Protein synthesis can be broken down into two main steps, transcription and translation.
Transcription
Transcription is the process by which an RNA copy of a gene is created in the nucleus of a cell.
Hydrogen bonds between the DNA strands of a gene break and the the DNA unwinds at this point. The DNA is left separated at the gene. This process involves an enzyme called RNA polymerase.
One of the strands becomes a template strand (also called the antisense strand) so that the RNA copy can be created. The other strand is called the sense strand and is not transcribed.
Free RNA mononucleotides line up alongside the antisense strand by complementary base pairing. Hydrogen bonds form between the bases. RNA bases pair up as follows:
- Adenine pairs with Uracil
- Cytosine pairs with Guanine
With the assistance of RNA polymerase bonds form between the mononucleotides to form a strand of RNA that that is a reverse copy of the antisense strand (the only difference being thymine is replaced by uracil).
This molecule of RNA is called mRNA which stands for messenger RNA. Once formed the hydrogen bonds between the bases break and it is released from the DNA. It leaves the nucleus through one of the nuclear pores and is transported to a ribosome for translation.
Hydrogen bonds reform between the bases of the DNA molecule and it re-coils back to its original double helix structure.
Translation
Translation is the process by which amino acids are brought together to form a polypeptide chain – the primary structure of a protein.
The mRNA molecule created in the nucleus attaches itself to a ribosome.
Three bases on the mRNA molecule make up what is known as a codon.
Codon – a sequence of three bases that correspond to a specific amino acid.
Transfer RNA (tRNA) molecules carry amino acids to the mRNA. The structure of tRNA is shown below:
tRNA molecules are composed of a single polynucleotide strand. At one end are three specific bases (an anticodon) and at the other end is a binding site for a specific amino acid.
Anticodon – a sequence of three bases that is complementary to an mRNA codon.
A tRNA molecule that has complementary bases to the first codon attaches itself to the mRNA by hydrogen bonding. Another tRNA molecule does the same with the second codon.
The two amino acids are then joined together by the formation of a peptide bond. Once the bond has formed the first tRNA molecule leaves the ribosome detaching itself from the amino acid and the mRNA.
The ribosome moves one codon along and a third tRNA molecule binds to the mRNA.
A peptide bond forms between the second and third amino acid and then the second tRNA molecule leaves the ribosome.
This process continues until a stop codon is reached. Once complete the polypeptide chain (protein) moves away from the ribosome. Translation is complete.
After translation the polypeptide chain may undergo further folding or join with other polypeptides to form its final structure.
Please let me know if you have any questions in the comments below or if you have any requests for topics that aren’t currently covered on The Science and Maths Zone. If you would like to stay up to date with the latest posts follow me on Instagram or Pinterest. To search for more posts use the search bar at the bottom of the page or click on one of the following categories: