Molecular Structure of DNA and RNA: Foundations of Genetic Material

Molecular Structure of DNA and RNA

Introduction to Molecular Genetics

Molecular genetics is the branch of genetics that studies the structure and function of DNA and RNA at the molecular level. Advances in molecular techniques have greatly expanded our understanding of how genetic information is stored, transmitted, and expressed in living organisms.

• Molecular genetics underpins our knowledge of transmission genetics and population genetics.

• Most of our understanding of genetics is based on the molecular structure of DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

Identification of DNA as the Genetic Material

Essential Properties of Genetic Material

For a molecule to serve as the genetic material, it must fulfill several key criteria:

• Information: Must contain the instructions necessary to construct an entire organism. Example: DNA stores codons (each 3 nucleotides) that code for amino acids in proteins.

• Transmission: Must be passed from parent to offspring during reproduction.

• Replication: Must be capable of being copied so it can be transmitted to the next generation.

• Variation: Must be able to change to account for phenotypic diversity within and between species.

Experimental Evidence for DNA as Genetic Material

While genetic crosses supported these properties, the chemical identity of the genetic material was established through a series of classic experiments.

Frederick Griffith's Experiments with Streptococcus pneumoniae

• Griffith studied two strains of Streptococcus pneumoniae:

  • Type S (Smooth): Pathogenic, secretes a polysaccharide capsule (protects from immune system), forms smooth colonies.

  • Type R (Rough): Non-pathogenic, lacks capsule, forms rough colonies.

• Key experiments:

  • Injecting mice with live S: mice died, S recovered.

  • Injecting mice with live R: mice survived, no bacteria recovered.

  • Injecting mice with heat-killed S: mice survived, no bacteria recovered.

  • Injecting mice with live R + heat-killed S: mice died, S recovered.

• Conclusion: A "transforming principle" from dead S cells converted R cells into S cells, indicating transfer of genetic information.

Avery, MacLeod, and McCarty's Experiments

• Purified different macromolecules (DNA, RNA, protein) from S cells and tested their ability to transform R cells.

• Only DNA could transform R cells into S cells.

• Treatment with DNase (destroys DNA) prevented transformation; RNase or protease did not.

• Conclusion: DNA is the hereditary material responsible for transformation.

Hershey and Chase Experiment

• Used T2 bacteriophage (virus that infects bacteria) labeled with radioactive isotopes:

  • 32P labels DNA (contains phosphorus).

  • 35S labels protein (contains sulfur).

• After infection of Escherichia coli, only 32P (DNA) entered the bacterial cells, not 35S (protein).

• Conclusion: DNA, not protein, is the genetic material of phages.

Structure of DNA and RNA

Nucleic Acids: DNA and RNA

DNA and RNA are large macromolecules known as nucleic acids. They are polymers made up of repeating units called nucleotides.

• Nucleotide: Consists of a phosphate group, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base.

• Nucleoside: A base plus a sugar (no phosphate).

• Examples: Adenosine (adenine + ribose), Deoxyadenosine (adenine + deoxyribose).

• Nucleotide Examples: AMP (adenosine monophosphate), ADP, ATP.

Components of Nucleotides

Component	DNA	RNA
Pentose Sugar	Deoxyribose	Ribose
Nitrogenous Bases	A, T, G, C	A, U, G, C
Phosphate Group	Present	Present

Structure of a DNA Strand

• Nucleotides are linked by phosphodiester bonds between the 5' carbon of one sugar and the 3' carbon of the next.

• DNA strands have directionality: 5' to 3'.

• The backbone is formed by sugars and phosphates; bases project from the backbone.

Discovery of the Double Helix

• Watson and Crick (1953) proposed the double helical structure of DNA, integrating data from:

  • Rosalind Franklin: X-ray diffraction showed DNA is helical, has more than one strand, and 10 base pairs per turn.

  • Erwin Chargaff: Base composition analysis led to Chargaff's rules: %A = %T, %C = %G.

• Double helix is right-handed, with two antiparallel strands (one 5'→3', one 3'→5').

• Bases pair via hydrogen bonds: A with T (2 bonds), G with C (3 bonds).

• There are about 10 base pairs per complete turn (3.4 nm per turn), and the helix is about 2 nm wide.

Stabilization of the Double Helix

• Stabilized by hydrogen bonding between complementary bases and base stacking (hydrophobic interactions between stacked bases).

• Major and minor grooves on the helix allow proteins to interact with specific base sequences.

Structure of RNA

• RNA is typically single-stranded but can form double-stranded regions via complementary base pairing (A-U, C-G).

• RNA uses ribose sugar and uracil instead of thymine.

• Secondary structures (hairpins, loops) are common and often right-handed, with 11-12 base pairs per turn in double-stranded regions.

Summary Table: DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Strandedness	Double-stranded (usually)	Single-stranded (usually)
Function	Genetic information storage	Information transfer, catalysis, regulation

Additional info: The notes above expand on brief slide points to provide full academic context, including definitions, examples, and summary tables for clarity.