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Biochemistry-II - RNA and Transcription

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    Study Guide Biochemistry - II – RNA and Transcription 1. RNA Functions 1.1 Role of RNA in the Cell Most RNA molecules in a cell are involved in carrying, processing, or using genetic information . These roles fit into the Central Dogma of Molecular Biology , which describes how information flows in a cell: DNA → RNA → Protein This means: • DNA stores genetic information • RNA transfers and interprets that information • Proteins carry out most cellular functions 1. 2 RNA as an Information Carrier RNA acts as a temporary 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 itself 1. 3 RNA as a Translator or Adaptor RNA also plays a key role in translating 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 Guide 1.4 RNA as a Catalyst Some RNA molecules act as catalysts . • They help form peptide bonds between amino acids • This step is essential for building proteins • RNA molecules that act as catalysts are sometimes called ribozymes Key Takeaways RNA is not just a messenger molecule. It plays multiple important roles: • Carries genetic information • Helps translate RNA into protein • Catalyzes key steps in protein synthesis Together, these functions make RNA central to how genetic information is expressed in the cell. 2. Messenger RNA (mRNA) 2.1 Differences Between Prokaryotic and Eukaryotic mRNA Messenger RNA (mRNA) carries genetic information from DNA to the ribosome, where proteins are made. However, prokaryotic and eukaryotic mRNAs differ in structure and organization . • Prokaryotic mRNAs are often polycistronic o One mRNA molecule carries information for more than one protein • Eukaryotic mRNAs are usually monocistronic o Each mRNA molecule codes for a single protein 2.2 Structural Features of Eukaryotic mRNA Eukaryotic mRNAs contain special structural modifications that are not found in prokaryotic mRNAs.

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    Study Guide While prokaryotic mRNAs usually contain only the four standard bases (A, C, G, and U), eukaryotic mRNAs have additional features at both ends. The 5′ Cap Structure At the 5′ end of eukaryotic mRNA is a modified structure called the cap . • The cap contains a 7 - methylguanosine nucleotide • It is attached by an unusual 5′ – 5′ phosphodiester linkage • This type of linkage does not occur elsewhere in RNA In addition: • One or two nearby nucleotides are methylated at the 2′ oxygen • These methylations form cap 0, cap 1, or cap 2 structures

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    Study Guide Figure 1 The 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 Guide 3′ Untranslated Region and Poly(A) Tail Eukaryotic mRNAs often contain a long 3′ untranslated region (3′ UTR) . • This region comes after the stop codon • It does not code for protein • It helps regulate mRNA stability and translation Most eukaryotic mRNAs end with a polyadenylic acid (polyA) tail : • A chain of up to 200 adenosine (A) residues • Added after transcription • Not encoded directly by the DNA template 2.3 Exceptions and Function of the Poly(A) Tail Not all mRNAs have polyA tails. • Histone mRNAs , for example, lack polyA sequences Polyadenylation is important because: • It increases mRNA stability • It helps control how long an mRNA remains functional An early step in the degradation of many mRNAs is the removal of the polyA tail , which often leads to rapid breakdown of the rest of the molecule. Key Takeaways Messenger RNA shows clear differences between prokaryotes and eukaryotes: • Prokaryotic mRNA: often polycistronic, simpler structure • Eukaryotic mRNA: monocistronic, contains a 5′ cap , 3′ untranslated region , and usually a polyA tail These structural features help regulate mRNA stability, translation, and lifespan in the cell.

