Microbial Genetic pdf - D. Zainab

Microbial Genetics

The science of genetics defines and analyzes heredity of the vast array of

structural and physiologic functions that form the properties of organisms.

The basic unit of heredity is the gene, a segment of deoxyribonucleic acid (DNA)

that encodes in its nucleotide sequence information for a specific physiologic

property.

DNA as the fundamental element of heredity was suggested in the 1930s

from a seminal experiment performed by Frederick Griffith. MacLeod, and

McCarty discovered that DNA was the transforming agent.

Nucleic acid Structure

Genetic information in bacteria is stored as a sequence of DNA bases, The

orientation of the two DNA strands is antiparallel. One strand is chemically

oriented in a 5′→3′ direction, and its complementary strand runs 3′→5′. The

complementarity of the bases enables one strand (template strand) to provide the

information for copying or expression of information in the other strand (coding

strand).

Microbiology

Medical bacteriology

Dr. Zainab D. Degaim



Nucleic acids are large polymers consisting of repeating nucleotide units.



Each nucleotide contains one phosphate group, one pentose or

deoxypentose sugar, and one purine or pyrimidine base.



In DNA the sugar is D-2-deoxyribose; in RNA the sugar is D-ribose.



In DNA the purine bases are adenine (A) and guanine (G), and the

pyrimidine bases are thymine (T) and cytosine (C).



In RNA, uracil (U) replaces thymine.



The repeating structure of polynucleotides involves alternating sugar and

phosphate residues, with phosphodiester bonds linking the 3'-hydroxyl

group of one nucleotide sugar to the 5'- hydroxyl group of the adjacent

nucleotide sugar. A purine or pyrimidine base is linked at the 1'-carbon

atom of each sugar residue and projects from the repeating sugarphosphate

backbone.



Double-stranded DNA is helical, and the two strands in the helix are

antiparallel. The double helix is stabilized by hydrogen bonds between

purine and pyrimidine bases on the opposite strands.



A on one strand pairs by two hydrogen bonds with T on the opposite strand,

or G pairs by three hydrogen bonds with C. The two strands of double-

helical DNA are, therefore, complementary.



Ribonucleic acid (RNA) most frequently occurs in single-stranded form.

The uracil base (U) replaces thymine base (T) in DNA, so the

complementary bases that determine the structure of RNA are A-U and C-



The most general function of RNA is communication of DNA gene

sequences in the form of messenger RNA (mRNA) to ribosomes. These

processes are referred to as transcription and translation. mRNA (referred

to as +ssRNA) is transcribed as the RNA complement to the coding DNA

strand. This mRNA is then translated by ribosomes. The ribosomes, which

contain both ribosomal RNA (rRNA) and proteins, translate this message

Figure 2

into the primary structure of proteins via aminoacyl-transfer RNAs

(tRNAs).

The Eukaryotic Genome



The genome is the totality of genetic information in an organism. Nearly all

of the eukaryotic genome is carried on two or more linear chromosomes

separated from the cytoplasm within the membrane of the nucleus. Diploid

eukaryotic cells contain two homologues (divergent evolutionary copies) of

each chromosome.



Eukaryotic cells contain mitochondria and, in the case of plants,

chloroplasts. Within each of these organelles is a circular molecule of DNA

that contains a few genes whose function relates to that particular organelle.



Many yeast contain an additional genetic element, an independently

replicating 2-μm circle containing about 6.3 kbp of DNA. Such small

circles of DNA, termed plasmids, are frequently associated with

prokaryotes.

The Prokaryotic Genome



Most prokaryotic genes are carried on the bacterial chromosome. And with

few exceptions, bacterial genes are haploid.



The most prokaryotic genomes (>90%) consist of a single circular DNA

molecule. A few bacteria (eg, Brucella melitensis, Burkholderia

pseudomallei, and Vibrio cholerae) have genomes consisting of two

circular DNA molecules.



Many bacteria contain additional genes on plasmids that range in size from

several to 100 kbp. In contrast to eukaryotic genomes, 98% of bacterial

genomes are coding sequences.



Covalently closed DNA circles (bacterial chromosomes and plasmids),

which contain genetic information necessary for their own replication, are

called replicons or episomes.



Some bacterial species are efficient at causing disease in higher organisms

because they possess specific genes for pathogenic determinants. These

genes are often clustered together in the DNA and are referred to as

pathogenicity islands.



Transposons are genetic elements that contain several genes, including

those necessary for their migration from one genetic locus to another.



The short transposons (0.75–2.0 kbp long), known as insertion elements,

produces the majority of insertion mutations. These insertion elements

(also known as insertion sequence [IS] elements) carry only the genes for

enzymes needed to promote their own transposition to another genetic

locus but cannot replicate on their own.

Replication



The replication of bacterial DNA begins at one point and moves in both

directions (bidirectional replication).



In the process, the two old strands of DNA are separated and used as

templates to synthesize new strands (semiconservative replication).



The structure where the two strands are separated and the new synthesis is

occurring is referred to as the replication fork.



Replication of the bacterial chromosome is controlled, and the number of

each chromosomes (when more than one is present) per growing cell falls

between one and four.



