The Central Dogma of Molecular Biology: From DNA to Protein and the Impact of Mutations

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 and finally to protein, which 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:

• DNA → RNA → Protein

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

Gene Structure and Transcription

Gene Organization

A gene is a segment of DNA that contains the instructions for making an RNA molecule. Genes are flanked by regulatory sequences that control their expression.

• Promoter: A DNA sequence that defines the start site for RNA synthesis by RNA polymerase.

• Terminator: A DNA sequence that signals the end of RNA synthesis.

Example: The lac operon in E. coli contains a promoter, operator, and structural genes for lactose metabolism.

Regulation of Prokaryotic Transcription

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

• RNA polymerase binds to the promoter (a specific DNA sequence).

• Transcription begins at the promoter and proceeds in the 5' to 3' direction using the template strand of DNA.

• Transcription stops at the terminator sequence, releasing the newly synthesized RNA.

Equation:

• $\text{DNA (template strand)} \xrightarrow{\text{RNA polymerase}} \text{mRNA}$

Translation: From mRNA to Protein

Initiation of Translation

Translation begins when the ribosome assembles at the start codon (AUG) on the mRNA, with the help of the methionine-initiator tRNA and initiation factors.

• Start Codon (AUG): Defines the beginning of translation and the reading frame.

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

• The small ribosomal subunit, initiation factors, and Met-initiator tRNA form a complex at the 5' cap of the mRNA and scan for the start codon.

• Upon recognition of the start codon, the large ribosomal subunit joins, and translation begins.

Equation:

• $\text{mRNA} + \text{ribosome} + \text{Met-tRNA}^{\text{init}} \rightarrow \text{Initiation complex}$

Elongation and Termination

During elongation, amino acids are added one by one to the growing polypeptide chain. Translation ends when a stop codon is reached.

• Elongation: Each new aminoacyl-tRNA enters the A site of the ribosome, and a peptide bond is formed.

• Termination: When a stop codon enters the A site, release factors bind, causing the ribosome to release the completed polypeptide.

Equation:

• $\text{Polypeptide chain elongation:} \quad \text{Amino acid}_{n} + \text{Amino acid}_{n+1} \rightarrow \text{Amino acid}_{n}-\text{Amino acid}_{n+1}$

Example: The sequence AUG-GUU-GGC codes for Met-Val-Gly.

Mutations and Their Effects

Types of Mutations

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

• Point Mutation: A change in a single nucleotide.

• Silent Mutation: A point mutation that does not alter the amino acid sequence due to the redundancy of the genetic code.

• Missense Mutation: A point mutation that results in a different amino acid being incorporated.

• Nonsense Mutation: A point mutation that creates a premature stop codon.

• Deletion: Loss of one or more nucleotides.

• Insertion: Addition of one or more nucleotides.

• Chromosomal Mutation: Large-scale changes affecting chromosome structure.

Example: Sickle cell anemia is caused by a missense mutation in the beta-globin gene.

Consequences of Mutations

• Silent mutations do not change the protein sequence.

• Missense mutations can alter protein function or stability.

• Nonsense mutations can truncate proteins, often leading to loss of function.

• Mutations in the start codon can prevent translation initiation, while mutations in the stop codon can result in abnormally long proteins.

Additional info: Some amino acids (e.g., methionine and tryptophan) are encoded by a single codon, so any mutation in their codons will not be silent.

Protein Quality Control and Degradation

Ubiquitin-Proteasome System

Cells use the ubiquitin-proteasome system to degrade misfolded, damaged, or unneeded proteins. Proteins are tagged with ubiquitin and directed to the proteasome for degradation.

• Ubiquitination: The process of attaching ubiquitin molecules to a substrate protein.

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

Equation:

• $\text{Protein} + \text{Ubiquitin} \rightarrow \text{Ubiquitinated protein} \xrightarrow{\text{Proteasome}} \text{Peptides}$

Example: Defective proteins resulting from missense or nonsense mutations are often degraded by this system.

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

• Product: Shorter sugar chains (oligosaccharides)

• Lysozyme lowers the activation energy of the hydrolysis reaction.

Equation:

• $\text{Polysaccharide} + \text{H}_2\text{O} \xrightarrow{\text{Lysozyme}} \text{Oligosaccharides}$

Example: Lysozyme is found in human tears and saliva, providing antimicrobial protection.

Importance of Amino Acid Residues in Enzyme Activity

Specific amino acids in the active site of enzymes are critical for their catalytic function. For lysozyme, Glu35 and Asp52 are essential for activity.

• Targeted mutation of Glu35 to Ala35 (E35A) results in loss or reduction of enzymatic activity.

• Enzyme structure and the properties of amino acid side chains (hydrophobic, hydrophilic, acidic, basic) determine function.

Additional info: There are 20 standard amino acids, each with unique side chains that influence protein structure and function.

Summary Table: Types of Point Mutations and Their Effects

Mutation Type	Definition	Effect on Protein	Example
Silent	Change in nucleotide does not alter amino acid	No change in protein sequence	GAA to GAG (both code for Glu)
Missense	Change in nucleotide alters amino acid	Possible change in protein function	GAG to GTG (Glu to Val)
Nonsense	Change in nucleotide creates stop codon	Truncated protein	CAG to TAG (Gln to Stop)