he culture media are classified in many different ways: Based on the physical state Liquid media Solid media Semisolid media Based on the presence or absence of oxygen Anaerobic media Aerobic media Based on nutritional factors Simple media Synthetic media Complex

1. Bacterial Genetics
LECTURE 12

2. Objectives:
• Definitions
• Structure & functions of genetic material
• Protein synthesis
• Transformations
• Mutation
• Types of mutations
• Transduction
• Conjugation

3. DEFINITIONS
Genetics: The study of heredity and variation, of the
reasons for similarity and differences between
parents and their offspring.
Gene :The unit of hereditary is the gene, a segment of
DNA that carries in its nucleotide sequence
information for a specific biochemical or physiological
property.
Genome: The sum total of all the genetic information
contained in a cell is its genome.

4. • DNA: or Deoxyribonucleic acid - the principal molecule
responsible for the storage of genetic information.
• Chromosome: The single largest DNA molecule in a cell
that contains the information necessary for the cell’s survival.
• Replicon: Any segment of DNA that is capable of encoding
its own replication in the cell. Thus, the chromosome is the single
largest replicon in the cell.
• Plasmid: Any extra-chromosomal DNA that is capable of
encoding its replication. In other words, non chromosomal
replicons.

5. • PHENOTYPE:-Characteristics expressed by a cell
in a given environment. Adaptable in response to
environment changes.
• GENOTYPE:-Collection of genes encoding the
characteristics. Remains Unchanged.
• RECESSIVE :- A gene that does not achieve
phenotypic expression in the presence of its
homologue is recessive.
• DOMINANT:- A gene that over rides the effect Of
its homologue is dominant.

6. Structure and Function of Genetic
Material:
DNA & RNA
• DNA=deoxyribonucleic acid
• RNA=ribonucleic acid
• Basic building blocks:
• Nucleotides
• Phosphate group
• Pentose sugar
• Nitrogenous base

7. Structure of DNA
• Double stranded (double helix)
• Chains of nucleotides
• 5’ to 3’ (strands are anti-parallel)
• Complimentary base pairing
• A-T
• G-C

8. DNA Structure:
Phosphate-P
Sugar-blue
Bases-ATGC

9. DNA Replication:
• Bacteria have closed, circular DNA
• Genome: genetic material in an
organism
• E. coli
• 4 million base pairs
• 1 mm long (over 1000 times larger that
actual bacterial cell)
• DNA takes up around 10% of cell volume

10. DNA Replication-occurs at the replication
fork :
• 5’ to 3’
• DNA helicase-unzips + parental DNA strand
that is used as a template
• Leading stand(5’ to 3’-continuous)
- DNA polymerase-joins growing DNA strand
after nucleotides are aligned (complimentary)
• Lagging strand(5’ to 3’-not continuous)
- RNA polymerase (makes short RNA primer)
- DNA polymerase (extends RNA primer then digests RNA
primer and replaces it with DNA)
- DNA ligase (seals Okazaki fragments-the newly
formed DNA fragments)

11. Replication Fork

12. Protein Synthesis:
• DNA------- mRNA------ protein
transcription translation
Central Dogma
of Molecular Genetics

13. Transcription:
• One strand of DNA used as a template to
make a complimentary strand of mRNA
• Promoter/RNA polymerase/termination site/5’
to 3’
• Ways in which RNA & DNA differ:
• RNA is ss
• RNA sugar is ribose
• Base pairing-A-U

14. Transcription:

15. Typesof RNA:
• Three types:
• mRNA: messenger RNA
• Contains3bases( codon)
• rRNA: ribosomal RNA
• Comprisesthe70S ribosome
• tRNA: transfer RNA
• Transfersaminoacidstoribosomesforprotein
synthesis
• Containstheanticodon(3 basesequencethat
iscomplimentarytocodononmRNA)

16. Genetic Code:
• DNA: tripletcode
• mRNA: codon(complimentaryto
tripletcodeof DNA)
• tRNA: anticodon(complimentary
tocodon)

17. GeneticCode:
• Codons:codefortheproductionof a
specificaminoacid
• 20amino acids
• 3base code
• Degenerative: morethan1codon
codesforanaminoacid.
• Universal: inallliving organisms.

