Central Dogma, DNA Replication, and Gene Regulation in Bacteria

Central Dogma of Molecular Biology

Overview

The central dogma of molecular biology describes the flow of genetic information within a biological system. It outlines the processes by which genetic information is transferred from DNA to RNA and then to protein, which ultimately determines cellular function.

• DNA Replication: The process by which DNA makes a copy of itself, ensuring genetic information is passed to daughter cells.

• Transcription: The synthesis of RNA from a DNA template, catalyzed by RNA polymerase.

• Translation: The synthesis of proteins from an mRNA template, occurring at the ribosome.

Example: In Escherichia coli, the lac operon is transcribed into mRNA, which is then translated into enzymes required for lactose metabolism.

DNA Structure and Nucleotides

Nucleotides and Nucleosides

• Nucleotide: Consists of a nitrogenous base, a five-carbon sugar (deoxyribose in DNA), and one or more phosphate groups.

• Nucleoside: Consists of a nitrogenous base and a sugar, without phosphate groups.

• Base Pairing: Purines (adenine [A], guanine [G]) pair with pyrimidines (thymine [T], cytosine [C]) via hydrogen bonds.

• Base Pairing Rules: A pairs with T (2 H-bonds), G pairs with C (3 H-bonds).

Example: The DNA double helix is stabilized by complementary base pairing between strands.

DNA Replication

Mechanism and Enzymes

DNA replication is a highly regulated, semi-conservative process, meaning each new DNA molecule contains one parental and one newly synthesized strand.

• Origin of Replication (oriC): Specific chromosomal location where replication begins.

• Initiator Proteins: DnaA binds fully methylated DNA at oriC to initiate replication.

• Helicase (DnaB): Unwinds the DNA double helix.

• Primase: Synthesizes short RNA primers required for DNA polymerase to begin synthesis.

• DNA Polymerase III: Main enzyme for DNA synthesis; adds nucleotides in the 5' to 3' direction.

• Sliding Clamp (DnaN): Increases processivity of DNA polymerase.

• Single-Stranded Binding Proteins (SSB): Stabilize unwound DNA.

• DNA Ligase: Seals nicks between Okazaki fragments on the lagging strand.

• Proofreading: DNA polymerase has exonuclease activity (DnaQ subunit) to correct errors.

Replication Fork: The Y-shaped region where DNA is actively being unwound and replicated.

• Leading Strand: Synthesized continuously in the 5' to 3' direction.

• Lagging Strand: Synthesized discontinuously as Okazaki fragments (~1,000 bp each), later joined by ligase.

Example: In E. coli, replication starts at oriC and proceeds bidirectionally until the termination site (ter).

Additional info: DNA methylation (by Dam methylase) regulates initiation; SeqA binds hemi-methylated DNA to prevent reinitiation.

Mutations and Genetic Variation

Types of Mutations

• Point Mutation: Single base change; can be silent, missense, or nonsense.

• Silent Mutation: No change in protein sequence.

• Missense Mutation: Alters one amino acid in the protein.

• Nonsense Mutation: Introduces a premature stop codon, truncating the protein.

• Frameshift Mutation: Insertion or deletion of bases that shifts the reading frame.

Genotype vs. Phenotype:

• Genotype: The complete genetic makeup of an organism.

• Phenotype: Observable characteristics resulting from genotype and environment.

• Genotypic changes do not always result in phenotypic changes (e.g., silent mutations).

Transcription

Process and Regulation

Transcription is the synthesis of RNA from a DNA template, catalyzed by RNA polymerase. It is regulated at multiple levels to ensure proper gene expression.

• Initiation: Sigma factor recognizes and binds the promoter, recruiting RNA polymerase.

• Elongation: RNA polymerase synthesizes RNA in the 5' to 3' direction; sigma factor dissociates after initiation.

• Termination: RNA polymerase stops transcription at specific sequences.

• Promoter Elements: -10 and -35 regions are recognized by sigma subunit.

• Rifamycin: Antibiotic that inhibits bacterial RNA polymerase initiation.

Example: The lac operon is transcribed when lactose is present and glucose is absent.

Gene Regulation in Bacteria

Operons and Regulons

• Operon: A cluster of genes transcribed as a single mRNA, regulated together (e.g., lac operon).

• Regulon: A group of operons or genes regulated by the same regulatory protein.

Lac Operon

• Contains three structural genes: lacZ, lacY, lacA.

• Regulated by the lacI repressor and the CRP-cAMP complex.

• Transcription occurs only when lactose is present and glucose is absent.

Glucose	Lactose	Lac Operon Expression
High	High	No
High	Low	No
Low	High	Yes
Low	Low	No

Maltose and Arginine Operons

• Maltose Operon: Contains malE, malF, malG; regulated by an activator protein and inducer.

• Arginine Operon: Contains argC, argB, argH; regulated by a repressor protein and corepressor.

Two-Component Regulation

• Sensor Kinase: Detects environmental signals and autophosphorylates at a histidine residue.

• Response Regulator: Receives phosphate and regulates transcription by binding DNA.

Example: Chemotaxis in bacteria is regulated by methyl-accepting chemotaxis proteins (MCPs), CheA, and CheY.

Quorum Sensing

• Bacteria communicate via small molecules called autoinducers (e.g., AHL, AI-2, short peptides).

• Regulates group behaviors such as bioluminescence, virulence, and biofilm formation.

Example: Quorum sensing was first discovered in bioluminescent bacteria.

Endospores

Formation and Regulation

• Endospore: A differentiated, highly resistant cell type produced by certain Gram-positive bacteria (mainly soil bacteria).

• Formation is regulated by sigma factors and allows survival in extreme conditions.

• Endospores can survive for extended periods and germinate under favorable conditions.

Example: Bacillus and Clostridium species form endospores.