BACTERIAL GENETICSBACTERIAL GENETICS

DR. A.S.AL-KHATTAFDR. A.S.AL-KHATTAF

Structure and Function of the Genetic Structure and Function of the Genetic MaterialMaterial

Chromosomes Chromosomes are cellular structures made up are cellular structures made up of genes that carry hereditary information.of genes that carry hereditary information.

GeneticsGenetics is the study of genes carry is the study of genes carry information, how they are replicated and passed information, how they are replicated and passed to other generations, and how they affect the to other generations, and how they affect the characteristics of an organism.characteristics of an organism.

A geneA gene is a segment of DNA that codes for a is a segment of DNA that codes for a functional product.functional product.

The genetic information in a cell is the The genetic information in a cell is the genome. genome.

NucleotidesNucleotides

DNA is composed of repeating DNA is composed of repeating nucleotidesnucleotides containing the bases containing the bases adenine (A), thymine (T), cytosine adenine (A), thymine (T), cytosine (C), or guanine (G); a deoxyribose (C), or guanine (G); a deoxyribose sugar; and a phosphate group.sugar; and a phosphate group.Bases occur is specific Bases occur is specific complementarycomplementary base pairs,base pairs, the the hydrogen bonds from which hydrogen bonds from which connect strands of connect strands of DNADNA: adenine : adenine with thymine, and cytosine with thymine, and cytosine

with guanine.with guanine.

Information on DNA can be Information on DNA can be transcribed into RNA transcribed into RNA ((transcription)transcription) and in turn, and in turn, translated into protein translated into protein ((translation). translation).

Genotype and PhenotypeGenotype and Phenotype

The The genotypegenotype is an organism’s genetic makeup, the is an organism’s genetic makeup, the

information that codes for all the characteristics and information that codes for all the characteristics and

potential potential properties of the organism.properties of the organism.

The genotype is its gene collection- its DNA.The genotype is its gene collection- its DNA.

The The phenotypephenotype refers to an organism’s refers to an organism’s actual expressedactual expressed

properties, such as its ability to perform a chemical reaction.properties, such as its ability to perform a chemical reaction.

The phenotype is the collection of enzymatic or structural The phenotype is the collection of enzymatic or structural

proteins.proteins.

DNA and ChromosomesDNA and Chromosomes

DNA in chromosomes is in the form of DNA in chromosomes is in the form of long double helix.long double helix.

In prokaryotes, DNA is not found within a In prokaryotes, DNA is not found within a nuclear membrane.nuclear membrane.

The chromosome takes up only about The chromosome takes up only about 10% of the cell’s volume because the DNA 10% of the cell’s volume because the DNA is is supercoiled supercoiled by an enzyme called by an enzyme called DNA DNA gyrase.gyrase.

DNA replicationDNA replicationIn DNA replication, the two In DNA replication, the two helical strands unravel and helical strands unravel and separate from each other at a separate from each other at a replication fork,replication fork, where the where the synthesis of new strand synthesis of new strand

beginsbegins..The complementary pairing of The complementary pairing of bases yields a complementary bases yields a complementary copy of the original DNA.copy of the original DNA.

Segments of new nucleotides Segments of new nucleotides are joined to form short are joined to form short strands of DNA by strands of DNA by DNA DNA

polymerasepolymerase enzymes. enzymes.

DNA ReplicationDNA Replication

Short strands of DNA are then joined into continuous DNA by Short strands of DNA are then joined into continuous DNA by DNA DNA

ligaseligase enzymes.enzymes.

Because each new double-stranded DNA molecule has one original Because each new double-stranded DNA molecule has one original

strand and one new strand, the process is called strand and one new strand, the process is called semi-semi-

conservative replication.conservative replication.

In bacteria, replication begins at an In bacteria, replication begins at an origin of replicationorigin of replication and in some and in some

cases two replication forks move in opposite directions.cases two replication forks move in opposite directions.

DNA replication makes few mistakes, largely due to the DNA replication makes few mistakes, largely due to the

proofreading capability of DNA proofreading capability of DNA polymerasepolymerase..

