Microbial Genetics: Structure, Function, and Expression of Genetic Material

Microbial Genetics

Introduction to Genetics

Genetics is the study of inheritance or heredity, focusing on how organisms transmit their genetic information to offspring. The genome is the sum total of genetic material in an organism, organized into chromosomes, which are discrete cellular structures composed of neatly packaged DNA molecules.

• Eukaryotic chromosomes: Located in the nucleus; linear and double-stranded.

• Bacterial chromosomes: Located in the nucleoid region; usually single, circular, and double-stranded.

Heredity Basics

Genes are segments of DNA containing the code to make proteins. The genotype is the sum of all genes constituting an organism’s distinctive genetic makeup, while the phenotype is the expression of the genotype, resulting in observable structures or functions.

• Cells: Have DNA genomes and RNA.

• Viruses: May have either DNA or RNA genomes.

Structure and Function of DNA

DNA Structure

The basic unit of DNA is the nucleotide, which consists of a phosphate group, deoxyribose sugar, and a nitrogenous base. Nitrogenous bases are classified as purines (adenine, guanine) or pyrimidines (thymine, cytosine).

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

• Double helix: DNA is a double-stranded helix, with nucleotides connected by phosphodiester bonds.

• Hydrogen bonds: Hold the two strands together between nitrogenous bases.

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

Function of DNA

DNA contains instructions for an organism to develop, survive, and reproduce. Each gene is a DNA sequence coding for a protein, and genes are located on chromosomes. Chromosomes may contain hundreds to thousands of genes.

Structure and Function of RNA

RNA Structure and Function

The primary function of RNA is to create proteins by translating genetic information from DNA. RNA nucleotides (ribonucleotides) consist of a phosphate group, ribose sugar, and nitrogenous bases (adenine, guanine, cytosine, uracil).

• Uracil (U): Replaces thymine (T) in RNA.

• Single-stranded: RNA is often single-stranded and can fold into helical and loop structures.

Types of RNA

• Messenger RNA (mRNA): Carries genetic code from DNA to ribosomes.

• Transfer RNA (tRNA): Brings amino acids to ribosomes during protein synthesis.

• Ribosomal RNA (rRNA): Forms the core of ribosome structure and catalyzes protein synthesis.

Comparison of DNA and RNA

• DNA: Double-stranded, contains thymine, deoxyribose sugar.

• RNA: Single-stranded, contains uracil, ribose sugar.

Flow of Genetic Information

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA directs the production of RNA, which then directs the assembly of proteins.

DNA Replication

Overview of DNA Replication

DNA replication is the process by which a cell copies its genome before division. It involves unwinding the original DNA, copying it, and rewinding the parent and newly made DNA. Each new DNA helix contains one original strand and one newly synthesized strand (semi-conservative replication).

Key Enzymes in DNA Replication

• Topoisomerase: Relieves tension in the DNA strand.

• Helicase: Unwinds the DNA by breaking hydrogen bonds.

• Single-strand binding protein: Stabilizes separated strands.

• DNA polymerase III: Synthesizes new DNA strand, requires a 3’ OH group provided by RNA primer.

• RNA primase: Adds RNA primer as an anchoring point.

• Ligase: Joins Okazaki fragments on the lagging strand.

• DNA polymerase I: Removes RNA primer and repairs mismatches.

Leading and Lagging Strands

• Leading strand: Synthesized continuously in the 5’ to 3’ direction.

• Lagging strand: Synthesized in short segments (Okazaki fragments), each primed with RNA primer.

Protein Synthesis (Gene Expression)

Transcription

Transcription is the process by which genetic information from one strand of DNA is copied to messenger RNA (mRNA). RNA polymerase binds to promoter regions, separates DNA strands, and lays down complementary ribonucleotides until reaching a termination sequence.

Translation

Translation is the decoding of mRNA by ribosomes to build proteins. It involves three main steps: initiation, elongation, and termination.

• Initiation: mRNA, tRNA, and rRNA assemble; tRNA with start amino acid binds to P-site.

• Elongation: New tRNA binds to A-site, amino acids are joined, tRNA moves through ribosome sites (A, P, E).

• Termination: Stop codon causes release factor to bind, releasing the polypeptide chain.

Genetic Code

• Codons: Nucleotides are grouped in triplets, each coding for an amino acid.

• Redundancy: Multiple codons can code for the same amino acid, protecting against mutations.

Differences Between Prokaryotic and Eukaryotic Transcription/Translation

• AUG: Codes for different forms of methionine in eukaryotes.

• Eukaryotic mRNAs: Usually code for one protein; bacterial mRNAs may code for several.

• Introns and Exons: Eukaryotic genes contain introns (non-coding) and exons (coding).

Antibacterial and Antiprotozoan Drugs Targeting DNA, RNA, and Protein Synthesis

Drugs Targeting DNA or RNA

• Fluoroquinolones: Inhibit DNA unwinding by topoisomerase or helicases, stopping transcription. Examples: ciprofloxacin, levofloxacin.

• Antiprotozoan drugs: Quinine (chloroquine) for malaria; metronidazole (Flagyl) for protozoan infections and some bacteria.

Drugs Targeting Protein Synthesis

• Aminoglycosides: Bind to 30S ribosome, causing misreading of mRNA. Examples: gentamicin, kanamycin, streptomycin.

• Tetracyclines: Block tRNA binding to ribosome. Side effects include tooth discoloration and GI disruption.

• Macrolides: Broad spectrum; used for respiratory, skin, and ear infections. Examples: erythromycin, clarithromycin, azithromycin.

Genetic Recombination and Horizontal Gene Transfer

Binary Fission and Mutations

Bacteria reproduce by binary fission, resulting in genetically identical cells. Mutations may occur during DNA replication, leading to genetic diversity.

Horizontal Gene Transfer

• Plasmids: Small DNA molecules used to share genetic information; R plasmids confer antibiotic resistance.

• Conjugation: Transfer of plasmid or genetic material via sex pili from donor to recipient cell.

• Transformation: Uptake of small DNA fragments from the environment; demonstrated by Griffith’s experiment with Streptococcus pneumoniae.

• Transduction: Transfer of DNA by bacteriophages; can be generalized (random fragments) or specialized (specific regions).

Mutations

Types of Mutations

• Spontaneous mutations: Random changes due to replication errors.

• Induced mutations: Caused by mutagens (chemical, physical, biological agents).

• Carcinogens: Mutagens that promote cancer development.

Mutation Effects

• Base-substitution: Single nucleotide change; can be silent, missense, or nonsense.

• Insertions and deletions: Can cause frame-shift mutations, leading to extensive missense or nonsense.

• Insertion/deletion of three nucleotides: Does not cause frame-shift but may cause missense or nonsense.

Summary Table: Horizontal Gene Transfer Mechanisms

Mechanism	Description	Key Features
Plasmids	Small, circular DNA molecules	Antibiotic resistance, virulence factors
Conjugation	Direct transfer via sex pili	Requires F-plasmid, donor and recipient
Transformation	Uptake of environmental DNA	Used in recombinant DNA technology
Transduction	DNA transfer by bacteriophage	Generalized or specialized

Additional info: This guide expands brief points into full academic explanations, adds definitions, examples, and organizes content for microbiology students.