Cracking the AP Biology Exam


Molecular Genetics


The DNA molecule consists of two strands that wrap around each other to form a long, twisted ladder called a double helix. The structure of DNA was brilliantly deduced in 1956 by two scientists named Watson and Crick.


DNA is made up of repeated subunits of nucleotides. Each nucleotide has a five-carbon sugar, a phosphate, and a nitrogenous base. Take a look at the nucleotide below. This particular nucleotide contains a nitrogenous base called adenine:

The name of the pentagon-shaped sugar in DNA is deoxyribose. Hence, the name deoxyribonucleic acid. Notice that the sugar is linked to two things: a phosphate and a nitrogenous base. A nucleotide can have one of four different nitrogenous bases:

  • Adenine—a purine (double-ringed nitrogenous base)
  • Guanine—a purine (double-ringed nitrogenous base)
  • Cytosine—a pyrimidine (single-ringed nitrogenous base)
  • Thymine—a pyrimidine (single-ringed nitrogenous base)

Any of these four bases can attach to the sugar. As we’ll soon see, this is extremely important when it comes to the “sense” of the genetic code in DNA.

The nucleotides can link up in a long chain to form a single strand of DNA. Here’s a small section of a DNA strand:

The nucleotides themselves are linked together by phosphodiester bonds.


Now let’s look at the way two DNA strands get together. Again, think of DNA as a ladder. The sides of the ladder consist of alternating sugar and phosphate groups, while the rungs of the ladder consist of a pair of nitrogenous bases:

The nitrogenous bases pair up in a particular way. Adenine in one strand always binds to thymine (A–T or T–A) in the other strand. Similarly, guanine always binds to cytosine (G–C or C–G). This predictable matching of the bases is known as base pairing.

The two strands are said to be complementary. This means that if you know the sequence of bases in one strand, you’ll know the sequence of bases in the other strand. For example, if the base sequence in one DNA strand is A–T–C, the base sequence in the complementary strand will be T–A–G.

The two DNA strands run in opposite directions. You’ll notice from the figure above that each DNA strand has a 5′ end and a 3′ end, so-called for the carbon that ends the strand (i.e., the fifth carbon in the sugar ring is at the 5′ end, while the third carbon is at the 3′ end). The 5′ end has a phosphate group and the 3′ end has an OH, or “hydroxyl,” group. The 5′ end of one strand is always opposite to the 3′ end of the other strand. The strands are therefore said to be antiparallel.

The DNA strands are linked by hydrogen bonds. Two hydrogen bonds hold adenine and thymine together and three hydrogen bonds hold cytosine and guanine together.

Before we go any further, let’s review the base pairing in DNA.

  • Adenine pairs up with thymine (A–T or T–A) by forming two hydrogen bonds.
  • Cytosine pairs up with guanine (G–C or C–G) by forming three hydrogen bonds.