DNA Structure, Function, and the Molecular Basis of Heredity

History of DNA and Chromosomes in Heredity

Seminal Discoveries in Chromosome Theory

The identification of chromosomes as carriers of genetic information was a major milestone in genetics. Early researchers such as Flemming, Boveri, Sutton, Morgan, Stevens, Bridges, and Sturtevant established the physical linkage of genes to chromosomes and their role in heredity.

• Chromatin: Highly stainable nuclear material that condenses into chromosomes during cell division.

• Nuclein: The original term for DNA, discovered by Friedrich Miescher in 1869.

• Chromosomes are composed of protein and DNA, and their number and shape are species-specific but stable across generations due to mitosis and meiosis.

• Genes are mapped to specific chromosomes, allowing prediction of recombination events.

Experimental Evidence for the Nucleus as the Hereditary Agent

Joachim Hammerling's experiments with Acetabularia (unicellular green alga) demonstrated that the nucleus contains the genetic material responsible for heredity.

• Removal and transplantation of the nucleus affected the morphology of the alga, confirming the nucleus's role in heredity.

Discovery of DNA as the Genetic Material

Griffith's Transformation Principle (1928)

Frederick Griffith's experiments with Streptococcus pneumoniae revealed that genetic traits could be transferred between bacteria, suggesting a 'transforming principle'.

• S strain (III-S): Virulent, smooth colonies with a polysaccharide capsule.

• R strain (II-R): Non-virulent, rough colonies lacking a capsule.

• Live R strain could be transformed into virulent S strain by exposure to heat-killed S strain, indicating transfer of genetic material.

Avery-MacLeod-McCarty Experiment (1944)

This experiment identified DNA as the 'transforming principle' by showing that only DNA, not protein, could transform non-virulent bacteria into virulent forms.

• Extracts from heat-killed S strain were treated with DNase (destroys DNA) or protease (destroys protein).

• Transformation occurred only when DNA was intact, confirming DNA as the genetic material.

Hershey-Chase Experiment (1952)

Alfred Hershey and Martha Chase used bacteriophages to demonstrate that DNA, not protein, is the genetic material transmitted to bacteria during infection.

• Bacteriophages were labeled with radioactive isotopes: 35S for protein and 32P for DNA.

• After infection and centrifugation, only 32P (DNA) entered the bacterial cells, proving DNA is the genetic material.

Nucleic Acid Structure

Nucleosides and Nucleotides

Nucleic acids are polymers of nucleotides, which are composed of a pentose sugar, a nitrogenous base, and a phosphate group.

• Nucleoside: Pentose sugar + nitrogenous base.

• Nucleotide: Nucleoside + phosphate group.

• Nucleotides are phosphorylated nucleosides.

Sugar Component

• Ribose: Sugar in RNA.

• Deoxyribose: Sugar in DNA (lacks an oxygen atom at the 2' position).

Nitrogenous Bases

• Pyrimidines: Cytosine (C), Thymine (T), Uracil (U).

• Purines: Adenine (A), Guanine (G).

• DNA contains A, T, G, C; RNA contains A, U, G, C.

Chargaff's Rules and Base Pairing

Erwin Chargaff discovered that the amount of adenine equals thymine, and the amount of guanine equals cytosine in DNA.

• Base Pairing: A pairs with T, G pairs with C.

• Complementary base pairing is essential for DNA replication and transcription.

Organism/Source	A	T	G	C
Ox thymus	26	25	21	16
Human sperm	29	31	18	18
Yeast	31	32	18	19
Avian tubercle bacilli	15	14	28	43

Sugar-Phosphate Backbone

The backbone of DNA and RNA consists of alternating sugars and phosphate groups, connected by phosphodiester linkages.

• Phosphate groups link the 3' carbon of one sugar to the 5' carbon of the next.

• This structure gives nucleic acids directionality (5' to 3').

Double Helical Structure of DNA

Watson and Crick Model

The double helix model of DNA was proposed by Watson and Crick, based on X-ray crystallography data from Rosalind Franklin and Maurice Wilkins.

• DNA is a right-handed double helix with antiparallel strands.

• Bases are planar and stacked perpendicular to the helix axis.

• Major and minor grooves allow protein interactions for gene regulation.

• 10 base pairs per turn; base-to-base distance is 3.4 Å.

Hydrogen Bonding and Base Pairing

Hydrogen bonds stabilize the pairing between complementary bases.

• A-T pairs form two hydrogen bonds.

• G-C pairs form three hydrogen bonds, making them more stable.

• Hydrogen bonds are non-covalent interactions between donor and acceptor atoms.

DNA Conformations: A-DNA, B-DNA, Z-DNA

DNA can adopt several conformations depending on environmental conditions.

Form	Helix Type	Grooves	Features
A-DNA	Right-handed	Deep major, shallow minor	Forms under dehydrating conditions
B-DNA	Right-handed	Wide major, narrow minor	Most common in cells
Z-DNA	Left-handed	Flat major, narrow minor	Forms in GC-rich regions, may regulate gene expression

Physical Properties and Experimental Analysis of DNA

DNA Melting and Absorbance

DNA denaturation (melting) refers to the separation of the two strands, which can be induced by heat or chemical agents.

• Single-stranded DNA (ssDNA) absorbs ultraviolet light at 260 nm more strongly than double-stranded DNA (dsDNA).

• The melting temperature (Tm) increases with higher GC content due to stronger hydrogen bonding.

Formula for DNA melting temperature:

$T_m = 2(A+T) + 4(G+C)$

Experimental Techniques

• X-ray Crystallography: Used to determine the helical structure of DNA.

• UV Absorbance: Used to quantify DNA concentration and monitor melting.

Summary Table: Key Features of DNA Structure

Feature	Description
Polymer Type	Polynucleotide
Monomer	Nucleotide (sugar, base, phosphate)
Strand Directionality	5' to 3'
Base Pairing	A=T, G≡C
Helix Type	Right-handed (B-DNA)
Grooves	Major and minor
Stability	GC pairs more stable than AT pairs

Conclusion

The molecular structure of DNA underpins its role in heredity, genetic information storage, replication, and transcription. Understanding the physical and chemical properties of DNA is essential for further studies in genetics and molecular biology.