DNA Polymorphism Lecture Notes

Introduction

• Despite global diversity, 99.9% of human DNA is similar.
• Variations among individuals arise from the remaining 0.1% of DNA.
• DNA polymorphism refers to DNA that exists in multiple forms within a species.

Definition and Examples

• Polymorphism: "Poly" means many, "morphism" means forms.
• Example: If a DNA sequence A, T, T, A, G, C, A is present in 99% of a population, and A, T, T, G, G, C, A in 1%, it constitutes a polymorphism.
• Single Nucleotide Polymorphism (SNP): Variation occurs at a single nucleotide level, e.g., A→G.

Criteria for Polymorphism

• Must have at least two variants.
• Each variant should be present in at least 1% of the population.
• Example: If a variant exists in only 0.5% of the population, it isn’t considered a polymorphism.

Types of DNA Polymorphism

1. Single Nucleotide Polymorphism (SNP)
2. Tandem Repeats
   • Sequences repeated consecutively, e.g., A, T, T, C.
   • Repeats can be short or extensive (hundreds/thousands of bases).

Origin of Polymorphisms

• Arise from mutations.
• Affect two cell types:
  • Somatic Cells: Any body cell except germ cells; mutations here don't matter for inheritance.
  • Germ Cells: Give rise to gametes (sperm and ova); mutations here are inherited.
• For a mutation to contribute to polymorphism:
  • It must occur in germ cells.
  • It should not be lethal nor affect reproductive potential.

Mutations and DNA

• Central Dogma: DNA → mRNA (Transcription) → Protein (Translation).
• Only 1% of DNA is coding (protein-coding); 99% is non-coding.
• Mutations can occur in both coding and non-coding regions.
• Mutations are more likely in non-coding regions:
  • Greater DNA volume (99%).
  • Less likely to be disastrous as they don't directly produce proteins.

Importance of DNA Polymorphism

• Drives variation essential for evolution.
• Has practical applications, e.g., DNA fingerprinting.

Note

• DNA fingerprinting will be covered in subsequent lectures.