3.5 Transcription and Translation

3.5.1  Compare the structure of DNA and RNA

3.5.2  Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase

Transcription is the process by which an RNA sequence is produced from a DNA template:

  • RNA polymerase separates the DNA strands and synthesises a complementary RNA copy from one of the DNA strands
  • It does this by covalently bonding ribonucleoside triphosphates that align opposite their exposed complementary partner (using the energy from the cleavage of the additional phosphate groups to join them together)
  • Once the RNA sequence has been synthesised, RNA polymerase will detach from the DNA molecule and the double helix will reform
  • The sequence of DNA that is transcribed into RNA is called a gene
  • Transcription occurs in the nucleus (where the DNA is) and, once made, the mRNA moves to the cytoplasm (where translation can occur)

Three main types of RNA are predominantly made:

  • Messenger RNA (mRNA):  A transcript copy of a gene used to encode a polypeptide
  • Transfer RNA (tRNA):  A clover leaf shaped sequence that carries an amino acid
  • Ribosomal RNA (rRNA):  A primary component of ribosomes


3.5.3  Describe the genetic code in terms of codons comprised of triplets of bases

The genetic code is the set of rules by which information encoded in mRNA sequences is converted into proteins (amino acid sequences) by living cells

  • Codons are a triplet of bases which encodes a particular amino acid
  • As there are four bases, there are 64 different codon combinations (4 x 4 x 4 = 64)
  • The order of the codons determines the amino acid sequence for a protein 
  • The coding region always starts with a START codon (AUG) and terminates with a STOP codon

                                                                                              The Genetic Code

The genetic code has the following features:

  • It is universal - every living thing uses the same code (there are only a few rare and minor exceptions)
  • It is degenerate - there are only 20 amino acids but 64 codons, so more than one codon may code for the same amino acid (this allows for silent mutations whereby a change in the DNA sequence does not affect the polypeptide sequence)

3.5.4  Explain the process of translation, leading to polypeptide formation

Translation is the process of protein synthesis in which the genetic information encoded in mRNA is translated into a sequence of amino acids in a polypeptide chain

  • Ribosomes bind to mRNA in the cell's cytoplasm and move along the mRNA molecule in a 5' - 3' direction until it reaches a start codon (AUG)
  • Anticodons on tRNA molecules align opposite appropriate codons according to complementary base pairing (e.g. UAC will align with AUG)
  • Each tRNA molecule carries a specific amino acid (according to the genetic code)
  • Ribosomes catalyse the formation of peptide bonds between adjacent amino acids (via a condensation reaction)
  • The ribosome moves along the mRNA molecule synthesising a polypeptide chain until it reaches a stop codon, at this point translation stops and the polypeptide chain is released

The Process of Translation

3.5.5  Explain the relationship between one gene and one polypeptide

  • A gene is a sequence of DNA which encodes a polypeptide sequence
  • A gene sequence is converted into a polypeptide sequence via the processes of transcription (making an mRNA transcript) and translation (polypeptide synthesis)
  • Translation uses tRNA molecules and ribosomes to join amino acids into a polypeptide chain according to the mRNA sequence (as read in codons) 
  • The universality of the genetic code means all organisms show the same relationship between genes and polypeptides (indicating a common ancestry and allowing for transgenic techniques to be employed)
  • Some proteins may consist of a number of polypeptide chains and thus need multiple genes (e.g. haemoglobin consists of four polypeptide subunits encoded by two different genes)
  • When a gene is mutated it may lead to the synthesis of a defective polypeptide, hence affecting protein function

The 'One Gene - One Polypeptide' Rule

There are two exceptions to the 'one gene - one polypeptide' rule:

  • Genes encoding for tRNA and rRNA do not code for polypeptide sequences (only mRNA sequences code for polypeptides)
  • A single gene may code for multiple polypeptides if alternative splicing occurs (the removal of exons as well as introns)