Microbial Genetics
Medical Microbiology (Jawetz, etal.)
chapter 7 (p 97-120) ed.24 (2007)
What is Genetics?
The science of Genetics defines and analyzes heredity, or constancy and
change in the vast arrangement of physiologic functions that form the
properties of organisms.
* The unit of heredity is the Gene :
- a segment on a DNA molecule.
- usually at a specific location (locus) on a chromosome or plasmid.
- characterized by its nucleotide sequence.
* Genotype:- alteration in the sequence of DNA within a gene or in the
organization of genes .
* Phenotype:- the collective structural and physiologic properties of a
cell or an organism, ex. eye color in a human or resistance to an antibiotic
in a bacterium.
Genetic Elements:The structure of nucleic acids (DNA & RNA):
1. composed of chains of nucleotides.
2. a nucleic acid molecule may contain several thousands or
millions of nucleotides.
3. nucleic acid molecules are usually composed of
4 different nucleotides.
4. each nucleic acid molecule has its own order, or “sequence,”
of nucleotides.
5. The correct sequence of nucleotides is essential for the
nucleic acid’s function.
Nucleotide structure:- A nucleotide consists of:
• nitrogenous base
• pentose sugar
• phosphate group
- Nitrogenous bases:
• purines: adenine & guanine
• pyrimidines: cytosine, thymine (in DNA), & uracil (in
RNA)
– Pentose sugars:
• Ribose (found in RNA)
• Deoxyribose (found in DNA)
(1)
DNA: Deoxyribonucleic acid:– nitrogenous bases:
adenine and guanine (purines)
cytosine and thymine (pyrimidines)
– pentose sugar: 2’-deoxyribose.
– structure is typically a double-stranded helix.
– nucleotide sequences of the strands are complementary to
each other, A pairing with T and C pairing with G.
RNA: Ribonucleic acid:– pentose sugar: Ribose
– nitrogenous bases:
adenine and guanine (purines)
cytosine and uracil (pyrimidines)
– structure is typically single-stranded.
* Genome: The entire nucleotide sequence of an organism; transmitted to
offspring during reproduction.
– Deoxyribonucleic acid (DNA): DNA molecules serve as the
genome for the proteins of all cellular organisms, both
eukaryotic and prokaryotic. DNA also serves as the genome
for certain viral groups.
– Ribonucleic acid (RNA): RNA molecules serve as an
intermediate in gene expression in eukaryotic and
prokaryotic organisms, as well as some viruses. RNA serves
as the genome for certain viral groups.
Eukaryotic genome (ex. human)
Prokaryotic genome(ex. bacteria)
1.
--Is carried on two or more liner --Is carried on the chromosome (a single
chromosomes separated from the cytoplasm circular DNA molecule) do not separate
within the membrane of the nucleus
from cytoplasm
2.
--Diploid eukaryotic cells contain two --Bacterial genes are haploid
homologues of each chromosome
3.
--Mutation cannot be detected in diploid --Mutation can be detected in haploid cells
cells (two copies of gene)
(single copy of gene)
4.
--No pathogenicity islands
--Pathogenicity islands are clustered genes
in the DNA and can code for virulence
genes
(2)
Replication:double-strand DNA is synthesized by semiconservative replication.
Eukaryotic DNA
Bacterial DNA
--Replication begins at several points along --Replication begins at one point and
the linear chromosome
moves in both directions
--Eukaryotes have evolved specialized --Absent of spindle,
machinery, called a spindle, that pulls division by binary fission
daughter chromosomes into separate nuclei
newly formed by the process of mitosis
--Meiosis (halves the chromosomal number --Absent meiosis
of diploid cells to form haploid cells
Plasmids: are extra chromosomal material, small genetic elements
capable of independent replication in bacteria and yeasts.
• F (fertility) factor or F plasmid is a conjugative plasmid, carries
genes for sex pili and for transfer.
• R factors- resistance to antibiotics, heavy metals or cellular toxins.
Importance of plasmids:1. Enhancing pathogenicity.
2. Antibiotic resistance:-many antibiotic resistance genes are carried on
plasmids which can be rapidly transferred to other bacteria, resulting in
widespread resistance to antibiotics and strains that resistant to multiple
antibiotics such as methicillin resistant Staphylococcus aureus or Golden
Staph.
Transposons :Segment of DNA moves from one place in chromosome to another:
-rare event.
-insert within a gene & inactivate it.
-complex transposons carry genes for enterotoxins or antibiotic
resistance.
(3)
Genetic Recombination:
Physical exchange of genes between 2 homologous DNA molecules.
Horizontal Gene Transfer
• microbes of same generation.
• Involves a donor cell - gives DNA to recipient cell.
• Part of DNA incorporated into recipient’s DNA.
1. Transformation:- include: Free (naked) DNA in solution or environment
 Cells after death, release DNA
 Cells may take up DNA
 Only in certain stage of cell cycle.
 Enzymes cut DNA into small pieces
 Recombination between donor & recipient
 Competence :-cells able to take up DNA & be transformed.
Release competence factor that helps in uptake
2. Conjugation:• Mediated by one kind of plasmid F plasmid or F factor
• Donor cells must have F plasmid
F Plasmid:• Plasmid integrates into the chromosome converts cell to Hfr cell (
high frequency of recombination)
• F factor DNA can separate and become plasmid
3. Transduction:• Bacterial DNA is transferred via a virus (Bacteriophage)
• Generalized Transduction all genes are equally likely to be
packaged inside phage.
• Specialized Transduction-only certain bacterial genes transferred.
Mutation:• A mutation is a change in the base sequence of DNA that usually
results in insertion of a different amino acid into a protein and the
appearance of an altered phenotype.
Types of Mutations
1. Spontaneous mutations:– DNA replication errors.
– occur at low frequencies.
2. Induced mutations-mutagens:– alter structure of bases.
– errors in base pairing.
(4)
Mutations result from three types of molecular changes:1)- Base substitution:- this occurs when one base is inserted in
place of another. It takes place at the time of DNA replication.
1)- a)- Missense Mutations
• Change in amino acid.
• Can result in significant changes in polypeptide.
• Examples of Missense, Sickle cell anemia- hemoglobin
– Change from glutamic acid (hydrophilic) to valine
(hydrophobic). Change in shape of protein.
1)- b)- Nonsense Mutation
• Base-pair substitution.
• Create stop codon in middle of mRNA.
2)- Frame Shift Mutation This occurs when one or more base pairs are
added or deleted, which shifts the reading frame on the ribosome and
results in incorporation of the wrong amino acids.
3)- Transposons
Mutation occurs when transposons are integrated into the DNA.
Induced Mutations
Mutations can be caused by chemicals, radiation, or viruses. Mutagens
increase mutation rate.
Chemical mutagens
– ex. nitrous acid
– ex. benzpyrene in tobacco smoke these chemicals are
carcinogens.
Radiation mutagens
- UV light.
- X- ray
Certain viruses such as bacterial virus cause a high frequency of
mutations when their DNA is inserted into the bacterial chromosome.
Conditional –lethal mutations:• are of medical interest because they may be useful in vaccines, eg,
influenza vaccine. For example temperature sensitive mutation in
influenza virus. This vaccine contain a virus that can not grow at
37°c and cannot infect the lungs and cause pneumonia, but it can
grow at 32°c in the nose, where it can replicate and induce
immunity.
(5)