Title: Introduction to Microbial Genetics
1Introduction to Microbial Genetics
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3A Historical Overview
- The scientists who provided the clues to the
nature of DNA - Friederich Meischer DNA isolated
- Luria and Delbruck Bacteriophages
- Stanely Giffiths( 1928) The idea of the
transforming substance Avery, MacLoed, and
McCarty( 1944) the nature of transformation - Hershey and Chase Bacteriophage DNA as the
hereditary material - Chargaff A T and CG
- Maurice Wilkins and Rosalind Franklin x-ray
crystallography of DNA - Watson and Crick Double helix
4Griffiths
5Luria and Delbruck at Cold Spring Harbor in 1953
- Luria and Delbruck studied bacterial mutations
and resistance to infection with bacteriophages - The characterized the virus and its life cycle
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7Alfred Hershey and Martha Chase and the Blender
Experiment
- Hershey and Chase wanted to verify that DNA was
the hereditary material - They used a bacteriophage for their study
- They labeled the DNA with Radioactive P( P32) and
the protein with radioactive sulfur( S35)
8Results of the Experiment
- Proved that the radioactivity from the labeled
DNA was present in the progeny phage produced
from infection of the bacteria.
9The Race for the Double Helix
- Rosalind Franklin and Maurice Wilkins at Kings
College - Studied the A and B forms of DNA
- Rosalinds famous x-ray crystallography picture
of the B form held the secret, but she didnt
realize its significance
10The Race for the Double Helix
- Watson and Crick formed an unlikely partnership
- A 22 year old PhD and a 34 year old want to be
PhD - embarked on a model making venture at Cambridge
- Used the research of other scientists to
determine the nature of the double helix
11Nucleic Acid CompositionDNA and RNA
- DNA Basic Molecules
- Purines adenine and guanine
- Pyrmidines cytosine and thymine
- Sugar Deoxyribose
- Phosphate phosphate group
- http//www.dnai.org/index.htm -Â DNA background
12Double Helix
- Two polynucleotide strands joined by
phosphodiester bonds( backbone) - Complementary base pairing in the center of the
molecule - A T and C G base pairing. Two
hydrogen bonds between A and T and three hydrogen
bonds between C and G. - A purine is bonded to a complementary pyrimidine
- Bases are attached to the 1 C in the sugar
- At opposite ends of the strand one strand has
the 3hydroxyl, the other the 5 hydroxyl of the
sugar molecule
13DNA Structure
http//www.johnkyrk.com/DNAanatomy.html - DNA
structure
14Double helix( continued)
- The double helix is right handed the chains
turn counter-clockwise. - As the strand turn around each other they form a
major and minor groove. - The is a distance of .34nm between each base
- The distance between two major grooves is 2.4nm
or 10 bases - The diameter of the strand is 2nm
15Complementary Base Pairing
- Adenine pairs with Thymine
- Cytosine pairs with Guanine
16The end view of DNA
- This view shows the double helix and the outer
backbone with the bases in the center. - An AT base pair is highlighted in white
17Double helix and anti-parallel
- DNA is a directional molecule
- The complementary strands run in opposite
directions - One strand runs 3-5
- The other strand runs 5 to 3
- ( the end of the 5 has the phosphates attached,
while the 3 end has a hydroxyl exposed)
18RNA structure
- Polynucleotide nucleic acid - Single stranded
molecule that can coil back on itself and produce
complementary base-pairing ( t- RNA) - Four bases in RNA are Adenine and Guanine (
purines) and Cytosine and Uracil( pyrimidines) - Sugar ribose
- Phosphates
19RNA
- Three types of RNA
- Messenger
- Transfer
- Ribosomal
- nc- non coding RNAs
20Prokaryote DNA
- Tightly coiled
- Coiling maintained by molecules similar to the
coiling in eukaryotes - Circular ds molecule
- Borrelia burgdoferi ( Lyme Disease )has a linear
chromosome - Other bacteria have multiple chromosomes
- Agrobacterium tumefaciens ( Produces Crown Gall
disease in plants) has both circular and linear
21Prokaryote chromosomes
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23E. coli most often studied in molecular biology
of prokaryotes
- The genes of E. coli are located on a circular
chromosome of 4.6 million basepairs. This 1.6 mm
long molecule is compressed into a highly
organized structure which fits inside the 1-2
micrometer cell in a format which can still be
read by the gene expression machinery. - Bacterial DNA is supercoiled by DNA gyrase.
