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Chapter 17: Outline

Chapter 17: Outline. DNA Mutation Chromosomes and Variations Chromatin Supercoiling Genome Structure RNA (We are here.) Transfer, Ribosomal, Messenger Heterogeneous and Small Nuclear Viruses. 17.2 Kinds of RNAs. transfer RNA (tRNA)

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Chapter 17: Outline

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  1. Chapter 17: Outline • DNA • Mutation Chromosomes and • Variations Chromatin • Supercoiling Genome Structure • RNA (We are here.) • Transfer, Ribosomal, Messenger • Heterogeneous and Small Nuclear • Viruses

  2. 17.2 Kinds of RNAs • transfer RNA (tRNA) • transfers amino acids to the site of protein synthesis (ribosomes) • ribosomal RNA (rRNA) • rRNA forms ribosomes by reacting with proteins • messenger RNA (mRNA) • mRNA directs the AA sequence of proteins

  3. tRNA • There is at least one tRNA (and often several) for each AA to be incorporated into a protein. • tRNA is single stranded with typically about 75 nucleotides. • Intrachain hydrogen bonding (A=U and G=C) occurs to gives regions called stems with an a-helix • The overall structure is called a cloverleaf in a L-shaped conformation.

  4. tRNA-2 • Two structural motifs in tRNA allow it to perform its critical function. • The 3’ terminus forms a covalent bond with a specific amino acid. • The anticodon loop contains a three-base-pair sequence complimentary to the DNA triplet code for an amino acid.

  5. tRNA-3 • tRNA also has: • A D loop (contains dihydrouridine) • A TYC loop which contains the base sequence thymine, pseusouridine, cytosine. • A variable loop with usually 4-5 nucleotides but up to 20. • The features above presumably relate to binding to a protein or to the ribosome.

  6. tRNA: cloverleaf; “L” model Attachment to mRNA here AA attaches here Stem region

  7. tRNA: Schematic View Fig17.22b

  8. rRNA • Ribosomal RNA is the most abundant form and makes up 80% cellular RNA. • Ribosomal RNA molecules are large and are found in the ribososmes. • Ribosome composition: • 60-65% mass in rRNA • 35-40% mass in protein • Ribosome subunits: • two: a large and a small

  9. Ribosomal subunits: 1 • 1. Small: 1 RNA + ~20 proteins • 2. Large: 2 RNA + ~35 proteins(procaryotes) • 3 RNA + ~50 proteins(eucaryotes) • Subunits easily separated by lowering the Mg2+ concentration. • Analytical ultracentrifuge separates subunits based on size and shape. • The Svedberg (S) measures the sedimentation coefficient of the subunit

  10. mRNA • mRNA is the least abundant RNA (5-10%) • mRNA is the carrier of the genetic message from DNA. A three base sequence on the mRNA codes for the AA to be incorporated in the growing protein chain. • Shows frequent turnover. • Chain length varies with the length of the protein to be coded.

  11. mRNA-2 • Many prokaryotic mRNAs are poly-cistronic, they contain information for several poylpeptides. Eukaryotic mRNA is typically monocistronic. • Prokaryotic mRNAs are translated in to proteins immediately after synthesis but eukaryotic mRNAs are extensively processed before translation.

  12. Other RNAs • Heterogeneous RNA (hnRNA) are the primary transcripts of DNA and the precursors of mRNA. • Small nuclear ribonuclearprotein particles (snRNP or “snurps”) are involved in splicing mRNA by removing the introns from the hnRNA.

  13. 17.3 Viruses • Viruses lack most properties that are associated with life. • A virus is a mobile genetic element. • A piece of nucleic acid with a protective coat. • It’s nucleic acid hijacks a cells machinery for making nucleic acids and proteins.

  14. Viral Structure • Virons (complete viral particles) range from 10 to 400 nm in diameter. • A simple viron consists of: • Capsid: an interlocking protein coat • Most capsids are helical or icosahedral • Nucleic acid: DNA or RNA • dsDNA, ssDNA, ssDNA, dsRNA • (most are double stranded DNA (dsDNA))

  15. Viral Structure-2 • Two types of ssRNA genomes • Positive-sense acts as a giant RNA molecule and directs synthesis of long viral polypeptide which is cleaved to smaller pieces. • Negative-sense is complimentary in base sequence to the mRNA directing viral protein synthesis. (Need transcriptase enzyme) • In complex viruses, the nucleocapsid is surrounded by a membrane envelope.

  16. Viral Structure-3 • Representative viral structures.

  17. The HIV Virus • HIV is a retrovirus that contains reverse transcriptase activity enabling it to make DNA from vRNA. • vDNA inserts into the host cell genome, destroying the immune system. • The next two slides depict the HIV virus and its life cycle.

  18. Fig 17M

  19. Fig 17N

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