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

The Structure of mRNA, tRNA and DNA


DNA structure

In 1953, James Watson and Francis Crick, working at the Cavendish Institute in Cambridge put forward their model for the structure of DNA. DNA is a polymer of nucleotides.Nucleotides are made up of:


Sugar (deoxyribose).

And a base (either adenine, guanine, thymine or cytosine).

DNA is a macromolecule polymer made of subunits called nucleotides. The nucleotides are arranged in two chains which are coiled into a spiral shape called a double helix.


RNA Structure

Above shows a messenger RNA molecule, it consists of thousands on mononucleotides. mRNA is a single helical molecule, it is manufactured when DNA forms a mirror copy of one of its two strands. Once formed, mRNA leaves the nucleus via pores in the nuclear envelope. It then enters the cytoplasm, where it associates with ribosomes. Here, it acts as a template where upon where proteins are built.

mRNA is suited to function as it possesses all of the correct sequences of numerous triplets of organic bases that code for polypeptides. mRNA can also be easily broken down, and only exists while it is need to manufacture a specific protein.


tRNATransfer RNA is a small molecule made up of approximately 80 nucleotides. It is a single strand chain folded into a clover-leaf shape, with one end extending beyond the other. This is the part of the tRNA molecule where amino acids attach. There are several types of RNA, each able to carry a single amino acid. At the other end of the tRNA molecule there is a sequence of three organic bases, this is known as the anti-codon. for each amino acid the anti-codon differs.

In RNA the base pair Thymine is replaced with Uracil, therefore:

A binds to U

C binds to G.

During protein synthesis the anti-codon pairs with the three complimentary organic bases that make the triplet bases on mRNA. The tRNA structure is structurally suited to lining up the amino acids on the mRNA template during protein synthesis.