What is Gene Expression?
Gene expression is the process by which the information encoded in a gene is turned into a function. This mostly occurs via the transcription of RNA molecules that code for proteins or non-coding RNA molecules that serve other functions.
Gene expression be thought of as an “on/off switch” to control when and where RNA molecules and proteins are made and as a “volume control” to determine how much of those products are made.
The process of gene expression is carefully regulated, changing substantially under different conditions and cell types. The RNA and protein products of many genes serve to regulate the expression of other genes.
Where, when, and how much a gene is expressed can also assessed by measuring the functional activity of a gene product or observing a phenotype associated with a gene.
How Genes are Expressed?
Most genes contain the information needed to make functional molecules called proteins. (A few genes produce regulatory molecules that help the cell assemble proteins.)
The journey from gene to protein is complex and tightly controlled within each cell. It consists of two major steps: transcription and translation. Together, transcription and translation are known as gene expression.
1. Transcription.
It is the first step of gene expression in which the genetic information encoded in DNA is transcribed into RNA. RNA polymerase reads the DNA template during transcription and synthesizes a complementary RNA strand.
In prokaryotes, transcription occurs in the cytosol, while eukaryotic transcription occurs in the nucleus, where DNA is stored. In eukaryotes, the resulting RNA molecule, the RNA transcript, carries the information from the nucleus to the cytoplasm.
Transcription takes place in three steps: initiation, elongation, and termination.
Step 1: Initiation
Initiation is the beginning of transcription. It occurs when the enzyme RNA polymerase binds to a region of a gene called the promoter.
This signals the DNA to unwind so the enzyme can ‘‘read’’ the bases in one of the DNA strands. The enzyme is now ready to make a strand of mRNA with a complementary sequence of bases.
Step 2: Elongation
Elongation is the addition of nucleotides to the mRNA strand. RNA polymerase reads the unwound DNA strand and builds the mRNA molecule, using complementary base pairs. During this process, an adenine (A) in the DNA binds to an uracil (U) in the RNA.
Step 3: Termination
Termination is the ending of transcription, and occurs when RNA polymerase crosses a stop (termination) sequence in the gene. The mRNA strand is complete, and it detaches from DNA.
2. Translation.
In the cytoplasm, the mature mRNA serves as a template for translation, the process by which proteins (the final product of gene expression) are synthesized.
Ribosomes, the cellular machinery accountable for protein synthesis, read the mRNA codons (three-base sequences) and match them with transfer RNA (tRNA) molecules carrying specific amino acids.
61 codons specify amino acids, where one is the start codon, the site of initiation, and three stop codons signal the end of the polypeptide synthesis.
As the ribosome moves along the mRNA, amino acids are joined together, forming a polypeptide chain, eventually folding into a functional protein.
In prokaryotes, like transcription, translation also occurs in the cytoplasm.
Step 1: Initiation
The translation process begins with the initiation phase, where a small subunit of the ribosome binds to the mRNA and a special initiator tRNA brings in the first amino acid at the AUG start codon.
The ribosome scans the mRNA until it finds the start codon, AUG, and the small ribosomal subunit binds to the mRNA, followed by the larger subunit.
Step 2: Elongation
The ribosome reads the sequence of codons on the mRNA. It matches them to the corresponding tRNAs, bringing in the correct amino acids to add to the growing polypeptide chain.
The ribosome moves along the mRNA, reading the codons and adding the matching amino acids to the growing protein chain.
Step 3: Termination
Finally, the ribosome reaches a stop codon, which signals the end of translation. The completed protein is then released into the cytoplasm.
The ribosome reaches one of the three stop codons (UAA, UAG, or UGA). These release factors bind to these codons, releasing the newly synthesized protein into the cytoplasm.
How Is Gene Expression Regulated?
The amounts and types of mRNA molecules in a cell reflect the function of that cell. In fact, thousands of transcripts are produced every second in every cell.
Given this statistic, it is not surprising that the primary control point for gene expression is usually at the very beginning of the protein production process the initiation of transcription.
RNA transcription makes an efficient control point because many proteins can be made from a single mRNA molecule.
Transcript processing provides an additional level of regulation for eukaryotes, and the presence of a nucleus makes this possible. In prokaryotes, translation of a transcript begins before the transcript is complete, due to the proximity of ribosomes to the new mRNA molecules.
In eukaryotes, however, transcripts are modified in the nucleus before they are exported to the cytoplasm for translation.
Eukaryotic transcripts are also more complex than prokaryotic transcripts. For instance, the primary transcripts synthesized by RNA polymerase contain sequences that will not be part of the mature RNA. These intervening sequences are called introns, and they are removed before the mature mRNA leaves the nucleus.
The remaining regions of the transcript, which include the protein-coding regions, are called exons, and they are spliced together to produce the mature mRNA. Eukaryotic transcripts are also modified at their ends, which affects their stability and translation.
Of course, there are many cases in which cells must respond quickly to changing environmental conditions. In these situations, the regulatory control point may come well after transcription.
For example, early development in most animals relies on translational control because very little transcription occurs during the first few cell divisions after fertilization.
Eggs therefore contain many maternally originated mRNA transcripts as a ready reserve for translation after fertilization.
On the degradative side of the balance, cells can rapidly adjust their protein levels through the enzymatic breakdown of RNA transcripts and existing protein molecules. Both of these actions result in decreased amounts of certain proteins.
Often, this breakdown is linked to specific events in the cell. The eukaryotic cell cycle provides a good example of how protein breakdown is linked to cellular events.
This cycle is divided into several phases, each of which is characterized by distinct cyclin proteins that act as key regulators for that phase.
Before a cell can progress from one phase of the cell cycle to the next, it must degrade the cyclin that characterizes that particular phase of the cycle. Failure to degrade a cyclin stops the cycle from continuing.