Bacterial Genetics

BACTERIAL GENETICS Rainelda Uy-Veloso, M.D.

GENETICS: = science of heredity = study of what genes are, how they carry information, how they are replicated and cells, information

passed to subsequent generations of and how the expression of their within an organism determines the

particular characteristics of that organism * ability to maintain viability, adapt, multiply and cause disease is founded in Genetics

Nucleic acid structure and organization: = where hereditary information resides = 2 major classes: 1. DNA = most common macromolecule that encodes genetic information 2. RNA = may encode genetic information for various viruses = other forms play an essential role in several genetic process

Nucleotide structure and sequence: = DNA consists of deoxyribose sugars connected by phosphodiester bonds = 4 bases include: a) 2 purines: adenine (A) and guanine (G) b) 2 pyramidine: cytosine (C) and thymidine (T) * in RNA, uracil replaces thymidine Sugar, phosphate and base, form a single unit → NUCLEOTIDE

= Order of bases along a DNA or RNA strand is known as BASE SEQUENCE → provides the information that specifies the

DNA molecular structure: = composed of two nucleotide polymers = the strands are also complementary as the A base of one strand always binds via 2 hydrogen bond to the T base of the

A

T

other G

C

A Likewise C

T

strand and vice versa.

G

G base of one strand always binds by 3 hydrogen bonds

to the C base of the other and vice

2 major classes of Nucleic Acids 1. Deoxyribonucleic acid (DNA) = most common macromolecule that encodes genetic information = intact DNA is composed of 2 nucleotide polymers = double stranded 2. Ribonucleic acid (RNA) = in some forms, it encodes genetic information = in other forms, plays an essential role in several genetic processes = rarely double stranded = 3 major types: Messenger RNA (mRNA)

GENE – a segment of DNA ( a sequence of nucleotides in DNA) that codes for a functional product GENOME – all genes taken together within an organism * size of the gene and entire genome is expressed in the number of base pairs CHROMOSOME – are cellular structures that physically carry hereditary information; contain the genes information Ex: human cell contains 23 pairs (diploid) of chromosome while bacteria contain

REPLICATION: a) bacteria multiply by cell division that produces two daughter cells from one parent cell b) genome must be replicated so that each daughter cell receives the same complement of functional DNA 4 STAGES: 1. unwinding or relaxation of the chromosome’s supercoiled DNA 2. unzipping or disconnecting, the complementary strands of parental DNA so that each may serve as a template for synthesis of new DNA strands 3. synthesis of new DNA strands 4. termination of replication with release of 2

GENE EXPRESSION OF GENETIC INFORMATION = processing of information encoded in genetic elements that results in the formation of biochemical products = 2 stages: 1) Transcription – converts the DNA base sequence of the gene into mRNA molecule. Produces mRNA, tRNA and rRNA 2) Translation – involves production of proteins (genetic code within mRNA molecule is translated into specific amino acid

GENETIC EXCHANGE = accomplished by 3 basic mechanisms: 1. mutation 2. genetic recombination 3. genetic exchange between bacteria, w/ or w/o recombination MUTATION = a change in the original nucleotide of a gene or genes within an organisms genome = this may involve: ♦a single DNA base within a gene ♦an entire gene ♦several genes

= causes: 1. may occur spontaneously → due to an error made during DNA replication 2. induced by chemical or physical factors (mutagens) in the environment 3. biologic factors such as introduction of foreign DNA into the cell ♦base pair substitution: Types: 1. TRANSITION: ♦pyrimidine is changed to other pyrimidine or a given purine is changed to the other purine T C A G

2. TRANSVERSION: ♦change from a purine to either of the two pyrimidine or change of a pyrimidine to either of the two purine T A A T C

G

G

C

♦deletion or breakage ♦addition ♦covalent bonding- a chemical bonding in which the electrons of one atom are shared with