RNA and Protein SynthesisRNA and Protein SynthesisTranscriptionTranscriptionIn transcription, a strand of In transcription, a strand of messenger RNA (mRNA)messenger RNA (mRNA) is is synthesized from the genetic synthesized from the genetic information in DNA.information in DNA. Adenine in the DNA dictates the Adenine in the DNA dictates the location of uracil, which replaces location of uracil, which replaces thymine in mRNA.thymine in mRNA.If DNA has the bases sequence If DNA has the bases sequence ATGCAT, the mRNA will have ATGCAT, the mRNA will have UACGUA.UACGUA.The region where The region where RNA RNA polymerasepolymerase (needed for synthesis) (needed for synthesis) binds to DNA and transcription binds to DNA and transcription begins is known as begins is known as promoter site.promoter site.The The terminator site terminator site is where the is where the RNA polymerase and newly RNA polymerase and newly formed mRNA are released from formed mRNA are released from the DNA.-Endpoint.the DNA.-Endpoint.

TranslationTranslationProtein synthesis is called Protein synthesis is called translation.translation.

The language of mRNA is in The language of mRNA is in codons,codons, groups of three groups of three nucleotides such as AUG.nucleotides such as AUG.

Each codon codes for a Each codon codes for a particular amino acid.particular amino acid.

There are 64 possible codons, There are 64 possible codons, but only 20 amino acids.but only 20 amino acids.

An amino acid has more than An amino acid has more than one codon (one codon (degeneracy)degeneracy) of of the code.the code.

Sense codonsSense codons code for amino code for amino acids; acids; nonsense (stop) nonsense (stop) codonscodons signal the end of signal the end of synthesis of a protein.synthesis of a protein.

TranslationTranslationThe site of translation are The site of translation are ribosomesribosomes that move that move along mRNA.along mRNA.The amino acids are The amino acids are transported to the transported to the ribosome by ribosome by transfer transfer RNA (tRNA).RNA (tRNA).Each tRNA molecule is Each tRNA molecule is made specific for an made specific for an amino acid by an amino acid by an anticodon that is anticodon that is complementary to a complementary to a codon.codon.

The codon AUG would The codon AUG would

attach to anticodon UAC.attach to anticodon UAC.

Repression and InductionRepression and Induction

An An inducer inducer is a substance is a substance (substrate) whose presence (substrate) whose presence results in the formation, or results in the formation, or increase in the amount of an increase in the amount of an enzyme.enzyme.Such enzymes are called Such enzymes are called inducible enzymes;inducible enzymes; this this genetically controlled response genetically controlled response is termed is termed enzyme induction.enzyme induction.Lactase Lactase production in production in response to lactose – response to lactose – example.example.Genetic regulation that Genetic regulation that decreases enzymes synthesis decreases enzymes synthesis is is enzyme repression.enzyme repression.

Mutation: Change in Genetic MaterialMutation: Change in Genetic Material

A mutation A mutation is a change in the is a change in the bases sequence of DNA.bases sequence of DNA.The most common mutation is a The most common mutation is a base substitution-base substitution- a single base a single base in DNA is replaced with different in DNA is replaced with different one.one.This may create a stop codon, This may create a stop codon, that stop protein synthesis before that stop protein synthesis before completion- completion- a nonesense a nonesense mutation.mutation.Deletion Deletion or or additionaddition of base of base pairs results in a pairs results in a frame-shift frame-shift mutation.mutation.Leads to shift in translational Leads to shift in translational reading frame (three-by-three).reading frame (three-by-three).

MutagensMutagensMany chemicals and Many chemicals and radiation bring about radiation bring about mutations; these are called mutations; these are called mutagens.mutagens.

Nitrous acid (HNONitrous acid (HNO22) is a ) is a

base pair mutagen.base pair mutagen.

It cases Adenine to pair with It cases Adenine to pair with Cytosine not Thymine.Cytosine not Thymine.

Other mutagens are Other mutagens are nucleoside analogsnucleoside analogs- - structurally similar to bases structurally similar to bases and incorporated into DNA and incorporated into DNA by error.by error.

