Biochemistry-II - RNA and Transcription - Quick Guides | Biochemistry

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Study GuideBiochemistry-II–RNA and Transcription1.RNA Functions1.1Role of RNA in the CellMost RNA molecules in a cell are involved incarrying, processing, or using genetic information.These roles fit into theCentral Dogma of Molecular Biology, which describes how information flowsin a cell:DNA → RNA → ProteinThis means:•DNA stores genetic information•RNA transfers and interprets that information•Proteins carry out most cellular functions1.2RNA as an Information CarrierRNA acts as atemporary copy of genetic information.•Information stored in DNA is copied into RNA•This RNA carries the instructions from the DNA to other parts of the cell•This allows the information to be used without damaging the DNA itself1.3RNA as a Translator or AdaptorRNA also plays a key role intranslating genetic information into protein.•RNA molecules help convert the nucleotide sequence of RNA•Into the amino acid sequence of a protein•This translation step ensures that genetic information is expressed correctly

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Study Guide1.4RNA as a CatalystSome RNA molecules act ascatalysts.•They help formpeptide bondsbetween amino acids•This step is essential for building proteins•RNA molecules that act as catalysts are sometimes calledribozymesKey TakeawaysRNA is not just a messenger molecule. It plays multiple important roles:•Carries genetic information•Helps translate RNA into protein•Catalyzes key steps in protein synthesisTogether, these functions make RNA central to how genetic information is expressed in the cell.2.Messenger RNA (mRNA)2.1Differences Between Prokaryotic and Eukaryotic mRNAMessenger RNA (mRNA) carries genetic information from DNA to the ribosome, where proteins aremade.However,prokaryotic and eukaryotic mRNAs differ in structure and organization.•Prokaryotic mRNAsare oftenpolycistronicoOne mRNA molecule carries information formore than one protein•Eukaryotic mRNAsare usuallymonocistronicoEach mRNA molecule codes fora single protein2.2Structural Features of Eukaryotic mRNAEukaryotic mRNAs containspecial structural modificationsthat are not found in prokaryoticmRNAs.

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Study GuideWhile prokaryotic mRNAs usually contain only the four standard bases (A, C, G, and U), eukaryoticmRNAs have additional features at both ends.The 5′ Cap StructureAt the5′ endof eukaryotic mRNA is a modified structure called thecap.•The cap contains a7-methylguanosinenucleotide•It is attached by anunusual 5′–5′ phosphodiester linkage•This type of linkage does not occur elsewhere in RNAIn addition:•One or two nearby nucleotides aremethylated at the 2′ oxygen•These methylations formcap 0, cap 1, or cap 2 structures

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Study GuideFigure 1The 5′ cap plays important roles in:•Protecting mRNA from degradation•Helping ribosomes recognize and bind to the mRNA•Promoting efficient translation

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Study Guide3′ Untranslated Region and Poly(A) TailEukaryotic mRNAs often contain along 3′ untranslated region (3′ UTR).•This region comesafter the stop codon•It does not code for protein•It helps regulate mRNA stability and translationMost eukaryotic mRNAs end with apolyadenylic acid (polyA) tail:•A chain of up to200 adenosine (A) residues•Addedafter transcription•Not encoded directly by the DNA template2.3Exceptions and Function of the Poly(A) TailNot all mRNAs have polyA tails.•Histone mRNAs, for example, lack polyA sequencesPolyadenylation is important because:•It increasesmRNA stability•It helps controlhow long an mRNA remains functionalAn early step in the degradation of many mRNAs is theremoval of the polyA tail, which often leadsto rapid breakdown of the rest of the molecule.Key TakeawaysMessenger RNA shows clear differences between prokaryotes and eukaryotes:•Prokaryotic mRNA: often polycistronic, simpler structure•Eukaryotic mRNA: monocistronic, contains a5′ cap,3′ untranslated region, and usually apolyA tailThese structural features help regulatemRNA stability, translation, and lifespanin the cell.