19. Stop codons:
-Actaspunctuation mark.
-Terminationthemessageforthesythesisof the
polypeptide.
- UAA,UAG, UGA

20. Translation:
• Three parts:
• Initiation-startcodon(AUG)
• Elongation-ribosomemovesalongmRNA
• Termination: stopcodon
reached/polypeptidereleasedandnew
protein forms
• rRNA=subunitsthatformthe70S
ribosomes(proteinsynthesisoccurs
here)
• tRNA=transfersaminoacidsto
ribosomesforprotein synthesis.

25. Mutation:
• Defination:
• Itisaheritablechangeingenotype,thatcanoccur
spontaneouslyorinducedbyphysicalorchemical
treatments.
• Organismselectedasreferancestrainiscalledas
wildtype.
• Theirprogenywithmutationiscalledmutagen.
• Processof mutationiscalledasMutagenesis.
• Theagentinducingmutationiscalledasmutagen.

26. • Spontaneousmutationoccursnaturallyaboutonein
everymilliontooneineverybilliondivisions&occurs
duringtheDNAreplication.
• Mutationratesof individualgenesinbacteriarange
from10‾2 to10‾10 /bactrium/division.

27. Mechanismof division:
• SubstitutionofNucleotides:
- Alsocalledaspointmutation.
- MC mechanismof themutation.
-Itinvovles the changing of single base in the DNA
sequence,whichiscopiedduringreplication,to
producepermanentchanges.
-If onepurineorpyrimidineisreplacedbytheother,
thesubstitutioniscalledas Transition.
- If apurineisreplacedbypurineorvice-versa,the
substitutioniscalledasTransversion.

28. • Deletionoradditionof nucleotide:
- OccursduringtheDNA replication.
-Whenatransposon(jumpinggene)insertsitself in
toagene,itleadstodistruptionof gene, calledas
Insertional mutation.

29. Typesofthe mutation:
• Missense mutation:
- CausedbyDNAmutatinswhichleadstochanges
intoaminoacidsequencesof theprotein products
-Caused bysinglepointmutationorseriesof
mutations.
• Nonsense mutation:
-formationof stopcodon,whichcausesterminationof protein
synthesisleadstoincompleteproteinproducts.
• Silent mutation:
-SinglesubstitutionmutationchangeintheDNAsequence
resultinanewcodon, stillcodingforthesameaminoacid&
thereisanochangeintheproduct.

30. • Frameshift mutation:
• Theadditionordeletionof basepaircausingashiftinthe
“readingframe”of thegene.
• Thesescauses areadingframe shift &allof the codon &all
amino acids afterthatmutationareusually wrong.
• Lethalmutation:
• Somemutationaffectthevitalfunctionof thecell,&bacterial
cellbecomenonviable,&whichcankillthecell.
• Suppressor mutation:
• It is a reversal of the mutantphenotype by another mutation at
apositionontheDNA, differfromthatof original mutation.
• Truereversionorbackmutationresultsinreversionof mutant to
originalform,which occurs asaresult of mutationoccurring
atthesamespotonceagain.

31. • Conditionallethal mutation:
• Mutationmayoccurinsuchawaythatmutationmay
surviveincertainenviromentalcondition.
• Inversion mutation:
• Segmentof theDNAisremoved&reinsertedinthereverse
direction.

32. NormalDNA/Missense Mutation

33. NonsenseMutation/Frameshift Mutation:

34. GeneticTransferinBacteria:
• Genetictransfer-resultsingeneticvariation
• Geneticvariation-neededforevolution
• Three ways:
• Transformation: genestransferredfromone
bacteriumtoanotheras“naked”DNA.
• Conjugation: plasmidstransferredonebacteriato
anotherviaapilus.
• Transduction: DNAtransferredfromonebacteria
toanotherbya virus.

35. Transformation:
•Recipientcellmustbecompetentforuptake
of DNA.
•Naturalcompetenceversusartificial
competence.
•Onlycertainbacteriaarenaturally
transformable- Streptococcuspneumoniae,
aemophilus influenzae, Neisseria gonorrhoeae,
Vibrio.

