Gene Expression

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By aaron

Gene expression is the process by which #genetic information in #DNA is transferred to #RNA, which then gets turned into some product that is needed by the cell. In the vast majority of cases, the output of gene expression is a protein. In fact, the flow of genetic information from DNA to RNA to #Protein is such a common pattern that it has become known as the central dogma of #biology. Let's have a look at each step in this process.

Phase 1: Transcription

The first step in gene expression is called transcription. During #transcription, a gene encoded by DNA is copied into RNA by an enzyme called RNA polymerase. The general process looks like this:

First, RNA polymerase has to bind to the DNA strand. Transcription wouldn't work if the RNA polymerase just attached to any random segment of the DNA. Instead, it binds to a special segment of DNA called a promoter which marks the correct starting point for transcription.

Once RNA polymerase is bound to the promoter, it temporarily unzips the DNA helix into its two strands. The RNA polymerase then begins to travel along one of the strands to each nucleotide, matching the DNA nucleotide with its complementary RNA nucleotide. For example, if the DNA nucleotide is Adenine, the complementary RNA nucleotide-- Uracil-- will match up with it and be added to the growing RNA strand. As this process continues, the sugar-phosphate backbone of the RNA strand is also being added to link the RNA nucleotides together. When it is finished, the RNA strand breaks free. What happens next will depend on whether the cell is eukaryotic or prokaryotic.

In #eukaryotic cells, the RNA product goes through some additional processing. If the gene encodes for structural RNA like the RNA that is used to build a ribosome, then the RNA itself is the final product. But normally, the final product will be a protein, so the finished RNA is exported from the nucleus out into the cytoplasm.

In #prokaryotes, it is a bit different. Because of structural differences in prokaryotic DNA compared to eukaryotic DNA, the RNA in prokaryotic cells does not need any additional processing. In addition, because prokaryotic cells have no nucleus, there is no need to export the RNA to the cytoplasm; it's already there.

At this point, the RNA is sitting in the cytoplasm. In its current form, it is known as messenger RNA, or mRNA for short. The structure of the mRNA is a bit different depending on whether the cell is eukaryotic or prokaryotic, but in either case it has a section of nucleotides that hold the instructions to build a protein. It will now enter the second main phase of gene expression, known as translation.

Phase 2: Translation

This phase is carried out by a cell component called a ribosome, which will read the mRNA nucleotides 3 at a time. Each set of 3 nucleotides is called a codon and has a corresponding amino acid. The amino acid is paired up with the help of another type of RNA called transfer RNA, or tRNA, which has two halves. The first half is a section of nucleotides corresponding to the nucleotide sequence in the mRNA. This tRNA sequence is called an anticodon. The second half of the tRNA is the amino acid that corresponds to the mRNA codon. Let's look at how all of this comes together.

First, the #ribosome attaches to the mRNA. It then travels along the mRNA strand until it hits the first codon. A tRNA with the correct anticodon then enters the ribosome and binds to the codon on the mRNA. The amino acid on the other end of the tRNA is then transferred to the growing amino acid chain, and the ribosome progresses to the next codon. The first tRNA breaks free, making room for the next one to match up. This process continues until a special codon called a stop codon is hit, at which point the polypeptide chain breaks free.

Phase 3: Folding

At this point, the output from the ribosome is just a long polypeptide chain. It still has to be folded into a stable 3D structure. The really interesting part of this step is that there is no organelle or cell component that does it. The molecular forces of the amino acids interact with each other to fold the protein into its final 3 dimensional structure. What this means is that the correct 3D structure of the protein is actually encoded within the amino acid sequence itself. This principle is known as Anfinsen's dogma, coined by Christian B. Anfinsen, a discovery for which he won a Nobel Prize in 1972.

Once the protein has folded into its final structure, it is transported off to wherever it needs to be. Usually this is somewhere within the cell, but sometimes it can be exported from the cell entirely and used somewhere else in the body. An example of this would be digestive enzymes, which leave the cells and enter the stomach.

So that's the basic process of gene expression. But how does the cell “know” when or when not to express a gene? In other words, how is gene expression regulated?

Regulating gene expression

Regulation of gene expression can occur at just about any stage of the gene expression process.

Transcriptional regulation

One method is called transcriptional regulation. Some genes have binding sites around the coding region of the gene that allow specific proteins to bind to the DNA, which can regulate expression of the gene in a variety of different ways. For example, the bound protein can decrease expression of that gene by blocking RNA polymerase from binding to the DNA. In other cases, it can increase expression of that gene by helping RNA polymerase bind to the DNA.

Post-transcriptional regulation

Eukaryotic cells have another method called post-transcriptional regulation. With this method, the cell simply blocks the RNA from leaving the nucleus. Because RNA is single-stranded, it is not as stable as DNA. If the cell blocks RNA from leaving the nucleus, it just sits there and breaks down without ever being translated into a protein.

Protein degradation

Another method of gene expression regulation is called protein degradation. Basically, this just means that when a protein is no longer needed, the cell can destroy it to control the amount of that protein in the cell.

You'll notice that none of these methods rely on direct regulation of translation. While that does happen, it is pretty rare in normal regulation. But I do mention it because this is how some antibiotics work. For example, an antibiotic called streptomycin works by binding to the ribosomes of bacterial cells and interfering with protein translation. Without the ability to synthesize proteins, the cell dies. Because the ribosomes of prokaryotic and eukaryotic cells have structural differences, the antibiotic does not bind to the eukaryotic ribosomes and the eukaryotic cell remains unaffected.

Genetic information almost always follows this same basic pattern, starting in DNA, being transcribed into RNA, and then translated into proteins in the process of gene expression.

#science #biology #genetics

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