Transcription vs. Translation

DNA is transcribed into RNA during the first step of gene expression
DNA Transcription. National Human Genome Research Institute

 

Evolution, or the change in species over time, is driven by the process of natural selection. In order for natural selection to work, individuals within a population of a species must have differences within the traits they express. Individuals with the desirable traits and  for their environment will survive long enough to reproduce and pass down the genes that code for those characteristics to their offspring.

Individuals that are deemed “unfit” for their environment will die before they are able to pass down those undesirable genes to the next generation. Over time, only the genes that code for the desirable adaptation will be found in the gene pool.

The availability of these traits are dependent upon gene expression.

Gene expression is made possible by the proteins that are made by cells during  and translation. Since genes are coded for in the DNA and the DNA is transcribed and translated into proteins, the expression of the genes are controlled by which portions of the DNA get copied and made into the proteins.

Transcription

The first step of gene expression is called transcription. Transcription is creation of a messenger RNA molecule that is the complement of a single strand of DNA. Free floating RNA nucleotides get matched up to the DNA following the base pairing rules. In transcription, adenine is paired with uracil in RNA and guanine is paired with cytosine.

The RNA polymerase molecule puts the messenger RNA nucleotide sequence in the correct order and binds them together.

It is also the enzyme that is responsible for checking for mistakes or mutations in the sequence.

Following transcription, the messenger RNA molecule is processed through a process called RNA splicing.

Parts of the messenger RNA that do not code for the protein that needs to be expressed are cut out and the pieces are spliced back together.

Additional protective caps and tails are added to the messenger RNA at this time as well. Alternative splicing can be done to the RNA to make a single strand of messenger RNA able to produce many different genes. Scientists believe this is how adaptations can occur without mutations happening at the molecular level.

Now that the messenger RNA is fully processed, it can leave the nucleus through the nuclear pores within the nuclear envelope and proceed to the cytoplasm where it will meet up with a ribosome and undergo translation. This second part of gene expression is where the actual polypeptide that will eventually become the expressed protein is made.

In translation, the messenger RNA gets sandwiched between the large and small subunits of the ribosome. Transfer RNA will bring over the correct amino acid to the ribosome and messenger RNA complex. The transfer RNA recognizes the messenger RNA codon, or three nucleotide sequence, by matching up its own anit-codon complement and binding to the messenger RNA strand. The ribosome moves to allow another transfer RNA to bind and the amino acids from these transfer RNA create a peptide bond between them and severing the bond between the amino acid and the transfer RNA.

The ribosome moves again and the now free transfer RNA can go find another amino acid and be reused.

This process continues until the ribosome reaches a “stop” codon and at that point, the polypeptide chain and the messenger RNA are released from the ribosome. The ribosome and messenger RNA can be used again for further translation and the polypeptide chain can go off for some more processing to be made into a protein.

The rate at which transcription and translation occur drive evolution, along with the chosen alternative splicing of the messenger RNA. As new genes are expressed and frequently expressed, new proteins are made and new adaptations and traits can be seen in the species. Natural selection then can work on these different variants and the species becomes stronger and survives longer.

Translation

The second major step in gene expression is called translation. After the messenger RNA makes a complementary strand to a single strand of DNA in transcription, it then gets processed during RNA splicing and is then ready for translation. Since the process of translation occurs in the cytoplasm of the cell, it has to first move out of the nucleus through the nuclear pores and out into the cytoplasm where it will encounter the ribosomes needed for translation.

Ribosomes are an organelle within a cell that helps assemble proteins. Ribosomes are made up of ribosomal RNA and can either be free floating in the cytoplasm or bound to the endoplasmic reticulum making it rough endoplasmic reticulum. A ribosome has two subunits - a larger upper subunit and the smaller lower subunit.

A strand of messenger RNA is held between the two subunits as it goes through the process of translation.

The upper subunit of the ribosome has three binding sites called the “A”, “P” and “E” sites. These sites sit on top of the messenger RNA codon, or a three nucleotide sequence that codes for an amino acid. The amino acids are brought to the ribosome as an attachment to a transfer RNA molecule. The transfer RNA has an anti-codon, or complement of the messenger RNA codon, on one end and an amino acid that the codon specifies on the other end. The transfer RNA fits into the “A”, “P” and “E” sites as the polypeptide chain is built.

The first stop for the transfer RNA is a “A” site. The “A” stands for aminoacyl-tRNA, or a transfer RNA molecule that has an amino acid attached to it.

This is where the anti-codon on the transfer RNA meets up with the codon on the messenger RNA and binds to it. The ribosome then moves down and the transfer RNA is now within the “P” site of the ribosome. The “P” in this case stands for peptidyl-tRNA. In the “P” site, the amino acid from the transfer RNA gets attached via a peptide bond to the growing chain of amino acids making a polypeptide.

At this point, the amino acid is no longer attached to the transfer RNA.

Once the bonding is complete, the ribosome moves down once again and the transfer RNA is now in the “E” site, or the “exit” site and the transfer RNA leaves the ribosome and can find a free floating amino acid and be used again.

Once the ribosome reaches the stop codon and the final amino acid has been attached to the long polypeptide chain, the ribosome subunits break apart and the messenger RNA strand is released along with the polypeptide. The messenger RNA may then go through translation again if more than one of the polypeptide chain is needed. The ribosome is also free to be reused. The polypeptide chain can then be put together with other polypeptides to create a fully functioning protein.

The rate of translation and the amount of polypeptides created can drive evolution. If a messenger RNA strand is not translated right away, then its protein it codes for will not be expressed and can change the structure or function of an individual. Therefore, if many different proteins are translated and expressed, a species can evolve by expressing new genes that may not have been available in the gene pool before.

Similarly, if an is not favorable, it may cause the gene to stop being expressed. This inhibition of the gene may occur by not transcribing the DNA region that codes for the protein, or it could happen by not translating the messenger RNA that was created during transcription.