100809-BCH311

100809-BCH311 - Fig. 9-5, p.219 A Section of an RNA Chain...

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Unformatted text preview: Fig. 9-5, p.219 A Section of an RNA Chain ACGU Major differences between RNA and DNA 1) Sugar - D-Ribose in RNA 2) Uracil instead of thymine Fig. 9-19, p.231 Six kinds of RNA 1. transfer RNA (tRNA) 2. ribosomal RNA (rRNA) 3. messenger RNA (mRNA) 4. small nuclear RNA (snRNA) 5. micro RNA (miRNA) 6. short interfering (siRNA) RNA Table 9-1, p.233 Table 9-1, p.233 Six different types of RNA Fig. 9-21, p.233 Transfer RNA Transports amino acids to sites of protein synthesis Smallest of the three most important RNAs At least one (and often many) tRNA binds specically to an amino acid Single-stranded polynucleotide chain, between 73 and 94 nt long, and a molecular mass of ~25 kDa Interchain H-bonding occurs between A-U and G-C, enabling the formation of a cloverleaf structure (which can be considered the secondary structure of tRNA) The hydrogen-bonded portions of the molecule are called stems, and the non-hydrogen-bonded poortions are called loops The tRNA duplexes adopt an A-helical form Fig. 9-23, p.234 Transfer RNA tertiary structure A particular tertiary structure is necessary for tRNA to interact with the enzyme aminoacyl-tRNA synthetase, the enzyme that attaches an amino acid to the 2 or 3 end To produce this tertiary structure, the tRNA folds into an L-shaped conformation that has been determined by X-ray diffraction Fig. 9-25, p.235 Ribosomal RNA In contrast to tRNA, rRNA molecules tend to be quite large, and only a few types exist in a cell rRNA combines with proteins to form ribosomes, the sites of protein synthesis (rRNA comprises ~65% of the total weight of ribosomes) In both prokaryotes and eukaryotes the ribosomes consist of two subunits Prokaryotes 70S bacterial ribosome comprises two subunits i. 50S - 23S rRNA (~2500 nt), 5S rRNA (~120 nt), and 34 different proteins ii. 30S - 16S rRNA (~1500 nt) and 21 different proteins N.B. S values are not additive S, Svedberg unit, a sedimentation coefFcient calculated from the weight and shape of an object Fig. 9-26, p.235 Proposed Secondary Structure for 16S rRNA Messenger RNA (mRNA) Least abundant, comprises approximately 5-10% total cellular RNA The sequences of the bases in mRNA specify the order of amino acids in proteins The sequence of mRNA bases (that encodes for the amino acids in a protein) reFects the sequence of DNA bases in the gene mRNAs are extremely heterogeneous in length, e.g. the BRCA1 mRNA precursor molecule is 81,154 bp ( or 81 kb ) while the FANCF mRNA precursor molecule is only 3,308 bp ( or 3.3 kb ) Highest turnover (least stable) of most RNAs, and the nucleotides can be recycled In eukaryotes, mRNA is initially synthesized as a larger precursor molecule called heterogeneous nuclear RNA (hnRNA), which is processed extensively to produce the mRNA The Behavior of Proteins: Enzymes Of all of the functions of proteins, catalysis is probably the most important In the absence of catalysis, most biological reactions would take place far too...
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100809-BCH311 - Fig. 9-5, p.219 A Section of an RNA Chain...

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