TYPES OF MUTATION 1. Spontaneous mutation = 1 for every 10M division = exemplified by fluctuation test of Luria and Delbruck 2. Induced mutagenesis = mutagenes – variety of physical and chemical agents that damage the DNA a) Radiation = ultraviolet light → most extensively studied = ionizing radiation have greater penetration than UV radiation →

b) Chemical mutagenesis = 3 categories 1. agents that cause covalent modification of DNA bases Ex.: = alkylating agents such as ethylmethane sulfonate and nitrosoguanidine = modified guanine mispair with thymine resulting in GC to AT transitions = deamination by hydroxylamine or nitrous acid = deaminate – cytosine into uracil → CG to TA transition adenine into

2. agents that interact noncovalently with DNA, intercalating between base pairs Ex.: = intercalating agents like proflavine and acridine orange = contain planar aromatic rings that intercalate between the stacked of DNA → distort DNA structure = anti-tumor or antimalarial agents or anti-viral 3. base analogues that are incorporated during replication = compounds that resemble normal bases of DNA but can base pair with two

GENE TRANSFER = when a genetic material is transferred from a donor cell to its recipient *RECOMBINATION* = when the genetic material being transferred is incorporated to the genetic material of the recipient = Three major pathways: transformation, transduction and conjugation

Transformation = involves the uptake and assimilation of naked DNA released = most important mechanism into the environment of genetic exchange for certain bacterial species; Streptococcus pneumoniae, Strepsanguis, Bacillus subtilis, Hemophilus influenza and Neisseria gonorrheae = common steps: binding of exogenous DNA of the cell surface ↓ transport of DNA into the cell ↓

= things transferred: ♦structural characteristics ♦essential enzymes ♦morphological characteristics ♦biochemical and serologic characteristics = play a major role in the development of antibiotic resistance and in the dissemination of genes that encode factors essential to the organisms ability to cause disease = not limited to organisms of the same specie

Transduction = involves two distinct mechanism whereby a bacteriophage can carry bacterial genes from one to another 1. generalized transduction = allows the transfer of any bacterial gene 2. specialized transduction = can operate only on particular gene Bacteriophage – a virus that multiplies in bacterial cells

2 types: 1. Temperate phage = when transferred to a recipient, it recombines with the genetic material but remains inactive = Ex.: Corynebacterium diphtheriae → gained a phage that promotes the formation of a diphtheria toxin 2. Virulent phage = every infected cell is taken over by the virus

Conjugation = this occurs between two living cells, involves cell to cell contact and requires mobilization of the donor’s bacterium chromosome = transfer of genetic materials in a process that requires direct contact between two cells and a special fertility (F) factor in the donor cell = critically dependent on the production of a specialized appendage, the F pilus

BACTERIAL PLASMIDS = ancillary genetic element of bacteria that generally replicate as duplex DNA circles independent of the bacterial chromosome = self replicating, gene containing circular pieces of DNA about 1%-5% the size of the bacterial chromosomes 1. Drug resistance plasmids = Japan 1950’s - Shigella flexneri - 4 Antibiotics: - Chloramphenicol, Tetracycline, Streptomycin, Sulfonamide = R plasmid can be transferred between different

= 2 functionally distinct units: 1. RTF(Resistance Transfer Factor) – contains genes for autonomous replication and conjugation 2. r - determinant → specifies antibiotic resistance 2. Bacteriocinogenic factors = code for synthesis of bactericidal proteins known as bacteriocins Ex.: Colicins → E. coli Pyocins → P. aeruginosa 3. Plasmids carrying genes determining metabolic pathways 4. Plasmids coding for toxin production and

Transposons = called “jumping genes” because of their ability to change location within and even between the genomes of the bacterial cells = do not exist independently within a cell and must be incorporated into the chromosome and or plasmid = carry genes whose products help mediate the transpositional process as well as genes that encode for some other characteristics such as antibiotic resistance = play a key role in genetic diversity and disseminate