RNA.docx

Structure of RNA molecules Sugar-phosphate backbone is constant. Bases vary. Polymer made up of nucleotides- ribose sugar, phosphate group and nitrogenous base. Sequence of bases provides coding information. Single stranded Form’s secondary structure= structures that form through complementary...

Structure of RNA molecules Sugar-phosphate backbone is constant. Bases vary. Polymer made up of nucleotides- ribose sugar, phosphate group and nitrogenous base. Sequence of bases provides coding information. Single stranded Form’s secondary structure= structures that form through complementary base pairing within same RNA molecule. Mismatches from base paring results in ‘bulges. Different secondary structures it can form are: Stem= where bases within the RNA polymer are complementary- binds to each other and forms stem structure. Hairpin loop= where there is no complementary base pairing, bulges form due to mismatches. These secondary structures above form because RNA is single stranded When some bases within the RNA molecule are complementary to other bases within the same RNA molecule. RNA is less stable than DNA because: Single stranded therefore more susceptible to cellular damage. Has a OH group- more reactive as it can be hydrolysed. This is important for RNA to be less stable RNA structures are stablised by metal ions eg Mg2+ RNAs can form structural components such as ribosomes. RNAs can also form ribozymes- possess enzymatic acitivty and can cleave either themselves or other RNA molecules. Different types of RNA species mRNA Acts as template for protein synthesis- product of gene transcription. Can take 2 forms, can be monocistronic where one RNA encodes for one protein (both prokaryotes and eukaryotes). Can be polycistronic where one RNA can encode for more than one protein (only prokaryotes). tRNA carries individual amino acids to the ribosomes for peptide bond formation during protein synthesis. At least one tRNA for each amino acid Very short- only 75 bases long Forms 3D ‘cloverleaf’ structure Ribosomal RNA (rRNA) Forms ribosomes Catalyst- forces mRNA and tRNA through ribosome which allows to process and translate into functioning protein. Subunits vary in both prokaryotic (70S) and eukaryotic (80S). Small nuclear RNAs (snRNAs) Found in ONLY eukaryotes. Has highly conserved secondary structure. Forms small nuclear ribonucleoprotein particles (snRNPs) Involved in RNA splicing. RNA composition example from E-coli. Different types of RNA are no present in the same amount in any given cell. RNA synthesis- transcription The RNA grows in the 5’ to 3’ direction. The OH group on 3’ of the last nucleotide in the chain nucleophilically attacks the phosphate group of incoming nucleotides. This releases energy and energy is used to form the phosphodiester bond. Enzyme responsible for RNA synthesis is RNA polymerase which requires Mg2+ as a cofactor. RNA polymerase needs the DNA template to read the sequence from. RNA polymerase reads the sequence from 3’ to 5’ although the sequence form from 5’ to 3’ direction. Mg2+ is used to stabilise the interaction between RNA polymerase and the growing nucleotide chain in order for phosphodiester bond formation to occur. There are 2 other amino acid residues within RNA polymerase. Tyr639 His- 784 These are called sugar discriminators as they discriminate between DNA and RNA nucleotides. RNA synthesis is a 3-stage process. Initiation= assemble RNA polymerase and other cofactors at the regulatory region of gene for transcription to start. Elongation= DNA needs to be unwound; RNA polymerase reads the sequence of DNA template to generate mRNA Termination= once RNA is made, RNA polymerase is released. All the stages above can be regulated. RNA polymerase Large, multi-subunit enzyme Prokaryote has a single RNA polymerase whereas eukaryotic have 3. Types of RNA polymerases Type Location Transcripts (produces) Effects of a-amanitin. 1 Nucleolus 18S, 5.8S and 28S rRNA insensitive 2 Nucleoplasm mRNA precursors and snRNA Strongly inhibited 3 Nucleoplasm tRNA and 5S rRNA Inhibited by high concentration a-amanitin is a toxin by poisonous mushrooms, it binds tightly to type 2 of RNA polymerase. Its toxic because it blocks RNA synthesis during the elongation phase. Rifampicin is an antibiotic which works by inhibition of bacterial transcription by inhibiting the initiation phase of RNA synthesis. Some viral genomes such as corona virus use RNA as genetic material and require RNA-directed RNA polymerase for replication.