CHAPTER 17 FROM GENE TO PROTEIN Section A: …lhsteacher.lexingtonma.org/Pohlman/17A-GenesAndProteins.pdfenzymes and yet their synthesis depends on speciﬁc genes. •This tweaked

CHAPTER 17FROM GENE TO PROTEIN

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Section A: The Connection Between Genesand Proteins

1. The study of metabolic defects provided evidence that genes specifyproteins

2. Transcription and translation are the two main processing linking gene toprotein: an overview

3. In the genetic code, nucleotide triplets specify amino acids4. The genetic code must have evolved very early in the history of life

• The information content of DNA is in the form ofspecific sequences of nucleotides along the DNAstrands.

• The DNA inherited by an organism leads to specifictraits by dictating the synthesis of proteins.

• Proteins are the links between genotype andphenotype.• For example, Mendel’s dwarf pea plants lack a

functioning copy of the gene that specifies the synthesis ofa key protein, gibberellins.

• Gibberellins stimulate the normal elongation of stems.

Introduction


• In 1909, Archibald Gerrod was the first to suggestthat genes dictate phenotype through enzymes thatcatalyze specific chemical reactions in the cell.

• The symptoms of an inherited disease reflect aperson’s inability to synthesize a particular enzyme.

• Gerrod speculated that alkaptonuria, a hereditarydisease, was caused by the absence of an enzymethat breaks down a specific substrate, alkapton.

• Research conducted several decades later supportedGerrod’s hypothesis.

1. The study of metabolic defects providedevidence that genes specify proteins


• Progress in linking genes and enzymes rested onthe growing understanding that cells synthesizeand degrade most organic molecules in a series ofsteps, a metabolic pathway.

• In the 1930s, George Beadle and Boris Ephrussispeculated that each mutation affecting eye colorin Drosophila blocks pigment synthesis at aspecific step by preventing production of theenzyme that catalyzes that step.

• However, neither the chemical reactions nor theenzymes were known at the time.


• Beadle and Edward Tatum were finally able toestablish the link between genes and enzymes intheir exploration of the metabolism of a breadmold, Neurospora crassa.• They mutated Neurospora with X-rays and screened the

survivors for mutants that differed in their nutritionalneeds.

• Wild-type Neurospora can grow on a minimal mediumof agar, inorganic salts, glucose, and the vitamin biotin.

• Most nutritional mutants can survive on a completegrowth medium which includes all 20 amino acids.


• One type of mutant required only the addition ofarginine to the minimal growth medium.• Beadle and Tatum concluded that this mutant was

defective somewhere in the biochemical pathway thatnormally synthesizes arginine.

• They identified three classes of arginine deficientmutants, each apparently lacking a key enzyme at adifferent step in the synthesis of arginine.• They demonstrated this by growing these mutant strains

in media that provided different intermediate molecules.• Their results provided strong evidence for the one

gene - one enzyme hypothesis.



Fig. 17.1

• Later research refined the one gene - one enzymehypothesis.

• First, it became clear that not all proteins areenzymes and yet their synthesis depends on specificgenes.• This tweaked the hypothesis to one gene - one protein.

• Later research demonstrated that many proteins arecomposed of several polypeptides, each of whichhas its own gene.

• Therefore, Beadle and Tatum’s idea has beenrestated as the one gene - one polypeptidehypothesis.


• Genes provide the instructions for making specificproteins.

• The bridge between DNA and protein synthesis isRNA.

• RNA is chemically similar to DNA, except that itcontains ribose as its sugar and substitutes thenitrogenous base uracil for thymine.• An RNA molecules almost always consists of a single

strand.

2. Transcription and translation are the twomain processes linking gene to protein: anoverview


• In DNA or RNA, the four nucleotide monomersact like the letters of the alphabet to communicateinformation.

• The specific sequence of hundreds or thousands ofnucleotides in each gene carries the information forthe primary structure of a protein, the linear orderof the 20 possible amino acids.

• To get from DNA, written in one chemicallanguage, to protein, written in another, requirestwo major stages, transcription and translation.


• During transcription, a DNA strand provides atemplate for the synthesis of a complementaryRNA strand.• This process is used to synthesize any type of RNA

from a DNA template.

• Transcription of a gene produces a messengerRNA (mRNA) molecule.

• During translation, the information contained inthe order of nucleotides in mRNA is used todetermine the amino acid sequence of apolypeptide.• Translation occurs at ribosomes.


• The basic mechanics of transcription and translationare similar in eukaryotes and prokaryotes.

• Because bacteria lack nuclei, transcription andtranslation are coupled.

