DNA & Genes

• genome = full set of genes in a cell
• a gene is the code for a single polypeptide
• a chromosome carries the genetic code for a large number of polypeptides
• each gene has a specific position on a chromosome called a locus
• intron - DNA that does not code for the sequences of amino acids in a polypeptide
• exon - base sequence that codes for the sequence of amino acids in a polypeptide
• only eukaryotes have introns
• non-coding regions between genes are multiple repeated sequence of bases
• homologous chromosomes:

  • chromosomes in a cell that pair up during meiosis
  • contain the same genes at the same loci
  • may have different forms of the gene (alleles)
• proteome = full range of proteins made by a cell
• post-transcriptional modifications:

  • alternative splicing - e.g reordering exons -> increases the number of proteins made
• post-translational modifications:

  • the polypeptide chain is modified
  • modification happens in the golgi
• DNA comparison: prokaryotes

  • relative length - shorter (fewer genes)
  • shape of molecule - circular
  • number of DNA mols. per cell - one
  • association with proteins - not associated with proteins
  • non-coding DNA - absent
• DNA comparison: eukaryotes

  • relative length - longer (many genes)
  • shape of molecule - linear (chromosomes)
  • number of DNA mols. per cell - more than one
  • association with proteins - associated with histone proteins
  • non-coding DNA - introns + multiple repeated sequences
• how is a gene a code for a polypeptide?

  • the base sequence
  • read in triplets
  • determines the sequence of amino acids in a polypeptide
• DNA similarities: eukaryotes and prokaryotes

  • both have identical nucleotide structures
  • adjacent nucleotides joined together by phosphodiester bonds in both
  • chloroplast/mitochondria DNA have a similar structure to prokaryotic DNA
• homologous pair of chromosomes - two chromosomes that carry the same genes at the same loci
• difference in the base sequence in a gene may lead to to the production of a non-functional protein or enzyme
• nucleosomes are histone proteins with DNA wrapped around them
• in eukaryotic cells, DNA is wrapped around histone proteins

  • provides structural support for the chromosome
  • efficient storage