Microbial Genetics

Key Vocabulary

This section introduces essential terms for understanding microbial genetics, including DNA structure, replication, gene expression, mutations, and genetic exchange in bacteria.

• Gene: A segment of DNA that encodes a functional product, usually a protein.

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

• Chromosome: A DNA molecule with part or all of the genetic material of an organism.

• Plasmid: Small, circular, double-stranded DNA molecules found in bacteria, separate from chromosomal DNA; often carry genes for antibiotic resistance.

• Central Dogma: The flow of genetic information: DNA → RNA → Protein.

• DNA Replication: The process by which DNA makes a copy of itself.

• Transcription: The synthesis of RNA from a DNA template.

• Translation: The synthesis of proteins from an mRNA template.

• Mutation: A change in the DNA sequence.

• Conjugation, Transduction, Transformation: Mechanisms of horizontal gene transfer in bacteria.

• Codon: A sequence of three nucleotides in mRNA that specifies an amino acid.

• Anticodon: A sequence of three nucleotides in tRNA complementary to a codon in mRNA.

• Exons/Introns: Exons are coding regions; introns are non-coding regions in eukaryotic genes.

• Semiconservative: Each new DNA molecule consists of one old and one new strand.

• Mutation Types: Point, insertion, deletion, frameshift, missense, nonsense, silent.

• Enzymes: DNA polymerase, RNA polymerase, helicase, ligase, primase, gyrase.

• Ribosome: The cellular machinery for protein synthesis.

• mRNA, tRNA, rRNA: Messenger, transfer, and ribosomal RNA, respectively.

• Replication Fork: The area where DNA is actively being unwound and replicated.

• Okazaki Fragments: Short DNA fragments synthesized on the lagging strand during replication.

• 5’ cap, Poly-A tail: Post-transcriptional modifications in eukaryotic mRNA.

Structure of Genes, Genomes, and Bacterial Chromosomes

Genes and Genomes

Genes are the basic units of heredity, encoding proteins or functional RNAs. The genome encompasses all genetic material, including chromosomes and plasmids.

• Bacterial Chromosomes: Typically a single, circular DNA molecule located in the nucleoid region.

• Plasmids: Extra-chromosomal DNA elements; can carry genes for antibiotic resistance, virulence, or metabolic functions.

• Contribution to Genome: Plasmids enhance genetic diversity and adaptability.

• Example: Escherichia coli can possess plasmids encoding resistance to multiple antibiotics.

DNA Replication

Semiconservative Replication

DNA replication is semiconservative, meaning each new DNA molecule contains one original strand and one newly synthesized strand. This ensures genetic continuity.

• Steps:

  1. Initiation: DNA unwinds at the origin of replication; helicase separates strands.

  2. Elongation: DNA polymerase III synthesizes new DNA; primase adds RNA primers; ligase joins Okazaki fragments on the lagging strand.

  3. Termination: Replication ends when forks meet or at specific termination sequences.

• Enzymes: Helicase, primase, DNA polymerase III, DNA polymerase I, ligase, gyrase.

• Replication Fork: The site of active DNA synthesis.

• Okazaki Fragments: Short DNA segments synthesized on the lagging strand.

Equation:

Additional info: DNA replication proceeds bidirectionally in bacteria.

Transcription

Process and Enzymes

Transcription is the synthesis of RNA from a DNA template, enabling gene expression.

• Steps:

  1. Initiation: RNA polymerase binds to promoter with help from sigma factor.

  2. Elongation: RNA polymerase synthesizes RNA complementary to DNA template.

  3. Termination: RNA polymerase releases the RNA transcript at terminator sequences.

• Enzymes: RNA polymerase, sigma factor.

• Products: mRNA, tRNA, rRNA.

Equation:

Post-Transcriptional Modifications (Eukaryotes)

RNA Processing

In eukaryotes, primary RNA transcripts undergo modifications to become mature mRNA.

• 5’ Capping: Addition of a modified guanine nucleotide to the 5’ end.

• Polyadenylation: Addition of a poly-A tail to the 3’ end.

• Splicing: Removal of introns and joining of exons.

• Example: Human pre-mRNA is processed to mature mRNA before translation.

Translation

Protein Synthesis

Translation is the process by which ribosomes synthesize proteins using mRNA as a template.

• Steps:

  1. Initiation: Ribosome assembles at start codon (AUG).

  2. Elongation: tRNA brings amino acids; peptide bonds form; ribosome moves along mRNA.

  3. Termination: Ribosome reaches stop codon; polypeptide is released.

• Key Sites: A site (aminoacyl), P site (peptidyl), E site (exit).

• Enzymes: Peptidyl transferase (ribosome component).

• Materials: mRNA, tRNA, rRNA, amino acids.

Equation:

Genetic Code and Codons

Understanding Codons

The genetic code is the set of rules by which nucleotide sequences are translated into amino acids.

• Codon: Three-nucleotide sequence in mRNA specifying an amino acid.

• Start Codon: AUG (methionine).

• Stop Codons: UAA, UAG, UGA.

• Example: Given DNA sequence ATG, the RNA sequence is AUG, coding for methionine.

Table: Example Codon-Amino Acid Mapping

Mutations

Types and Effects

Mutations are changes in the DNA sequence that can affect protein function and contribute to genetic diversity and drug resistance.

• Point Mutation: Single nucleotide change.

• Insertion/Deletion: Addition or removal of nucleotides; may cause frameshift.

• Frameshift Mutation: Alters reading frame, often resulting in nonfunctional protein.

• Missense Mutation: Changes one amino acid.

• Nonsense Mutation: Creates a premature stop codon.

• Silent Mutation: No change in amino acid sequence.

• Example: Mutations in bacterial genes can confer resistance to antibiotics.

Additional info: Mutations can be spontaneous or induced by mutagens.

Genetic Exchange in Bacteria

Mechanisms of DNA Exchange

Bacteria can exchange genetic material through three main mechanisms, increasing genetic diversity and adaptability.

• Conjugation: Direct transfer of DNA via sex pili between F+ and F- cells.

• Transformation: Uptake of free DNA from the environment.

• Transduction: Transfer of DNA by bacteriophages (viruses that infect bacteria).

• Significance: Enables rapid spread of traits such as antibiotic resistance.

• Example: Streptococcus pneumoniae can acquire capsule genes via transformation.

Table: Bacterial DNA Exchange Mechanisms