The replication of circular double-stranded bacterial DNA begins at the ori

locus and involves interactions with several proteins. In E coli, chromosome

replication terminates in a region called ter. The two daughter chromosomes

are separated, before cell division, so that each progeny cell gets one of the

daughter DNAs.



Similar processes used in the replication of bacterial chromosomes are used

in the replication of plasmid DNA.

Gene expression



In both eukaryotic and prokaryotic genomes, genetic information is encoded

in DNA, transcribed into mRNA, and translated on ribosomes through

tRNA into the structure of proteins. The mechanism by which the sequence

of nucleotides in a gene determines the sequence of amino acids in a protein

is largely similar in prokaryotes and eukaryotes and is as follows:

1. RNA polymerase forms a single polyribonucleotide strand, called mRNA, using

DNA as a template; this process is called transcription. The mRNA has a

nucleotide sequence complementary to a template strand in the DNA double helix

if read in the 3′–5′ direction. Thus, an mRNA is oriented in a 5′–3′ direction.

2. Amino acids are enzymatically activated and transferred to specific adapter

molecules of RNA, called tRNA. Each adapter molecule has a triplet of bases

(anticodon) complementary to a triplet of bases on mRNA, and at one end its

specific amino acid. The triplet of bases on mRNA is called the codon specific for

that amino acid.

3. mRNA and tRNA come together on the surface of the ribosome. As each tRNA

finds its complementary nucleotide triplet on mRNA, the amino acid that it carries

is put into peptide linkage with the amino acid of the preceding tRNA molecule.

The ribosome moves along the mRNA with the nascent polypeptide growing

sequentially until the entire mRNA molecule has been translated into a

corresponding sequence of amino acids. This process, called translation.

Genes of higher eukaryotes are interrupted by introns, (long sequences that do not

code for proteins) which are spliced out of the RNA before translation (post-

transcription process).

Mechanisms of Gene Transfer

The maintains of genetic variability in microbes through the exchange and

recombination of allelic forms of genes. Recombination refers to the acquisition

of new segments of DNA from the local environment or provided by another

microorganism. When the recombination takes place, new genes or new

recombination are added to those already present in a microorganism.

DNA can be transferred from one organism to another, and that DNA can be

stably incorporated in the recipient, permanently changing its genetic composition,

this process is called horizontal gene transfer (HGT) to differentiate it from the

inheritance of parental genes, a process called vertical inheritance. Three broad

mechanisms mediate efficient movement of DNA between cells: conjugation,

transduction, and transformation.

Conjugation



is a one-way transfer of genetic material (usually plasmids) from donor to

recipient by means of physical contact. Conjugation requires donor cell to-

recipient cell contact to transfer only one strand of DNA.



The recipient completes the structure of dsDNA by synthesizing the strand

that complements the strand acquired from the donor. F

possesses a fertility

(F) plasmid, mediating the creation of a sex pilus necessary for conjugal

transfer of the F plasmid to the recipient.



It can integrate into chromosomal DNA, creating high-frequency

recombination (HFR) donors from which chromosomal DNA is readily

transferred. R factor contain genes conferring drug resistance. Frequently,

the resistance genes are carried on transposons. express resistance

phenotype through natural selection.

2- Transduction: is phage-mediated genetic recombination in bacteria. DNA is

carried by a phage coat and is transferred into the recipient cells.

3- Transformation: the direct uptake of “naked” donor DNA by the recipient

cell, may be natural or forced. Forced transformation is induced in the laboratory,

where, after treatment with high salt and temperature shock, many bacteria are

rendered competent for the uptake of extracellular plasmids.

Mutation

is the permanent alteration of the nucleotide sequence of the genome of an

organism, virus, or extrachromosomal DNA or other genetic elements. Mutation

occurs approximately once for any gene in every 1 million cells.

It is an induced

or spontaneous heritable alteration of the DNA sequence. The spontaneous

mutation is a random change in the DNA arising from errors in replication that

occur without a known cause. Induced mutations result from exposure to known

mutagens, which are primarily physical or chemical agents that damage DNA and

interfere with its functioning. The frequency of spontaneous mutations has been

measured for a number of organisms. It introduces variability into the gene and

changes in the phenotype.

Mutation may be caused by various mutagens,

including ultraviolet light, acridine dyes, base analogues, and nitrous acid. The

mutations include base substitutions, deletions, insertions, and rearrangements.

Nucleotide substitutions: arise from mutagenic activity or the mis-pairing of

complementary bases during DNA replication. It's often do not significantly

disrupt the function of gene products.

Frameshift mutations: result from the insertion or deletion of one or two base

pairs, disrupting the phase of the triplet-encoded DNA message.

Rearrangements are the result of deletions that remove large portions of genes or

even sets of genes.

Deletions: are usually large excisions of DNA, dramatically altering the sequence

of coded proteins. It may also result in frameshift mutations.

Insertions: change genes and their products by integration of new DNA via

transposons.

— The genetic systems (genome) of different living things

Structure

Virus

Bacteria

Fungus

Plant

Animal

Human

Nucleus

— -

— +

Nuclear membrane

— -

— +

chromosome

genome

single

multiple

Extra chromosome

— -

Plasmid

mitochondria chloroplast

mitochondria

mRNA

mature

immature

DNA

— +

/
-

— +

Intron

— -

— +

Histone

— -

— +