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Chapter 17: OutlineDNA

Mutation Chromosomes andVariations ChromatinSupercoiling Genome Structure

RNA (We are here.)Transfer, Ribosomal, MessengerHeterogeneous and Small Nuclear

Viruses

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17.2 Kinds of RNAstransfer RNA (tRNA)

transfers amino acids to the site of protein synthesis (ribosomes)

ribosomal RNA (rRNA)rRNA forms ribosomes by reacting

with proteinsmessenger RNA (mRNA)

mRNA directs the AA sequence of proteins

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tRNAThere 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 -helix

The overall structure is called a cloverleaf in a L-shaped conformation.

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tRNA-2Two 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.

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tRNA-3tRNA also has:

A D loop (contains dihydrouridine)A TC 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.

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tRNA: cloverleaf; “L” modelAttachment tomRNA here

Stemregion

AAattacheshere

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tRNA: Schematic View

Fig17.22b

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rRNARibosomal 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 rRNA35-40% mass in protein

Ribosome subunits:two: a large and a small

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Ribosomal subunits: 11. Small: 1 RNA + ~20 proteins2. 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

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mRNAmRNA 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.

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mRNA-2Many 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.

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Other RNAsHeterogeneous 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.

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17.3 VirusesViruses 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.

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Viral StructureVirons (complete viral particles) range

from 10 to 400 nm in diameter.A simple viron consists of:

Capsid: an interlocking protein coatMost capsids are helical or icosahedral

Nucleic acid: DNA or RNAdsDNA, ssDNA, ssDNA, dsRNA(most are double stranded DNA (dsDNA))

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Viral Structure-2Two types of ssRNA genomes1. Positive-sense acts as a giant RNA

molecule and directs synthesis of long viral polypeptide which is cleaved to smaller pieces.

2. 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.

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Viral Structure-3Representative viral structures.

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The HIV VirusHIV 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.

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Fig 17M

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Fig 17N