DNA, genes and chromosomes

• Gene


  A base sequence of DNA that contains a code for

  • The amino acid sequence of a polypeptide
  • A functional RNA- including ribosomal RNA and tRNA

  
• Locus

  • The fixed position/ location of a particular gene on a chromosome/ DNA molecule

  
• Allele


  - a different version of the same gene

  
• Chromosome


  - threadlike structure

  - composed of tightly coiled DNA wrapped around histones (if it is a eukaryotic cell)

  
• Homologous chromosome


  - a pair of chromosomes that have the same genes

  - therefore, they are the same size

  
• Eukaryotic DNA


  - DNA is stored as chromosomes inside the nucleus

  - linear chromosomes

  - DNA is tightly coiled and wrapped around proteins called histones
• Prokaryotic DNA


  - DNA molecules are shorter and circular

  - DNA is not wound around histones. Instead, it supercoils to fit in the cell
• Codon


  - 3 bases on mRNA that code for an amino acid
• Start codon


  - AUG (methionine)

  - 3 bases at the start of an mRNA sequence which help to initiate translation
• Stop codon


  - 3 bases at the end of every gene that do not code for an amino acid

  - this causes ribosomes to detach

  - and therefore stops translation
• Genetic code


  - an amino acid is coded for by 3 DNA bases which are described as the "triplet code"
• What is meant by 'the genetic code is degenerate??

  - each amino acid is coded for by more than one triplet of bases
• What is the advantage of the genetic code being degenerate?

  - if a substitution mutation occurs, the new triplet of bases may still code for the same amino acid

  - therefore the mutation will have no impact on the final protein produced
• What is the advantage of the genetic code being universal?

  - genetic engineering is possible

  - a human gene can be inserted into another organism

  - e.g human gene for insulin inserted into bacteria to make insulin
• What is the advantage of the genetic code being nonoverlapping?

  • If a point mutation occurs, it will only affect one codon and therefore one amino acid
• Triplet code


  • 3 bases on DNA which code for one amino acid
• Mutation


  • A change in the DNA base sequence
  • Can be a gene or chromosome mutation
• What is meant by 'the genetic code is universal?

  • The same triplet of bases codes for the same amino acid in all organisms
• What is meant by 'the genetic code is non-overlapping?

  • Each base in a gene is only part of one triplet of bases that codes for one amino acid
  • Therefore each codon is read as a discrete unit
• Introns


  • DNA base sequences which DO NOT code for the sequence of amino acids in a polypeptide chain
• Exons

  • DNA base sequences that code for the sequence of amino acid in a polypeptide chain
• What is splicing?

  • Post-transcription modification
  • Involved removing introns - non-coding regions of DNA
  • Done by a spliceosome

  
• Genome


  • The complete set of genes in a cell
• Proteome


  • The full range of proteins that a cell is able to produce
• Non coding repeats of of base sequences (introns) are BETWEEN genes
• Within a gene - exons (code for amino acid sequence in polypeptide) are SEPERATED by one or more introns (non-coding DNA)