Messenger RNA (mRNA) — Overview & Role in Translation

From Genes to mRNA to Protein: Transcription and Translation

Genes encode protein structure. When the cell needs to make a specific protein, the gene for that protein needs to be transcribed. During transcription, the molecule called RNA polymerase reads the DNA sequence of the gene and creates a complementary mRNA molecule. This molecule consists of just one strand and serves as a temporary copy of the gene.

Immediately after transcription, what we get is pre-mRNA. This molecule contains two parts: exons and introns. This molecule needs to be edited to cut out the introns, which do not contribute to the final protein. The final mRNA strand contains only the coding regions, the exons.

Once this is done, the mRNA can be used to make a protein. First, it needs to move out of the nucleus and into the cytoplasm of the cell. Here, it can be translated. In this process, an organelle called the ribosome reads the sequence of the messenger RNA strand, three bases at a time. As this happens, another type of RNA, tRNA, brings in amino acids that correspond to the sequence. Each codon, a group of three bases, on the mRNA strand corresponds to one amino acid.

As the process goes on, the amino acids are bonded together into long chains. After translation is complete, the chains are modified and folded until they make a fully functional protein.

(Video) mRNA Translation

by DNA Learning Center

Transcription

Messenger RNA is a single-stranded copy of a gene and a crucial part of gene expression. Essentially, when the cell needs to make a protein, the gene that codes for that protein is transcribed into an mRNA molecule. This process is carried out by a molecule called RNA polymerase. RNA polymerase reads a segment of DNA and creates a complementary strand of mRNA to temporarily store a copy of the genetic information that is needed. The original transcript, called pre-mRNA, is modified and then sent to the cytoplasm.

Translation

In the cytoplasm, the ribosome proceeds to read the code of the messenger RNA molecule in order to know what amino acids need to be put in place during protein synthesis. This process is called translation. Just like how mRNA is complementary to the DNA strand, tRNA has nucleotides that are complementary to the mRNA strand. tRNA and the ribosome carry out the process by adding amino acids to a chain that will eventually become a full protein.

The ribosome travels along the messenger RNA strand reading its base pairs in groups of three called codons. Depending on the sequence, a new tRNA will enter the ribosome carrying the amino acid it matches.

What Does mRNA Do?

Messenger RNA is a temporary copy of part of a DNA strand. Basically, the sequence of mRNA encodes instructions for the cell to build polypeptide chains, or in other words, how to build proteins.

This strand of information is made during transcription, where a molecule called RNA polymerase reads the genetic code of a DNA segment and creates a complementary mRNA molecule. This molecule only has one strand, whereas DNA has two. It also has a slightly different structure, so it uses the base uracil instead of thymine.

Once complete, the mRNA is released into the cytoplasm of the cell from the nucleus. Once in the cytoplasm, it attaches to a ribosome, which uses the instructions to help build a protein.

The messenger RNA is decoded in the ribosome. Here, another type of RNA, the tRNA, makes sure the correct amino acids are bonded together to form a protein. The mRNA passes through the ribosome and is read three bases at a time. A set of three bases is called a codon, and each codon corresponds to an amino acid. The tRNA brings in the amino acids that match the codons and strings them together into long chains. These chains will later be modified and folded so that they make complete and functional protein products.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Translation}}$ : The cell uses the information from RNA to make proteins

$\color{#8925AE}{\text{mRNA}}$ : Messenger RNA. Its nucleobases act as instructions for the cell to build polypeptides

$\color{#8925AE}{\text{tRNA}}$ : Transfer RNA. Carries a specific amino acid to the ribosomes to help build proteins

$\color{#8925AE}{\text{Codon}}$ : A group of three nucleobases that code for a specific amino acid

The Cellular Blueprint

Messenger RNA holds a crucial role during transcription and translation. The process of making proteins is similar to building a house. Ribosomes can be thought of as construction workers taking all the necessary materials, the amino acids, and putting them together to form a whole structure. If we think about making proteins this way, we could say that mRNA is equivalent to a blueprint. It tells the construction worker what materials it needs and what order to put those materials in. mRNA is responsible for telling the ribosomes what amino acids are needed in a specific protein and what order to put them in.

RNA Structure

RNA is composed of four different nucleic acids: Adenine, Guanine, Cytosine, and Uracil, usually written as A, G, C, and U respectively. The ribosome is able to read the mRNA sequence in groups of three nucleic acids. A group of three nucleic acids is called a codon. In total, there are 64 different codon combinations. This number comes from the fact that in each of the three nucleic acid locations there are four possible nucleic acids that can be placed there. Therefore: $$4\times4\times4=64$$

Of these 64 possible codon combinations, 61 of them code for 20 different amino acids. This means that different codons can code for the same amino acid. For example: AUU, AUC, and AUA all code for the amino acid isoleucine.

The three codons which do not code for an amino acid are: UAA, UAG, and UGA. These codons are referred to as stop codons. This is because instead of telling the ribosome to add another amino acid, it tells the ribosome the protein is complete. This causes the ribosome to release the amino acids so they can then be used by the cell. Below is a chart giving the codes for all 64 codons and what they code for. An important thing to note is that while multiple codons can code for a single amino acid, a single codon can not code for multiple amino acids. What this means is that the codon AUU can code for only isoleucine.