Reading: Transcription from RNA to Protein

Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
http://www.ck12.org/saythanks

• Define translation.
• Explain how the RNAs interact during translation.
• Summarize the process of translation.

RNA to proteins. How?

You must translate. To go from one language to another. Spanish to English, French to German, or nucleotides to amino acids. Which type is the translation of molecular biology? Obviously, the type of translating discussed here translates from the language of nucleotides to the language of amino acids.

Translation

Translation is the second part of the central dogma of molecular biology: RNA→Protein. It is the process in which the genetic code in mRNA is read, one codon at a time, to make a protein. Figure 1.1 shows how this happens. After mRNA leaves the nucleus, it moves to a ribosome, which consists of rRNA and proteins. The ribosome reads the sequence of codons in mRNA. Molecules of tRNA bring amino acids to the ribosome in the correct sequence.

To understand the role of tRNA, you need to know more about its structure. Each tRNA molecule has an anticodon for the amino acid it carries. An anticodon is a sequence of 3 bases, and is complementary to the codon for an amino acid. For example, the amino acid lysine has the codon AAG, so the anticodon is UUC. Therefore, lysine would be carried by a tRNA molecule with the anticodon UUC. Wherever the codon AAG appears in mRNA, a UUC anticodon on a tRNA temporarily binds to the codon. While bound to the mRNA, the tRNA gives up its amino acid. Bonds form between adjacent amino acids as they are brought one by one to the ribosome, forming a polypeptide chain. The chain of amino acids keeps growing until a stop codon is reached.

After a polypeptide chain is synthesized, it may undergo additional processes. For example, it may assume a folded shape due to interactions among its amino acids. It may also bind with other polypeptides or with different types of molecules, such as lipids or carbohydrates. Many proteins travel to the Golgi apparatus to be modified for the specific job they will do.

FIGURE 1.1
Translation of the codons in mRNA to a chain of amino acids occurs at a ribosome. Notice the growing amino acid chain attached to the tRNAs and ribosome. Find the different types of RNA in the diagram. What are their roles in translation?

FIGURE 1.2
The tRNA structure is a very important aspect in its role. Though the molecule folds into a 3-leaf clover structure, notice the anticodon arm in the lower segment of the molecule, with the amino acid attached at the opposite end of the molecule (acceptor stem). It is the anticodon that determines which codon in the mRNA the tRNA will bind to.

Summary

• Translation is the RNA !Protein part of the central dogma.
• Translation occurs at a ribosome.
• During translation, a protein is synthesized using the codons in mRNA as a guide.
• All three types of RNA play a role in translation.

Review

1. Outline the steps of translation.
2. Discuss the structure of a tRNA molecule, and its role in translation.
3. How are transcription and translation related to the central dogma of molecular biology?