Microbial Genetics: Structure, Function, and Regulation

Microbial Genetics

Overview of Genetics

Microbial genetics is the study of genes, their functions, and how genetic information is transferred and expressed in microorganisms. Understanding microbial genetics is essential for comprehending microbial diversity, adaptation, and the mechanisms underlying disease and biotechnology.

• Genetics: The science of heredity, including the study of genes, gene expression, and gene replication.

• Genome: All genetic material in a cell.

• Chromosome: Structure containing DNA that carries hereditary information.

• Gene: Segment of DNA encoding a functional product, usually a protein.

• Genetic code: Set of rules determining how nucleotide sequences are converted to amino acid sequences.

• Genotype: Genetic makeup of an organism.

• Phenotype: Expression of the genes as observable traits.

• Genomics: Study of an organism's entire genome.

The Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA is transcribed into RNA, which is then translated into protein, resulting in cellular function. Mutations can alter this flow, leading to changes in protein function.

• DNA → RNA → Protein → Function

• Mutations: Changes in DNA can lead to altered mRNA, proteins, and functions.

Structure and Function of DNA

DNA is a double helix composed of two antiparallel strands. The backbone consists of deoxyribose sugars and phosphates, with nitrogenous bases (A, T, G, C) paired by hydrogen bonds. The sequence of bases encodes genetic instructions.

• Antiparallel strands: One strand runs 5' to 3', the other 3' to 5'.

• Base pairing: Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C).

• Genetic instructions: The order of bases determines the genetic code.

Chromosomes in Bacteria

Bacterial chromosomes are typically single, circular DNA molecules associated with proteins. Short tandem repeats (STRs) are noncoding, repeating sequences found in DNA.

Flow of Genetic Information

Genetic information can be transferred vertically (from parent to offspring) or horizontally (between cells of the same generation). Expression, recombination, and replication are key processes.

• Vertical gene transfer: Parent to offspring.

• Horizontal gene transfer: Between cells of the same generation.

• Expression: Use of genetic information to produce proteins.

• Recombination: Exchange of genetic material.

• Replication: Copying DNA for cell division.

DNA Replication

Mechanism of DNA Replication

DNA replication is the process by which DNA is copied before cell division. It is highly accurate due to proofreading by DNA polymerase.

• Double helix: DNA forms a double helix.

• Antiparallel strands: Strands run in opposite directions.

• Replication fork: Created by helicase separating strands.

• Leading strand: Synthesized continuously.

• Lagging strand: Synthesized discontinuously, forming Okazaki fragments.

• Enzymes: DNA polymerase, primase, ligase, helicase, gyrase, and others facilitate replication.

Important Enzymes in DNA Replication

Several enzymes are involved in DNA replication, expression, and repair. Their functions are summarized below:

Enzyme	Function
DNA Gyrase	Relaxes supercoiling ahead of the replication fork
DNA Ligase	Joins DNA strands; Okazaki fragments and excision repair
DNA Polymerase	Synthesizes DNA; proofreads and repairs DNA
Helicase	Unwinds double-stranded DNA
Primase	Makes RNA primers from a DNA template
Topoisomerase	Relaxes supercoiling; separates DNA circles at replication end
Photolyase	Uses light to repair UV-induced pyrimidine dimers
Exonuclease/Endonuclease	Cut DNA for repair
Methylase	Adds methyl groups to DNA bases
Ribozyme	Removes introns and splices exons
RNA Polymerase	Copies RNA from DNA template
snRNP	Removes introns, splices exons
Transposase	Cuts DNA backbone, leaves sticky ends

Energy Requirements

Energy for DNA replication is supplied by nucleotides. Hydrolysis of two phosphate groups from ATP provides the energy for the reaction.

RNA and Protein Synthesis

Types of RNA

RNA is a single-stranded nucleotide with a ribose sugar and uracil instead of thymine. There are three main types:

• mRNA (Messenger RNA): Carries coded information from DNA to ribosomes.

• rRNA (Ribosomal RNA): Integral part of ribosomes.

