What are Genes?

Genes are the functional units of heredity. They transfer characteristic information from parents to the offspring.

What is meant by Genetics?

It is the study of genes and cell characters that pass from one generation to the other generation. It is one of the main forms of biology and is interlinked with taxonomy and agriculture.

What is meant by Microbial Genetics?

It deals with studying hereditary material in different microorganisms such as viruses, bacteria, protozoa, archaea, and fungi.

Prokaryotic Genome 

In prokaryotes, the genetic material is a circular, double-stranded DNA (deoxyribonucleic acid) molecule. The characteristics of the prokaryotic genome are:

• The bacterial chromosome is present within the nucleoid: The nucleoid is an irregularly-shaped region within the cytoplasm of a prokaryote that contains all or most of the genetic material.
• Supercoiling: It refers to the over-or under-winding of the DNA strands. It generates an expression of strain on the strands.
• Size variation and ORF (open reading frame)contents in the genome: In molecular genetics, an ORF is the part of a reading frame that contains no stop codons. The site for the termination of transcription lies after the ORFto prevent the formation of an incomplete protein if the process terminates before the stop codon. The ORF is a part of the reading frame of different sizes and content in the bacterial genome.
• Bioinformatic analyses and gene distributions: The branch of science that deals with studying methods for storing, retrieving, and analyzing biological data are called bioinformatics.

Aspects of Microbial Genetics

The following are the four different aspects of microbial genetics:

• Gene expression
• Gene regulation
• DNA replication and error
• Gene transfer mechanism

Gene Expression

It involves two processes: transcription and translation.

Transcription

In this genetic process, the genes on the DNA sequence are transcribed to form RNA (ribonucleic acid) molecules. It is a unidirectional process. The transcribed gene products can be rRNA (ribosomal RNA), tRNA (transfer RNA), or a polypeptide gene product like mRNA (messenger RNA).

Steps involved in gene transcription:

1. Initiation: The enzyme RNA polymerase binds to the initiation site of the promoter region. After binding, the RNA polymerase adds on ribonucleotides to form a new complementary RNA. 
2. Elongation: The polymerase enzyme synthesizes ten base pairs of long RNA. It continuously synthesizes RNA in the 5'-3' direction.
3. Termination: Specific DNA sequences trigger it. It is of two types: Rho-dependent and Rho-independent.

Types of RNAs and their function:

• mRNA: It serves as a template for protein synthesis.
• rRNA: It provides structure to the ribosome.
• tRNA: It transfers correct amino acids at the time of translation.

Translation

In microbial genetics, translation is the process by which a polypeptide is formed.

Translation involves three steps:

1. Initiation: The 30S subunit of the ribosome binds at the start codon (AUG) on the mRNA. The tRNA places a formylated methionine at the P site with the help of GTP (guanosine triphosphate). The 50S ribosome binds with 30Sto form the 70S ribosomal subunit.
2. Elongation: The incoming tRNA and amino acid are placed next to the P site that is the A site. As the mRNA moves, the tRNA from the A site is released, emptying space for the next incoming tRNA, and the process continues.
3. Termination: The stop codons trigger it, that is, UUA, UGA, and UAG. The polypeptide synthesis stops when the ribosome encounters the stop codon on the mRNA. 

Gene Regulation

This process regulates gene expression.

• The bacterial genes are generally aggregated in an operon. The operon is controlled by one promoter. The eukaryotes lack operons.
• Some operons are generally turned "off" but can be turned "on" by binding specific molecules. These molecules are called inducers. For example, the lac operon is turned on in the presence of allolactose. The high concentration of allolactose induces the regulation. However, if the concentration decreases, the operon is turned off.
• In some cases, the operons are generally on; the binding of specific molecules turns them off. An example of such an operon is the trp operon. When the tryptophan concentration is high, it binds to the promoter region and stops the gene expression. Alternatively, when the concentration of tryptophan is low, the gene expression takes place.

DNA Replication and Mutation

DNA replication : This phase of microbial genetics deals with the process of synthesizing new DNA from the old DNA. 

