29 microbial genetics

7/23/2019 29 Microbial Genetics

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MICROBIAL GENETICS



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.



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.



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.



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



99

eon eonintronupstream downstream

4nitiation codon termination codon



Circular shape of microbial DNA



"&% 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.



Genetics codes



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.



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



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'&%.



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



*ontaneous mutation



A

IS

A’

IS

A’’IS’

$ovement of insertion sequence (I*)I* element relicates and inserts a coy +ithin the A #ene



$ismatch durin# relication



$uta#enic a#ent caused mutation



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*



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.



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



Con'u$ation



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



DNA $iruses



RNA $iruses



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



Positi$e Strand RNA %irus Genome



Negati$e strand RNA %irus Genome



Segmen

RNA &#'

Segmen

RNA &('

Segmen

RNA &#'

%irus )aru

%irus en*yme

mRNA

Strand &(' Protein

29 microbial genetics

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