inheritance, variation and evolution

• what does DNA stand for?

  Deoxyribonucleic acid
• DNA is a polymer made up of two strands coiled together in the shape of a double helix
• a gene is a small section of DNA that codes for a specific protein.
• only 20 amino acids are used but can make up thousands of different proteins
• what is a genome?

  an entire set of genetic material in an organism
• DNA strands are polymers made up of lots if repeating units called nucleotides
• each nucleotide consists of a sugar, a phosphate group and one 'base'
• the sugar and phosphate groups in the nucleotides form a 'backbone' to the DNA strands. The sugar and phosphate groups alternate
• one of four different bases - A , T , C or G - joins to each sugar, each base links to a base on the opposite strand in the helix
• What base doe A pair with?

  T
• what base does C pair up with?

  g
• the order of the bases in the gene decide the order of amino acids in a protein
• proteins are made in the cell cytoplasm on the ribosomes
• to make proteins, ribosomes use the code in the DNA
• the code is moved from the DNA to the ribosome by a molecule called mRNA
• mRNA is made by copying the code from DNA
• the mRNA acts as a messenger between the DNA and the ribosome
• the correct amino acids are brought to the ribosomes in the correct order by carrier molecules
• examples of types of proteins:

  • enzymes
  • hormones
  • structural proteins
• mutations change the sequence of the DNA bases in a gene, which produces a genetic variant
• most mutations have very little or no effect on the protein, however some can seriously effect a protein
• insertions are where a new base is inserted into the DNA base sequence where it shouldn't be. An insertion changes the way groups of bases are 'read' which can change the amino acids they code for
• deletions are when one or more bases are deleted from the DNA base sequence. This also alters how groups of bases are read, changing the amino acids that are coded for
• substitutions occur when one base is replaced with another. this will alter the amino acid produced
• sexual reproduction is where genetic information from two organisms is combined to produce offspring which are genetically different to either parent
• in sexual reproduction, the mother and father produce gametes by meiosis
• in humans, each gamete contains 23 chromesomes
• sexual reproduction involves the fusion of male and female gametes. Because there are two parents, the offspring contain a mixture of their parents genes
• in asexual reproduction there is only one parent so the offspring are genetically identical to that parent
• asexual production happens by mitosis - an ordinary cell makes a new cell by dividing in two
• in asexual reproduction the offspring are clones of the parent
• what are the Stages of meiosis?

  1. cell duplicates its genetic information, forming two armed chromosomes
  2. in the first division the chromosome pairs line up in the centre of the cell
  3. these pairs are then pulled apart so each new cell only has one copy of each chromosome
  4. in the second division, the chromosomes line up again and are pulled apart
  5. you get four gametes, each with only a single set of chromosomes in it. each gamete is genetically different from the others
• after two gametes have fused during fertilisation, the resulting new cell divides by mitosis to make a copy of itself, this repeats many times to produce lots of new cells in an embryo
• advantages of sexual reproduction

  • variation increases the chance of species survival
  • individuals are better adapted to environments
  • can use selective breeding to speed up natural selection
• advantages of asexual reproduction

  • only needs one parent
  • uses less energy
  • faster than sexual reproduction
  • offspring can be produced in favourable conditions
• males have an X and a Y chromosome
• females have two X chromosomes
• some characteristics are controlled by a single gene, but most characteristics are controlled by several genes interacting
• all genes exist in different versions called alleles
• if an organism has two alleles for a particular gene that are the same, the it is homozygous for that trait