3.1.8.1 = THE GENETIC CODE DNA IS UNIVERSAL, NON-OVERLAPPING & DEGENERATE: nature of genetic code

3.1.8.1 = THE GENETIC CODE


DNA IS UNIVERSAL, NON-OVERLAPPING & DEGENERATE: nature of genetic code


Genetic code: the sequence of DNA triplets bases (or mRNA codons) that code for a specific amino acid.


triplet refers to DNA whereas a codon refers to RNA


The nature of genetic code:


  • Triple code / 3 bases to each code (each triplet of bases codes for one amino acid)

  • DNA triplet: sequence of 3 bases coding for specific amino acid, E.g. UAU codes for tyrosine


  • Universal: The same specific DNA base triplets codes for the same amino acids in all living organisms e.g. UAU codes for tyrosine in all organisms

  • Non-overlapping: each DNA triplet or mRNA codon contains three bases that codes for a unique amino acid. DNA is discrete; each base from any triplet / codon gene is separate from each other and can only be used once and in only one triplet

  • Degenerate: The same amino acid can be coded for by more than one base triplet, e.g. tyrosine can be coded for by UAU or UAC

  • Codon: sequence of three bases (triplet)


Protein synthesis: 


Transcription (like a scribe): DNA (nucleotides) → mRNA (nucleotides) 

Translation (translating from one language to another): mRNA (nucleotides) → amino acids / protein / polypeptides (amino acids) 


3.1.8.2 = POLYPEPTIDE SYNTHESIS


Protein synthesis overview: 2 stages


  1. Transcription; Production of mRNA from DNA

  • Nucleus 

  1. Translation; Production of polypeptides form the sequence of codons carried by mRNA

  • Cytoplasm on ribosomes


Messenger RNA (mRNA)


  • Made by transcription in the nucleus

  • Acts as a template for translation in the cytoplasm

  • Sequence of bases on RNA determines sequence of amino acids in polypeptide chain

  • Straight chain molecule

  • Sequence of bases on RNA determined by sequence of bases on DNA

    • Triplet code = codon

  • Chemically unstable

    • So breaks down after a few days


Transfer RNA (tRNA)


  • Involved in translation

  • Carries an amino acid

    • Amino acid binding site

  • Anticodon = 3 bases

    • Anticodon bases complementary to mRNA codon

  • Each tRNA specific to one amino acid, specific to its anticodon

  • Single polynucleotide strand

    • Folded - 3 hairpin loops = three-leafed clover shape

    • Hydrogen between specific base pairs holds molecule in this shape


Similarities / difference between structure of mRNA and tRNA molecules


  • Similarities

    • Both single polynucleotide strand

  • Differences

    • mRNA single helix / straight, whereas tRNA folded into clover shape

    • mRNA is longer, variable length, whereas tRNA is shorter

    • mRNA contains no paired bases or hydrogen bonds, whereas tRNA has some paired bases and hydrogen bonds


Role of ATP, tRNA and ribosomes in translation


  • ATP

    • Hydrolysis of ATP, to ADP + Pi, releases energy

    • For the bond between the amino acid and its corresponding tRNA molecule

      • Amino acid attaches at amino acid binding site 

    • For peptide bond formation between amino acids


  • tRNA

    • tRNA attaches to and transports a specific amino acid, in relation to its anticodon

    • tRNA anticodon complementary base pairs to mRNA codon, forming hydrogen bonds

    • Two tRNAs bring amino acids together for the formation of a peptide bonds

    • About 60 types of tRNAS to carry 20 different amino acids

      • Genetic code is degenerate


  • Ribosomes

    • Attaches to mRNA and houses tRNA, allowing codon-anticodon complementary base pairing

    • Allows peptide bonds to form between amino acids


Relating the base sequence of nucleic acids to the amino acid sequence of polypeptides, when provided with suitable data about the genetic code


  • tRNA anticodons are complementary to mRNA codon

    • Eg mRNA codon = ACG; tRNA anticodon = UGC

  • mRNA sequence of bases / codons are complementary to sequence of bases / triplets on DNA template strand

