Chapter 17—From Gene To Protein - Weebly

Chapter 17—From Gene toProtein“Let me out of the nucleus, finally!”orThe functional definition of a gene

I. The Connection Between Genes & ProteinsMetabolic defects non-functional enzymes Studying metabolic diseases suggested thatgenes specified proteins- alkaptonuria (black urine from alkapton)- PKU (phenylketonuria) Conclusion: Genes dictate phenotype

Beadle and Tatum (1941)Experiments with Neurospora

ARGININE MUTANTS

One Gene-One Enzyme Hypothesis (Beadle & Tatum)The function of a gene is to dictate the production of a specific enzymeOne Gene—One Enzyme but not all proteins are enzymes those proteins are coded by genes tooOne Gene—One Protein but many proteins are composed of several polypeptides, each of whichhas its own geneOne Gene—One Polypeptide

CENTRAL DOGMA ofMolecular BiologyHow does the informationget from nucleus tocytoplasm?Messenger RNA (mRNA) bridge between DNA &protein

DNA vs. RNASugarDNARNANucleotidesStranded

Transcription—synthesis of RNAunder the direction of DNA 1 DNA strand is template strand complementary RNA strandis made– messenger RNA (mRNA) Enzyme RNA polymeraseTranslation: Synthesis of a polypeptide under thedirection of mRNADNA RNA Protein

Prokaryotes vs. Eukaryotes

The GeneticCode3 letter codon genetic “word”Each word standsfor an amino acid

AUG start codon(starts a protein,adds a methionine)UAA/UAG/UGA stop codons(signals end ofprotein)Code is universal—from bacteria toplants & animals(evolved very earlyin the history of life)“Redundancy without Ambiguity”

II. The Synthesis & Processing of RNATranscriptionStep 1 Initiation:RNA polymerasebinds to promotersequence on DNA

Step 2 Elongation:RNA is made5 3

Step 3 Termination:RNA polymerasestops at terminatorsequencemRNA leavesnucleus throughpores afterprocessing

Transcription Animationhttp://www-class.unl.edu/biochem/gp2/m biology/animation/gene/gene a2.html

Initiation of Transcription at aEukaryotic PromoterTranscription factors bind toDNATHEN RNA Polymerase bindsTranscription Factors proteins which bind to DNA & turntranscription “on” or “off” (masterregulators)Transcription Unit length of DNAthat is transcribed into RNA

RNA Processing (only in eukaryotes!)Step 1: Alteration of mRNA ends-- add 5’ cap (modified G (GTP))-- protects RNA from degradation (hydrolytic enzymes in cytoplasm)-- “attach here” sign for ribosomes-- add polyA tail (50-250 A’s)-- same as above and -- helps mRNA export from nucleus

Step 2: RNA Splicing (cut & paste)-- Pre-mRNA mRNA-- edit out introns (noncoding regions)-- splice together exons (coding sequences)In higher eukaryotes 90% or more of gene can be intronno one knows why yet

Spliceosome snRNPs &proteins Recognize splice sites Ribozymes catalyze splicingprocess

Exon protein domainAlternative RNAsplicing

III. Synthesis of Protein

How are the codons read?

Translation Basics Ribosome reads mRNA in codons tRNA anticodon base pairs withcodon of mRNA tRNA brings in correct amino acid Amino acids assembled intopolypeptide chain

Structure of Transfer RNA(tRNA)“clover leaf” structure anticodon on “clover leaf”end amino acid on 3’ end

Aminoacyl-tRNA SynthetasesEnzyme that bonds aminoacids to tRNAs

RibosomesConsist of proteins andribosomal RNA (rRNA)2 subunits (large & small)Made in nucleolus (eukaryotes)and then exportedFigure 17.15 pg. 316

P site (peptidyl-tRNA site)holds tRNA carrying growing polypeptide chainA site (aminoacyl-tRNA site)holds tRNA carrying next amino acid to be added to chainE site (exit site)discharged tRNA leaves ribosome from exit siteTRANSLATION DETAILSStep 1: Initiation—requires small ribosomal subunit, Met tRNA, large ribosomalsubunit, initiation factors (proteins), & GTP (energy)

Step 2:ElongationAmino acids areadded one byone to thepreceding A.A.Occurs in a 3-stepcycle with thehelp ofelongationfactors(proteins):1. Codonrecognition(A site)2. Peptide bondformation3. Translocation (Ato P site)Ribosomemoves 5’ 3’on mRNA