Chemical inhibition of gyrase without allowing
the cells to reprogram gene expression relaxes
supercoiling and expands the nucleoid, suggesting
that supercoiling is one of the tools used to
compress the genome
24Coiling
- Coiling maintained by Gyrase
- Relaxation of the coils by Topoisomerase
25Nucleosome formation
- DNA is more highly organized in eukaryote cells
- The DNA is associated with proteins called
histones.( eukaryotes) - These are small basic proteins rich in the amino
acids lysine and/or arginine - There are five histones in eukaryote cells, H1,
H2A, H2B,H3 and H4. - .
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27Beads on a String
- The DNA coils around the ellipsoid approximately
1 ¾ turns or 166 base pairs before proceeding to
the next. - The DNA the histone proteins arranged in this
formation are referred to as a nucleosome. - The stretch of DMA between the beads varies in
length from 14 to 100 base pairs. - H1 appears to associate with the linker regions
to enable the nucleosome to supercoil - When folding of the structure reaches a maximum,
the chromosomes can be visualized
28Chromosome structure
- http//www.johnkyrk.com/chromosomestructure.html
29Eukaryote replication
- The nature of DNA replication was elucidated by
Meselson and Stahl
30Meselson and Stahl experiment
- Grew bacteria in heavy Nitrogen N-15
- Transferred bacteria to N-14
- Before bacteria reproduce in new media, all
bacteria contain heavy DNA - Samples were taken after one round of replication
and two round of replication
31Semiconservative replication
- Each original strand serves a template or pattern
for the replication of the new strand. - The new strand contains one original and a newly
synthesized strand
32Eukaryote replication
- Multiple linear chromosomes
- Each chromosome has more than one origin of
replication - Approximately 1400 x as long as bacterial DNA
- Multiple replicons on a chromosome
- Oris along the length every 10 to 100 um
- Replication forks and bubbles are formed.
Replication proceeds bidirectionally until the
bubbles meet - This shortens the length of time necessary to
replicate eukaryote chromosomes - The process of elongation occurs at a speed of
50-100 base pairs/minute as compared to 750 to
1000 base pairs/ minute - http//www.johnkyrk.com/DNAreplication.html
33The origin of replication and replication forks
34Eukaryote replication
- During the S phase, there are 100 replication
complexes and each one contains as many as 300
replication forks. These replication complexes
are stationary. The DNA threads through these
complexes as single strands and emerges as double
strands.
35DNA Polymerases
- Fourteen DNA polymerases have been observed in
human beings as compared to three in E. coli.
36Prokaryote Replication
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38Bidirectional replication
- There is an origin of replication
- Two replication forks are formed
- Replication occurs around the circle until they
have opened and copied the entire chromosome - Replicon- contains an origin and is replicated as
a unit
39Ori Origin of replication
- Characteristics used to define Origins
- The position on the DNA at which replication
start points (see right) are found. - A DNA sequence that when added to a
non-replicating DNA causes it to replicate. - A DNA sequence whose mutation abolishes
replication. - A DNA sequence that in vitro is the binding
target for enzyme
40Topoisomerases
- Topoisomerase
- When the double helix of DNA, which is composed
of two strands, separates, helicase makes these
two strands rotate around each other. - The DnaB protein is the helicase most involved in
replication, but the n protin may also
participate in unwinding. - The single stranded binding proteins SSBP help to
keep the strand open - But there is a problem due to the topological
reason that the unreplicated part ahead of the
replication fork will rotate around its helical
axis when the two strands separate at the
replication fork
41Topoisomerase action
- It causes strong strain in the helix (1). Thus,
it is impossible to unlink the double helical
structure of DNA without disrupting the
continuity of the strands. - In order to perform unraveling of a "compensating
winding up" DNA, enzymes are required (1).
