Biology 10-2

What piece of information did Franklin and Wilkins have that helped Watson and Crick determine the double helix structure of DNA?

Franklin and Wilkins had X-ray diffraction photographs that were crucial to determining the structure of DNA.

How do the base-pairing rules relate to the structure of DNA?

Hydrogen bonds between complementary base pairs help hold the strands together. In addition, a double-ringed purine on one strand bonds with a single-ringed pyrimidine on the opposite strand, so the chains are always the same distance apart.

purines

Nitrogenous bases that have a double ring of carbon and nitrogen atoms, such as adenine and guanine, are called purines.

pyrimidines

Nitrogenous bases that have a single ring of carbon and nitrogen atoms, such as cytosine and thymine, are called pyrimidines.

deoxyribose

The five-carbon sugar in a DNA nucleotide is called deoxyribose.

nitrogenous base

The nitrogenous base contains nitrogen atoms and carbon atoms and is a base. (accepts hydrogen ions)

base sequence

The order of nitrogenous bases on a chain of DNA is called its base sequence.

Summarize the locations of covalent bonds and hydrogen bonds in a DNA molecule.

The phosphates and the sugars form the "handrails" of the helix with covalent bonds, and the hydrogen bonds are in the "steps" of the DNA helix.

Name the three parts of a nucleotide.

a sugar, a phosphate, and a nitrogenous base

State the base-pairing rules in DNA.

guanine and cytosine pair; thymine and adenine pair

base-pairing rules

This observation was key to understanding the structure of DNA because it meant bases pair by base-pairing rules: in DNA, cytosine on one strand pairs with guanine on the opposite strand, and adenine pairs with thymine.

complementary base pairs

A-T G-C Each complementary base pairs contains one double-ringed purine and one single-ringed pyrimidine.

Describe why the two strands of the double helix are considered to be complementary.

DNA strands are complementary because only A and T pair and only G and C pair. If a DNA chain has the sequence of ATTC then the other chain must have the complementary sequence TAAG.

nucleotide

Each nucleotide consists of three parts: a five-carbon sugar, a phosphate group, and a nitrogenous base.