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

Microbial Genetics

Structure and Function of Genetic Material

Microbial genetics explores how genetic information is stored, replicated, and expressed in microorganisms. The genetic material, primarily DNA, determines the characteristics and functions of microbes.

• DNA is the genetic material in most organisms, with some viruses using RNA instead.

• Genome: The complete set of genetic material in an organism.

• Nucleotides: The building blocks of DNA, each consisting of a phosphate group, a 5-carbon sugar (deoxyribose), and a nitrogenous base (adenine, thymine, cytosine, or guanine).

• DNA is a double helix with two antiparallel strands held together by hydrogen bonds between complementary bases (A-T, C-G).

Example: In prokaryotes, the genome is typically a single, circular, double-stranded DNA molecule.

DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division, ensuring genetic continuity.

• Most bacteria have closed, double-stranded circular DNA.

• Replication is semiconservative: each new DNA molecule contains one original and one new strand.

• Replication begins at the origin of replication and proceeds bidirectionally.

• Major requirements:

  • Single-stranded DNA template

  • RNA primer (synthesized by primase)

  • Deoxynucleotide triphosphates (dNTPs)

  • Enzymes: DNA polymerase III (main enzyme), helicase (unwinds DNA), ligase (joins fragments)

• DNA polymerase adds nucleotides to the 3' end; synthesis occurs 5' to 3'.

• Leading strand: synthesized continuously.

• Lagging strand: synthesized discontinuously as Okazaki fragments, joined by ligase.

Example: For the DNA sequence 5'-ATGGAGGGGGTGCC-3', the complementary strand is 3'-TACCTCCCCCACG-5'.

Genetic Transfer and Recombination

Genetic transfer introduces genetic variation in bacteria through the movement of DNA between cells.

• Vertical gene transfer: DNA passed from parent to offspring.

• Horizontal gene transfer: DNA transferred between cells of the same generation.

• Three mechanisms in bacteria:

  1. Transformation: Uptake of free DNA fragments from the environment by a recipient cell.

  2. Conjugation: Direct transfer of DNA via cell-to-cell contact, typically involving an F plasmid and sex pilus.

  3. Transduction: Transfer of DNA from one cell to another via a bacteriophage (virus).

Example: In conjugation, an F+ donor cell transfers the F plasmid to an F- recipient, making both F+.

The Flow of Genetic Information (Protein Synthesis)

Genetic information flows from DNA to RNA to protein, following the central dogma of molecular genetics.

• Transcription: DNA is used as a template to synthesize a complementary RNA strand.

• Translation: mRNA is decoded to build a protein.

• Central Dogma: DNA → mRNA → Protein

Equation: $\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{Protein}$

Transcription

• Occurs in the cytoplasm of prokaryotes.

• RNA differs from DNA: single-stranded, ribose sugar, uracil replaces thymine.

• Types of RNA:

  • mRNA: Messenger RNA, template for protein synthesis.

  • rRNA: Ribosomal RNA, structural component of ribosomes.

  • tRNA: Transfer RNA, brings amino acids to ribosome; contains anticodon.

• Steps:

  1. Initiation: RNA polymerase binds to promoter, unwinds DNA.

  2. Elongation: RNA polymerase synthesizes RNA 5' to 3'.

  3. Termination: RNA polymerase reaches terminator sequence, transcription ends.

Example: For DNA 5'-ATGGAGGGGGTGCC-3', mRNA is 5'-AUGGAGGGGGUGCC-3'.

Genetic Code

• Set of rules for converting nucleotide sequence into amino acid sequence.

• Codons: Three-base sequences on mRNA coding for specific amino acids.

• 64 codons encode 20 amino acids; includes start (AUG) and stop codons.

Example: mRNA 5'-AUGGAGGGGGUGCCA-3' translates to a specific amino acid sequence.

Translation

• Occurs at ribosomes, involving mRNA, rRNA, and tRNA.

• Ribosome has small and large subunits; three sites: E (exit), P (peptidyl), A (aminoacyl).

• Steps:

  1. Initiation: Ribosome assembles at start codon (AUG); initiator tRNA brings methionine.

  2. Elongation: tRNAs bring amino acids; peptide bonds form; polypeptide grows.

  3. Termination: Stop codon signals release of polypeptide; ribosome disassembles.

Equation: $\text{mRNA} + \text{tRNA} + \text{rRNA} \xrightarrow{\text{translation}} \text{Protein}$

Regulation of Gene Expression in Prokaryotes

Gene expression in bacteria is tightly regulated to conserve resources and respond to environmental changes.

• Constitutive genes: Always expressed; essential for basic cell functions ("housekeeping" genes).

• Regulated genes: Expressed only when needed; controlled by operons.

• Operon: A cluster of genes under control of a single promoter and operator, functioning in a related process.

• Components of an operon:

  • Structural genes: Encode proteins.

  • Promoter: Site for RNA polymerase binding.

  • Operator: Regulatory site; repressor protein binds here to block transcription.

  • Regulatory gene: Encodes repressor protein.

• Inducible operon (e.g., lac operon):

  • Repressor binds operator in absence of inducer (e.g., lactose); gene is "off".

  • Inducer (lactose) binds repressor, inactivating it; gene is "on".

  • Glucose presence affects lac operon via cAMP and CAP protein.

Condition	cAMP Level	CAP Activity	Lac Operon Transcription
Lactose present, glucose absent	High	Active	High
Lactose present, glucose present	Low	Inactive	Low
Lactose absent	Variable	Inactive	None

Example: Bacteria preferentially use glucose; lac operon is activated only when glucose is depleted and lactose is available.

Errors and Mutations in Genes

Mutations are changes in the genetic material that can affect protein function and organismal traits.

• Mutagen: An agent that causes mutations (e.g., chemicals, radiation).

• Spontaneous mutations: Occur without external mutagen.

• Types of mutations:

  • Point mutations: Change in a single base pair.

    • Substitution: One base replaced by another; may be silent, missense (wrong amino acid), or nonsense (stop codon).

  • Insertions and deletions: Addition or loss of nucleotides; often cause frameshift mutations, altering the reading frame and resulting in nonfunctional proteins.

Mutation Type	Effect on Protein
Silent	No change in amino acid sequence
Missense	Wrong amino acid incorporated
Nonsense	Premature stop codon; truncated protein
Frameshift	Altered reading frame; usually nonfunctional protein

Example: A frameshift mutation in a gene can disrupt all downstream codons, leading to a nonfunctional protein.

Additional info: The Ames test is a rapid method for screening substances for mutagenic potential, using bacteria to detect mutation rates as a proxy for carcinogenic risk.