Polypeptide Synthesis – Translation

Once mRNA has passed out of the nuclear pore, it determines the synthesis of a polypeptide.


  1. A ribosome attaches to the starting codon, at one end of an RNA molecule.
  2. The tRNA molecule with a complementary anticodon sequence moves to the ribosome,  and pairs up with the sequence on the mRNA.
  3. A tRNA molecule with a complimentary anticodon pairs up with the next codon on the mRNA.
  4. The ribosome moves along the mRNA, bringing together two tRNA molecules, each pairing up with the corresponding two codons on the mRNA.
  5. By means of an enzyme, ATP and and two amino acids on the tRNA are joined by a peptide bond.
  6. The ribosome moves onto the third codon, on the mRNA linking the amino acids.
  7. As this happens the first tRNA is released from its amino acid, from the amino acid pool in the cell.
  8. This process continues until a complete polypeptide chain is created.

Polypeptide synthesis – Transcription and Splicing


Transcription is the process of making pre-mRNA using part of the DNA as a template.


  1. The Enzyme DNA Helicase acts on a specific region of the DNA molecule, this cases the hydrogen bonds to break between base pairs. this cause the two strands of DNA to separate, and expose the nucleotides.
  2. The enzyme RNA Polymerase then moves along the two strands of DNA, these are know as the template strands, and cause the nucleotides on the template strand to join to individual complimentary nucleotides in the nuclear pool.
  3. In this way, exposed Guanine bases are linked to the free Cytosine bases in the nuclear pool. in this way, cytosine links to guanine, thymine joins to adenine, and adenine joins to uracil.
  4. As the RNA polymerase adds the nucleotides one at a time to build a strand of of pre-mRNA, the DNA strands rejoin behind it. As a result, only approx. 12 base pairs on the DNA are exposed at any one time.
  5. when the RNA polymerase reaches a certain section of bases on the DNA strand, it recognises a stop triplet code, and detaches. the production of pre-mRNA is complete.



DNA is made up of sections called exons and introns.Exons code for proteins, and introns do not. In Eukaryotic cells, these non-functioning introns are removed. This process is called splicing. once the introns have been removed, the exons can be rejoined into a variety of combinations. a single strand of DNA can code for dozens of proteins. Disease such as alzheimer’s are caused by splicing failures.

The Structure of Ribosomes

A ribosome is a biological molecule made of RNA and proteins. The structure of a ribosome is complex, and it is responsible for making the millions of proteins that are needed by cells. 

A ribosome may be located in many places within the cell. Some are in the cytosol, others are bound to cellular membranes. Membrane-bound ribosomes are responsible for the characteristic roughness of the endoplasmic reticulum when seen under a microscope. 

A ribosome is made of two pieces (subunits). These two subunits are named according to their ability to sediment on a special gel. The bigger the number given to the subunit the bigger the molecule. A typical eukaryotic cell ribosome consists of two subunits named 60S (large subunit) and 40S (small). Image