Transcription and translation
Protein synthesis is made up of two stages: transcription and translation .
Transcription and translation are the processes that turn the instructions found in genes into the proteins they encode. Information is transcribed from DNA into RNA and then translated into protein structure. These processes differ between the simple cells of prokaryotes and the more complex cells of eukaryotes. The way genes are regulated also differs between these types of cells.
Use this page to revise the following concepts of transcription and translation:
Transcription
In eukaryotes, transcription occurs in the nucleus of the cell, where the DNA is found. Prokaryotes do not have a nucleus, thus transcription occurs in the cytoplasm. Click on the figure below for descriptions of the different stages of transcription.
RNA processing
In eukaryotes, genes contain exons (coding sequences of DNA) and introns (non-coding sequences). The RNA copy made in transcription , or pre-mRNA, includes both the exons and introns. While this RNA transcript is still in the nucleus, the introns are ‘spliced out’ or removed and the exons are joined together, creating an uninterrupted coding sequence (see part a. in the figure below). In addition, a ‘methyl cap’ is added to the 5’ end of the pre-mRNA and a ‘poly-A tail’ is added to the 3’ end. These additions help to protect the transcript from being degraded by enzymes and ensure it is able to reach the cytoplasm to be properly translated into a protein. After RNA processing, the RNA strand is now called messenger RNA (mRNA).
By joining the exons in different ways, cells can create more than one protein from one gene (see part b. in the figure below). This is called alternative splicing. Due to alternative splicing, the proteome (all proteins that are or can be expressed by a cell) is larger than the genome (all genes present in a cell).
RNA processing does not occur in prokaryotes. Their genes do not contain introns and are not given a 5’ cap or poly-A tail, and mRNA is the initial prodcut of transcription.
Translation
In eukaryotes, the mRNA leaves the nucleus and a ribosome (often on the rough endoplasmic reticulum) will begin translating it. The genetic code is read in groups of three nucleotides, called codons, each of which corresponds to a specific amino acid as can be seen in the genetic code table below. The genetic code is degenerate, meaning multiple codons can code for the same amino acid. For example the codons UUU and UUC both code for the amino acid phenylalanine. The code is also referred to as universal as almost all living organisms have the same code. For example, UUU codes for the amino acid phenylalanine in all organisms.
The ribosome moves along the mRNA from a START codon in a 5’ to 3’ direction and, at each codon, a tRNA with a complementary anticodon will attach. The amino acid carried by the tRNA at the opposite end is joined to the previous amino acid with a peptide bond. The tRNA can then detach from the amino acid. The chain of amino acids , or polypeptide chain, elongates until the ribosome reaches a STOP codon. At this point the ribosome releases the polypeptide chain and the primary structure of the protein is created.
In prokaryotes, translation can begin even before transcription is completed. This is because both processes occur in the cytoplasm where ribosomes have access to the mRNA being transcribed.
Gene Regulation
The mechanism of gene regulation is vital in ensuring that only genes that are needed are expressed to conserve energy for the cell and only make proteins when they are required. The mechanism differs in eukaryotes and prokaryotes.
Gene Regulation in Eukaryotes
Eukaryotes genes are regulated by transcription factors (a type of protein) which bind to a region known as the promoter region found just before the gene they regulate. This binding will either ‘turn on’ the gene, attracting RNA polymerase to begin transcription or will ‘turn off’ the gene by preventing RNA polymerase from binding to the promoter. These transcription factors are thus referred to as activators or repressors, depending on whether they turn the gene on or off.
While this is a more complex interaction than described here, interactions between enhancer regions, the transcription factors and activator proteins, and folding of the DNA, allow RNA polymerase to transcribe an activated gene. The diagram below despicts how these complex interactions can facilitate the binding of RNA polymerase to the gene.
Gene Regulation in Prokaryotes
In prokaryotes, a structure called an operon. An operon is a cluster of genes that are transcribed together, under the control of a single promoter and regulatory elements. These genes typically work together to carry out a related function, and they are regulated as a single unit. An example of this is the trp operon which contains structural genes coding for proteins that create tryptophan, an amino acid. The trp operon shown below illustrates a summary of how the transcription and translation of the structural genes are regulated. To understand this better, we will need to explore each of the elements of the operon and their function. We can do this by examining the trp operon in prokaryotes.
The trp operon is an example of a repressible operon (one which is ‘on’ but can be turned ‘off’ by an activated repressor) but there are operons that are inducible (ones which are ‘off’ but can be turned ‘on’ by molecules known as inducers). An example of an inducible operon would be the lac operon.
Investigate the function of the trp operon in the figure below.
Attenuation of trp operon
As transcription and translation can occur simultaneously in prokaryotes, ribosomes may start to translate the mRNA being produced while RNA polymerase is still in the process of transcription. In the case of the trp operon, in the leader sequence, there are segments on the mRNA being produced that also play a role in regulating the expression of the structural genes. The details of this process, known as attenuation, will not be covered here.
