Genetic of microorganisms

Advantages of bacteria and viruses as objects for genetic researches

Simplicity of genome structure Universality of gene codeLack of diploid set of chromosomes and dominant genes Відносна легкість культивуванняGrate rapid of reproduction and plurality of population Genetic heterogeneity of population Accessibility modern methods of genetic analysis as sequencing of DNA, polymerase chain reaction (PCR)

Competitive structure genome of E.coli and human cell

Properties E.coli Human cellPlace in cell Nucleoid Nucleus

Quantity of chromosomes 1 46

Quantity of genes 4000 100000

Molecular weight of DNA 2х109 2х1012

DNA length 0,1 см 183 см

DNA shape Supercoiled Linear

Introns Present Absent

Time existence of mRNA Minutes Hours

Genome type Haploid Diploid

Plasmid Present Absent

DNA combined in cell with Polyamines Histones

The genome

is the some total of genetic material of a cell

Semiconservative replication of DNA. Simplified steps

a. A helicase unwinds the double helix into 2 parent strands (P1 and P2).

b. The replication of new complementary strands proceeds though the action of an enzyme that attaches nucleotides, using the exposed strands as templates.

c. Each completed daughter molecule contains one strand that is newly synthesized and one of the original parent strands.

Replication of bacterial DNA

a. The theta stage of replication, in which one strand loops down as it grown in length.

b. Nicking, separation, repair, and release of two completed molecules that will be separated into daughter cells during binary fission.

Definitions

Together the structural and regulatory genes constitute the genotype

Phenotype is determined by genotype and is the actual appearance and activities of the organism.

Exons are coding regions that will be translated into product – proteins, enzymes

Introns are sequences of bases that do not code for product

Structure of bacterial genome

Structural genes are genes that code for proteins

Regulatory genes are genes that have regulatory function and act to control the expression of the structural genes

Operon is a specific unit of DNA that regulates genetic function in procaryotes

The lac operon

Regulator – composed of the gene that codes for a protein capable of repressing the operon (a repressor)

Control lorus - composed of two genes, the promoter (recognized by RNA polymerase) and the operator, a sequence where transcription of the structural genes is initiated

Structural locus – made up of 3 genes, each coding for a different enzyme (b-galactosidase, permease, and transacetylase) needed to catabolize lactose

Regulator Control locus Structural locus

Two possible ways work of operon

Inducible. The operon can be turned on (induced) by the substrate of the enzyme for which the structural genes code

Repressible. The operon can be turned off (repressed) by the product its enzymes synthesize

A model of inducible regulation. Lac operon

A model of repressible regulation. Arg operon

Types of changes in the genetic code

Mutation is a change in the nucleotide sequence of DNA that result is a recognizable change in the organism

Recombination means the addition of genes from an outside source, such as a virus or another cell

Categories based on causes of mutations

A spontaneous mutation is a random change in the DNA arising from mistakes in replication of the detrimental effects of natural background radiation on DNA

An induced mutations result from exposure of the cell to exogenous DNA modifiers such as radiation or chemical substances

Categories of mutation based in alteration of base sequence in DNA

A base substitution mutation occurs when one pair of nucleotide bases in the DNA is replaced by another that results in change in codon and synthesis different protein.

Transitions involves the raplacement of a purine by a different purine or pyrimidine by a different pyrimidine

Transversions occurs when purines replace pyrimidines and pyrimidines replace purines.

