Introduction

There are three types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA), each with specific functions (see module 2.1). RNA is important in protein synthesis, where cells express genes by converting the genetic message into protein. The process of protein synthesis occurs in two stages: transcription (DNA is copied to RNA) and translation (RNA is used to produce proteins).

Transcription and translation

Transcription: During gene expression in the nucleus, the base sequence of DNA is copied (transcribed) into mRNA (messenger RNA) ina series of stages:

RNA polymerase moves along the DNA and unwinds the strand
Hydrogen bonds between base pairs break which allows the double helix to be unzipped
As RNA polymerase breaks the bonds, it synthesises a primary transcript of mRNA using RNA nucleotides. These form hydrogen bonds with the exposed DNA strand by complementary base pairing
The primary transcript of mRNA is processed to produce a mature transcript of mRNA
The mature mRNA transcript leaves the nucleus and travels to the ribosome

Translation: The mRNA molecule travels through the cytoplasm and attaches to the ribosome where proteins are synthesised from amino acid building blocks

tRNA molecules transport specific amino acids to the ribosome
Each mRNA codon codes for a specific amino acid
The first codon of an mRNA molecule is a start codon which signals the beginning of translation
The anti-codons and codons match up and form complementary base pairs
Peptide bonds form between the adjacent amino acids to form the polypeptide (protein) (catalysed by rRNA)
Used tRNA molecules exit the ribosome and collect another specific amino acid
The last codon of an mRNA molecule is a stop codon which signals the end of translation

After translation, the polypeptide chain can be altered by cutting and combining polypeptide chains or by adding phosphate or carbohydrate groups to the protein. In order to function, proteins must fold into the correct three-dimensional shape and be targeted to the correct part of the cell.

Watch this video to help you understand the process of protein synthesis through transcription of DNA and translation of RNA.