Example- 2-Aminopurine Example- 2-Aminopurine analog to Adenine, 5-analog to Adenine, 5-Bromouracil to Thymine.Bromouracil to Thymine.

Mutagenic drugsMutagenic drugs

Some antiviral and anti-tumor drugs are nucleoside Some antiviral and anti-tumor drugs are nucleoside analogs.analogs.

AZT (azidothymindine), one of the primary drugs used to AZT (azidothymindine), one of the primary drugs used to treat HIV infection.treat HIV infection.

Other chemical mutagens cause small deletions or Other chemical mutagens cause small deletions or insertions, which can result in frame-shifts.insertions, which can result in frame-shifts.

Example- benzpyrene, which present in smoke and soot.Example- benzpyrene, which present in smoke and soot.

Aflatoxin – produced by Aflatoxin – produced by AspergillusAspergillus flavus is a frame flavus is a frame mutagen.mutagen.

Acridine dyes used against herpesvirus infections.Acridine dyes used against herpesvirus infections.

RadiationRadiation Ionizing radiation-Ionizing radiation- such such as X rays and gamma as X rays and gamma rays are mutagens and rays are mutagens and damage DNA.damage DNA.They cause electrons to They cause electrons to pop out of their usual pop out of their usual shells- producing ions shells- producing ions and free radicals.and free radicals.Ultraviolet light (Ultraviolet light (non-non-ionizing radiation)ionizing radiation) is is also mutagen.also mutagen.Light-repair enzymesLight-repair enzymes repair UV damage to repair UV damage to DNA.DNA.EnzymesEnzymes cut out cut out distorted DNA and distorted DNA and synthesize replacement.synthesize replacement.

Genetic Transfer and RecombinationGenetic Transfer and Recombination

Genetic recombination Genetic recombination is the is the rearrangement of genes to rearrangement of genes to form new combinations.form new combinations.

Crossing overCrossing over happens when happens when two chromosomes break and two chromosomes break and rejoined in such a way- genes rejoined in such a way- genes are reshuffled between the two are reshuffled between the two chromosomes.chromosomes.

The donor cellThe donor cell gives a portion gives a portion of its total DNA to a different of its total DNA to a different recipient cell (recombinant).recipient cell (recombinant).

Vertical gene transferVertical gene transfer occurs occurs from an organism to its from an organism to its offspring.offspring.

Horizontal gene transferHorizontal gene transfer from from bacteria to other microbe.bacteria to other microbe.

Transformation in BacteriaTransformation in Bacteria

TransformationTransformation –naked –naked DNA in solution is DNA in solution is transferred from one transferred from one bacteria to another.bacteria to another.

It occurs naturally among It occurs naturally among very few genera of very few genera of bacteria, when donor and bacteria, when donor and recipient are closely recipient are closely related- occurs in the log-related- occurs in the log-phase of growth.phase of growth.

Recipient cell must be Recipient cell must be competent- competent- cell wall cell wall permeable to large DNA permeable to large DNA molecules.molecules.

Conjugation in BacteriaConjugation in Bacteria

Conjugation requires contact Conjugation requires contact between living cells of opposite between living cells of opposite mating types.mating types.

The donor cell The donor cell FF+ + has plasmid has plasmid called called F F (fertility factor).(fertility factor).

When FWhen F++ cell mixed with F cell mixed with F--, , cells attach by sex pili.cells attach by sex pili.

F factor is duplicated by donor F factor is duplicated by donor and the new copy is and the new copy is transferred to Ftransferred to F- - cell.cell.

Transduction in BacteriaTransduction in Bacteria

In In generalized generalized transduction, transduction, the the phage phage (bacteriophage)(bacteriophage) attaches to the bacterial attaches to the bacterial cell wall and injects DNA cell wall and injects DNA into bacteria.into bacteria.Normally this synthesis Normally this synthesis new viruses.new viruses.Occasionally part of Occasionally part of bacterial chromosome is bacterial chromosome is taken by the viral DNA.taken by the viral DNA.When new bacteria are When new bacteria are infected by these viruses infected by these viruses old bacterial DNA is old bacterial DNA is incorporated in the new incorporated in the new bacterial genome. bacterial genome.