Topoisomerase changes the linking number as well
as catalyzes the interconversionn of other kinds
of topological isomers of DNA (2).
42Initiation
- Initiationa. oriC - origin of chromosomal
replicationRecognized by DnaA protein - only
recognizes if GATC sites are fully
methylatedBinding of DnaA allows DnaB to open
complexb. DnaB is the replication helicasec.
Strand separation by helicased. SSB
(single-stranded binding) protein keeps strands
aparte. DNA gyrase - a topoisomerase - puts
swivel in DNA which allows strands to rotate and
relieve strain of unwinding
43Explanation
- Recall that DNA double helix is tightly wound
structure and that bases lie between the two
backbones. If these bases are the template for
new strand, how do the appropriate enzymes reach
these bases? By the unwinding of the helix. - An enzyme called helicase catalyzes the unwinding
of short DNA segments just ahead of the
replication fork. The reaction is driven by the
hydrolysis of ATP.
44Explanation continued
- As soon as duplex is unwound, SSB
(single-stranded binding protein) binds to each
of the separated strands to prevent them from
base-pairing again. Therefore, the bases are
exposed to the replication system. - The unwinding of the duplex would cause the
entire DNA molecule to swivel except for the
action of a topoisomerase (DNA gyrase) which
introduce breaks in the DNA just ahead of the
unwinding duplex. These breaks are then rejoined
after a few revolutions of the duplex.
45The need for a primer
- When DNA template is exposed, DNA synthesis must
begin. But DNA polymerases not only need a
template but also a primer for replication to
proceed. Where does the primer come from? - After observations that RNA synthesis is required
for DNA synthesis, it was discovered that the
synthesis of DNA fragments requires a short
length of RNA as a primer.Primosome (complex of
20 polypeptides) makes RNA primers in E. coli
46Formation of the Primer
- Primosome contains primase
- Primosome moves along DNA duplex in 3'gt5'
direction (with respect to lagging strand
follows replication fork) even though primer is
made in 5'gt3' direction(Note The symbol "gt"
indicates the direction that is, the primer is
made from 5' to 3'.)n' protein removes SSB in
front of primosome - DnaB protein organizes some components of
primosome and prepares DNA for primasePrimase
forms the primer
47DNA POLYMERASE III
- Holoenzyme
- Complex that synthesizes most of the DNA copy
contains the DNA polymerase enzyme and other
proteins - The gamma delta complex and the B subunits of
the holoenzyme bind it to the template and the
primer - The alpha subunit carries out the actual
polymerization reaction - All of the proteins form a huge complex called
the replisome
48DNA polymerase III
- This is a stationary complex that probably
attached to the plasma membrane. - The DNA moves through the replisome and is copied
49Elongation of the chain
- dCTP dCMP
- PPi
- Energy is supplied for biosynthesis by the
cleaving of the phosphate bond
50Elongation( continued)
- Elongation proceeds in 5' gt 3' direction and
requires 1) all 4 deoxyribonucleoside
5'-triphosphates (dATP, dGTP, dCTP, dTTP), 2)
Mg ions, 3) a primer made of nucleic acid, and
4) a DNA template. - Rate of elongation 750 - 1000 nucleotides per
secondRate of formation of initiation complex
1-2 minutes
51Elongation
- ElongationDNA polymerase I, II and III in E
.coliDNA polymerase III holoenzyme - complex of
7 polypeptides - Replisome - primosome and 2 DNA polymerase III -
synthesizes DNA on both strands simultaneously
without dissociating from DNA - DNA polymerase III catalyzes the addition of
deoxyribonucleotide units to end of the DNA
strand with release of inorganic pyrophosphate
(PPi)(DNA)n residues dNTP ltgt (DNA)n 1
residues PPiAttachment of new units is by
their a-phosphate groups to a free 3'-hydroxyl
end of preexisting DNA chain.
