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What is gene expression?

picture of gene expression

Gene expression is the process our cells use to convert the instructions in our DNA into a functional product, such as a protein.

  • Our DNA stores the information our cells need to function. It is organised into small sections, called genes, which contain instructions for making a specific product, usually a protein.
  • Proteins carry out most of the active functions of a cell, such as protecting against disease or absorbing nutrients from food.
  • The conversion of genes into their functional products is called gene expression. It’s a tightly regulated process that enables a cell to respond to its changing environment by making more or less of a specific protein.

 

The steps of gene expression

 

There are two key steps involved in making a protein: transcription and translation.

 

Transcription

  • The first step of gene expression is called transcription. During transcription, the DNA of a gene is transcribed to produce an RNA transcript, called messenger RNA (mRNA).
  • RNA is chemically similar to DNA, but only has a single strand of bases, rather than two. And while DNA has the base thymine (T), RNA has a base called uracil (U).
  • Transcription takes place inside the nucleus, by an enzyme called RNA polymerase. Once the gene has been transcribed, the mRNA leaves the nucleus and delivers the message to the ribosomes – the cell’s protein factories.

 

An illustration showing the process of transcription - where the DNA of a gene is transcribed into messenger RNA (mRNA).
Image credit: Laura Olivares Boldú / Wellcome Connecting Science

 

Translation

  • The second step of gene expression is called translation. During translation, the mRNA is converted into a string of amino acids – the basic building blocks of a protein.
  • When the mRNA reaches the ribosomes, it is read by a carrier molecule, called transfer RNA (tRNA).
  • The tRNA reads the mRNA in chunks of three bases, known as a codon, which encodes one amino acid. For example, the codon ‘GGU’ codes for the amino acid glycine.
  • The tRNA temporarily binds to the mRNA codon and delivers the corresponding amino acid to the ribosome.
  • The ribosome binds the amino acids together, producing a long chain called a polypeptide.
  • This process continues until the ribosome reaches a ‘stop’ codon - a group of three bases that signal the end of the protein.

 

An illustration showing the process of translation - where the mRNA is translated into amino acids, creating a protein.
Image credit: Laura Olivares Boldú / Wellcome Connecting Science

There are 64 ways that the four bases A, U, C and G can be arranged in multiples of three and form a codon. But there are only 20 amino acids. This is because more than one codon can specify the same amino acid. For example, the codons ‘GGU’ and ‘GGC’ both code for glycine.

In the 1950s, James Watson posited that genetic material must flow from DNA in the direction of the functional product. This sequence is critical for life to function – but it’s not the only direction that genetic information can flow. Find out more on our Central Dogma fact page.