DNA Structure and Experimental Evidence

What is the overall structure of DNA

Two complementary polynucleotide strands arranged in a double helix.

What does it mean that DNA strands are antiparallel

The strands run in opposite directions: one 5′ to 3′ and the other 3′ to 5′.

What does it mean that DNA strands are complementary

Each base on one strand pairs with a specific base on the opposite strand.

What is the DNA sugar-phosphate backbone

The outer portion of the double helix, formed by alternating deoxyribose sugars and phosphate groups.

Where are the nitrogenous bases located in the DNA double helix

They face inward, toward the centre of the helix.

What is the approximate diameter of the DNA double helix in the lecture model

20 Å (2 nm).

How many nitrogenous base pairs occur per 360° turn in the lecture model

Approximately 10 base pairs.

Which bases pair together in DNA

Adenine pairs with thymine, and guanine pairs with cytosine.

How many hydrogen bonds form between adenine and thymine

Two hydrogen bonds.

How many hydrogen bonds form between guanine and cytosine

Three hydrogen bonds.

Why does a purine always pair with a pyrimidine

This pairing maintains a consistent width of the DNA double helix.

What did Chargaff's rules establish

The amount of adenine approximately equals thymine, and guanine approximately equals cytosine.

If a DNA sample contains 30% adenine, what percentage is thymine

30%, because A pairs with T.

If a DNA sample contains 20% guanine, what percentage is cytosine

20%, because G pairs with C.

If a DNA sample contains 30% adenine, what percentage is guanine

20%; the remaining 40% is divided equally between G and C.

What contribution is associated with Rosalind Franklin's work on DNA

X-ray diffraction evidence that helped reveal DNA's helical structure.

What type of evidence helped scientists determine DNA's structure

X-ray diffraction patterns, base-composition measurements, and chemical knowledge of nucleotides.

What was the significance of Watson and Crick's 1953 model

It explained DNA's double helix, complementary base pairing, and how DNA could be copied.

How does complementary base pairing explain DNA replication

Each parental strand provides a template for building a new complementary strand.