PYTR Class

Learning Objectives

1. Explain the role of complementary base pairing in genetic replication and expression.
2. Recognise the diversity of DNA base sequences and its limitless capacity for storing information.
3. Understand the conservation of the genetic code across all life forms as evidence of universal common ancestry.

Part 1: Importance of Base Complimentary

Role of Complementary Base Pairing in DNA Replication and Gene Expression

DNA Replication

• Complementary base pairing ensures that adenine (A) pairs with thymine (T) and cytosine (C) pairs with guanine (G).
• During DNA replication, the two strands of the double helix separate.
• Each original strand serves as a template for the formation of a new complementary strand.
• New nucleotides are added one by one, ensuring the correct base sequence is maintained.
• The result is two identical DNA molecules, each containing one original strand and one new strand (semi-conservative replication).

Gene Expression

• Genes are sections of DNA that contain genetic information.
• When a gene is expressed, its base sequence is copied into RNA (transcription).
• Only one DNA strand is used as a template for transcription.
  • Complementary base pairing applies, but adenine (A) pairs with uracil (U) instead of thymine (T) in RNA.
• The RNA molecule produced can serve regulatory, structural, or protein synthesis roles.
• In protein synthesis, the RNA sequence is translated into an amino acid sequence, again involving complementary base pairing.

Part 2: DNA as Universal Genetic Materials

Diversity of DNA Base Sequences and Limitless Information Storage

• Genetic information is stored in the base sequence of DNA.
• Any sequence of four bases (A, C, G, T) is possible.
• The number of possible sequences increases exponentially:
  • 4² = 16 possible two-base sequences
  • 4³ = 64 possible three-base sequences
  • 4ⁿ possible sequences for n bases, making the potential combinations immense.
• DNA molecules vary in length, further increasing sequence diversity.
• DNA is extremely compact (2 nm in diameter), allowing vast amounts of information to be stored efficiently.
• Compared to human-made storage systems, DNA is highly space-efficient and uses minimal material.

Conservation of the Genetic Code and Universal Common Ancestry

• DNA and RNA store genetic information in a coded form.
• Information is decoded during protein synthesis, where:
  • Three-base units (codons) encode amino acids.
  • There are 64 possible codons.
  • Most codons specify an amino acid.
  • One codon signals start of protein synthesis.
  • Three codons signal stop.
• The genetic code is nearly universal, used by all living organisms and viruses.
• Despite billions of years of evolution, the code has remained almost unchanged, supporting the idea of a universal common ancestor.
• Minor exceptions exist, where stop codons have been repurposed, but the core structure remains the same.