The Central Dogma of Molecular Biology

Overview of the Central Dogma

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

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

• Transcription: The synthesis of RNA from a DNA template, producing messenger RNA (mRNA).

• Translation: The process by which ribosomes synthesize proteins using the sequence encoded in mRNA.

Key Equation:

Example: The gene for hemoglobin is transcribed into mRNA, which is then translated into the hemoglobin protein.

Structure of a Gene

A gene is a segment of DNA that contains the instructions for making an RNA molecule. Genes are composed of several key regions:

• Promoter: Defines the start site for RNA synthesis by RNA polymerase.

• Coding Region: Contains the sequence that will be transcribed and translated.

• Terminator: Defines the stop site for RNA synthesis.

Example: In the gene diagram, the promoter is upstream of the coding region, and the terminator is downstream.

Regulation of Prokaryotic Transcription

Initiation of Transcription

Transcription in prokaryotes is regulated by the binding of RNA polymerase to the promoter region of DNA, often with the help of transcription factors such as the sigma factor.

• Promoter: A specific DNA sequence where RNA polymerase binds to initiate transcription.

• Sigma Factor: A protein in prokaryotes that helps RNA polymerase recognize the promoter.

Direction of Synthesis: RNA polymerase synthesizes RNA in the 5' to 3' direction using the template strand of DNA.

Termination: RNA polymerase stops at the terminator sequence, releasing the newly synthesized RNA.

Translation: From mRNA to Protein

Initiation of Translation

Translation begins when the methionine-initiator tRNA binds to the start codon (AUG) on the mRNA. This process is facilitated by initiation factors and the small ribosomal subunit.

• Met-initiator tRNA: The only tRNA that can initiate translation by binding to the start codon.

• Initiation Complex: Includes Met-initiator tRNA, initiation factors, and the small ribosomal subunit.

• 5' Cap: In eukaryotes, the 5' cap of mRNA helps the ribosome locate the start codon.

Process: The initiation complex scans the mRNA until it finds the start codon. Initiation factors then dissociate, allowing the large ribosomal subunit to bind and translation to proceed.

Elongation and Termination

• Elongation: The ribosome moves along the mRNA, adding amino acids to the growing polypeptide chain.

• Termination: Translation ends when a stop codon is reached. Release factors bind to the stop codon, causing the ribosome to release the completed protein.

Key Equation:

Example: The sequence AUG codes for methionine and serves as the universal start codon for translation.

Genetic Mutations

Types of Mutations

Mutations are permanent changes in the nucleotide sequence of DNA. They can occur naturally or be engineered in the laboratory.

• Point Mutations: Changes in a single nucleotide.

• Silent Mutations: Point mutations that do not alter the amino acid sequence due to the redundancy of the genetic code.

• Missense Mutations: Point mutations that result in the incorporation of a different amino acid.

• Nonsense Mutations: Point mutations that create a premature stop codon.

• Insertions/Deletions: Addition or loss of one or more nucleotides, which can cause frameshift mutations.

• Chromosomal Mutations: Large-scale changes to chromosome structure.

Example: A mutation in the start codon can prevent translation initiation, while a mutation in the stop codon can result in a longer, nonfunctional protein.

Consequences of Mutations

• Silent Mutations: No change in protein sequence or function.

• Missense Mutations: May alter protein function or stability.

• Nonsense Mutations: Usually result in truncated, nonfunctional proteins.

• Frameshift Mutations: Change the reading frame, often leading to nonfunctional proteins.

Additional info: Some amino acids are encoded by only one codon (e.g., methionine and tryptophan), so mutations in their codons cannot be silent.

Protein Folding and Degradation

Protein Folding

Proteins must fold into specific three-dimensional structures to function properly. Mutations can interfere with folding, leading to loss of function or aggregation.

• Primary Structure: The sequence of amino acids in a protein.

• Folding: Driven by interactions among side chains and the peptide backbone.

Protein Degradation: Ubiquitin-Proteasome System

Cells degrade misfolded or damaged proteins using the ubiquitin-proteasome system.

• Ubiquitination: Proteins are tagged with ubiquitin molecules for degradation.

• Proteasome: A large protein complex that degrades ubiquitinated proteins into peptides using ATP hydrolysis.

Example: Truncated or misfolded proteins resulting from mutations are often targeted for degradation.

Enzyme Function: Lysozyme Example

Lysozyme and Polysaccharide Hydrolysis

Lysozyme is an enzyme that catalyzes the hydrolysis of polysaccharides in bacterial cell walls, aiding in bacterial cell lysis.

• Substrate: Polysaccharide chains in the bacterial cell wall.

• Product: Shorter polysaccharide fragments after hydrolysis.

• Mechanism: Lysozyme lowers the activation energy for the hydrolysis reaction.

Key Equation:

Example: Mutation of a key active site residue (e.g., Glu35 to Ala35) can abolish lysozyme activity.

Amino Acids in Proteins

Proteins are composed of 20 different amino acids, each with unique side chains that determine their properties.

• Hydrophobic (nonpolar): Water-fearing side chains.

• Hydrophilic (polar): Water-loving side chains.

• Acidic and Basic: Charged side chains at physiological pH.

Additional info: The specific sequence and properties of amino acids determine protein structure and function.