6.1 - Central Dogma of Gene Expression

UNIT 3: MOLECULAR GENETICS

Lesson 4: Central Dogma of Gene Expression

Key Concepts

Building Blocks of Proteins

• Amino acids: Fundamental building blocks of all proteins.

• Levels of Protein Structure:

  • Primary Structure: Sequence of a chain of amino acids.

  • Secondary Structure: Local folding into shapes like B-pleated sheets and alpha helices.

  • Tertiary Structure: Three-dimensional folding of a polypeptide chain.

  • Quaternary Structure: Assembly of multiple polypeptide chains into a functional protein.

DNA and Protein Synthesis

Locations of DNA and Protein Synthesis

• DNA Location: Found within the nucleus of the cell.

• Protein Synthesis: Occurs in the cytoplasm.

• Organelle for Protein Production: Ribosomes are responsible for protein production.

Beadle & Tatum's Experiments

Overview

• Investigated Neurospora crassa (red bread mould), which can survive on minimal media with sugar and salt.

• Explored metabolic pathways for the synthesis of arginine, which requires multiple enzymes and intermediates.

Supplementation and Results

• Systematically supplemented growth media with different intermediates to identify arginine pathway mutants:

  • arg1: Mutant could grow on ornithine or citrulline, indicating a defect early in the pathway.

  • arg2: Mutant could grow on citrulline, indicating the defect is between ornithine and citrulline.

  • arg3: Mutant could grow on arginine, indicating a defect in the final step of the pathway.

One Gene-One Enzyme Hypothesis

• Beadle and Tatum proposed the one gene-one enzyme hypothesis, suggesting each gene is responsible for one enzyme's production.

• Results: Identified four genes, each producing different enzymes involved in arginine synthesis.

• Subsequently refined to the one gene-one polypeptide hypothesis.

The Central Dogma of Gene Expression

Gene Expression Process

• Transfer of Genetic Information:

  • DNA ➔ mRNA ➔ Protein

• Transcription:

  • Converts DNA sequence to mRNA.

  • Example: DNA sequence ATGATCTCGTAA transcribes to mRNA AUGAUCUCGUAA.

• Translation:

  • mRNA is translated into a polypeptide sequence.

  • Example: mRNA AUG codes for Methionine (Met).

    

Transcription and Translation

Transcription Details

• Synthesis of messenger RNA (mRNA) from a DNA template.

• mRNA carries genetic information to the protein synthesis machinery and determines the amino acid sequence of proteins.

Translation Mechanics

• Synthesis of proteins from mRNA templates.

• Process involves ribosomes and various types of RNA.

The Genetic Code

Characteristics

1. Redundant:

   • More than one codon can code for the same amino acid due to limited nitrogenous bases.

   • Example: 64 total codons correspond to only 20 amino acids.

2. Continuous:

   • The genetic code is read in triplets (codons) without gaps.

   • A shift in nucleotide sequences can alter amino acid sequences.

3. Nearly Universal:

   • Most organisms use the same genetic code, making it possible to transfer genes across species with consistent protein expression.