number 3

Done by Neda’a Baniata

Corrected by Waseem Abu Obeida

Doctor Hamed Al Zoubi

Note: it is important to refer to slides.

Bacterial genetics

*The main concepts we will talk about in this lecture:

We will talk about genome replication briefly, mutations,

genetic exchange between bacteria, and finally we will talk

about transposons briefly.

Bacterial genome and replication:

-Bacterial genome (chromosome): single circular double

stranded DNA, found in a region called nucleoid.

Bacterial genetic structure is represented by a genetic

material consisting of:

1-chromosome: must be found in every bacterial genome.

2- Plasmids.

3-bacteriophage.

4-transposons.

The last three are accessory components that might not be

found in bacterial genetic material.

*why we are interested in study of bacterial

genetics?

1-Firstly, bacterial genetics is important in determining the

structure of bacteria: genes are translated into proteins

(sequence of amino acids).these proteins can have a structural

(being a component of the cell wall giving it more rigidity) or

functional role (as enzymes or as pathogenic function like

toxins).

2-It is important in determining antibiotic resistance of

bacteria: to understand some of their mechanisms later on.

3-molecular cloning :sometimes we use bacterial genetics to

clone some genes into the bacteria –then bacteria will express

it into proteins –usually these proteins are useful to us, so we

can extract it for using it for various aspects (vaccines

,antibiotics ,antigens, or for diagnosis).

*Bacterial chromosome:

-1300Mm long, super coiled.

-it has (2-5) million nucleotide bases (nucleotide bases are: A,

T, G, C base pairing: A—T, G—C)

-Have enough DNA to encode around 1-3 thousand different

genes according to bacteria types.

-genes are expressed as required depending on the

surroundings (nutrients, metabolites, temperature and

pressure),or if some of the bacterial proteins are recognized by

the immune system ,the bacteria turns off the expression for

these proteins.

-Gene expression is controlled by many factors such as

amount of end product metabolism and presence of repressors

(a repressor is a DNA- or RNA-binding protein that inhibits

the expression of one or more genes by binding to the

operator or associated silencers.)

Genotype /wild type

Represent the genetic

structure of bacteria.

• All Inherited essential

biological features &

growth patterns.

phenotype

represent the formal

characteristics of bacteria

-phenotype determine a lot of

characteristics of bacteria:

biochemical rxns, irreversible

factors that can be used to

find bacteria.

*not every genotype is expressed to phenotype (they are

expressed upon need as we mentioned earlier).

Mutations:Mistakes or inaccuracies during DNA

replication.

Spontaneous mutations: it

occurs naturally (not induced

by an external factor), where

its occurrence may result in a

protein defect and the

outcome will be significant.

Induced Mutations:

1-chemicals, X-rays, UV light

and viruses.

2-direct damage of nucleotides

or alteration of nucleotide

bonds.

-these mutations could be

induced in vitro and lead to

dimerization of base pairs and

this can affect replication.

*In vitro: in laboratory or in

animals.

*In vivo: in human being.

-Remember that:

-A codon is a triplet of nucleotides that codes for one amino

acid only, the same amino acid may be coded by more than

one codon.

- The translation begins from the start codon (AUG) and ends

at the stop codon (UAA, UAG, UGA).

-we call a triplet of nucleotides a codon if they exist on the

mRNA strand and we call it the reading frame.

Types of the mutation:

1-Multisite: affects many nucleotides by inversion,

duplication or deletion.

-Inversion for example occurs in bacteria to avoid recognition

by the immune system. The gene which was recognized by

the immune system will be changed by inversion, when threat

disappears reinversion occurs.

2-Point mutation: affects single or few nucleotides by

insertion, substitution, deletion.

Outcome of mutations:

Outcomes of Base substitution

which occurs due to DNA

polymerase error or mutagens.

1)missense mutation :

GGC—Glycine

AGC—Serine

DNA sequences change –

RNA sequences change—

Codes for different amino

acids.

2)nonsense mutation :

AAG—lysine

UAG—stop codon

DNA sequences change –

RNA sequences change –

Early stop codon introduced.

(Translating stops—protein

incomplete so the bacteria loses

the protein).

3)silent mutation:

GGC, GGU—both code for the

same amino acid (glycine),

So no effect on the amino acids

sequence would occur.

Frame-shift mutation :

-One or more bases are

added or deleted

-Shift in the reading frame,

Corrupting the reading of

codons downstream leading

to inactive protein.

-Change in protein structure is significant to bacteria –for

example-if it is a toxin and after mutation become nontoxic –it

loses the pathogenicity trait- Can be lethal to bacteria.

Or change in protein structure which is an antibiotic target

causing resistance to antibiotics.

Genetic exchange:

The importance of genetic exchange :

1-moving antibiotic resistance genes among bacteria.

2-moving virulence genes among bacteria.

3-changing the genetic makeup to avoid immunity.

Mechanisms:

1-transformation: the uptake of naked DNA from the

environment.