Process of DNA replication: 

• Both the DNA strands act as a template.
• The new DNA synthesis is always in the 5’-3' direction.
• The helicase enzyme forms a nick, and the DNA double helix unwinds. The SSB (single-stranded binding) proteins bind to the strands and prevent them from rewinding.
• The enzyme DNA polymerase synthesizes the leading strand continuously in the 5’-3' direction. However, the other strand is known as the lagging strand, and its synthesis is discontinuous.
• The RNA polymerase synthesizes a short primer in a lagging strand; the DNA polymerase attaches to the primer and adds on the nucleotides. This type of DNA synthesis is discontinuous.
• The enzyme DNA ligase joins the fragments on the lagging strand to form a continuous strand.

Mutation : The changes in a genome sequence due to the deletion or duplication of genes or DNA fragments is called a mutation.

Sources of mutation:

• Errors in DNA replication and DNA repair.
• Chemical mutagens.
• High energy electromagnetic radiation like ultraviolet rays and X rays.
• Environmental factors.

In genetics, Neurospora, a fungus, is used as a model microorganism to study microbial mutations.

Mechanisms of Gene Transfer

The microbes acquire different modes of gene transfer. One method is gene recombination through homologous recombination, site-specific recombination, transposons, and merozygote revertant transformation.

Bacteria can acquire foreign DNA in three basic ways:

Transformation : In this process, the bacteria acquire naked DNA from the environment. The recipient bacteria are often referred to as competent cells that readily uptake the foreign DNA. When a bacterium dies, its genetic material is released into the environment. This genetic material enters the competent cell and gets inserted within the host genome through homologous recombination.

Conjugation : Plasmids are extra circular chromosomal DNA that is capable of self-replicating. The plasmids of some bacteria, known as F (fertility) plasmid, can move between cells through conjugation. The bacterial cell that contains the F factor is called the F⁺ cell, and the one that doesn't contain the F factor is called the F^- cell.

• The donor cell or the F⁺ cell forms a sex pilus that helps it attach to the F^- cell.
• The plasmid replicates itself and is transferred to the F^- cell through the pilus.

Certain bacterial cells have the F factor integrated within their genome. These cells are referred to as Hfr (high frequency of recombination) cells.

 Conjugation in an Hfr cell

1. The F factor of the F⁺ cell gets integrated into the bacterial chromosome, forming an Hfr cell.
2. The Hfr cell forms a conjugative tube or pilus and attaches to the F^- cell.
3. The Hfr chromosome replicates itself and is transferred into the F^- cell.
4. The Hfr chromosome divides in the F^- cell to form a plasmid. The F^- cell is now called F' cell.

Transduction : In this process, certain viruses called bacteriophages infect the bacteria. These viruses use the bacteria to produce new offspring.

The bacteriophages follow two types of the life cycle:

1. Lytic cycle: In this process, the phage penetrates the host cell and releases its genetic material inside the bacterium. The phage DNA replicates and is packaged inside the coat proteins, producing several progeny phages. These new phages lyse the bacterial cell and are released into the environment, capable of infecting other bacteria.
2. Lysogenic cycle: In this process, the phage doesn't destroy the host cell. After the phage DNA is released into the host cell, it gets incorporated within the bacterial genome, forming a prophage. The prophage remains dormant for some time and can also be transferred to the bacterial offspring. Under favorable conditions, the prophage detaches from the host chromosome and follows the lytic cycle.

Applications of Microbial Genetics

Microbes have various roles in different aspects of science. These include:

• Gene therapy: Microbial genetics plays a great role in gene therapy or the treatment of genetic disease. But its implication in humans is restricted for various reasons.
• Genomics: It involves the study of the entire genome of an organism. Studying the microbial genome help the medical industries to develop strategies against various diseases caused by pathogenic microbes.
• Metagenomics: It is the study of genetic material obtained directly from an environmental sample. Microbial organisms or microbiome.
• RNA interference (RNAi): It is a process of gene silencing that is an effective method in treating viral diseases.
• Transcriptomics and proteomics: This branch of biology compares the gene expression of different microbial cells and helps collect the data for bioinformatics.

Microbial genetics has major applications in modern microbiology, genetic recombination of organisms, microbial genetic studies, biotechnology, molecular cloning, genomic library, gel electrophoresis, DNA sequencing, and various other fields. 

Context and Applications

This topic is helpful in future professional exams like

• Bachelor of Science in Microbiology
• Bachelor of Science in Genetics
• Bachelor of Technology in Genetic Engineering
• Master of Science in Microbiology
• Master of Science in Genetics
• Master of Science in Molecular Biology