Science is complex. It’s understandable that scientists will develop a jargon, a specialized language of their discoveries. But this forms a frustrating and completely unnecessary barrier to public understanding. In this series of posts I explain fundamental processes in molecular biology, the better to breach that jargon-wall.
Gene expression is one of the most important processes in the biology of a cell. It is a two-stage process; first, a messenger RNA (mRNA) molecule is transcribed from the DNA double helix. Next, this mRNA molecule is translated into amino acids; the linked amino acids form proteins. How does the cell translate the information encoded in an mRNA molecule into a chain of amino acids?
Translating the Message
Transcription, which is the process of converting the information encoded in DNA into mRNA is easy to understand. The DNA acts as a direct template for the mRNA, and since both DNA and mRNA molecules are chemically and structurally similar it is easy to understand how this happens, as I described previously. It’s as if a handwritten note is transcribed into typed text; the language and the form of the message does not change. But protein molecules, made up of specific sequences of amino acids, are completely different from the nucleotide molecules that make up DNA and mRNA. The typewritten text has to be converted into a completely different format while retaining the same information; think of it as a movie script that is transformed into a movie. How is this “code” translated from mRNA to protein?
The Genetic Code
RNA molecules are made up of four different nucleotides linked together like a daisy chain; Adenine (A), Guanine (G), and Cytosine (C), and Uracil (U). Protein molecules are made up of twenty different amino acids. Therefore, the translation of the RNA code is not a direct one-to-one conversion from a nucleotide in the mRNA to an amino acid in the protein. The rules for translating this code were deciphered in the 1960s and comprise the universal genetic code used by all living organisms. We know that the sequence of nucleotides on the mRNA molecule is ‘read’ in groups of three, or triplets., there are 4×4×4 = 64 possible combinations of nucleotide triplets;
AGA etc. However, through some extremely clever and painstaking experiments, early molecular biologists realised that the genetic code has redundancy; some amino acids are specified by more than one triplet. Each group of three consecutive nucleotides in the mRNA molecule is known as a codon, and each codon specifies one amino acid or a stop to the translation when the end of the protein sequence is reached. Most proteins start with the amino acid Methionine, and have the start codon
AUG, which signals the initiation of translation.
Adaptor molecules: tRNAs
The codons on the mRNA molecule cannot directly recognise the amino acid molecules they specify. An adaptor molecule, capable of binding to the codon on one end and hooking the corresponding amino acid on the other is required. These adaptors are made of RNA too, and are known as Transfer RNAs (tRNAs, to distinguish them from mRNAs). tRNA molecules are short pieces of RNA that fold up into a clover-leaf shape. Because the genetic code has redundancy, meaning that more than one codon can specify a single amino acid, so too can more than one tRNA molecule latch onto a single amino acid. In order to translate the genetic code from nucleotides to amino acids, the cell makes a series of different tRNA molecules. Specific enzymes, known as aminoacyl-tRNA synthetases catalyse the ‘loading’ of the tRNA molecule with the amino acid. Since there are twenty standard amino acids, there are twenty different aminoacyl-tRNA synthetases. For example, alanyl-tRNA synthetase is responsible for attaching the amino acid alanine to all tRNAs that recognise the code for alanine, which could be any of
Thus the first step of translating the genetic code involves linking amino acids to tRNA molecules. Next, these amino acids are linked to each other in a chain to form a protein molecule. These reactions occur within structures known as ribosomes. A ribosome is a large, complex machine made up of more than 50 different proteins and catalytic RNA molecules; it is a veritable beast of a molecule and is considered the protein-making factory within the cell. A typical human cell can contain several million of these ribosomes in order to keep up with the constant protein synthesis demands expected of it.
For a great visual summary of this process, check out this short YouTube video:
The chain of amino acids ordered in the sequence dictated by the genetic code, assembled within the ribosome, represents how every single protein in every single living organism is made. Given how important it is, there are quality control checkpoints at various steps of this process to ensure that the linear amino acid chain is folded correctly. This ensures that the freshly produced protein has the correct structure that allows it to carry out its function within the cell. Protein synthesis is a fundamental process in molecular biology, and indeed, of life itself.