Molecular Structure of DNA and RNA Flashcards

Goal of Molecular Genetics

• The goal of molecular genetics is to use our knowledge of DNA structure to understand how DNA functions as a genetic material.

Disciplines Supporting Molecular Genetic Technology

• Biochemistry, cell biology, and microbiology support molecular genetic technology.

Four Criteria for Genetic Material

• The four criteria that genetic material must meet are:
  • Information – contains information to construct an organism.
  • Transmission – must be passed from parents to offspring.
  • Replication – must be copied for cell division.
  • Variation – must account for phenotypic differences.

Chromosome Theory and Early Discoveries

• August Weismann and Carl Nägeli proposed that a chemical substance in cells transmits traits.
• The chromosome theory of inheritance states that chromosomes are carriers of genetic material.

Griffith’s Experiment (1928)

• Frederick Griffith studied Streptococcus pneumoniae (formerly pneumococci).
• The types of S. pneumoniae used in his experiment were Type S (smooth, encapsulated) and Type R (rough, non-encapsulated).
• Griffith concluded that a “transforming principle” from dead Type S converted Type R into Type S.
• The transformation in genetic terms meant:
  • Type R acquired information to make a capsule.
  • The genetic material must be replicated and transmitted.
  • Dead bacteria transferred genetic material to living bacteria.

Avery, MacLeod, and McCarty Experiment (1940s)

• They used biochemical purification of bacterial extracts to identify the genetic material.
• The result of mixing Type R with DNA extract from Type S was that Type R converted to Type S.
• They confirmed DNA was the genetic material by:
  • Treating extract with DNase, RNase, and protease.
  • Only DNase prevented transformation.
  • Conclusion: DNA is responsible for transformation.

Hershey and Chase Experiment (1952)

• E. coli and T2 bacteriophage were used.
• The T2 phage is made up of: Head, sheath, tail fibers, base plate (composed of protein); DNA inside the head.
• Radioactive labels used:
  • ^{35}S for proteins
  • ^{32}P for DNA
• The results showed:
  • ^{35}S in supernatant → proteins remained outside
  • ^{32}P in pellet → DNA entered bacterial cells
• Conclusion: DNA is the genetic material.

DNA and RNA Overview

• Friedrich Meischer discovered DNA in 1869 (called it “nuclein”).
• Nucleic acids have acidity because they release H^+ and are negatively charged at neutral pH.

Four Levels of Nucleic Acid Complexity

• The 4 levels of complexity in DNA/RNA are:
  1. Nucleotides
  2. Linear strand
  3. Double helix
  4. 3D folding with proteins

Nucleotide Components

• The three components of a nucleotide are:
  1. Phosphate group(s)
  2. Pentose sugar
  3. Nitrogenous base
• Sugars found in DNA and RNA:
  • DNA: Deoxyribose
  • RNA: Ribose
• Bases are purines and pyrimidines:
  • Purines: Adenine (A), Guanine (G)
  • Pyrimidines: Thymine (T), Cytosine (C), Uracil (U)

Nucleosides (Examples)

• Examples of nucleosides:
  • Ribose-based:
    • ribose + adenine = adenosine
    • ribose + guanine = guanosine
    • ribose + cytosine = cytidine
    • ribose + uracil = uridine
  • Deoxyribose-based:
    • deoxyribose + adenine = deoxyadenosine
    • deoxyribose + guanine = deoxyguanosine
    • deoxyribose + cytosine = deoxycytidine
    • deoxyribose + thymine = deoxythymidine

Nucleotides

• A nucleotide is formed by attaching one or more phosphate groups to a nucleoside via an ester bond.
• Examples of nucleotides:
  • Adenosine monophosphate (AMP): ribose + adenine + 1 phosphate
  • Adenosine triphosphate (ATP): ribose + adenine + 3 phosphates

Structure of DNA Strand

• A phosphodiester linkage links nucleotides in a DNA or RNA strand.
• Sugar and phosphate groups form the backbone of the DNA/RNA strand.
• The backbone is negatively charged because each phosphate group carries a negative charge.

Discovery of the Double Helix

• Linus Pauling built early structural models of DNA (built α helix models).
• Watson and Crick discovered the DNA double helix structure using model building and X-ray diffraction data.
• Rosalind Franklin contributed X-ray diffraction images suggesting:
  1. Helical structure
  2. Double-stranded width
  3. 10 base pairs per turn

Chargaff’s Rules

• Erwin Chargaff discovered:
  • Amount of adenine ≈ thymine
  • Amount of guanine ≈ cytosine
• This implies:
  • Adenine pairs with thymine, and guanine pairs with cytosine (Chargaff’s rule).

Watson and Crick’s Model

• A key feature of Watson and Crick’s DNA model was:
  • Two backbones outside with bases pointing inward
  • Hydrogen bonding between A-T and G-C
  • Antiparallel strands with 10 bp per turn
• Watson, Crick, and Maurice Wilkins received the 1962 Nobel Prize for this discovery.

Rosalind Franklin’s Legacy

• Rosalind Franklin was not credited in the original publication.
• Her contributions were recognized posthumously; she died in 1958 and couldn’t share the Nobel Prize.

DNA Double Helix Details

• Pairing of purine with pyrimidine (A-T and G-C) keeps the width of the DNA double helix constant.
• The strands of DNA are oriented antiparallel.
• B-DNA (right-handed helix) is the predominant DNA form in living cells.
• Z-DNA is an alternative left-handed conformation of DNA.
• Triplex DNA is a triple-helical DNA structure formed in vitro (synthetically).
  • Discovered in 1957 by Rich, Davies, and Felsenfeld
  • Likely has little biological relevance