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Study Guide3.Transfer RNA (tRNA)3.1Role of tRNA in Protein SynthesisTransfer RNAs (tRNAs) play acentral role in linking genetic information to proteins.They connect the language of nucleic acids (RNA) with the language of proteins (amino acids).Each tRNA hastwo main functions:1.Binding a specific amino acid2.Placing that amino acid at the correct position in a growing proteinAccurate Amino Acid AttachmentEach tRNA carriesone specific amino acidout of the 20 commonly used in proteins.•This process is extremely accurate—greater than 99.99% precision•tRNAs can even distinguish between amino acids that arechemically very similarThis high accuracy is essential for making correct proteins.Shared Features Among All tRNAsAlthough each tRNA carries a different amino acid,all tRNAs must work equally well on theribosome.•The ribosome must accept any tRNA•Each amino acid must be added withsimilar efficiency•Otherwise, protein production would be limitedBecause of this, all tRNAs sharecommon structural featuresthat are recognized by the ribosome.3.2Secondary Structure of tRNA (Cloverleaf Model)At thesecondary structure level, all tRNAs fold into acloverleaf shape.Key features include:

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Study Guide•The5′ and 3′ endsare base-paired to form the acceptor stem•Three additional stem-loop structures are formed by internal base pairing•Only a few nucleotide sequences are identical across all tRNAs, but the overall shape isconservedAll tRNAs end with the sameacceptor sequence: CCA.Figure 13.3The Acceptor Stem and Amino Acid AttachmentTheCCA sequence at the 3′ endis where the amino acid attaches.•The CCA sequence may be:oDirectly transcribed from DNA, oroAdded later by a specific enzyme•The terminaladenosine (A)does not have a 2′ or 3′ phosphate•The amino acid attaches via anester bond•Depending on the tRNA, attachment occurs at either:oThe2′ hydroxyl, oroThe3′ hydroxylof the terminal A

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Study GuideThe Anticodon: Reading the Genetic CodeAnother critical region of tRNA is theanticodon.•The anticodon is athree-base sequence•It base-pairs with thecodon on mRNA•This interaction ensures the correct amino acid is placed in the proteinThe anticodon is located:•At the same relative position in all tRNAs•Far away from the acceptor stem in the cloverleaf structureFunctional Meaning of tRNA StructureThe structure of tRNA reflects its function:•Theacceptor stembinds a specific amino acid•Theanticodondetermines where that amino acid is added in the proteinThis dual role allows tRNA to act as amolecular adaptorduring translation.Tertiary Structure of tRNAAt thetertiary structure level, all tRNAs have a similarL-shaped structure.•One end of the L contains theanticodon•The other end contains theacceptor stem•Each arm of the L is formed by stacking two stems from the cloverleaf structureRegions that are not base-paired participate intertiary interactions, helping stabilize the 3D shape.Importance of the L-Shaped StructureThe L-shaped structure keeps the anticodon and amino acid attachment sitewell separated.

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Study GuideThis arrangement:•Allows two tRNAs to bind toadjacent codons on an mRNA•Enables smooth and accurate protein synthesis on the ribosomeKey TakeawaysTransfer RNA is essential for translation because it:•Accurately binds specific amino acids•Recognizes mRNA codons using its anticodon•Has conserved secondary and tertiary structures•Acts as a bridge between RNA information and protein structuretRNA structure and function are tightly linked, ensuringprecise and efficient protein synthesis.4.Ribosomal RNA (rRNA)4.1Role of rRNA in Protein SynthesisRibosomal RNA (rRNA) isessential for making proteins.In fact, rRNA is believed to be thecatalytically active componentof the ribosome—the largemolecular machine that assembles proteins.Although ribosomes contain both proteins and RNA, it is theRNA part that carries out key catalyticfunctions, especially during peptide bond formation.Structure of RibosomesRibosomes arecomplex structuresmade of:•Ribosomal RNA (rRNA)•Ribosomal proteinsRibosomes can be separated intotwo subunits:•Asmall subunit•Alarge subunit

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Study GuideEach subunit contains:•One or more rRNA molecules•Several proteinsThe rRNAs in the small and large subunits aredifferent in sizeand structure.4.2Similarities and Differences in rRNARibosomal RNAs vary greatly in size between organisms and between ribosomal subunits.However, they sharecommon secondary structural features.Ineukaryotes, rRNAs are larger than those in prokaryotes.This larger size is not due to a completely new structure. Instead:•Extrastructural domainsare inserted into the basic rRNA framework•The overall folding pattern remains similar to that of smaller rRNAsAntibiotics and rRNAManyantibioticsare natural substances produced by soil bacteria or molds.They work byinhibiting the growth of other bacteria, often by targeting ribosomal RNA.Example: Streptomycin•Streptomycin is an antibiotic used to treattuberculosis•It binds to a specific region ofbacterial 16S rRNA•This binding interferes with protein synthesis in bacteriaImportantly:•Streptomycin doesnotsignificantly affect human ribosomes•This selectivity gives the drug ahigh therapeutic indexoMeaning it is effective at doses that are not very harmful to humans

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