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Translation involves three steps: 

  1. Initiation 

  1. Elongation 

  1. Termination 

Initiation 

Translation begins with the binding of the small ribosomal subunit to a specific sequence on the mRNA chain. In eukaryotes. initiation begins with the ribosomal recognition of the GTP cap. The small subunit binds via complementary base pairing between one of its internal subunits and the ribosome binding site, a sequence of about 10 nucleotides on the mRNA. 

A graphic depicts the initiation of m R N A translation. A strand of m R N A is positioned with the 5 prime end to the left and the 3 prime end to the right. A ribosome is centralized along the m R N A strand, with the large 50 S subunit above the strand and the smaller 30 S subunit below. Three compartments within the 50 S subunit are labeled E, P, and A, from left to right. The E and A compartments are empty but the P compartment contains a structure carrying an amino acid. Other amino acids are positioned above the m R N A.

Figure 6.14: Initiation 

Once the small subunit has bound, the large subunit binds, forming what is known as the initiation complex. This entire initiation process is facilitated by extra proteins, called initiation factors that help with the binding of ribosomal subunits and tRNA to the mRNA chain. 

Elongation 

With the formation of the complex containing Met-tRNA in the peptidyl site, an aminoacyl tRNA with the complementary anticodon sequence can bind to the mRNA passing through the acceptor site. This binding is aided by elongation factors that are dependent upon the energy from the hydrolysis of GTP. Elongation factors go through a cycle to regenerate GTP after its hydrolysis. 

Now, with tRNA bearing a chain of amino acids in the P site and tRNA containing a single amino acid in the A site, the addition of a link to the chain can be made. This addition occurs through the formation of a peptide bond, the nitrogen-carbon bond that forms between amino acid subunits to form a polypeptide chain.  

The peptide bond occurs between the carboxyl group on the lowest amino acid in the peptide chain located at the P site and the amine group on the amino acid in the A site. As a result, the peptide chain shifts over to the A site, with the original amino acid on the A site as the lowest link in the chain. The tRNA in the A site becomes peptidyl RNA, and shifts over to the P site. Meanwhile, the ribosome engages in a process called translocation: spurred by elongation factors, the ribosome moves three nucleotides in the 3' prime direction along the mRNA. In other words, the ribosome moves so that a new mRNA codon is accessible in the A site. 

A diagram depicts translocation during the elongation phase of m R N A translation. A t R N A strand enters the A site of the 50 S ribosome subunit. In the A site and the P site, codons within the t R N A strand are checked against codons along the m R N A strand. The m R N A codons are labeled 1, 2, and 3, with 1 located below the P site, 2 located below the A site, and 3 located below the E site. After translocation, the A site is empty, and amino acids enter at the P site. m R N A codons are labeled 1 through 4, with 1 positioned below the E site, 2 below the P site, 3 below the A site, and 4 to the right of the third. Spent t R N A is expelled from the E site.

Figure 6.15: Translocation 

With the A site open again, the next appropriate aminoacyl tRNA can bind there and the same reaction takes place, yielding a three-amino acid peptide chain. This process repeats, creating a polypeptide chain in the P site of the ribosome. A single ribosome can translate 60 nucleotides per second. This speed can be vastly augmented when ribosomes link up to form polyribosomes. 

Termination 

Translation ends when one of three stop codons, UAA, UAG, or UGA, enters the A site of the ribosome. There are no aminoacyl tRNA molecules that recognize these sequences. Instead, release factors bind to the P site, catalyzing the release of the completed polypeptide chain and separating the ribosome into its original small and large subunits. 

Retroviruses 

We just went through the typical flow of information from DNA to RNA. Before we move on to discussing gene expression, it is worth noting that there are some instances when information flows from RNA to DNA. Retroviruses, for instance, utilize reverse transcriptase to copy their viral RNA into DNA. This DNA is then inserted into the host genome. Once a part of the host genome, it is processed like the rest of the DNA and goes through transcription and translation to create new viral progeny. 

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