Course: SAT 170 Introductory Biotechnology

Unit 4 Learning Module

SubTabs

- Unit 4
- Table of Contents (Selected)
- Unit 4 Glossary (Selected)

Page 2 of 4

Gene Expression (continued)

Translation is the second step of gene expression where the mRNA is translated into a protein. The direct product of gene expression is a chain of amino acids, known as a polypeptide. The polypeptide will be folded into functional proteins. We will discuss the process of protein folding in our next unit.

There are several key players involved in the process of translation including: mRNA, transfer RNA (tRNA) and the ribosomes.

- mRNA is “read” in triplets called codons. Each codon codes for one amino acid; this is known as the genetic code. There is some flexibility in this code since several codons can code for the same amino acid. The start codon initiates translation, while the stop codons are responsible for the termination of translation.

See Figure 4: This image illustrates the complementary nature of mRNA to the DNA template. Notice the presence of uracil (U) in the mRNA. You can also see how the mRNA is arranged into sets of three nucleotides (shown here in different colors) known as codons. During translation, each codon specifies for an amino acid in the protein (the amino acids are illustrated here by their single letter abbreviations).

Figure 4

- tRNA is a key player in the process of translation as it contains a three-nucleotide sequence called an anticodon that binds to the codon on the mRNA. This anti-codon is complementary to the mRNA molecule. Thus, if the mRNA codon is AUG, the corresponding tRNA anti-codon is UAC (since tRNA is a type of RNA, we still use uracil rather than thymine). The other end of the tRNA contains the amino acid corresponding to that codon. Thus, tRNA functions as an interpreter for the process as it is able to read both the nucleic acid message (through the pairing of the anticodon to the codon) and the protein message (through the amino acid binding site).

Figure 5

Figure 6

- Ribosomes are another key player in the process of translation as they bring tRNA and mRNA together. The ribosome is composed of both protein subunits as well as a special type of RNA known as ribosomal RNA (rRNA). The mRNA is interpreted one codon at a time in the ribosomes as the tRNA anti-codons bind to those codons bringing in the appropriate amino acid. Ribosomes contain three important sites for the process of translation: the P site, the A site and the E site. The P site is what I refer to as the “peptide-bond site”, because it is the site which binds the tRNA that is carrying the chain of amino acids (chains of amino acids are linked by special chemical bonds known as peptide bonds). The A site, or the “amino acid site) is where the tRNA carrying the next amino acid in the chain binds. The E site, or “empty/exit site.

See Figure 5: The ribosome (shown here in two shades of green) is able to bring the mRNA (shown in pink and blue) together with the tRNA (shown in various shades of blue). These tRNA molecules bring in the amino acids (shown as different colored circles) of the protein product.

Translation occurs in three general steps: initiation, elongation, and termination.

- During translation Initiation, the translation complex is formed. This complex consists of the ribosome, the mRNA and an initiator tRNA. The mRNA is positioned such that the start codon is located in the P site, this is where the initiator tRNA binds.
- Translation Elongation occurs in three separate steps: codon recognition, peptide bond formation and translocation.
  - The tRNA carrying the next amino acid will bind to the mRNA codon in the A site during codon recognition.
  - Amino acids from the tRNA in the P site will be transferred and bound to the amino acid on the tRNA in the A site during peptide bond formation.
  - Translocation occurs as the empty tRNA moves from the P site to the E site to exit. At the same time the tRNA, now carrying the chain of amino acids, can move from the A site to the P site leaving the A site open for a new round of codon recognition, peptide bond formation and translocation.
  - These three steps continue as the mRNA is interpreted one codon at a time, bringing in one amino acid at a time.
- Translation Termination occurs once the stop codon has been reached. A release factor binds to the A site of the ribosome causing the ribosomes, mRNA, and tRNA to disassemble, releasing the protein product. The ribosomes, tRNA, and mRNA can be involved in subsequent rounds of translation (although not necessarily all together), while the protein product will move on to be processed and folded into functional proteins.

See Figure 6: The tRNA molecules (dark green) are able to bring in the amino acids(lighter shades of green) in the appropriate order during translation as the ribosome (purple) holds the mRNA (pink) in the correct sites.