Molecular Structure of DNA and RNA

Molecular Structure of DNA and RNA

Identification of DNA as the Genetic Material

• Criteria for Genetic Material:

  • Information: Must contain information necessary to create an entire organism.

  • Transmission: Must be passed from parent to offspring.

  • Replication: Must be capable of being copied for transmission.

  • Variation: Must allow for changes accounting for phenotypic variation across species.

Historical Context

• Data from various geneticists, notably Mendel, supported characteristics of genetic material.

• Chemical nature of genetic material cannot be identified solely through genetic crosses.

• Identification of DNA involved a range of experimental approaches.

Griffith's Experiments with Streptococcus pneumoniae

• Streptococcus pneumoniae: A bacterium studied by Griffith, exhibiting two strains:

  • Type S (Smooth):

    • Secretes polysaccharide capsule.

    • Protects from the immune system, producing smooth colonies on media.

  • Type R (Rough):

    • Cannot secrete capsule, resulting in rough colonies.

Griffith’s Experiment Process

• Griffith injected mice with various combinations of live and dead bacteria:

  • (a) Live type S → Mouse died, type S recovered.

  • (b) Live type R → Mouse survived, no bacteria recovered.

  • (c) Dead type S → Mouse survived, no bacteria recovered.

  • (d) Live type R + Dead type S → Mouse died, type S recovered.

Transforming Principle

• Griffith concluded something from dead type S transformed type R into type S.

• This process was termed transformation; the transforming substance was later studied.

• The capsule formation fulfills the four properties of genetic material (information, variation, replication, transmission).

Avery, MacLeod, and McCarty's Experiments

• Conducted in the 1940s using Griffith's findings to identify genetic material.

• Major constituents known: DNA, RNA, proteins, carbohydrates.

Experimental Approach

• Prepared extracts from type S cells and purified macromolecules.

• Only DNA extract converted type R to type S.

• Treatments:

  • RNase/protease did not eliminate transformation.

  • DNase treatment did eliminate transformation, proving DNA is genetic material.

Hershey and Chase Experiments

• Provided further evidence that DNA is the genetic material using T2 bacteriophage.

• Distinctive labeling:

  • 32P for DNA.

  • 35S for proteins.

• Phages infected non-radioactive E. coli cells, evidencing that DNA entered cells while proteins did not.

Overview of DNA and RNA Structure

• Definition: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are nucleic acids, first termed nuclein by Friedrich Miescher in 1869.

• Nucleotides are the basic repeating units linked to form strands.

• In DNA, two strands cohere into a double helix with additional complexity through folding and bending with protein interactions.

Nucleotide Structure

• Components of a nucleotide:

  • Phosphate Group

  • Pentose Sugar: Ribose in RNA, Deoxyribose in DNA.

  • Nitrogenous Base: Composed of purines (double ring) and pyrimidines (single ring).

Terminology of Nucleic Acid Units

• Nucleoside: Base + Sugar (e.g., Adenosine = Adenine + Ribose).

• Nucleotide: Base + Sugar + Phosphate (e.g., ATP = Adenosine Triphosphate).

Structure of a DNA Strand

• Nucleotides linked by covalent bonds (ester bonds), forming a phosphodiester linkage.

• Directionality: DNA strands run in a 5’ to 3’ direction, with a backbone formed from alternating phosphates and sugars.

Discovery of the Double Helix

• 1953: Watson and Crick revealed the double helical structure of DNA, aided by previous scientists such as Rosalind Franklin.

• Chargaff’s rules established base pairing: A-T and C-G.

Stabilization of the Double Helix

• Stabilized through hydrogen bonding between bases (2 between A and T; 3 between C and G) and base stacking interactions.

Grooves on the DNA Double Helix

• Major and minor grooves allow protein interactions with specific base sequences.

RNA Structure

• RNA differs from DNA by using uracil instead of thymine and ribose instead of deoxyribose.

• RNA can form secondary structures through base-pairing.

Types of RNA Structures

• Different types of structures include bulge loops, internal loops, multibranched loops, and stem loops.