A2 Edexcel Biology: Protein Synthesis

DNA molecules are found in the nucleus of the cell, but the ribosomes needed in protein synthesis are found in the cytoplasm.
The DNA is too large to move out of the nucleus.
Instead a section is copied into RNA.
This process is called transcription.
The RNA leaves the nucleus and joins with a ribosome, where it can be used to synthesise a protein.
This process is called translation.

There are 2 types of RNA:

2. Transfer RNA (tRNA):

This carries the amino acids to the ribosomes.
It has an amino acid building site at one end and a sequence of 3 bases at the other end called an anticodon.

RNA polymerase attaches to the DNA double helix at the beginning of a gene.
This causes the hydrogen bonds between the 2 DNA strands to break, separating the strands, and uncoiling the DNA molecule.
One of the strands is then used as a template to make an mRNA copy, the strand is called the antisense strand.
The RNA polymerase lines up free RNA nucleotides alongside the template strand.
Complementary base pairing means that the mRNA strand is a reverse copy of the antisense strand.
Except that the base T on the antisense strand is replaced by base U in RNA.
As soon as the RNA nucleotides have paired with their complementary bases, they're joined together, forming an mRNA molecule.
RNA polymerase moves along the DNA strand, separating the strands and assembling the mRNA strand.
The hydrogen bonds between the uncoiled strands of DNA re-form once the RNA polymerase has passed by and the strands coil back into a double helix.
RNA polymerase stops making mRNA and detaches from the DNA once is reaches a stop codon.
The mRNA then moves out of the nucleus through a nuclear pore and attaches itself to a ribosome in the cytoplasm.

Genes contain sections that don't code for amino acids, called introns.
All the sections that do code for amino acids are called exons.
During transcription, both introns and exons are copied into mRNA.
A process then occurs called splicing, this is when introns are removed and exons are joined forming mRNA strands.
This takes place in the nucleus.
The exons are then joined together in different orders to form different mRNA strands.
This means more than 1 amino acid sequence, and therefore, more than 1 protein, can be produced from 1 gene.
After splicing the mRNA leaves the nucleus for the next stage of protein synthesis.

The mRNA attaches itself to a ribosome.
A tRNA molecule carrying an amino acid, with an anticodon that's complementary to the 1st codon on the mRNA, attaches itself to the mRNA by complementary base pairing.
A 2nd tRNA molecule attaches itself to the next codon on the mRNA in the same way.
The 2 amino acids attached to the tRNA molecules are joined by a peptide bond.
The 1st tRNA molecule moves away, leaving its amino acid behind.
A 3rd tRNA molecule binds to the next codon on the mRNA.
Its amino acid binds to the first 2 and the 2nd tRNA molecule moves away.
This process continues, producing a chain of linked amino acids (a polypeptide chain), until there's a stop codon on the mRNA molecule.
The polypeptide chain (protein) then moves away from the ribosome.

A2 Edexcel Biology