Translation

figure 10-16

During translation, amino acids are assembled from information encoded in mRNA. As the mRNA codons move through the ribosome, tRNAs add specific amino acids to the growing polypeptide chain. The process continues until a stop codon is reached and the newly made protein is released.

Nuclear envelope

Although the instructions for making a protein are copied from DNA to mRNA, all three major types of RNA are involved in translation—the making of a protein.

Protein Structure

Every protein is made of one or more polypeptides. Polypeptides are chains of amino acids linked by peptide bonds. There are 20 different amino acids found in the proteins of living things. Each polypeptide chain may consist of hundreds or thousands of the 20 different amino acids, arranged in a sequence specific to each protein. The amino acid sequence determines how the polypeptides will twist and fold into the three-dimensional structure of the protein. The shape of the protein is critical to its function.

Steps of Translation

The translation or decoding of the genetic instructions to form a polypeptide involves five main steps, as shown in Figure 10-16. In step Q, two ribosomal subunits, tRNA, and an mRNA join together. Enzymes first attach a specific amino acid to one end of each tRNA according to the genetic code. The other end of each tRNA contains the anticodon, three nucleotides on the RNA that are complementary to the sequence of a codon in mRNA.

Nuclear pore

Peptide bond tRNA

Nuclear pore

Peptide bond tRNA

Initiation

The ribosomal subunits, the mRNA, and the tRNA carrying methionine bind together.

Elongation

The tRNA carrying the amino acid specified by the next codon binds to the codon. A peptide bond forms between adjacent amino acids. The ribosome moves the tRNA and mRNA.

Initiation

The ribosomal subunits, the mRNA, and the tRNA carrying methionine bind together.

Elongation

The tRNA carrying the amino acid specified by the next codon binds to the codon. A peptide bond forms between adjacent amino acids. The ribosome moves the tRNA and mRNA.

A tRNA carrying the amino acid methionine at one end and the anticodon UAC at the other end pairs with the start codon AUG on the mRNA. The first amino acid in nearly all polypeptides is methio-nine, but this amino acid may be removed later.

In step ©, the polypeptide chain is put together. A tRNA carrying the appropriate amino acid pairs its anticodon with the second codon in the mRNA. The ribosome then detaches methionine from the first tRNA, and a peptide bond forms between methionine and the second amino acid. The first tRNA then exits the ribosome. The ribosome then moves a distance of one codon along the mRNA.

During step ©, the polypeptide chain continues to grow as the mRNA moves along the ribosome. A new tRNA moves in, carrying an amino acid for the next mRNA codon. The growing polypeptide chain moves from one tRNA to the amino acid attached to the next tRNA.

The polypeptide grows one amino acid at a time until step ©. At this step, the ribosome reaches the stop codon. The newly made polypeptide falls off.

During step ©, the components of translation come apart. The last tRNA leaves the ribosome, and the ribosome moves away from the mRNA. The translation machinery is now free to translate the same or another mRNA.

Comparing and Contrasting RNA Types

Materials paper and pencil Procedure Create a chart that compares and contrasts the different forms of RNA. Include descriptions of each form's structure and function.

Analysis Which types of RNA are alike structurally? What might happen if one type of RNA were missing?

Large ribosomal subunit

Large ribosomal subunit

Small ribosomal subunit

Stop codon

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