What Is Translation? — Overview & Stages

Making Proteins

There is a central dogma of biology which helps explain the flow of genetic information in organisms. It states: DNA $\rightarrow$ RNA $\rightarrow$ Protein. There are a few exceptions to this central dogma, but overall most organisms follow this flow of genetic information.

The first segment of the central dogma DNA $\rightarrow$ RNA represents transcription, which is the process by which DNA is transcribed or converted into RNA. The second segment of the central dogma RNA $\rightarrow$ Protein represents translation, which is the process by which the code of RNA is made into a protein.

In prokaryotes DNA $\rightarrow$ RNA and RNA $\rightarrow$ Protein occur almost simultaneously, while in eukaryotes these two segments of the central dogma are separated in time and space. This is because prokaryotes have no nucleus, so their DNA is free-floating in their cytosol. This means that the ribosomes, which are responsible for translating RNA into protein, have immediate accesses to the RNA. However, in eukaryotes there is a nucleus which separates the DNA from the cytosol. Therefore, the RNA must first travel to the cytosol where the ribosomes are then able to translate the RNA into proteins.

Types of RNA

There are several types of RNA in the cell which are used for various processes. The two major types of RNA used in translation are mRNA (messenger RNA) and tRNA (transfer RNA). mRNA acts as the middleman between DNA and proteins, and each mRNA has a unique combination of four different amino acids–CGAU (Cytosine, Guanine, Adenine, Uracil). tRNA works as the middleman between mRNA and amino acids. On one end of tRNA is an amino acid, while the opposite end has an anticodon which corresponds to a codon on the mRNA. Therefore, each codon on mRNA has a matching anticodon on a tRNA molecule, which corresponds to a particular amino acid.

Steps of Translation

There are three major steps to translation: Initiation, Elongation, and Termination.

1) Initiation

• The ribosome is made of two separate subunits: the small subunit and the large subunit. During initiation the small subunit attaches to the 5' end of mRNA. It then moves in the 5' $\rightarrow$ 3' direction. Once the small subunit hits the start codon (AUG), the corresponding tRNA will attach followed by the large subunit. The first tRNA molecule always carries the amino acid methionine.

2) Elongation

• In the elongation phase of translation, the tRNA with the correct corresponding anticodon will match with the corresponding mRNA codon. A peptide bond, which is the type of bond that holds amino acids together, is formed between the methionine from the first tRNA with the second amino acid from the second tRNA. The ribosome then shifts down moving in the 5' $\rightarrow$ 3' direction, making space for another tRNA to match with its corresponding codon and thereby allowing another peptide bond to form. This process continues with the ribosome moving down the mRNA strand, growing its amino acid chain the further it goes.
• An important thing to note is that once a tRNA transfers its amino acid to the amino acid chain, it then leaves the ribosome to be "recharged" with another amino acid.

3) Termination

• The final phase of translation is termination. When the ribosome reaches a stop codon (UAG, UAA, or UGA), a release factor will bind to the stop codon and cause the amino acid chain to be released and the ribosome subunits to separate.

Translation (Protein Synthesis)

Translation is the process of converting mRNA into an amino acid chain. DNA stores the information for proteins in its nucleotide sequence. During transcription, the genetic code (DNA) was turned into mRNA and modified. Now, the modified RNA is ready to be turned into a protein.

Translation occurs on ribosomes which are either floating in the cytoplasm or chilling on the surface of the rough endoplasmic reticulum.

There are three steps to translation:

1. Initiation

• In this step the small subunit part of the ribosome attaches to the 5' end of the mRNA strand.

• The small subunit then reads the mRNA nucleotides in groups of three, called codons, until it runs into the start codon which is always AUG.

• It then recruits the large ribosomal subunit and the transfer RNA (tRNA).

tRNA is a type of RNA that contains an area called an anticodon. This area is a section of three nucleotides that are complementary to the mRNA strand. The tRNA also carries a specific amino acid.

1. Elongation

• During elongation, the now complete ribosome with its large and small subunits travels along the mRNA strand and reads the nucleotides in groups of three, called codons.

• It then grabs tRNAs from the surrounding areas and matches their anticodons to the respective codons, following base-pairing rules.

• The amino acids are joined together by peptide bonds and the tRNAs leave. This creates a string of amino acids coming out of the ribosome.

• Elongation always goes from the 5' end of the mRNA molecule towards the 3' end.

1. Termination

• The third step of translation is when the ribosome runs into a stop codon.

• A stop codon is either UAA, UAG or UGA.

• The ribosome places the last amino acid and severs the protein. It is now as long as it needs to be and will go into the cytoplasm for modification.

• The ribosome breaks off from the mRNA strand.

What's Translation?

Translation happens when the cell uses the information from RNA to make proteins. It is one of the major steps in gene expression. Basically, the sequence of mRNA nucleotides acts as instructions for the cell to build polypeptides, or in other words, how they build proteins.

Before this process occurs, the mRNA made during transcription 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. You can think of the ribosome like a screwdriver that helps you build a complex structure.

The mRNA is decoded on the ribosome and the tRNA and mRNA work together to make sure the correct amino acids are bonded together to form a protein. As the mRNA passes through the ribosome one codon at a time, the tRNA brings in the amino acids that match the instructions. Each tRNA can only carry one specific type of amino acid, so many tRNAs are needed to build the whole polypeptide. After an amino acid is brought to the ribosome, ribosomal RNA (rRNA) connects it to the growing chain.

The ribosome then does its duty as a tool to link the amino acids together with peptide bonds. At the same time, it breaks the bond between the tRNA and the amino acid which allows the tRNA to go grab another amino acid. This process continues until a stop codon shows up, at which point protein synthesis is complete. Neat, huh?

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

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

$\color{#8925AE}{\text{Nucleic Acid}}$ : A nucleic acid is a complex polymer that stores information in a cell in the form of a code. DNA and RNA

$\color{#8925AE}{\text{mRNA}}$ : Messenger RNA. mRNA 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 codes for a specific amino acid

(Video) Transcription & Translation | From DNA to RNA to Protein

by 2 Minute Classroom

Check out this video to see how codons (and their anticodons) are used to build polypeptide chains.

(Video) DNA, Hot Pockets, and the Longest Word Ever

by CrashCourse

Watch this video to see how ribosomes synthesize proteins with the help of tRNA in the process known as translation.