• Ribosomes attach to the leading end of a mRNAmolecule while transcription is still in progress.


Fig. 17.2a

• In a eukaryotic cell, almost all transcription occursin the nucleus and translation occurs mainly atribosomes in the cytoplasm.

• In addition, before theprimary transcriptcan leave the nucleusit is modified invarious ways duringRNA processingbefore the finishedmRNA is exportedto the cytoplasm.


Fig. 17.2b

• To summarize, genes program protein synthesis viagenetic messenger RNA.

• The molecular chain of command in a cell is :

DNA -> RNA -> protein.


• If the genetic code consisted of a single nucleotide oreven pairs of nucleotides per amino acid, there wouldnot be enough combinations (4 and 16 respectively)to code for all 20 amino acids.

• Triplets of nucleotide bases are the smallest units ofuniform length that can code for all the amino acids.

• In the triplet code, three consecutive bases specifyan amino acid, creating 43 (64) possible code words.

• The genetic instructions for a polypeptide chain arewritten in DNA as a series of three-nucleotide words.

3. In the genetic code, nucleotide tripletsspecify amino acids


• During transcription, one DNA strand, the templatestrand, provides a template for ordering thesequence of nucleotides in an RNA transcript.• The complementary RNA

molecule is synthesizedaccording to base-pairingrules, except that uracil isthe complementary baseto adenine.

• During translation, blocksof three nucleotides,codons, are decoded intoa sequence of amino acids.


Fig. 17.3

• During translation, the codons are read in the 5’->3’direction along the mRNA.

• Each codon specifies which one of the 20 aminoacids will be incorporated at the correspondingposition along a polypeptide.

• Because codons are base triplets, the number ofnucleotides making up a genetic message must bethree times the number of amino acids making upthe protein product.• It would take at least 300 nucleotides to code for a

polypeptide that is 100 amino acids long.


• The task of matching each codon to its amino acidcounterpart began in the early 1960s.

• Marshall Nirenberg determined the first match, thatUUU coded for the amino acid phenylalanine.• He created an artificial mRNA molecule entirely of uracil

and added it to a test tube mixture of amino acids,ribosomes, and other components for protein synthesis.

• This “poly(U)” translated into a polypeptide containing asingle amino acid, phenyalanine, in a long chain.

• Other more elaborate techniques were required todecode mixed triplets such a AUA and CGA.


• By the mid-1960s the entire code was deciphered.• 61 of 64 triplets code

for amino acids.

• The codon AUG notonly codes for theamino acid methioninebut also indicates thestart of translation.

• Three codons donot indicate aminoacids but signalthe terminationof translation.


Fig. 17.4

• The genetic code is redundant but not ambiguous.• There are typically several different codons that would

indicate a specific amino acid.

• However, any one codon indicates only one amino acid.

• [If you have a specific codon, you can be sure of thecorresponding amino acid, but if you know only theamino acid, there may be several possible codons.]

• Both GAA and GAG specify glutamate, but no otheramino acid.

• Codons synonymous for the same amino acid often differonly in the third codon position.


• To extract the message from the genetic coderequires specifying the correct starting point.• This establishes the reading frame and subsequent

codons are read in groups of three nucleotides.

• The cell’s protein-synthesizing machinery reads themessage as a series of nonoverlapping three-letter words.

• In summary, genetic information is encoded as asequence of nonoverlapping base triplets, or codons,each of which is translated into a specific amino acidduring protein synthesis.


• The genetic code is nearly universal, shared byorganisms from the simplest bacteria to the mostcomplex plants and animals.

• In laboratory experiments,genes can be transcribed andtranslated after they aretransplanted from one speciesto another.• This tobacco plant is expressing

a transpired firefly gene.

4. The genetic code must have evolved veryearly in the history of life

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 17.5

• This has permitted bacteria to be programmed tosynthesize certain human proteins after insertion ofthe appropriate human genes.

• This and other similar applications are excitingdevelopments in biotechnology.

• Exceptions to the universality of the genetic codeexist in translation systems where a few codonsdiffer from standard ones.• These occur in certain single-celled eukaryotes like

Paramecium.

• Other examples include translation in certainmitochondria and chloroplasts.


• The near universality of the genetic code must havebeen operating very early in the history of life.

• A shared genetic vocabulary is a reminder of thekinship that bonds all life on Earth.


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CHAPTER 17 FROM GENE TO PROTEIN Section A: …lhsteacher.lexingtonma.org/Pohlman/17A-GenesAndProteins.pdfenzymes and yet their synthesis depends on speciﬁc genes. •This tweaked