. Fertilisation forms a diploid zygote, if a heterozygous zygote is formed often only one characteristic is shown. This allele is said to be dominant and the other recessive.

  The alleles in an organism are its genotype.

  What an organism looks like is its phenotype.