-DNA source can be from dead bacteria cell and its DNA is

free in the environment.

-usually this mechanism occurs between the related bacteria.

-a recipient cell must be competent; competent means: it has

structures on its cell surface that can bind to DNA and take it

up intracellularly).

-Naked DNA will be up taken by competent bacterial cell

through pores, a semi-permeable cytoplasmic membrane

-then the DNA might be degraded or incorporated with

bacterial chromosome to become a part of it.

*normal competence: certain bacteria are naturally

transformable. Example: streptococcus pneumoniae that has

two populations classified according to the differences in their

cellular Capsule: 1-encapsulated bacteria: stronger and more

pathogenic 2-non encapsulated Bacteria: weak, is able to

cause disease only if the immune system efficiency decreased.

So the non-encapsulated kind can be transformed into the

encapsulated, as a result the coming generation of these

transformed encapsulated bacteria will be stronger and more

pathogenic.

2-trasduction: in this mechanism, bacteriophage is the vector

of the genetic material.

-Bacteriophage structure: head (contains the DNA), tail and

fibres.

-bacteriophages have two life cycles:

1-Lytic cycle: the phage injects its DNA in the bacterium

resulting in the lysis of the bacterium and replication of its

DNA material.

2-Lysogenic cycle: the phage injects its DNA and merges it

with the bacterium DNA, the DNA of the virus replicates

when the bacterium divides until there are better conditions

for the bacterium DNA to separate from the bacterial genome

and enter the lytic cycle to replicate and infect other bacteria.

-in the case of the lysogenic cycle, the virus genome is called

prophage and the bacterium is called a lysogen, if the phage

genome encodes an observable function, the lysogen will be

altered in its phenotype (lysogenic conversion, example:

cholera toxin).

Types of transduction:

1-generalized transduction (happens in the lytic cycle):

usually when the phage attacks the bacterium and replicates,

the newly synthesized bacteriophages contain only the DNA

of the virus, but a few bacteriophages could carry the bacterial

DNA in their capsids, and when attacking a new bacterium by

lysogenic cycle the new host will contain part of the old

bacterial genome adding a new property to the new host, for

example adding antibiotic and toxin resistance.

2-specialized transduction (happens in the lysogenic

cycle):

The prophage integrates into the bacterial genome at a

specific location, when a prophage is induced to lytic phase, it

may drag along a piece of the bacterial genome next to the

integration site and move that bacterial sequence into the new

recipient host cell changing its genome(usually non medically

significant).

3-conjugation:

The importance of conjugation is to move certain features

among bacteria such as moving antibiotic resistance genes

that exist in the plasmids of certain bacteria(R plasmids).

- mediated by plasmids: circular double-stranded DNA

molecules that lie outside the chromosome and can carry

many genes including those for drug resistance.

-there are three possible states for plasmid conjugation:

1-conjugative: the plasmid contains a set of genes which

promote sexual conjugation by sexual pilus with other

bacteria, and the plasmid can move itself from the donor to

the recipient cell.

2-mobilizable: the plasmid can only move with the help of a

conjugative plasmid in the recipient cell.

3-non-transmissible: can’t move by conjugation.

*steps of conjugation in general:

1-synthesis of the sex pilus.

2-cell to cell contact.

3-copying plasmid DNA and transfer of the copy into

recipient cell.

-bacteria containing a conjugative plasmid are called donor,

male (F+).

-bacteria receiving the plasmid are called recipient, female (F-

).

*types of conjugation:

1) F+ conjugation: Genetic recombination in which there is a transfer of an F+ plasmid but not chromosomal DNA from a male donor bacterium to a female recipient bacterium. Other plasmids present in the cytoplasm of the bacterium, such as those coding for antibiotic resistance, may also be transferred during this process.

2)High frequency recombination (Hfr conjugation):

In this type the plasmid ( F factor) integrates with

the chromosome of the same bacteria(donor)

before transport to the recipient cell, in this case

the donor cell is called Hfr instead of F+, now the

genome of the donor bacteria has the plasmid and

the chromosome genes connected, the plasmid

genes will initiate connection to other bacterium

and try to transport the F factor to the other cell

,but because the F factor is connected to the

chromosome it will move the bacterial genes with

it, but usually the connection breaks before moving

all the genes, so the recipient bacteria will have

some of the donor genes in addition to the F factor

3)F’ conjugation: in this type of conjugation the F factor

merges with the chromosome of the same bacterium, picks

up some genes and then breaks apart from the

chromosome and now it is called the F’ factor, when the

donor connects to a recipient cell it moves a copy of the F’

factor.

4-transposons: Jumping genes/copy and paste (Class1) or cut

and paste (2) between plasmids or between chromosomes

and plasmids.

• Medical importance: since many antibiotic resistance genes are encoded by transposons in antibiotic

resistance. *it is considered fundamental transmission between plasmids and chromosomes.

plasmids

THE END

GOOD LUCK DOCTOR 2016