The DNA Double Helix

MCAT Biochemistry - Chapter 4 - Section 1.2 - Nucleotides and Nucleic Acids - The DNA Double Helix

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Sample MCAT Question - The DNA Double Helix

The base pair between adenine and thymine involve how many hydrogen bonds?

a) 2

b) 3

c) 4

d) 1

Click to Reveal the Answer

A is correct. 2 hydrogen bonds.

Watson-Crick base pairing refers to the interaction between the nitrogenous bases of DNA. The base pairs between adenine and thymine and adenine and uracil involve only two hydrogen bonds. However, the base pair between cytosine and guanine involve three hydrogen bonds. Due to the higher number of bonds, a cytosine and guanine base pair is more durable than either an adenine and thymine or adenine and uracil base pair.

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Watson and Crick

In April of 1953, Francis Crick and James D. Watson published an article in the journal Nature entitled, “Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid.” In this article, they detailed their discovery of the structure of DNA. The Watson-Crick model of DNA, as it has since been called, helps to explain how DNA functions and illustrates the fundamental interactive forces that hold DNA molecules together.

Watson-Crick Base Pairing

Both DNA and RNA are made from nucleotides, which include phosphates, five-carbon sugars, and nitrogenous bases. The interactions between the nitrogenous bases are termed Watson-Crick base pairing. Figure 1 shows the possible combinations of base pairing: adenine with thymine, adenine with uracil, and cytosine with guanine.

It is important to note that the adenine and thymine/uracil base pairs involve only two hydrogen bonds, whereas the cytosine and guanine base pair involves three hydrogen bonds. This difference in hydrogen bonds means that the cytosine and guanine base pair is held together more strongly than either the adenine and thymine or adenine and uracil base pairs. In this way, it would take more energy to break the cytosine and guanine base pair bonds compared to the adenine and thymine or adenine and uracil base pair bonds.

DNA as an Antiparallel Double Helix

The model for the structure of DNA that Francis Crick and James D. Watson developed consists of a double helix formed between two antiparallel, single-stranded DNA molecules (Figure 2).

The so-named “double-helix” refers to the fact that in this model, there are two single-stranded molecules that are bound in a helical shape. Also, it is important to note that the two strands of the double helix molecule are antiparallel. This means that strand runs in the 5′ to 3′ direction, whereas the complementary strand runs from the 3′ to 5′ direction. In other words, the strands go in opposite directions. Furthermore, on the side of the double helix molecule is what is termed the sugar-phosphate backbone, referring to alternating molecules of sugar and phosphate on the outside or “backbone” of the molecule.

Base Pair Interactions

The Watson-Crick DNA model displays how the nitrogenous bases of both DNA strands interact with each other at the center of the double helix. This interaction is favorable because the two strands are complementary to each other based on Watson-Crick base pairing. Where adenine is present on one strand, thymine is on the other strand. Similarly, where guanine is on one of the single-stranded DNA molecules, cytosine is on the other. The strands are held together by the hydrogen bonds between the base pairs.

Also, aside from hydrogen bonds, the two strands of DNA are held together by what is termed base stacking. Base stacking refers to the fact that the nitrogenous bases are stacked on top of each other. Since the nitrogenous bases are aromatic, they can form favorable interactions with the bases stacked above and below them.

Mechanism of DNA Replication

The Watson-Crick model of DNA is important because it implies a simple mechanism for DNA replication. DNA is genetic material, and in order to pass on genetic material to offspring, there has to be some way to replicate it. The double-helix model consists of two single-stranded, complementary DNA molecules. In this way, it would be possible to separate the two strands and use them as templates to create copies of DNA. Doing so would produce two identical copies of DNA, which is what happens during DNA replication.