recap

13.2 recap

DNA is a double helix made up of two antiparallel polynucleotide chains. The two chains are joined by hydrogen bonds between the nucleotide bases, which pair specifically: A with T, and G with C. Chemical groups on the bases that are exposed in the grooves of the helix are available for hydrogen bonding with other molecules, such as proteins. Various molecules that interact with DNA can recognize specific sequences of nucleotide bases because the surfaces of the base pairs are chemically distinct.

learning outcomes

You should be able to:

  • Explain how Watson and Crick developed their model of DNA.

  • Relate structural aspects of the DNA molecule to its biological functions.

Question 1

Describe the evidence that Watson and Crick used to come up with the double helix model for DNA.

X-ray diffraction indicated that DNA is double-stranded and twisted into a helix. There were indications that the bases were inside the helix, and the sugars and phosphates on the outside. Base composition data from many organisms showed that the percentages of the purine A = those of the pyrimidine T, and that the percentages of the purine G = those of the pyrimidine C. This suggested that A might be opposite T on the inside of the double helix, and G opposite C. When Watson and Crick built molecular models with the atoms and bonds of polynucleotide strands, the base pairing was confirmed, as the A-T and G-C pairs fit nicely together.

Question 2

How does the two-stranded structure of DNA relate to its functions?

The double-stranded structure is essential in the replication of DNA, as the opposite strands can each act as a template for a new strand, so that two new identical strands are made. This is key in the replication of the genetic material when cells divide. The two strands can unravel at places, exposing the bases in the inside for gene expression. This is important because genes must be expressed for the phenotype.

Question 3

A key to the function of DNA in directing information in the cell is the binding of other molecules such as proteins to the double helix at specific base sequences. Recalling the forces involved in protein interactions (see Figure 3.12) and the chemical groupings exposed in base pairs (see Figure 13.7), how do specific interactions between proteins and DNA occur?

The bases in DNA expose chemical groupings that can interact with groups on proteins. These include polar groups (e.g., C=O) on the bases that can attract oppositely polar groups (e.g., NH2) on proteins, as well as form hydrogen bonds with groups on proteins.

Once the structure of DNA was understood, it was possible to investigate how DNA replicates itself. Next we will examine the experiments that taught us how this elegant process works.