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    Study Guide 3. Transfer RNA (tRNA) 3.1 Role of tRNA in Protein Synthesis Transfer RNAs (tRNAs) play a central role in linking genetic information to proteins . They connect the language of nucleic acids (RNA) with the language of proteins (amino acids). Each tRNA has two main functions : 1. Binding a specific amino acid 2. Placing that amino acid at the correct position in a growing protein Accurate Amino Acid Attachment Each tRNA carries one specific amino acid out 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 are chemically very similar This high accuracy is essential for making correct proteins. Shared Features Among All tRNAs Although each tRNA carries a different amino acid, all tRNAs must work equally well on the ribosome . • The ribosome must accept any tRNA • Each amino acid must be added with similar efficiency • Otherwise, protein production would be limited Because of this, all tRNAs share common structural features that are recognized by the ribosome. 3.2 Secondary Structure of tRNA (Cloverleaf Model) At the secondary structure level , all tRNAs fold into a cloverleaf shape . Key features include:

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    Study Guide • The 5′ and 3′ ends are 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 is conserved All tRNAs end with the same acceptor sequence: CCA . Figure 1 3.3 The Acceptor Stem and Amino Acid Attachment The CCA sequence at the 3′ end is where the amino acid attaches. • The CCA sequence may be: o Directly transcribed from DNA, or o Added later by a specific enzyme • The terminal adenosine (A) does not have a 2′ or 3′ phosphate • The amino acid attaches via an ester bond • Depending on the tRNA, attachment occurs at either: o The 2′ hydroxyl , or o The 3′ hydroxyl of the terminal A

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    Study Guide The Anticodon: Reading the Genetic Code Another critical region of tRNA is the anticodon . • The anticodon is a three - base sequence • It base - pairs with the codon on mRNA • This interaction ensures the correct amino acid is placed in the protein The anticodon is located: • At the same relative position in all tRNAs • Far away from the acceptor stem in the cloverleaf structure Functional Meaning of tRNA Structure The structure of tRNA reflects its function: • The acceptor stem binds a specific amino acid • The anticodon determines where that amino acid is added in the protein This dual role allows tRNA to act as a molecular adaptor during translation. Tertiary Structure of tRNA At the tertiary structure level , all tRNAs have a similar L - shaped structure . • One end of the L contains the anticodon • The other end contains the acceptor stem • Each arm of the L is formed by stacking two stems from the cloverleaf structure Regions that are not base - paired participate in tertiary interactions , helping stabilize the 3D shape. Importance of the L - Shaped Structure The L - shaped structure keeps the anticodon and amino acid attachment site well separated .

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    Study Guide This arrangement: • Allows two tRNAs to bind to adjacent codons on an mRNA • Enables smooth and accurate protein synthesis on the ribosome Key Takeaways Transfer 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 structure tRNA structure and function are tightly linked, ensuring precise and efficient protein synthesis . 4. Ribosomal RNA (rRNA) 4.1 Role of rRNA in Protein Synthesis Ribosomal RNA (rRNA) is essential for making proteins . In fact, rRNA is believed to be the catalytically active component of the ribosome — the large molecular machine that assembles proteins. Although ribosomes contain both proteins and RNA, it is the RNA part that carries out key catalytic functions , especially during peptide bond formation. Structure of Ribosomes Ribosomes are complex structures made of: • Ribosomal RNA (rRNA) • Ribosomal proteins Ribosomes can be separated into two subunits : • A small subunit • A large subunit

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    Study Guide Each subunit contains: • One or more rRNA molecules • Several proteins The rRNAs in the small and large subunits are different in size and structure. 4.2 Similarities and Differences in rRNA Ribosomal RNAs vary greatly in size between organisms and between ribosomal subunits. However, they share common secondary structural features . In eukaryotes , rRNAs are larger than those in prokaryotes. This larger size is not due to a completely new structure. Instead: • Extra structural domains are inserted into the basic rRNA framework • The overall folding pattern remains similar to that of smaller rRNAs Antibiotics and rRNA Many antibiotics are natural substances produced by soil bacteria or molds. They work by inhibiting the growth of other bacteria , often by targeting ribosomal RNA. Example: Streptomycin • Streptomycin is an antibiotic used to treat tuberculosis • It binds to a specific region of bacterial 16S rRNA • This binding interferes with protein synthesis in bacteria Importantly: • Streptomycin does not significantly affect human ribosomes • This selectivity gives the drug a high therapeutic index o Meaning it is effective at doses that are not very harmful to humans

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