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54The lagging strand and discontinuous replication
- The replication on the 5 to 3 strand differs
- The template strand still must be read from 3 to
5 - The reading begins at the replication fork
- Occurs at the same time as the synthesis of the
lagging strand - Same steps in synthesis of DNA
- But DNA is synthesized in pieces about 1000 to
2000 bases in length. These are known as Okazaki
fragments
55Okazaki fragments
- After the lagging strand has been duplicated by
the formation of Okazaki fragments, DNA
Polymerase I or RNase H removes the RNA primer.
Polymerase I synthesizes the complementary DNA to
fill the gap resulting from the RNA delection. - The polymerase removes one nucleotide at a time
and then replaces it - AMP( RNA nucleotide) replaced by dAMP( DNA
nucleotide)
56DNA ligase
- Ligase can catalyze the formation of a
phosphodiester bond given an unattached but
adjacent 3'OH and 5'phosphate. - This can fill in the unattached gap left when the
RNA primer is removed and filled in. - The DNA polymerase can organize the bond on the
5' end of the primer, but ligase is needed to
make the bond on the 3' end.
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59The End of Replication
- DNA replication stops when the polymerase complex
reaches a termination site on the DNA in E. coli - The Tus protein binds to the ter site and halts
replication. - In many prokaryotes the replication process stops
when the replication forks meet
60Plasmid replication
- ColE1 is a naturally occurring plasmid of E.
coli. Its replication is controlled independently
of the replication of the host chromosome. - Two plasmids with the same origin of replication
can not coexist in the same cell. - The ColE1 origin, defined by molecular genetic
methods, is in a region from which two RNAs are
transcribed. - An active RNase H gene is required for ColE1
replication. RNase H cleaves the RNA II
transcript. The remaining RNA serves as primer
for initiation of replication. - RNA I binds to 5' sequences of RNA II via
pseudoknots and regular complementary pairing.
This binding is stabilized by the ROP or ROM
protein. - The binding prevents changes in the conformation
of RNA II that would otherwise result in RNAse H
cleavage.
61Rolling Circle Replication Occurs in
Conjugation in E. coli.
62How can one account for the high fidelity of
replication?
- The answer is based on the fact that DNA
Polymerase absolutely requires 3'-OH end of
base-paired primer strand on which to add new
nucleotides. - DNA polymerase III has 3' gt 5' exonuclease
activity. It was discovered that DNA polymerase
III actually proofreads the newly synthesized
strand before continuing with replication. When
incorrect nucleotide is incorporated, DNA
polymerase III, by means of the 3' gt 5'
exonuclease activity, "backs up" and hydrolyzes
off the incorrect nucleotide. The correct
nucleotide is then added to the chain and
elongation is resumed. - All 3 DNA polymerases have 3'gt5' exonuclease
activity - Proofreading ability - 1 error in 10 million
63Exonucleases and repair
- DNA polymerase I also has 5'gt3' exonuclease
activity which removes RNA primer and 5'gt3'
polymerase activity which fills in the gap - This causes a single-stranded break in the DNA -
called a nickDNA ligase repairs nick by creating
a phosphodiester bond
64Genes and Gene Expression
- Genes are written in a code consisting of groups
of three letters called triplets. - There are four letters in the DNA alphabet.
