3.4.1 DNA, genes and chromosomes

• A DNA molecule is a double helix structure made up of two polynucleotide strands held together by hydrogen bonds. They are made of monomers called nucleotides made of condensation reactions which form phosphodiester bonds. A nucleotide contains a phosphate group, nitrogen-containing base and deoxyribose sugar
• RNA - ribonucleic acid

  • Transfers genetic information from the nucleus to ribosomes
  • Single stranded and shorter than DNA
  • Has a ribose sugar
  • Contains uracil instead of thymine
• mRNA

  • Single stranded
  • Shorter than DNA
  • Contains uracil instead of thymine
  • Contains ribose instead of deoxyribose
  • Linear
  • Codons
• tRNA

  • Same structure as mRNA
  • Has amino acid attachment site
  • is single stranded but has double stranded sections
  • folded into a cloverleaf shape
  • Contains hydrogen bonds
  • Anticodons
• Gene


  A base sequence on a section of DNA that codes for a polypeptide or functional RNA molecule
• Functional RNA molecules


  rRNA

  tRNA
• A gene occupies a specific position on a DNA molecule on a chromosome called a locus
• In eukaryotic cells much of the DNA is non-coding sections called introns. Between genes and within genes, the coding sections are called exons
• In prokaryotic DNA there are no introns as all of it is coding section
• Genome

  complete set of genes in a cell
• Proteome

  Full range of proteins that a cell can produce
• The genetic code -

  • 1 DNA strand is the coding strand, the other will be non-coding
  • A set of bases called a triplet codes for 1 amino acid
  • Is universal
  • Is non-overlapping
  • Is degenerate
• Universal

  Triplets code for the same amino acid in every species
• Non-overlapping 


  Triplets must be read in sequence
• Degenerate

  Most amino acids are coded for by more than one triplet