Transcript chapt17b_lecture

Chapter 17: Outline
DNA
Mutation
Chromosomes and
Variations
Chromatin
Supercoiling Genome Structure
RNA (We are here.)
Transfer, Ribosomal, Messenger
Heterogeneous and Small Nuclear
Viruses
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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
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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.
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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 threebase-pair sequence complimentary to
the DNA triplet code for an amino
acid.
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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.
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tRNA: cloverleaf; “L” model
Attachment to
mRNA here
AA
attaches
here
Stem
region
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tRNA: Schematic View
Fig17.22b
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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
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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
2+
the Mg concentration.
Analytical ultracentrifuge separates
subunits based on size and shape.
The Svedberg (S) measures the
sedimentation coefficient of the
subunit
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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.
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mRNA-2
Many prokaryotic mRNAs are polycistronic, 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 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.
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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.
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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))
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Viral Structure-2
Two types of ssRNA genomes
1. 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-3
Representative viral
structures.
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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.
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Fig 17M
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Fig 17N
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