36. Transformation:

37. Transductionbya Bacteriophage:
• Bacteriophage(phage) arevirusesofbacteria- can
be eitherlyticortemperate
• 1. lytic- alwayslyse(kill)hostbacterialcell
temperate- canstablyinfectandcoexistwithin
bacterialcell(lysogeny)untilalyticphaseis induced
• 2. lysogeny
• Thephagegenomeduringlysogenyiscalledthe
prophage,andthebacterialcelliscalleda lysogen
• Ifthephagegenomeencodesanobservablefunction,
thelysogenwillbealteredinitsphenotype- lysogenic
conversion(e.g., diphtheriatoxininCorynebacterium
diphtheriae)

38. • 3. Specialized transduction:
• Someprophagesintegrateintothebacterialgenome
ataspecificlocation.
• Whenaprophageisinducedtolyticphase,itmay
dragalongapieceof thebacterialgenomenextto the
integrationsiteandmovethatbacterialsequence into
thenewrecipienthostcell,changingthe recipient's
genome.
not very important medically since only selected
genes can be transferred

40. • 4. Generalizedtransduction:
• whenaphagelysesthehostbacterialcell,itnormally
packagesphagegenomeintothe capsid.
• sometimesthecapsidisaccidentallyfilledwith
randompiecesof bacterialgenome,possibly
includingplasmids
• whenthecapsidinjectsthehostgenesintoanew
recipient,thenewgenecanrecombineintothe
recipientgenomeandcausea change.
• virulenceandantibioticresistancegenescanbe
movedbygeneralizedtransduction.

41. • Differencebetweenlysogenyandgeneralized
transduction
1.lysogeny- whenaphagelysogenizesahost
bacterialcellandbringsinanewgenethatchanges
thecharacteristicsof thecell- itisaphagegenethat
makesthechangeinlysogenicconversionandahost
geneinspecialized transduction.
2.Generalized transduction involves the phage particle
acting as a vehicle to move bacterial genes from one
celltoanother,thephagegenomeisnotmoved.

42. Transductionbya Bacteriophage:

43. conjugation:
Possiblestatesforconjugation
i.Conjugative- theplasmidencodesallof the
functionsforconjugationandcanmoveitself fromthe
donorcelltotherecipient cell.
ii.Mobilizable - the plasmid cannot move itself, but
can be moved with help from a conjugative plasmid.
Conjugation functions:
iii.synthesisof sex pilus.
iv. celltocellcontactviapilus.
v.copying plasmid DNA and transfer of copy into
recipient cell.

46. •
•
•
Bacteriacontainingaconjugativeplasmidarecalled
donor,male, (F+)
Bacteriareceivingtheplasmidarecalledrecipient,
female, (F-)
The F plasmid is a specific E. coli plasmid that has
nothing to do withmedicine other than its historical
andlaboratoryuse(andstandardizedexams)

47. i.
• Hfr- aplasmidintegratesintothechromosome,
conjugationwillmovepartof thechromosomeintothe
recipient(notof medicalrelevance)
• F' - plasmidintegratesintothechromosome,excision
dragsalongpieceof the chromosome.
• F' canmovehostDNAbetweencells(notof medical
relevance) (similartospecializedtransduction)
• Phenocopy- acellwiththeF plasmid,butlacking pili

48. •Unliketransductionbyphages,mostplasmids
are more promiscuous in theirhostrange
•Importanceof conjugation- movingplasmids
encodingmultipleantibioticresistancegenes(R
plasmids)among bacteria.

49. InsertionSequencesand Transposons:
1.Thesesequencesarepartof othergenetic
elements- chromosome,plasmid.
-They movefromonesiteinDNAtoanotherWITHIN
thesamecell(transposition)
2. Insertionsequence (IS)
-Gene encodingtranspositionenzyme
(transposase)flankedbyinvertedrepeatsof DNA
sequence.
- caninterruptgenesif theyinsertintothem.

50. ConjugationinE. coli:

51. Conjugationcont…

52. Conjugationcont…

53. Referances:
1. T
extbookof microbiology:anantnarayan.
2. NotesbyShridharrao