    • Eg mRNA base sequence = ACG UAG AAC; DNA base sequence = TGC ATC TTG

  • In RNA, uracil replaces thymine

  • You may then have to relate this to amino acid sequences


Interpreting data from experimental work investigating the role of nucleic acid in protein synthesis


Protein synthesis: transcription


  • In nucleus 

  • DNA double helix unzipped by DNA Helicase 

  • Breaking the hydrogen bonds 

  • Forming 2 template strands

  • Free RNA nucleotides attach to the template strand’s exposed nucleotides

  • By complementary base pairing (U replaces T in RNA)

  • H-bonds reform

  • RNA polymerase joins adjacent mRNA nucleotides together in a condensation reaction forming phosphodiester bonds to form premRNA

  • When RNA polymerase reaches stop codon, mRNA (prokaryotes) and pre-mRNA (eukaryotes) detaches from DNA


Post transcriptional modification:


  • Eukaryotic genes = pre-mRNA (messenger) containing exons and introns.

  • Exons = coding regions

  • Introns = non-coding regions

  • pre-mRNA goes into splicing

  • Introns removed

  • Exons spliced together in different combos for different proteins and exported

  • Leaving just the exons and forming mRNA

  • mRNA now leaves nucleus via nuclear pore for translation in cytoplasm


Prokaryotes vs Eukaryotes


  • Prokaryotes → transcription :DNA → mRNA 

    • No introns

    • No splicing → mRNA produced directly from DNA

  • Eukaryotes: DNA → premRNA → mRNA

    • Introns are removed (splicing) to form mRNA


Protein synthesis - translation


Codon: triplet of bases on mRNA that codes for a specific amino acid

Anticodon: triplet of bases on tRNA that is complementary to codon on mRA


  • Sequence of mRNA codons determines sequence of amino acids

  • tRNAS carry specific amino acids, in relation to their anticodon

  • At the ribosome, tRNA codon binds to mRNA codon at a start codon 

    • tRNA anticodon attach to mRNA codon  by complimentary base pairing

    • Hydrogen bonds formed

    • First codon = start codon

    • Ribosome holds tRNA in place

  • Two amino acids joined by condensation, forming a peptide bond

    • Using energy from ATP

  • tRNA detaches (without its amino acid), ribosome moves along mRNA to next codon

  • Continues until stop codon (polypeptide released)


ATP has two roles in translation. It is required to provide energy to attach amino acids to tRNA and also to attach amino acids together.


3.1.8.3 = PROTEIN FOLDING


Protein folding: process by which a polypeptide folds into its characteristic three-dimensional structure.


Protein folding is determined by the amino acid sequence of the polypeptide. Specialised proteins, called

chaperones, assist in the folding of other proteins.


Students should be able to:

• relate base sequences of nucleic acids to the amino acid sequence of a polypeptide when provided with suitable information relating to the genetic code.


Students will not be required to recall in written papers specific codons and the amino acids for which they code.


  • Primary structure of polypeptide is coiled / folded to produce secondary structure

  • Interactions between R-groups further coil / fold polypeptide to produce tertiary structure (3D shape)

  • Different polypeptide chains + non-protein groups link to form functional proteins

  • This is a quaternary structure


*** Some amino acids have neutral non-polar R groups that are hydrophobic, hydrophobic parts of a molecule interact with each other


The sequence of amino acids position of the R groups in the polypeptide determines how a protein is folded.


  • Disulphide bonds (eg in cystine with another cystine)

  • Acidic / basic R groups form ionic bonds

  • Hydrophobic R groups aggregate together / join other hydrophobic molecules → protein is mis-folded → non functional → build up of misfolded protein can cause disease eg alzheimer's disease


Chaperone proteins: ensure polypeptide chains synthesised on ribosomes fold correctly. Some found in endoplasmic reticulum → facilitate folding and assemlby of membrane and sexeerory proteins 


  • E.g. HSP 70. Bind to hydrophobic regions on polypeptides as they are being formed and prevent incorrect hydrophobic interactions occurring before polypeptide chain is completed

  • E.g. HSP 60: chaperonins, large cylindrical proteins with central compartment. Polypeptide chain fits into central compartment and isolated from ogre proteins molecules → prevented from interacting with them. Interaction between chaperonin and polypeptide enable the protein to fold correctly