• tRNA (Transfer RNA): Transports amino acids during protein synthesis.

Transcription

Transcription is the synthesis of a complementary mRNA strand from a DNA template. It begins when RNA polymerase binds to the promoter and ends at the terminator sequence.

• Promoter: DNA sequence where transcription starts.

• Terminator: DNA sequence where transcription ends.

• Direction: 5' to 3' direction; only one DNA strand is transcribed.

Translation

Translation is the process by which mRNA is decoded into a protein. Codons (three mRNA nucleotides) specify amino acids. The genetic code is degenerate, meaning multiple codons can code for the same amino acid.

• Start codon: AUG

• Stop codons: UAA, UAG, UGA

• tRNA: Brings amino acids to the ribosome; anticodon pairs with codon.

• Peptide bonds: Join amino acids.

Protein Synthesis in Prokaryotes vs. Eukaryotes

In prokaryotes, transcription and translation can occur simultaneously. In eukaryotes, transcription occurs in the nucleus and translation in the cytoplasm. Exons code for proteins, while introns do not. snRNPs remove introns and splice exons together.

Regulation of Gene Expression

Operons and Gene Regulation

Gene expression in bacteria is regulated by operons, which consist of promoter, operator, and structural genes. Genes can be constitutive (always expressed), inducible (turned on by inducers), or repressible (turned off by repressors).

• Inducible operon: Genes are off unless an inducer is present.

• Repressible operon: Genes are on unless a corepressor activates the repressor.

Positive Regulation and Catabolite Repression

Catabolite repression prevents cells from using carbon sources other than glucose. cAMP accumulates when glucose is absent, binds to CAP, and activates transcription of the lac operon.

Epigenetic and Post-Transcriptional Control

Methylation of nucleotides can turn genes off, and this modification can be inherited but is not permanent. Riboswitches and microRNAs regulate gene expression post-transcriptionally.

• Riboswitch: mRNA structure changes upon substrate binding, affecting translation.

• miRNA: Base pairs with mRNA, leading to its destruction and preventing protein production.

Mutations and Genetic Variation

Types of Mutations

Mutations are permanent changes in DNA sequence. They can be neutral, beneficial, or harmful. Mutagens are agents that cause mutations.

• Base substitution (point mutation): Change in one base.

• Missense mutation: Base substitution changes an amino acid.

• Nonsense mutation: Base substitution creates a stop codon.

• Frameshift mutation: Insertion or deletion shifts the reading frame.

Mutagenic Agents

Chemical mutagens include nitrous acid and nucleoside analogs. Radiation (ionizing and UV) can also cause mutations, such as thymine dimers.

Mutation Repair Mechanisms

Cells repair mutations using photolyases (for UV-induced dimers) and nucleotide excision repair (removal and replacement of incorrect bases).

Identifying Mutants and Carcinogens

Mutants can be identified by direct (positive) or indirect (negative) selection. The Ames test measures mutagenicity by exposing bacteria to mutagenic substances and observing mutation reversal rates.

Genetic Transfer and Recombination

Genetic Recombination

Genetic recombination is the exchange of genes between DNA molecules, creating diversity. Crossing over involves breaking and rejoining chromosomes.

Vertical and Horizontal Gene Transfer

Vertical gene transfer is from parent to offspring. Horizontal gene transfer occurs between cells of the same generation and includes transformation, conjugation, and transduction.

Plasmids and Transposons

Plasmids are self-replicating circular DNA pieces that often carry genes for antibiotic resistance or pathogenicity. Transposons are DNA segments that move within and between DNA molecules.

Mechanisms of Genetic Transfer

• Transformation: Uptake of naked DNA from the environment.

• Conjugation: Transfer of plasmids via cell-to-cell contact (sex pili).

• Transduction: DNA transfer via bacteriophage.

Genes and Evolution

Role of Mutation and Recombination in Evolution

Mutations and recombination generate genetic diversity, which is the raw material for evolution. Natural selection acts on populations, favoring organisms best suited to their environment.