Biochemistry I - Overview of Biological Information Flow - Quick Guides | Biochemistry

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Study GuideBiochemistry I–Overview of Biological InformationFlow1.RNA Carries Genetic InformationDNA stores genetic information in two complementary strands. Each strand carries information thatcan be used to build the other. However, duringtranscription, usuallyonly one DNA strandis usedas a template to make RNA.RNA is different from DNA in an important way—it issingle-stranded, not double-stranded.How DNA Is Copied into RNARNA is made by matching bases with the DNA template strand:•DNA baseApairs with RNA baseU•DNA baseTpairs with RNA baseA•DNA baseCpairs with RNA baseG•DNA baseGpairs with RNA baseCFor example:A DNA sequence3′ ATCCG 5′is transcribed into an RNA sequence5′ UAGGC 3′RNA Is Temporary and FlexibleUnlike DNA, RNA isnot permanent. A single DNA gene can producemany RNA copies, which areused to make proteins and then broken down into nucleotides again.This disposable nature of RNA allows cells torespond quickly to changesby making only the RNAthey need at a given time.

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Study GuidePromoters and Transcription Start SitesRNA synthesis does not begin randomly. Special DNA sequences calledpromoterstellRNApolymerase(the enzyme that makes RNA)where to start transcription.•Inprokaryotes, common promoter regions are the–35 regionand–10 region•Ineukaryotes, promoter regions often include the–70 regionand–25 (TATA box)Figure 1 Messenger RNA specifies the order of amino acids in proteins1.1Messenger RNA and the Genetic CodeProteins are long chains (polymers) ofamino acids.Theorder of amino acidsdetermines a protein’s structure and function.Codons: Reading the MessageThe message inmRNAis read in groups ofthree nucleotides, calledcodons.•There are4 RNA bases(A, U, G, C)•Possible codons =4³ = 64•Only20 amino acidsexist

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Study GuideThis means:•More than one codon can code for the same amino acid•The genetic code isredundantStart and Stop SignalsThe genetic code includes special signals, similar to punctuation in language:•AUG→ codes formethionine (Met)oActs as thestart codonoSignals where protein synthesis begins•UAA, UAG, UGA→stop codonsoSignal the end of protein synthesisMethionine can appearboth at the start and within proteins, just like a capital letter can appear atthe start or inside a sentence.

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Study Guide1.2Universality of the Genetic CodeAlmost all organisms use thesame genetic code, from bacteria to humans.However, small variations exist.Example: Mycoplasma•Mycoplasma DNA is rich inA and T•The codonUGA, which is normally a stop codon, codes fortryptophanin Mycoplasma•This happens because the usual tryptophan codon (UGG) is rare in its genomeAmino Acid AbbreviationsHere are standard abbreviations used for amino acids:•phe–phenylalanine•leu–leucine

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Study Guide•ile–isoleucine•met–methionine•val–valine•ser–serine•pro–proline•thr–threonine•ala–alanine•tyr–tyrosine•his–histidine•gln–glutamine•asn–asparagine•lys–lysine•asp–aspartic acid•glu–glutamic acid•cys–cysteine•trp–tryptophan•arg–arginine•gly–glycineTransfer RNA (tRNA)Transfer RNA (tRNA) acts as abridge between mRNA and proteins. It ensures that the correctamino acid is added for each codon.Two Key Functional Parts of tRNA1.AnticodonoA set ofthree nucleotidesoBase-pairs with the complementarymRNA codon

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Study Guide2.Acceptor EndoLocated at the3′ endoHolds the specificamino acidCharging the tRNAEach amino acid is attached to its correct tRNA by a specific enzyme calledaminoacyl-tRNAsynthetase.This step ensuresaccuracy in protein synthesis.Figure 2Ribosomes and translation1.3Ribosomes and TranslationRibosomesare the cellular machines that build proteins.Structure of Ribosomes•Composed of about2/3 RNAand1/3 protein•Made oftwo subunits:

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Study GuideoSmall subunitoLarge subunitFunctions of RibosomesRibosomes perform several key tasks:•Start protein synthesis (initiation)•Match mRNA codons with tRNA anticodons•Formpeptide bondsbetween amino acids•Move mRNA through the ribosome•Release the completed proteinRole of Ribosomal Subunits•Small subunitoBinds mRNAoHelps in codon–anticodon pairingoInvolved in initiation and termination•Large subunitoCatalyzes peptide bond formationoExtends the growing polypeptide chain

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