29 microbial genetics
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MICROBIAL GENETICS
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Two types of organism:
Eukaryotes; cells contain membrane-boundcompartments, including a nucleus and organelles.
Eukaryotes include: animals, plants, fungi, and
protozoa. Prokaryotes: Lack internal compartments etensi!ely.
"i!ided into two groups: bacteria and arc#aea.
$enome:T#e entire genetic complement of a li!ing organism.
$ene: % "&% segment containing biological information and
#ence coding for an '&% and ( or polypeptide molecule.
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Eukaryotes genomes; "i!ided into two or more linear "&% molecules.
Eac# contained in a different c#romosome. Possess smaller, usually circular, mitoc#ondrial
genomes. T#ird genome located in t#e c#loroplast )in plant and
ot#er p#otosynt#etic organism* + b +, b in lengt#. /ig#er eukaryotes need larger genome to
accommodate t#e etra genes. 0orrelation between genome size and compleity )111*
--- C-value paradox . 2pace is sa!ed in t#e gnomes of less comple
organism because t#e genes are more closely packed
toget#er.
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Prokaryotes genomes:
3#ole prokaryotes genomes are smaller t#aneukaryotes.
ost if not all -- are contained in a single "&%
molecules. T#e molecule is circular. /a!e second circular or linear genome, called
PL%24". /a!e fewer genes ore compact genome organization, more gene but
less space. T#ere is &5 4&T'5&2 )some eception is in
arc#aea.* 4nfre6uency of repetiti!e se6uences.
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7acterial "&% structure: 0onsists of two c#ains of c#emical units called
nucleotides. ay contain + 8 million nucleotides. % nucleotides consists of a base )purine and
pyrimidine* Purine : adenine )%* and guanine )$* Pyrimidine: T#ymine )T* and cytosine )0* "&% contains a specific se6uence of purine and
pyrimidine base.
% binds to T
$ binds to 0
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99
eon eonintronupstream downstream
4nitiation codon termination codon
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Circular shape of microbial DNA
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"&% and $enetics 0ode:
T#e se6uence of codons on t#e m'&% contain
t#e necessary information to :
4nitiate polypeptide synt#esis. "esignate t#e se6uence of amino acids in t#e
polypeptide. Terminate t#e synt#esis of t#e polypeptide. 'elease t#e completed polypeptide.
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Genetics codes
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DNA Replication:
T#e transmission of genetic material from parent to
progeny re6uires an eact duplication of t#e original
"&% strands. a<or errors in t#is process leading to pre!ent normal
cell growt# and de!elopment.
Eac# strand of t#e parent "&% ser!es as templatefor t#e formation of a complementary strand.
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DNA transcription:
T#e process in w#ic# one strand of "&% ser!es as a
template for t#e synt#esis a single-stranded
messenger '&% )m'&%*. 4t is t#e information on t#e '&% t#at can be
translated into usable information by t#e cell.
DNADNA
tRNA
rRNA
mRNA Protein
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Translation of the genetic code: Protein
synthesis
m'&% is translated into polypeptides in a process
t#at take place on t#e ribosome. 'ibosomes are small structural components in t#e
cell t#at are composed of r'&% and a !ariety of
proteins. 7efore protein synt#esis begins, eac# amino acids
was broug#t to t#e ribosome by t'&%.
T#e protein synt#esis of t#e polypeptide takes placeby t#e linear mo!ement of ribosomes along t#e
m'&%.
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MutationThe alterations in the base sequence of the hereditary
material (DNA)
Spontaneous mutationsOccur in the absence of environment intervention.It is occur in case of:
Purine or yrimidine bases temorarily e!ist in differentelectrochemical forms """""""" misairin#. Insertion sequence. $issmatch durin# relication
Induced mutationsPhysical a#ents: %ltraviolet li#ht (%&) '"rays amma"rayshemical a#ents
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*ontaneous mutation
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A
IS
A’
IS
A’’IS’
$ovement of insertion sequence (I*)I* element relicates and inserts a coy +ithin the A #ene
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$ismatch durin# relication
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$uta#enic a#ent caused mutation
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How mutations affect the genetics code ? *ilent mutation (has no effect) $issense mutation (chan#e to a different amino acids) Nonsense mutation (chan#e to a sto codon) ,rameshift mutation.
Open reading frame (ORF)-o+ to read the codons sequences.
DG!ANR"N
DG#!AN#R"N )=rame 4*
D#GS#ANR#"N )=rame 44*
D#G!A#NR"#N )=rame 444*
DG!ANR"N
DG#!AR#"N )=rames#ift*
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The effect of mutations on microorganism:
Auotrophs $icroor#anism that +ill only #ro+ +hen rovided +ith a
nutrient that is not roduced by it self. !rototrophs are the+ild tye microor#anism.
Con"itional#lethal mutants : ermissive condition
restrictive condition temerature"sensitive.
Inhibitor#resistant mutants: able to resist the to!ic effectsof an antibiotic or other tye of inhibitor.
Re$ulator% mutants& has mutation in the romoter or otherre#ulatory sequence. /': constitutive mutants of E. Coli e!ressed #enes all the times.
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Con'u$ation&Onl% one stran" of DNA is transferre"( The recipientcompletes the structure of "ouble stran"e" DNA(
Trans"uction&
Donor DNA )as carrie" in a pha$e coat an" istransferre" into the recipient b% the mechanism use"for pha$e infection(
Transformation&
Direct up ta*e of the "onor DNA b% the recipient cell+ma% be nature or force"(
Mechanism of $ene transfer in pro*ar%ote
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Con'u$ation
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Transosable elements :
Insertion se,uences -IS. Se$ment of DNA Appro( /000 bp Contain $enes nee" for transposition onl% Enco"e en1%me for site#specific recombination( 2as certain se,uence in its termini( Calle" replicati3e recombination
Transposons -Tn element. /0 fol" of IS in len$th( 2as more comple en1%me4proteins
Transposable pro$ha$es Certain pha$e+ e& Mu
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DNA $iruses
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RNA $iruses
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7altimore 0lassification of >iruses
2 ssDNA ParvovirusdsDNA mRNAssDNA
4 +ve ssRNA dsRNA +ve ssRNA [Acts as mRNA] Enterovirus
5 -ve ssRNA Influenza A
virus
dsRNA -ve ssRNA mRNA
ssRNA mRNAdsDNAssRNA Retrovirus!e"#" $I%&
' Nic(ed dsDNA $e)atitis *virus
nic(ed dsDNA intact dsDNA mRNA
RNA
oup enome Replication Example
dsDNA dsDNA mRNA $er)es sim)le,virus
dsRNA ReovirusdsRNA mRNA
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Positi$e Strand RNA %irus Genome
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Negati$e strand RNA %irus Genome
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Segmen
RNA &#'
Segmen
RNA &('
Segmen
RNA &#'
%irus )aru
%irus en*yme
mRNA
Strand &(' Protein