DNA Structure and Properties

What did Phoebus Levene contribute to the study of DNA and RNA?

He differentiated between DNA and RNA and identified the three major components of nucleic acids: phosphate, sugar, and base.

What structural features of DNA are associated with Levene's work?

DNA has a backbone containing deoxyribose and the bases T, A, C, and G.

What was the tetranucleotide hypothesis?

The idea associated with the early understanding of DNA structure in which DNA was viewed in terms of repeating nucleotide units.

WRONG → Chargaff

What was known about chromosomes?

Chromosomes were known to contain the hereditary material and were associated with both proteins and DNA.

What did William Astbury study?

He used X-ray diffraction studies on nucleic acids.

What did Astbury's X-ray diffraction studies suggest about DNA?

They showed that DNA had a regular, repeating structure.

How were the bases arranged according to Astbury's observations?

The bases were basically laid flat, like a stack of pancakes.

What are Chargaff's rules?

Guanine equals cytosine (G = C), and adenine equals thymine (A = T).

What did Chargaff discover about DNA composition between species?

The composition of DNA varies from species to species, demonstrating molecular diversity.

What aspects of DNA composition were important in Chargaff's work?

DNA components, heredity, base positions, and base ratios.

What did Pauling propose about DNA?

He proposed a triple-helix structure of deoxyribonucleic acid.

What limited Pauling's DNA model?

It was limited to models.

Why was X-ray diffraction used in DNA research?

X-rays were directed at a DNA sample, and the reflected pattern provided information about the structure.

Negative image → Crystalline

What did the crystalline structure of DNA indicate about its shape?

DNA needed to have a helical shape.

What did Rosalind Franklin study?

She used diffraction studies to examine DNA and found that her samples included a blend of DNA rather than completely pure DNA.

What did Franklin examine about DNA's dry and wet forms?

She studied both dry and wet DNA.

Why is water important to DNA structure?

DNA attracts water molecules and therefore exists in a hydrophilic environment.

Why must phosphates be on the outside of DNA?

Phosphates must be on the outside to attract water molecules.

What did Franklin focus on with DNA-A?

She began focusing on DNA-A, including the size of repeating units and the symmetry of DNA crystals.

What was Image B-51 (Wet-51)?

It was Franklin's clearest X-ray diffraction image of DNA and showed an obvious cross pattern.

What did Franklin's B-51 image reveal about DNA?

It showed that DNA has a helical shape and flat, stacked bases.

What structural measurements were obtained from Franklin's diffraction work?

DNA has about 10 bases per twist and a diameter of about 2 nm.

What was wrong with the early Watson and Crick model?

It used the incorrect tetranucleotide theory, placed phosphates on the inside, and had other incorrect structural assumptions.

Magnesium in the middle?

What was described as problematic about the development of the Watson and Crick model?

Stole the idea of SYMMETRY from Rosalind

Must be double helix with anti-parallel strands

What did the final DNA model need to contain?

A double helix with antiparallel strands.

How did Chargaff's work relate to the DNA model?

Base ratios, including G = C and A = T, provided information relevant to constructing the DNA structure.

Hydrogen bonds MUST be in middle to form bonds between bases

Why was the shape of the bases important to the DNA model?

The shape of the bases helped determine how they could fit together and form hydrogen bonds.

Especially with uneven twists later on

What properties of DNA were important in determining its final structure?

The shape of the bases, base ratios, hydrogen bonding between A-T and C-G, the helical shape, flat stacked bases, antiparallel arrangement, and the placement of phosphates on the outside.