Bacterial genetics deals with the study of heredity and gene variations seen in bacteria. All hereditary characteristics of the bacteria are encoded in their DNA (deoxyribonucleic acid). Bacterial DNA is present in chromosome as well in extra chromosomal genetic material as plasmid.
BACTERIAL DNA
Bacteria
possess a single haploid chromosome, comprising of super coiled circular double
stranded DNA of 1 mm length. The bacterial DNA lacks basic proteins. However, some
bacteria have a linear DNA chromosome and some have two chromosomes (e.g. Vibrio
cholerae). Bacteria do not
have a true nucleus; but the genetic material is located in an irregularly
shaped region called the nucleoid. There is no nuclear membrane or nucleolus.
Structure of Bacterial DNA (Watson
and Crick Model)
The
bacterial DNA molecule is composed of two strands of complementary nucleotides
that are coiled together in the form of a double helix (Fig. 6.1) as described
first by Watson and Crick.
- Each strand is composed of three elements: It has a backbone of deoxyribose sugar and phosphate groups.The nitrogenous bases are attached to the sugar group.The terms nucleotide and nucleoside are often used to describe the components of the DNA strand - Nucleoside = Sugar+ nitrogenous base, Nucleotide =Sugar+ nitrogenous base + phosphate. There are four nitrogenous bases:
- Two purines- adenine (A) and guanine (G)
- Two pyrimidines- thymine (T) and cytosine (C)
- Pairing: The two DNA strands are held together by hydrogen
bonds occurring between the nitrogenous bases on the opposite strands. The
pairing follows a specific rule-
- Adenine of one strand binds with thymine (A-T) of other strand by double hydrogen bonds.
- Guanine of one strand binds with cytosine (G-C) of other strand by triple hydrogen bonds.
- Hence, in a molecule of DNA, the number of adenine molecules is equal to that of thymine, and the number of guanines is equal to cytosines.
- The ratio of A + T to G + C is constant for each species but varies widely from one bacterial species to another.
DNA Replication in Bacteria
In eukaryotes, during DNA replication, the two strands of !he double helix unwind from one another and separate. Each strand acts as template for a new DNA strand which is synthesized through complementary base pairing- A with T, and G with C. In prokaryotic cells, DNA replication takes place in a similar way with some differences.
Bi-directional Replication in Bacteria
For example in E. coli, the replication begins at a single point, the origin. DNA helix is unwound at a region called replication fork. It is the site at which the DNA synthesis occurs and individual strands are replicated. Two replication forks move outwards from the origin until they have copied the whole replicon and a structure shaped like the Greek letter theta (Ɵ) is formed (Fig. 6.2). Finally, since the forks meet on the other side and two chromosomes are separated.
Rolling-circle Mechanism in Bacteria
This
is a different pattern of DNA replication which occurs during bacterial
conjugation and during the reproduction of viruses and bacteriophages.
- The outer strand is nicked and the free 3' end is extended by replication enzymes in a manner that the growing point rolls around the circular inner strand (Fig. 6.3).
- At the same time, the 5' end of the outer strand is displaced and forms a single-stranded tail which later may be converted to the dsDNA by complementary strand synthesis.
The
DNA replication in bacteria is catalyzed by several replication enzymes such
as-
- Helicase: It is responsible for DNA unwinding
- Topoisomerase (e.g. DNA gyrase in E. coli): It relieves the tension generated by rapid unwinding by removing the super-twists.
- DNA polymerase: It forms complementary strand synthesis by adding nucleotides to the growing end of the strand. It catalyzes the synthesis of DNA in the 5' to 3' direction while reading the DNA template in the 3' to 5' direction. DNA polymerase III plays the major role in replication, although it is probably assisted by polymerase I. It is thought that polymerases I and II participate in the repair of damaged DNA.
- DNA ligase: It helps in joining of the fragments.
BACTERIAL
RNA
RNA
(ribonucleic acid) is structurally similar to DNA, except for two differences.
- In sugar- ribose is present instead of deoxyribose and
- In nitrogenous base- uracil replaces thymine.
There
are three different types of RNA in a cell, messenger RNA (mRNA), ribosomal RNA
(rRNA) and transfer RNA (tRNA). The main function of RNA is protein synthesis.
POLYPEPTIDE
SYNTHESIS IN BACTERIA
Gene
is a segment of DNA that stores information for a particular polypeptide
synthesis. The genetic information that is stored in DNA is transcribed into
RNA and then translated to form the particular polypeptide.