A deletion mutation involves the removal of one or more nucleotide base pairs from the DNA

An insertion mutation involves the addition of one or more pairs to the DNA

A frameshift mutation are addition or loss of one or two bases in a gene. It can result in the misreading of large numbers of codons

Categories based in overall effect of mutation

A silent mutation alters a base but not change the amino acid and has no effect

A missense mutation is a change in the code that leads to placement of a different amino acid. It can do one of the following:

create a faulty, nonfunctional protein

produce a different but functional protein

cause no significant alteration in protein function

A nonsense mutation changes a normal codon into a stop codon that does not code for an amino acid and stops the production of the protein wherever it occurs. It almost always results in a nonfunctional protein

A back-mutation (reversion mutation) occurs when a gene that has undergone mutation reverse (mutates back) to its original base composition

Selected mutagenic agents and their effects

Agent EffectChemical

Nitrous acid, disulfite

Removes an amino group from some bases

Mustard gas Causes cross-linkage of DNA strands

Acridine dyes Cause frameshifts due to insertion between base pairs

Notrogen base analogs

Compete with natural bases for sites on replicating DNA

Radiation

Ionizing (gamma rays)

Form free radicals that cause single or double breaks in DNA

Ultraviolet Causes cross-links between adjacent pyrimidines

Transposable genetic elements

Insertion sequences (IS elements) Transposons Plasmids

Competitive characteristic IS, transposons, and plasmids

Genetic element

DNA size Quantity of genes

Self-dependent replication

IS 800-1400 pair nucleic bases

Only genes for transposase

No

Transposon 2000-2500 pair nucleic bases

Only few genes for transposase and resistance to antibiotics

No

Plasmid 3000-5000 pair nucleic bases

40-50 different genes

Yes

Transposable genetic elements

Plasmid

Chromosomal and plasmid DNA leaking out of a cell.

Cell functions coded for by some plasmids

Group Function Fertility plasmids (F)

Transfer of DNA from one cell to another via conjugation (F-pili)

Resistance plasmids (R)

Resistance to various antibiotics

Resistance to cadmium and mercury

Resistance to ultraviolet radiation

Col plasmids Bacteriocin production

Virulence factor plasmids

Enterotoxin production

Fimbriae production

Metabolic plasmids

Utilization of camphor

Formation of spores in streptomycetes

Intermicrobial DNA transfer and recombination

Transformation

Transduction

Conjugation

Plasmid transfer

Types of intermicrobial exchange

Mode Requirement Direct of indirect

Genes transferred

Conjuga-tion

Sex pilus on donorFertility plasmid in donorBoth donor and recipient Gram-negative cells

Direct Drug resistance, resistance to metal; enzymes; degradation of toxic substrate

Transfor-mation

Free donor DNA (fragment)Live, competent recipient cell

Indirect Polysaccharide capsule

Transduc-tion

Donor is lysed bacteriumDefective bacteriophage is carrier of donor DNALive, competent recipient cell of came species as donor

Indirect Toxins; enzymes for fermentation; drug resistance

Conjugation

Conjugation is mode of sexual process mating in which a plasmid or other genetic material is transferred by a donor to a recipient cell via a specialized appendage

Conjugation process

Sex, or F, pilus holding together donor and recipient cell of E.coli during DNA transfer

Participants of conjugation process

Gram-negative bacterium that produce F pili act as donors during conjugation. Donor strains are designated F+ if the F plasmid is independent

Bacteria lacking the F plasmid are recipients and designated

F-

If the F plasmid DNA incorporated into the bacterial chromosome the donor cell designated Hfr (high frequency recombination)

Cojugation – transfer of the F plasmid

Transformation F+ to Hfr cell

The F plasmid integrates at specific locations into the chromosome and F+ cell is transformed to Hfr cell.

The process is reversible.

Conjugation – transfer of chromosomal DNA

Transformation

In transformation a free DNA molecule is transferred from a donor to a recipient bacterium

Infecting mouse with virulent encapsulated streptococci

Infecting mouse with nonvirulent streptococci without capsule

Infecting mouse with killed virulent streptococci

Infecting mouse with mix killed virulent streptococci and alive nonvirulent

Transduction

is a process by which a bacteriophage serve a the carrier of DNA from a donor cell to a recipient cell.

There are 2 version of transduction:

Generalized

Specialized

Generalized transduction

Specialized transduction

In specialized transduction, a highly specific part of the host genome is regularly incorporated into the virus. It occurs only during infection caused by temperate phage

a)

b)

c)

d)

e)

f)