DNA and Genetics

• Nucleic acid structure

  Nucleic acids are long chains (polymers) of lots of nucleotide monomers joined together by phosphodiester bonds. Each nucleotide is made up of three components:

  1. Pentose sugar (either deoxyribose or ribose),
  2. Nitrogenous base (guanine, cytosine, adenine,thymine or uracil)

  3. Phosphate
• Image of Nucleic acid structure:

  
• DNA

  Two nucleic acid strands bonded together by complementary base pairing, the strands twisted around each other to form a double helix
• DNA strands

  • Have the same sequence running in opposite directions (anti-parallel)
  • Bonding between bases is predictable
• Guanine-Cytosine base pair


  Held together by three hydrogen bonds
• Adenine-Thymine (in DNA) or Adenine-Uracil (in RNA) base pair


  Held together by two hydrogen bonds
• Image of DNA pairings:

  
• DNA molecule

  • Contains deoxyribose sugar
  • Double-stranded
• RNA molecule

  • Contains ribose sugar
  • Single-stranded
  • Contains uracil instead of thymine
• DNA and RNA synthesis

  • Nucleotides are connected through the formation of phosphodiester bonds
  • Catalysed by DNA polymerase or RNA polymerase
  • Breaking phosphodiester bonds requires the addition of a water molecule (hydrolysis reaction)
• Types of RNA

  • Messenger RNA (mRNA)
  • Transfer RNA (tRNA)
• Messenger RNA (mRNA)

  • Produced during transcription
  • Carries the genetic code from the nucleus to the cytoplasm
  • Provides instructions for making a protein on the ribosome in translation
  • Made up of triplets of bases called codons
• Transfer RNA (tRNA)

  • Carries amino acids to the ribosome during translation
  • Contains an amino acid binding site at one end and an anticodon at the opposite end
  • Anticodons bind to complementary codons on mRNA to convert the mRNA sequence into a protein's primary sequence
• Protein synthesis


  1. DNA is converted into messenger RNA in transcription

  2. mRNA moves into the cytoplasm and binds to a ribosome

  3. mRNA is used to synthesise a protein in the process of translation
• Transcription


  1. RNA polymerase binds to the promoter region

  2. RNA polymerase separates the DNA strands

  3. RNA polymerase adds complementary nucleotides and connects them through phosphodiester bonds

  4. RNA polymerase reaches a stop codon

  5. mRNA molecule formed
• Promoter region


  Regulatory region which does not code for amino acids but facilitates the process of transcription by helping RNA polymerase bind to the gene
• Template strand


  The DNA strand that RNA polymerase uses as a template to synthesize the mRNA
• Coding strand


  The DNA strand that has an identical sequence to the newly synthesised RNA, except for the presence of thymine instead of uracil
• Stop codon


  A codon which does not code for an amino acid but tells the enzyme to stop transcribing
• Messenger RNA (mRNA)


  The molecule formed during transcription that leaves the nucleus and enters the cytoplasm
• Transcription:

  
• Translation


  1. Messenger RNA finds its way to ribosomes

  2. Ribosome attaches to RNA

  3. Ribosome reads mRNA in 3-base codons

  4. Transfer RNA (tRNA) carries amino acid to ribosome

  5. Ribosome catalyses peptide bond formation between amino acids to form polypeptide
• Codon


  A series of three bases in mRNA (e.g. AUG, CCA, GCU) that corresponds to a particular amino acid
• Anticodon


  The complementary base sequence to the mRNA codon, found on the tRNA molecule
• tRNA molecule


  • Has a clover-shaped structure formed by a single RNA strand folded over on itself through hydrogen bonding
  • Has an amino-acid binding site at one end
  • Has an anticodon at the other end
• Translation:

  
• Genetic code


  Can be described in a number of ways
• Ways the genetic code can be described


  • Triplet code
  • Non-overlapping
  • Degenerate
• Triplet code


  • Three nucleotide bases make up a codon, which code for a particular amino acid
• Non-overlapping code


  • The codons do not overlap. Once the ribosome has 'read' one codon and the appropriate amino acid has been recruited, the ribosome moves onto a new codon
• Degenerate code


  • Different codons can code for the same amino acid. For example, the codons CUU and CUC both code for the amino acid leucine. This means that some mutations will have no effect on the organism since the same protein will still be produced