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Lecture 7

BIOLOGY 2EE3 Lecture Notes - Lecture 7: Prokaryotic Large Ribosomal Subunit, Prokaryotic Small Ribosomal Subunit, Chloramphenicol

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Turlough Finan

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DNA Replication and Gene Expression
How do we know that DNA is the hereditary molecule?
Griffith experiment 1920s
oStreptococcus pneumoniae R strain (not virulent) and S strain (virulent)
oMice didn’t die if R strain ingested
oMice died if S strain or a mixture of R and S strain was being injected
oThus there was something that was transforming R —> S strain
Avery, MacLeod and McCarty experiment 1940s
oUsed mixtures from S strain, and digested away one component at a time
oThe mixture with DNA left was the only one that transformed R to S strain, thus DNA was
responsible for this transformation
oObservation - Bacteriophage T2 binds to E.coli and injects its genetic material into it and then
causes E.coli to produce new viral particles.
oT2 consists of DNA and protein, one of them must be the genetic material
oLabelled the proteins of some bacteriophages with radioactive P32 and some bacteriophages
with radioactive S35. After infecting it with E.coli cells, they separated the phage capsid from
infected cells via centrifuge
oThe radioactive phosphorous was found in cell, and radioactive sulfur did not
oThus, DNA is the genetic material
Structure of DNA
5C sugar
5’ phosphate
Synthesis 5’ to 3’
Hydrogen bases - T=A and C=G
DNA structure across domains
DNA structure is same but DNA packaging is not
Bacteria - single circular chromosome
Archaea - single circular chromosome packaged around histone proteins
Eukarya - multiple linear chromosomes packaged around histone proteins
Histone significance
Wrapping of dsDNA around histone helps keep the chromosomes compact and prevents it from
being degraded
Bacteria and archaea are almost always Haploid
DNA replication
oInitiation in bacteria starts at a specific site on the chromosome, origin of replication, oriC. This
sequence is a 245 bp sequence (repeats of 9 and 13 bp sequences), A-T rich.
oSame motif across bacteria, but specific spot may change.
oDnaA, a protein, begins denaturing the DNA, with other proteins such as (DnaB/helicase) and
DnaC(helicase loader)
oDnaG/primase, synthesize short segements of RNA needed to prime DNA replication.
oSingle-stranded DNA-binding proteins (SSB) attach to the newly formed single-stranded DNA to
keep strands from reannealing and a single-stranded replication bubble forms.
oDNA polymerase begins replicating DNA at both ends by adding nucleotides to the initial RNA
oA continuous leading and lagging strands are formed.
oTermination - Tus proteins bidn to ter sites on chromosomes, sites of stopping elongation.
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