Biology 1st Paper (Botany) · Classes 11-12 · Chapter 1 · 1.4

Translation.

The information sits in DNA, but the cell does its work with protein. So how does the one become the other? mRNA carries a copy, the ribosome reads it, and tRNA hands over amino acids one at a time. That whole job is called translation.

1. From the language of DNA to the language of protein

Translation is the making of a polypeptide chain, that is a protein, along a path that RNA lays down. Put simply, it turns the language of DNA into the language of protein, with mRNA as the go-between. It happens on the ribosome, and a polypeptide once chained up is the protein.

2. The five ingredients

Translation needs five ingredients. The first is mRNA. It carries the genetic code from DNA and serves as the template for building the protein.

The second is tRNA. It brings one specific amino acid, and every amino acid has at least one tRNA. The molecule is very small, only 75-80 nucleotides. The amino acid attaches at the 3' end, and in the middle sits a 3-base anticodon that pairs with mRNA.

To write an anticodon from a codon, use the complement rule: A-U, G-C, with mRNA and tRNA antiparallel. The book gives UAC as the anticodon of AUG. Apply the same rule to the other codons and see.

The third ingredient is the amino acids and their codons. There are usually twenty amino acids, and sixty-one kinds of codons code for them.

The fourth is the ribosome, the platform where tRNA sits. Each ribosome has two sites for tRNA, the A site and the P site. One ribosome can bind any mRNA and all the tRNAs.

The fifth is the activating enzyme, an aminoacyl-tRNA synthetase. Each one is specific for a single amino acid and a single tRNA. So it carries three functional sites: one for its amino acid, one for ATP, and one for its tRNA.

Keep the enzyme's order of work in mind. First it reacts with the amino acid and ATP to form an AA-AMP bond, and pyrophosphate leaves. Then it joins the tRNA. The energy for making the peptide bond comes from this stage.

3. Three stages: initiation, elongation, termination

In initiation, the charged tRNA carrying the first amino acid and the small ribosomal subunit bind the start point of mRNA. The start code is usually AUG, so the first amino acid is methionine, and its tRNA has the anticodon UAC. Together these form the initiation complex. Using GTP energy, initiation factors, a group of proteins, bring mRNA, tRNA and the ribosome together.

Then the large ribosomal subunit joins. It has two adjacent sites, the A site and the P site. A tRNA first binds the A site, then vacates it and moves to the P site. At the P site its amino acid joins the polypeptide chain, and a new tRNA enters the empty A site.

At the joined site, mRNA and tRNA are antiparallel and complementary by base pairing. After handing over its amino acid, the tRNA drifts back into the cytosol, ready to fetch another of the same kind.

In elongation the ribosome moves along the mRNA in the 5'-3' direction. The amino acids join by peptide bonds into a polypeptide, and elongation factors assist.

Termination comes when a stop codon, UAA, UAG or UGA, enters the A site of the ribosome. These codons encode no amino acid or tRNA; instead they bind a protein release factor. Translation then ends and the new protein is released from the ribosome. A polysome (polyribosome) greatly speeds up translation.

Now walk the ribosome yourself along the book's mRNA: AUG CCG UAU GCU UAU GCU CCU UAA. At each step watch who enters, where the bond forms, and who leaves.

Figure 1.35 is drawn here in its three stages. Tap the points and chips to see what each part does.

Now put the whole process in order yourself. The first two steps are the preparation before translation starts, but they belong in the sequence.

4. More about translation

Antibiotics disturb translation in bacteria, each at its own stage. Chloromycin (chloramphenicol) blocks peptide bond formation, erythromycin the movement of mRNA on the ribosome, neomycin the mRNA-tRNA interaction, streptomycin the initiation stage, and tetracycline the attachment of tRNA to the ribosome.

mRNA directly decides how many amino acids a protein has and in what order. And mRNA is an exact mirror of a part of DNA, so DNA controls them indirectly.

Prokaryotes have no nucleus. So transcription at one end of the mRNA and translation at the other can go on at the same time.

Only 20 amino acids, arranged in different orders, build the large proteins. Two amino acids are joined to each other by a peptide bond.

5. Transcription against translation

Transcription happens in the nucleus, translation in the cytoplasm. Transcription is not linked with the ribosome; translation is. The key enzyme of transcription is RNA polymerase; in translation it is the activating enzyme. Tap the cells and go through the rows.

These two processes together with replication make up the Central Dogma of biology: DNA makes RNA, RNA makes protein, and protein, that is enzymes, controls the making of both DNA and RNA. It was proposed in 1958.

6. Check yourself

Start with the first question.

Read the second one carefully.

One more to finish.

The whole note on six cards.

Source: Biology 1st Paper (Botany), Classes 11-12 (NCTB), Chapter 1 “Cell and Its Structure”, section 1.4.