DNA Structure and Synthesis

DNA Molecule Structure

Introduction to DNA

DNA: Deoxyribonucleic acid is frequently referred to as the blueprint of life. It holds the necessary genetic instructions for the development and maintenance of life, much like how blueprints guide construction.

Double Helix Model

• Watson-Crick Model: The structure of DNA is similar to a twisted ladder, termed a double helix.

• Components:

  • Sugar-Phosphate Backbone: Offers structural stability to the DNA molecule.
  • Nitrogenous Bases: Act as the ladder's rungs and include four types—Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).

• Base Pairing Rules:

  • Adenine pairs with Thymine.
  • Cytosine pairs with Guanine.
  • These bases are linked through hydrogen bonds, similar to a zipper mechanism.

Overview of Transcription

Transcription is a fundamental process in gene expression, whereby DNA is transcribed into RNA as a precursor to protein synthesis.

Transcription Process

• Initiation:
  • RNA polymerase attaches to the promoter region.
• Elongation:
  • RNA polymerase elongates the RNA strand by adding complementary nucleotides.
• Termination:
  • This phase indicates the cessation of transcription.

RNA Modifications

• 5' Capping: A modified guanine cap is added for RNA stability.

• Poly-A Tail Addition: A tail is attached to the 3' end to aid in stability and translation.

• Splicing: Introns are excised, and exons are joined.

Translation and Polypeptide Formation

Ribosome Function and tRNA Interaction

• Ribosomes: Function as biological machines that convert mRNA's instructions into proteins.
  • Composed of a large and small subunit.
• tRNA's Role: It transports specific amino acids to the ribosome, facilitating protein synthesis through precise codon-anticodon pairing.

Site	Function
A	Entry site for aminoacyl-tRNA
P	Forms peptide bonds between amino acids
E	Exit site for the tRNA

Amino Acids and Peptide Bonds

• Amino Acids: Recognised as the building blocks of proteins.
• Peptide Bonds: These link amino acids, forming polypeptides.

Codon-Anticodon Interaction

• Genetic Code Importance: Codons are pivotal in determining protein sequences.
• Anticodon Role: Ensures precise pairing with mRNA codons.

Codon	Anticodon	Amino Acid
AUG	UAC	Methionine
UUU	AAA	Phenylalanine

Worked Example

Given the mRNA sequence AUG-UUU-GCC, let's determine the resulting polypeptide:

1. AUG codes for Methionine (Met/M) - This is always the start codon
2. UUU codes for Phenylalanine (Phe/F)
3. GCC codes for Alanine (Ala/A)

Therefore, the polypeptide produced would be: Met-Phe-Ala

Practice Questions:

1. Describe the function of the P site during translation.

   • Solution: The P (Peptidyl) site holds the tRNA attached to the growing polypeptide chain. It's where peptide bonds form between amino acids as the ribosome moves along the mRNA.

2. Explain how anticodon pairing maintains translation accuracy.

   • Solution: Anticodon pairing maintains accuracy through complementary base pairing with mRNA codons. The precise matching of tRNA anticodons to mRNA codons ensures that the correct amino acid is added to the growing polypeptide chain, following the genetic instructions encoded in the DNA.

Gene and Environment Interaction

Overview

Understanding how genes interact with their environments is vital for predicting how organisms respond to environmental changes, such as those caused by climate shifts.

Genetic Influence

• Genes: DNA's encoded instructions determine traits, directing development.

• Environmental Variability: Influences gene expression, thereby causing variability.

Environmental Influence

• Factors: Light, temperature, and nutrition can impact gene expression.

Classic and Modern Examples

• Hydrangea pH Impact: Flower colour variation due to soil pH.
• Siamese Cat Enzyme Activity: Temperature-sensitivity affects fur colouration.
• Leaf Morphology: Sunlight exposure dictates adaptive traits.

Epigenetic Influences

• Methylation & Histone Modification: Alter gene expression without modifying the DNA sequence itself.

Ethical Considerations in Genetic Manipulation

Introduction

Genetic manipulation transforms industries such as medicine, necessitating ethical scrutiny.

Impact

• Biodiversity Loss: Diversity reduction possible due to genetic interventions.
• Ecological Effects: GMOs may disrupt ecosystems and affect gene flow.

Considerations in Gene Therapy

• Ethical Debates: Raises moral questions regarding genetic enhancements.

• Case Study: CRISPR:

  • Pros: Potential for curing diseases.
  • Cons: Ethical dilemmas surrounding enhancement.

Impact Across Generations

• Epigenetic Changes: Ethical considerations regarding heredity.
• Responsibility: Policies must reflect potential impacts.

Societal Implications

• Phenotype Control: Could increase disparities among populations.
• Access Inequities: Limited access to advancements may heighten existing inequalities.