There are 64 possible arrangements of the four
letters in groups of three - The triplets specify amino acids for the
synthesis of proteins from the information
contained in the gene - Genes can also specify t- RNA or r- RNAs
- The gene begins with a start triplet and ends
with a stop. The bases between the start and the
stop are called an open reading frame, ORF. - The information in the gene is transcribed by RNA
polymerase. - It reads the gene from 3 to 5
- The template strand is now referred to as the
CRICK strand and the nontemplate strand is now
known as the WATSON strand - DNA sequences are stored in data bases as the
WATSON strand - Reference - COLD SPRING HARBOR - 2003
65Promoters are at the beginning of the Gene
- RNA polymerase recognizes a binding site in front
of the gene. This is referred to as upstream of
the gene. - The direction of transcription is referred to as
downstream - Different genes have different promoters. IN E.
coli the promoters have two functions - The RNA recognition site for transcription which
is the consensus sequence for prokaryotes is - 5 TTGACA3 ( Watson strand) which means on the
reading strand 3 AACTGT5 ( Crick strand)
66The Pribnow Box and Shane -Dalgarno
- The RNA binding site has a consensus sequence of
- 5 TATAAT 3 ( -) and 3 ATATTA 5 ()
- This is where the DNA begins to become unwound
for transcription - The initially transcribed sequence of the gene
may not reflect doing but may be a leader
sequence. - The prokaryotes usually contain a consensus
sequence known as the Shane Delgarno which is
complememtary to the 16s rRNA on the ribosome - ( small subunit )
- The leader sequence also may regulate
transcription
67The structure of a prokaryote gene
68Prokaryote Genes are
- Continuous
- They do not contain introns like eukaryote genes
- The gene consists of codons that will determine
the sequence of amino acids in the protein - At the end of the gene there is a terminator
sequence rather than an actual stop - The terminator may be at the end of a trailer
sequence located downstream from the actual
coding region of the gene
69The Gene begins with
- DNA is read 3 to 5 and m RNA is synthesized 5
to 3 - 3 TAC is the start triplet
- This produces a complementary mRNA message 5 AUG
3 - Groups of three bases in the messenger RNA formed
are referred to as CODONS
70RNA POLYMERASE
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72- Wobble
- There is wobble in the DNA code This is a
protection from mutations - More than one codon can specify the same amino
acid - Note arginine - CGU, CGC,CGA, CGG all code for
arginine only the third base in the codon
changes - There are two additional codons for arginine as
well AGA and AGG these reflect the degenerate
nature of the code
73Codon chart
74Genes for t RNAs and r RNAs
- The genes for t RNAs have a promoter and
transcribed leader and trailer sequence that are
removed prior to their utilization in
translation. Genes coding for tRNA may code for
more than a single tRNA molecule - The segments coding for r RNAs are separated by
spacer sequencs that are removed after
transcription.
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76t-RNA
- The acceptor stem includes the 5' and 3' ends of
the tRNA. - The 5' end is generated by RNase P
- The 3' end is the site which is charged with
amino acids for translation. - Aminoacyl tRNA synthetases interact with both the
acceptor 3' end and the anticodon when charging
tRNAs. - The anticodon matches the codon on mRNA and is
read - 3 to 5
77t- RNA
- Found in the cytoplasm
- Amino acyl t- RNA synthetase is an enzyme that
enables the amino acid to attach to t-RNA - Also activates the t- RNA
- Clover leaf has a stem for attachment to the
amino acid and an anticodon on the bottom of the
clover leaf
78t- RNA
- Common Features
- a CCA trinucleotide at the 3' end, unpaired
- four base-paired stems, and
- One loop containing a T-pseudoU-C sequence and
another containing dihydroU.
