Chapter 17

three nucleotides code for a single amino acid

the process by which DNA directs the synthesis of proteins The DNA inherited by an organism leads to specific traits because DNA dictates the synthesis of proteins or, in some cases, just RNA molecules. In other words, proteins are the link between genotype and phenotype. The expression of genes that code for proteins includes two stages: transcription and translation.

the genetic code is nearly universal

do not code for amino acids that allow a releasing factor to bind to the A site of the ribosome

coupled transcription and translation

nonsense Substitution mutations are usually missense mutations; that is, the altered codon still codes for an amino acid and thus makes sense, although not necessarily the right sense. But a point mutation can also change a codon for an amino acid into a stop codon. This is called a nonsense mutation, and it causes translation to be terminated prematurely; the resulting polypeptide will be shorter than the polypeptide encoded by the normal gene. Nearly all nonsense mutations lead to nonfunctional proteins.

the addition of a 5ꞌ cap, a 3ꞌ poly-A tail, and the splicing out of introns

U; A

At least 1,350 three times as much or 450 x 3 = 1,350. Because codons are nucleotide triplets, the number of nucleotides making up a genetic message must be three times the number of amino acids in the protein product.

The three-nucleotide words of a gene are arranged in a nonoverlapping series on the DNA template strand. The genetic instructions for a polypeptide chain are written in the DNA as a series of nonoverlapping, three-nucleotide words. The series of words in a gene is transcribed into a complementary series of nonoverlapping, three-nucleotide words in mRNA, which is then translated into a chain of amino acids.

groups of ribosomes

that not all genes code for enzymes; some genes code for structural proteins such as keratin

promoter

hydrogen bonds

Cleavage of a polypeptide into two or more chains

deliver amino acids to their proper site during protein synthesis The message is a series of codons along an mRNA molecule, and the translator is called transfer RNA (tRNA). The function of tRNA is to transfer amino acids from the cytoplasmic pool of amino acids to a growing polypeptide in a ribosome. A cell keeps its cytoplasm stocked with all 20 amino acids, either by synthesizing them from other compounds or by taking them up from the surrounding solution.

translation During this stage, there is a change in language: The cell must translate the nucleotide sequence of an mRNA molecule into the amino acid sequence of a polypeptide. The sites of translation are ribosomes, molecular complexes that facilitate the orderly linking of amino acids into polypeptide chains.

RNA In a eukaryotic cell, by contrast, the nuclear envelope separates transcription from translation in space and time. Transcription occurs in the nucleus, and mRNA is then transported to the cytoplasm, where translation occurs.

signal peptide; signal-recognition particle

ribozymes The idea of a catalytic role for the RNAs in the spliceosome arose from the discovery of ribozymes, RNA molecules that function as enzymes. In some organisms, RNA splicing can occur without proteins or even additional RNA molecules: The intron RNA functions as a ribozyme and catalyzes its own excision! For example, in the ciliate protist Tetrahymena, self-splicing occurs in the production of ribosomal RNA (rRNA), a component of the organism's ribosomes. The pre-rRNA actually removes its own introns. The discovery of ribozymes rendered obsolete the idea that all biological catalysts are proteins.

nutritional mutants; biochemical pathway

frameshift

eukaryotic promoter; transcription factors; RNA polymerase II

missense mutation Substitutions that change one amino acid to another are called missense mutations. Such a mutation may have little effect on the protein: The new amino acid may have properties similar to those of the amino acid it replaces, or it may be in a region of the protein where the exact sequence of amino acids is not essential to the protein's function. Thus, the nucleotide-pair substitutions of greatest interest are those that cause a major change in a protein. The alteration of a single amino acid in a crucial area of a protein—such as in the part of the β-globin subunit of hemoglobin shown below—can significantly alter protein activity.

part of a spliceosome

3 2 1 4

In prokaryotic cells, the mRNA transcript is immediately available as mRNA without processing.

Transcription

RNA polymerase