INTRODUCTION

DNA or deoxyribonucleic acid is a double-stranded (ds) helically twisted polymer of nucleotides, which represents our genetic material responsible for inheritance and variations.

Friedrich Miescher in 1869 revealed nucleic acid from pus cells and furthermore reported DNA as an acidic substance (ph-2.3). The structure of DNA was detailed by Watson and Crick through its study by X-ray diffraction technique.

STRUCTURE OF DNA

DNA is a polynucleotide. Each nucleotide is a Pentose Sugar which is deoxyribose in nature along with a nitrogen base and phosphate.

DNA —- Pentose Sugar + Nitrogen base + Phosphate.

WATSON AND CRICK MODEL

Francis Crick and James Watson in 1953 concluded that DNA is made up of two antiparallel chains held together by hydrogen bonds. They were awarded a Nobel Prize in 1962 along with Wilkins.

OBSERVATIONS:

• DNA is made up of 2 strands in a helical fashion (plectonemically).
• Each strand is made up of a chain of nucleotides.
• The arrangement of these strands is antiparallel in nature.
• Purines always pair with Pyrimidines.
• The DNA duplex is 20 angstroms in diameter.
• One complete turn in a DNA duplex is of 34 angstroms (for 10 base pairs). Coiling results in the production of a major groove and a minor groove.
• Major groove – 22 angstroms in length.
• Minor groove – 12 angstroms in length.
• The distance between two consecutive base pairs = 34/10. = 3.4 angstroms.
• Pentose Sugar and Phosphate form the backbone of the structure.
• At 3’ the 3rd carbon of the pentose sugar is free and at 5’ the 5th carbon is free giving it a helical structure.

BASE PAIR-RULE

Pairing always occurs between a Purine (Adenine and Guanine) and a Pyrimidine (Cytosine and Thymine).

There are two Hydrogen bonds present between Adenine with Thymine and three Hydrogen bonds present between Cytosine and Guanine.

CHARGAFF’S RULE

• Given by E.E. Chargaff in (1950).
• Purines and Pyrimidines always occur in equal amounts.
• A + G = T + C.
• Sugar deoxyribose and phosphate residues also occur in equal amounts.
• The base ratio is fixed for a species.

BASE RATIO is the ratio between the sum total of Adenine + Thymine and the total of Cytosine + Guanine in a DNA sample.

TYPES OF DNA

FUNCTIONS OF DNA

• Being the genetic material; DNA is responsible for inheritance and variations.
• Cellular metabolism is controlled by DNA through transcription.
• DNA is used for resolving parenthood disputes and the identification of individuals (DNA Fingerprinting).
• In the late 1980s, DNA technology transformed the field of Forensics. DNA Fingerprinting did not only help in identifying the culprits but also revealed the loopholes in the judiciary system by bringing quintessential evidentiary values.
• DNA is autocatalytic i.e. it can replicate itself.

PARTS OF THE HUMAN BODY WHERE DNA IS NOT PRESENT

• Mature Red Blood Cells
• Cornified Cells in the skin
• Mature hairs

REFERENCES

• Urry, Cain, Wasserman, Minorsky, Reece (2015) Campbell biology, 11th edn,: Pearson.
• K. Bhatti (2013) Companion Biology, 27th edn.: Dinesh.