79tRNA
- tRNAs attach to a specific amino acid and carry
it to the ribosome - There are 20 amino acids
- 61 different codons for these amino acids and 61
tRNAs - The anticodon is complementary to the codon
- Binds to the codon with hydrogen bonds
80Ribosomal genes
- Very similar to the structure of protein genes
81tRNA and rRNA genes
- The genes for rRNA are also similar to the
organization of genes coding for proteins - All rRNA genes are transcribed as a large
precursor molecule that is edited by
ribonucleases after transcription to yield the
final r RNA products
82Ribosomal RNA
- Combines with specific proteins to form ribosomes
- Serves as a site for protein synthesis
- Associated enzymes and factors control the
process of translation
83Prokaryote ribosomes
- Ribosomes are small, but complex structures,
roughly 20 to 30 nm in diameter, consisting of
two unequally sized subunits, referred to as
large and small which fit closely together as
seen below. - A subunit is composed of a complex between RNA
molecules and proteins each subunit contains at
least one ribosomal RNA (rRNA) subunit and a
large quantity of ribosomal proteins. - The subunits together contain up to 82 specific
proteins assembled in a precise sequence. Â Â
84Prokaryote ribosomal RNA
85Prokaryote ribosomes polysomes- the process of
translation
86Prokaryote transcriptionand translation
- Prokaryote transcription and translation take
place in the cytoplasm - All necessary enzymes and molecules are present
for the transcription and translation to take
place
87Translation
- A molecule of messenger RNA binds to the 30S
ribosome - ( small ribosomal unit) at the Shine Dalgarno
sequence - This insures the correct orientation for the
molecule - The large ribosomal sub unit locks on top
88The Ribosome
- There are four significant positions on the
ribosome - EPAT
- When the 5 AUG 3 of the mRNA is on the P site
the t-RNA with the anticodon, 5UAG3 forms a
temporary bond to begin translation
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92From Gene to polypeptide
93E. Coli Gene Map
94Mutations in DNA
- May be characterized by their genotypic or
phenotypic change - Mutations can alter the phenotype of a
microorganisms in different ways - Mutations can involve a change in the cellular or
colonial morphology
95Types of Mutations
- Conditional mutations are those mutations that
are expressed only under specific environmental
conditions ( temperature) - Biochemical mutations are those that can cause a
change in the biochemistry of the cell - ( these may inactivate a biochemical pathway)
- These mutants are referred to as auxotrophs
because they cannot grow on minimal media - Prototrophs are usually wild type strains capable
of growing on minimal media
96Two types of mutations
- Spontaneous mutations These occur without a
causative agent during replication - Induced mutations are the result of a substance
referred to as a mutagen - Cairns reports that a mutant E. coli strain
unable to use lactose is able to regain its
ability to use the sugar again should this be
referred to as adaptive mutation?
97Hypermutation
- One possible explanation is hypermutation
- A starving bacterium has the ability to generate
multiple mutations with special mutator genes
that enable them to form bacteria with the
ability to metabolize lactose - This is an interesting theory still under
investigation
98Spontaneous mutations
- Types
- A purine substitutes for a purine or a pyrimidine
substitutes of a pyrimidine. This type of
mutation is referred ta as a transition. Most of
these can be repaired by proofreading mechanisms - A pyrimidine substituted for by a purine is
referred to as a transversion. These are rarer
due to steric problems in the DNA molecule such
as pairing purines with purines. - Insertions or deletions cause frame shifts the
code shifts over the number of bases inserted or
deleted
99Mutation Types
- Erors in replication due to base tautomerization
- AT and CG pairs are formed when keto groups
participate in hydrogen bonds - In contrast enol tautomers produce AC and GT base
pairing
100Spontaneous mutations another cause
- Depurination
- A purine nucleotide can lose its base
- It will not base pair normally
- It will probably lead to a transition type
mutation after the next round of replication. - Cytosine can be deaminated to uracil which can
then create a problem
101Frame Shifts
- Additions and deletions change the reading frame.
- The hypothetical origin of deletions and
insertions may occur during replication - If the new strand slips an insertion or addition
may occur - If the parental slips a deletion may occur
102Mutagenesis
- Any agent that directly damages DNA, alters its
chemistry, or interferes with repair mechanisms
will induce mutations - Base analogs
- Specific mispairing
- Intercalating agents
- Ionizing radiation
Base analogs are structurally similar to normal
nitrogenous bases and can be incorporated into
the growing polynucleotide chain during
replication.