Step 3: Terminationstop codon in mRNA reaches A siterelease factor (protein) binds to stop codonprotein is freed from the ribosomeother components disassemble

Polyribosomes—Clusters of ribosomesthat translate a singlemRNA simultaneouslyMany copies of aprotein are madequickly(prokaryotes &eukaryotes)

Coupled Transcription &TranslationProkaryotes—With no nucleus bacteriacan simultaneouslytranscribe & translate thesame gene

Signal peptide—(postal code)stretch of amino acids that targets the protein to a specificdestination in a eukaryotic cellPosttranslational Modifications—(Polypeptide Functional Protein)chemically modified amino acids, removal of amino acids, polypeptidecut into pieces, or several joined together

Point mutations can affect proteinstructure and functionMutations—changes in the genetic material of a cellPoint mutation–a change in 1 base pair of a geneExample—sickle cell anemia

2 Categories of point mutations:1. Substitutionssilent mutations—same A.A. insertedmissense mutations—different A.A. insertednonsense mutations—Stop codon formed

2. Insertions/Deletionsframeshift mutation—changes reading framecauses shortened,nonfunctional proteinsMutagens?

1. A mutation results in a defective enzyme a. In thefollowing simple metabolic pathway, what would bea consequence of that mutation?enzyme aabenzyme bca) an accumulation of A and no production of B and Cb) an accumulation of A and B and no production of Cc) an accumulation of B and no production of A and Cd) an accumulation of B and C and no production of Ae) an accumulation of C and no production of A and BCopyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

3. A portion of the genetic code is UUU phenylalanine,GCC alanine, AAA lysine, and CCC proline. Assumethe correct code places the amino acids phenylalanine,alanine, and lysine in a protein (in that order). Which of thefollowing DNA sequences would substitute proline d)AAA-GGG-TTTe)AAA-CCC-TTTCopyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

4. A particular triplet of bases in the coding sequenceof DNA is AAA. The anticodon on the tRNA thatbinds the mRNA codon isa) TTT.b) UUA.c) UUU.d) AAA.e) either UAA or TAA, depending on wobble in thefirst base.Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

5. A part of an mRNA molecule with the following sequence isbeing read by a ribosome: 5' CCG-ACG 3' (mRNA). Thefollowing activated transfer RNA molecules are available. Twoof them can correctly match the mRNA so that a dipeptide canform.The dipeptide that willform will betRNA AnticodonAmino AcidGGCProlineCGUAlanineUGCThreonineb) proline-threonine.CCGGlycinec) glycine-cysteine.ACGCysteined) alanine-alanine.CGGAlaninee) threonine-glycine.Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummingsa) cysteine-alanine.

7. Each of the following is a modification of the HERATWhich of the above is analogous to a frameshift mutation?a)Ab)Bc)Cd)De)ECopyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

8. Each of the following is a modification of the HERATWhich of the above is analogous to a single substitution mutation?a)Ab)Bc)Cd)De)ECopyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

9. What is the relationship among DNA, a gene, and achromosome?a)A chromosome contains hundreds of genes, which arecomposed of protein.b)A chromosome contains hundreds of genes, which arecomposed of DNA.c)A gene contains hundreds of chromosomes, which arecomposed of protein.d)A gene is composed of DNA, but there is no relationshipto a chromosome.e)A gene contains hundreds of chromosomes, which arecomposed of DNA.Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

10.A biologist inserts a gene from a human liver cell intothe chromosome of a bacterium. The bacterium then transcribesthis gene into mRNA and translates the mRNA into protein. Theprotein produced is useless.The biologist extracts the protein and mature mRNA that codesfor it. When analyzed you would expect which of the followingresults?a)the protein and the mature mRNA are longer than in humancellsb)the protein and mature mRNA are shorter than expectedc)the protein is longer and the mRNA is shorter than expectedd)the protein is shorter and the mRNA is longer than expectedCopyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

One Gene-One Enzyme Hypothesis (Beadle & Tatum) The function of a gene is to dictate the production of a specific enzyme One Gene—One Enzyme but not all proteins are enzymes those proteins are coded by genes too One Gene—One Protein but many proteins are composed of several polypeptides, each of which has its own gene One Gene—One Polypeptide