The DNA double helix looks like a twisted staircase with the sugar and phosphate backbone (banisters) surrounding complementary nitrogen bases (stairs). 

DNA usually exists in a form described by the Watson-Crick model. In this structure, DNA is made up of two strands that are twisted around each other to form a right-handed helix, called a double helix. The strands lie side by side in antiparallel 3′ → 5′ directions and are bound together by the hydrogen bonds between nitrogenous bases, forming a double-stranded structure. This base-pairing takes place between a purine and pyrimidine: adenosine (A) with thymine (T), and guanine (G) with cytosine (C). When two strands are complementary, they curl into a double helix with a major groove and minor groove. Each groove spirals around the double helix every ten base pairs.

Practice Questions

 

Khan Academy

 

MCAT Official Prep (AAMC)

Section Bank C/P Section Question 80

Section Bank B/B Section Passage 3 Question 18

Key Points

• Watson-Crick model suggests that DNA has a helical structure with the sugar and phosphate backbone outside, nitrogen bases inside. 

• Adenine always pairs with thymine and guanine with cytosine in the opposite strand of DNA. This is called base pairing.

• Hydrogen bonds between strands lock them together. 

• Due to the base pairing, the DNA strands are complementary to each other, run in opposite directions, and are called antiparallel strands.

Key Terms

Watson-Crick Model: A model of DNA structure in which the molecule is a cross-linked double-stranded helix, each strand is composed of alternating links of phosphate and deoxyribose, and the strands are cross-linked by pairs of purine and pyrimidine bases projecting inward from the deoxyribose sugars and joined by hydrogen bonds.

Nitrogenous bases: Organic molecules, which are part of the nucleotides in DNA, showing base-like chemical properties.

Genetic material: A material that carries hereditary information to build the organism in each generation.

Sugar-phosphate backbone: The outer support of the ladder, forming strong covalent bonds between monomers of DNA.

Base pairing: The specific way in which bases of DNA line up and bond to one another; A always with T and G always with C.

Double helix: The structure of DNA which looks like a twisted staircase.

Antiparallel: A term applied to two molecules that are side by side but run in opposite directions.