Dna and rna
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Dna and rna






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    Dna and rna Dna and rna Presentation Transcript

    • DNA and RNA June.19.2010
    • Chemical structure of DNA
    • The chemical nature of RNA differs from that of DNA RNA is a polymer of purine and pyrimidine ribonucleotides linked together by 3’, 5’- phosphodiester Bridges analogous to those in DNA. Although sharing many features with DNA, RNA possesses several specific differences
    • Types of RNA
      • Messenger RNA (mRNA)
      • Transfer RNA (tRNA)
      • Ribosomal RNA (rRNA)
      • Small stable RNA (sRNA)
    • Messenger RNA (mRNA)
      • The most heterogeneous in size and stability.
      • It is synthesized as large precursor forms known as heterogenous nuclear RNA or hnRNA .
      • All members of the class function as messengers conveying the information in a gene to the protein synthesizing machinery, where it serve as a template on which a specific sequence of amino acids is polymerized to form a specific protein molecule.
      • Newly synthesized mRNA combines with protein to form mRNA-protein complex
      • (mRNA), which come out through nuclear pore by simple diffusion into the cytoplasm
      • to serve as templates for protein synthesis.
      • The 5’ terminal of mRNA is “capped” by a 7-methylguanosine triphosphate that is linked to an adjuacent 2’ – O -methy ribonucleoside at its 5’-hydroxyl through the three
      • Phosphates.
      • The mRNA molecules frequently contain internal 6-methyladenylates and other 2’ – O -methylated nucletides.
      • The cap is involved in the recognition of mRNA by the translating machinery, and it
      • probably helps stabilize the mRNA by preventing the attack of 5’-exonucleases. The protein-synthesizing machinery begins translating the mRNA into proteins beginning downstream of the 5’ or capped terminal.
      • The other end of most mRNA molecules, the 3’-hydroxyl terminal, has an attached polymer of adenylate residuces 20-250 nucleotides in length. (Poly(A) tail).
    • Posttranscriptional modification of mRNA showing the 7-methylguanosine cap and poly-A tail.
    • Transfer RNA (tRNA)
      • tRNA molecules vary in length from 74 to 95 nucleotides, have clover leaf like
      • structure.
      • The tRNA molecule serve as adapters for the translation of the information in the
      • sequence of nucleotides of the mRNA into specific amino acids.
      • There are at least 20 species of tRNA molecules in every cell, at least one
      • corresponding to each of the 20 amino acids required for protein synthesis.
      • All tRNA molecules have four main arms. The acceptor arm terminates in the
      • nucleotides CCA.
    • Thymidine, pseudouridine, cytidine
    • A. Characteristic tRNA secondary structure. B. Folded (tertiary) tRNA structure found in cells. D = dihydro-uracil. ψ = pseudouracil.
    • Primary tRNA transcript. B. Functional tRNA after posttranscriptional modification. Modified bases include D (dihydrouracil), ψ ( pseudouracil), and  m , which means that the base has been methylated.
    • AUG
    • Genetic Code
    • Ribosomal RNA (rRNA)
      • Ribosomal RNAs are synthesized in large precursor form known as pre rRNA.
      • Four rRNA molecules 5, 5.8, 18 and 28 S are present in eukaryotes formed from
      • 40S and 60S subunit.
      • rRNA are required for the formation of ribosomes.
      • 16S RNA is involved in initiation of protein synthesis.
    • Posttranscriptional processing of eukaryotic ribosomal RNA by ribonucleases (RNases)
    • Prokaryotic and eukaryotic rRNAs
    • Structure of eukaryotic messenger RNA
    • Alternative splicing patterns in eukaryotic mRNA.
    • Differences between DNA and RNA DNA RNA 1. Sugar moiety is deoxyribose. Sugar moiety is ribose. 2. Uracil is absent. Thymine is absent. . 3. Double stranded molecules. Single stranded molecules. 4. Sum of purine bases is equal to sum of pyrimidine base. (A+G)=(C+T) Sum of purine bases is not equal to sum of pyrimidine base. (A+G)#(C+T) 5. Resistant to hydrolysis by alkali because of absence of hydroxyl group on 2 carbon atom of deoxyribose. Because of presence of hydroxyl group on 2 carbon atom of ribose, RNA is easily hydrolyzed by alkali.
    • 以上です。