Page 112 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

8.1 The Genetic Code 271

the process known as translation. It takes place on molecular workbenches called Figure 8.1 Gene expression: The
ribosomes, which are composed of proteins and ribosomal RNAs (rRNAs). Trans­ flow of genetic information from
lation depends on the dictionary known as the genetic code, which defines each DNA via RNA to protein. In
amino acid in terms of specific sequences of three nucleotides. Translation also re­ transcription, the enzyme RNA
quires transfer RNAs (tRNAs), small RNA adapter molecules that place specific polymerase copies DNA to produce an
RNA transcript. In translation, the
amino acids at the correct position in a growing polypeptide chain. cellular machinery uses instructions in
The Central Dogma does not explain the behavior of all genes. As Crick mRNA to synthesize a polypeptide,
himself realized, many genes are transcribed into RNAs that are never translated following the rules of the genetic code.
into proteins. You will see in this chapter that many nontranslated RNAs are DNA
critical to various steps of gene expression. The genes encoding rRNAs and
tRNAs belong to this group.
Four general themes emerge from our discussion of gene expression. First, Transcription
the pairing of complementary bases is key to the transfer of information from
DNA to RNA, and from RNA to protein. Second, the polarities (directionality)
of DNA, RNA, and polypeptides help guide the mechanisms of gene expres­ RNA transcript: serves directly as mRNA
sion. Third, like DNA replication and recombination, gene expression requires in prokaryotes; processed to become
an input of energy and the participation of specific proteins, RNAs, and mac­ mRNA in eukaryotes
romolecular assemblies, such as ribosomes. Finally, mutations that change ge­ Translation
netic information or obstruct the flow of its expression can have dramatic
effects on phenotype.

Polypeptide

8.1 The Genetic Code more letters exist than the two symbols (dot or dash), groups
of one, two, three, or four dots or dashes in various combi­
nations represent individual letters. For example, the sym­
bol for C is dash dot dash dot (– · – ·), the symbol for O is
learning objectives
dash dash dash (– – –), D is dash dot dot (– · ·), and E is a
1. Explain the reasoning that established a sequence of single dot (·). Because anywhere from one to four symbols
three nucleotides (a triplet codon) as the basic unit of specify each letter, the Morse code requires a symbol for
the code relating DNA to protein. pause (in practice, a short interval of time) to signify where
2. Summarize the evidence showing that the sequence of one letter ends and the next begins.
nucleotides in a gene is colinear with the sequence of
amino acids in a protein.
3. Define reading frame and discuss its significance to the Triplet Codons of Nucleotides Represent
genetic code. Individual Amino Acids
4. Describe experiments that determined which codons The language of nucleic acids is written in four nucleo­
are associated with each amino acid and which are tides—A, G, C, and T in the DNA dialect; A, G, C, and U
stop codons. in the RNA dialect—while the language of proteins is writ­
5. Explain how mutations were used to verify the genetic ten in amino acids. The first hurdle to be overcome in deci­
code. phering how sequences of nucleotides can determine the
6. Discuss evidence that the genetic code is almost order of amino acids in a polypeptide is to determine how
universal, and cite some exceptions. many amino acid “letters” exist.
Over lunch one day at a local pub, Watson and Crick
produced the now accepted list of the 20 common amino
A code is a system of symbols that equates information in acids that are encoded directly by DNA. They created the
one language with information in another. A useful anal­ list by analyzing the known amino acid sequences of a
ogy for the genetic code is the Morse code, which uses dots variety of naturally occurring polypeptides. Amino acids
and dashes to transmit messages over radio or telegraph present in only a small number of proteins or in only cer­
wires. Various groupings of the dot­dash symbols represent tain tissues or organisms did not qualify as standard
the 26 letters of the English alphabet. And because many building blocks; Crick and Watson correctly assumed