Genetic
Code in Bacteria
Codon:
It is a sequence of three nucleotide bases present on mRNA that stores the information
of an amino acid synthesis.
It was
discovered by Nirenberg and Khorana (1968).
- Sense codons: There are 64 codons, out of which 61 are sense codons, each directs the production of a single amino acid. As there are only 20 amino acids, so more than one codon exists for the same amino acid.
- Non-sense codons: The remaining three codons (UGA, UAG, and UAA) do not code for any amino acids and are involved in the termination of translation; hence called as stop codons.
- Start codon: It is the first codon of an mRNA from which the translation begins. The most common start codon is AUG which codes for methionine in eukaryotes and modified methionine (N-Formyl merhionine (fMet)J in prokaryotes.
- Anticodon: It is a set of three nucleotide bases present on tRNA that is complementary to the nucleotide bases of codon on rRNA.
Transcription in Bacteria
Transcription
is a process, in which a particular segment of DNA is copied into RNA by the
enzyme RNA polymerase.
Since
DNA acts as a template for synthesis of mRNA, therefore, the bases in mRNA are
complementary to that of DNA.
Translation
In
translation, the mRNA transcribed from DNA is decoded by a ribosome to produce a
specific amino acid chain, or polypeptide. It occurs in cytoplasm of the bacteria
and proceeds in four phases:
1. Initiation: The ribosome assembles around the target mRNA. The first tRNA is attached at the start codon of mRNA.
2. Elongation: The tRNA transfers an amino acid co the adjacent tRNA, corresponding to the next codon.
3. Translocation: The ribosome then moves (translocates) to the next mRNA codon co continue the process, creating an amino acid chain.
4. Termination: When a stop codon is reached, the ribosome releases the polypeptide.
PLASMID IN BACTERIA
Plasmids
are the extrachromosomal ds circular DNA molecules that exist in free state in
the cytoplasm of bacteria (Fig. 6.4A) and also found in some yeasts.
- Not essential: Plasmids are not essential for life; bacteria may gain or lose plasmid during their lifetime.
- Numbers: They may be present singly or in multiple numbers- up to more than 40 plasmids per cell.
- Independent replication: Plasmids are capable of replicating independently. They can behave as replicon, possessing an origin of replication and other genes that help in replication.
- Episome: Sometimes, the plasmid may integrate with chromosomal DNA of bacteria and such plasmids are called as episomes. They replicate along with bacterial chromosome (Fig 6.48).
- Curing: The process of eliminating the plasmids from bacteria is known as curing. It may occur spontaneously or may be induced by treatment of the host cells with substances that inhibit plasmid replication without affecting the host cell, such as acridine, radiations, thymine starvation, and growth at higher temperatures.
Classification
of Bacterial Plasmids
Bacterial plasmids
can be classified in many ways:
1. Based
on ability to perform conjugation:
- Conjugative plasmids: Some plasmids have an ability to transfer themselves to other bacteria by means of conjugation. These are called self-transmissible or conjugative plasmids.
- Non-conjugative plasmids: These are also called as non-transmissible plasmids. They can not transfer themselves.
2.
Based on compatibility between the plasmids, they can be grouped into:
- Compatible plasmids: Different plasmids can exist in a single bacterial cell only if they are compatible to each other.
- Incompatible plasmids: If two plasmids are not compatible, one or the other will be rapidly lose from the cell. They normally share the same replication or partition mechanisms, hence compete with each other.
3.
Based on function, there are five main classes of bacterial plasmids:
- Fertility or F-plasmids: They contain tra-genes, which code for the expression of sex pili that help in bacterial conjugation by forming the conugation tube.
- Resistance (R) plasmids: They contain genes that code resistance to various antibiotics.
- Col plasmids: They contain genes that code for bacteriocins (antibiotic-like protein substances produced by bacteria that can kill ocher bacteria).
- Virulence plasmids: They code for certain virulence factors and toxins that help in bacterial pathogenesis. Examples include:
- Heat
labile and heat stable toxin of E.
coli
- Siderophore production
- Adherence antigens (K88 plasmid in E. coli)
- Metabolic plasmids: They enable the host in various metabolic activities:
- Digestion of unusual substances, e.g. toluene and salicylate, camphor, etc.
- Urease synthesis
- Nitrogen fixation
Bacterial plasmids
by their ability to transfer DNA from one cell to another, they have become important vectors in genetic engineering. Bacterial plasmids contain certain sites where genes can be inserted artificially by
recombinant DNA technology. Such plasmids can be used for various purposes such
as protein production, gene therapy, etc.
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