103The expression of mutations
- Forward mutations a mutation from the wild type
to a mutant form is called a forward mutation - Reversion-If the organism regains its wild type
characteristics through a second mutation - Back mutation The actual nucleotide sequence is
converted back to the original - Suppressor mutation overcomes the effects of
the first mutation
104More on mutations
- Point mutations caused by the change in one DNA
base - Silent mutations mutations can occur which
cause no effect this is due to the degeneracy
of the code ( more than one base coding for the
same amino acid) - Missense mutation changes a codon for one amino
acid into a codon for another amino acid - Nonsense In eukaryotes the substitution of a
stop into the sequence of a normal gene
105Detection and isolation of mutants
- Requires a sensitive system
- Mutations are rare
- One in about every 107 1011
- Replica plating is a technique that is used to
detect auxotrophs - It distinguishes between wild type and mutants
because of their ability to grow in the absence
of a particular biosynthetic end product - Replica plating allows plating on minimal media
and enriched media from the same master plate
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107The selection of auxotorph revertants
- The lysine auxotrophs ( Lys-) are treated with a
mutagen such as nitroquanidine or uv light to
produce revertants
108Ames Test
- Developed by Bruce Ames
- Used to test for carcinogens
- A mutational reversion assay based upon mutants
of Salmonella typhimurium
109DNA repair mechanisms
- Type I -Excision repair
- Corrects damage which causes distortions in the
double helix - A repair endonuclease or uvr ABC endonuclease
removes the damaged bases along with some bases
on either side of thee lesion - The usual gap is about 12 nucleotides long. It
is filled by DNA polymerase and ligase joins the
fragments. - This can remove Thymine-Thymine dimers
- A special type of repair utilizes glycosylases to
remove damaged or unnatural bases yielding the
results discussed above
110Mutations and repair
- Type II Removal of lesion
- Thymine dimers and alkylated bases are often
repaired directly - Photoreactivation is the repair of thymine dimers
by splitting them apart into separate thymines
with the aid of visible light in a photochemical
reaction catalyzed by the enzyme photolyase - Light repair-phr gene - codes for
deoxyribodipyrimidine photolyase that, with
cofactor folic acid, binds in dark to T dimer.
When light shines on cell, folic acid absorbs the
light and uses the energy to break bond of T
dimer photolyase then falls off DNA
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112Dark repair of mutations
- Dark repairThree types1) UV Damage Repair (also
called NER - nucleotide excision
repair)Excinuclease (an endonuclease also
called correndonuclease correction endo.) that
can detect T dimer, nicks DNA strand on 5' end of
dimer (composed of subunits coded by uvrA, uvrB
and uvrC genes). UvrA protein and ATP bind to
DNA at the distortion. UvrB binds to the
UvrA-DNA complex and increases specificity of
UvrA-ATP complex for irradiated DNA. UvrC nicks
DNA 8 bases upstream and 4 or 5 bases downstream
of dimer. UvrD (DNA helicase II same as DnaB
used during replication initiation) separates
strands to release 12-bp segment. DNA polymerase
I now fills in gap in 5'gt3' direction and ligase
seals.
113The Effects of uv light
114Post replication repair
- If T dimer not repaired, DNA Pol III can't make
complementary strand during replication.
Postdimer initiation - skips over lesion and
leaves large gap (800 bases). Gap may be repaired
by enzymes in recombination system - lesion
remains but get intact double helix. - Successful post replication depends upon the
ability to recognize the old and newly
replicated DNA strands - This is possible because the newly replicated DNA
strand lack methyl groups on their bases, whereas
the older DNA has methyl groups on the bases of
both strands. - The DNA repair system cuts out the mismatch from
the non- methylated strand
115Recombination repair
- The DNA repair for which there is no remaining
template is restored - RecA protein cuts a piece of template DNA from a
sister molecule and puts it into the gap or uses
it to replace a damaged strand - Rec A also participates in a type of inducible
repair known as SOS repair. - If the DNA damage is so great that synthesis
stops completely leaving many gaps, the Rec A
will bind to the gaps and initiate strand
exchange. - It takes on a proteolytic funtion that destroys
the lexA repressor protein which regulates genes
involved in DNA repair and synthesis