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.

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.