Connecting Concepts Across Chapters 9

This chapter has focused on the process by which genetic information in an mRNA molecule is transferred to the amino acid sequence of a protein. This process is termed translation because information contained in the language of nucleotides must be "translated" into the language of amino acids.

The link between genotype and phenotype is usually a protein: most genes affect phenotypes by encoding proteins. How the presence of a protein produces a particular anatomical, physiological, or behavioral trait, however, is often far from clear, as was illustrated by the story of Lesch-Nyhan disease. The relation between genes and traits is the subject of much current research and will be explored further in Chapters 16 and 21.

In this chapter, we have examined the nature of the genetic code. It is a very concise code, with each codon consisting of three nucleotides, the minimum number capable of specifying all 20 common amino acids. Breaking the genetic code required great ingenuity and hard work on the part of a number of geneticists.

Much of this chapter has centered on protein synthesis. We learned that translation is a highly complex process: rRNAs, ribosomal proteins, tRNAs, mRNA, initiation factors, elongation factors, release factors, and aminoacyl-tRNA synthetases all help to assemble amino acids into a protein. This complexity might seem surprising, because the peptide bonds that hold amino acids together are simple covalent bonds. Translation is complex not because of any special property of the pep-tide bond, but rather because the amino acids must be linked in a highly precise order. The amino acid sequence determines the secondary and tertiary structures of a protein, which are critical to its function; so the genetic information in a mRNA molecule must be accurately translated. The complexity of translation has evolved to ensure that few mistakes are made in the course of protein synthesis.

An important theme in protein synthesis is RNA- RNA interaction, which takes place between tRNAs and mRNA, between mRNA and rRNAs, and between tRNAs and rRNAs. The prominence of these RNA - RNA interactions in translation reinforces the proposal that life first evolved in an RNA world, where flexible and versatile RNA molecules carried out many life processes (Chapter 13).

This chapter has built on our understanding of other processes of information transfer covered earlier in the book: replication (Chapter 12), transcription (Chapter 13), and RNA processing (Chapter 14). It also provides a critical foundation for later discussions of gene regulation (Chapter 16), gene mutations (Chapter 17), and the advanced topics of developmental genetics, cancer genetics, and immunological genetics (Chapter 21).

CONCEPTS SUMMARY)_

• Genes code for phenotypes by specifying the amino acid sequences of proteins.

• The relation between genes and proteins was first suggested by Archibald Garrod.

• Beadle and Tatum developed the one gene, one enzyme hypothesis, which proposed that each gene specifies one enzyme; this hypothesis was later modified to become the one gene, one polypeptide hypothesis.

• Proteins are composed of 20 different amino acids, several or many of which are linked together by peptide bonds. Chains of amino acids fold and associate to produce the secondary, tertiary, and quaternary structures of proteins.

• The genetic code is the way in which genetic information is stored in the nucleotide sequence of a gene.

• Solving the genetic code required several different approaches: the use of synthetic mRNAs with random sequences; short mRNAs that bind tRNAs with their amino acids; and long synthetic mRNAs with regularly repeating sequences.

• The genetic code is a triplet code: three nucleotides specify a single amino acid. It is also degenerate, nonoverlapping, and universal (almost).

• The degeneracy of the code means that more than one codon may specify an amino acid. Different tRNAs (isoaccepting tRNAs) may accept the same amino acid, and different anticodons may pair with the same codon through wobble, which can exist at the third position of the codon and which allows some nonstandard pairing of bases in this position.

• The reading frame is set by the initiation codon.

• The end of the protein-coding section of an mRNA is marked by one of three termination codons.

• Protein synthesis comprises four steps: (1) the binding of amino acids to the appropriate tRNAs, (2) initiation, (3) elongation, and (4) termination.

• The binding of an amino acid to a tRNA requires the presence of a specific aminoacyl-tRNA synthetase and ATP. The amino acid is attached by its carboxyl end to the 3' end of the tRNA.

• In bacterial translation initiation, the small subunit of the ribosome attaches to the mRNA and is positioned over the initiation codon. It is joined by the first tRNA and its associated amino acid (N-formylmethionine in bacterial cells) and, later, by the large subunit of the ribosome. Initiation requires several initiation factors and GTP.

• In elongation, a charged tRNA enters the A site of a ribosome, a peptide-bond is formed between amino acids in the A and P sites, and the ribosome moves (translocates) along the mRNA to the next codon. Elongation requires several elongation factors and GTP.

• Translation is terminated when the ribosome encounters one of the three termination codons. Release factors and GTP are required to bring about termination.

• Like RNA processing, translation requires a number of RNA - RNA interactions.

• Each mRNA may be simultaneously translated by several ribosomes, producing a structure called a polyribosome.

• Many proteins undergo posttranslational modification.

[important terms_

auxotroph (p. 000) one gene, one enzyme hypothesis (p. 000) one gene, one polypeptide hypothesis (p. 000) amino acid (p. 000) peptide bond (p. 000) polypeptide (p. 000) sense codon (p. 000) degenerate genetic code (p. 000) synonymous codons (p. 000)

isoaccepting tRNAs (p. 000) wobble (p. 000) nonoverlapping genetic code (p. 000) reading frame (p. 000) initiation codon (p. 000) stop (termination or nonsense) codon (p. 000) universal genetic code (p. 000) aminoacyl-tRNA synthetase (p. 000) tRNA charging (p. 000)

initiation factors (IF-1, IF-2,

IF-3) (p. 000) 30S initiation complex (p. 000) 70S initiation complex (p. 000) aminoacyl (A) site (p. 000) peptidyl (P) site (p. 000) exit (E) site (p. 000) elongation factor Tu (EF-Tu) (p. 000)

elongation factor Ts (EF-Ts) (p. 000) peptidyl transferase (p. 000) translocation (p. 000) elongation factor G (EF-G) (p. 000) release factors (RF1, RF2, RF3) (p. 000) polyribosome (p. 000) molecular chaperone (p. 000) signal sequence (p. 000)

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