recap

4.1 recap

The nucleic acids DNA and RNA are polymers made up of nucleotide monomers. The sequence of nucleotides in DNA carries the information that is used by RNA to specify primary protein structure. The genetic information in DNA is passed from generation to generation and can be used to understand evolutionary relationships.

learning outcomes

You should be able to:

  • Explain how purines and pyrimidines differ.

  • Explain how biological information is encoded in DNA.

  • Identify cellular functions of nucleotides other than information processing.

Question 1

What are the key differences between purines and pyrimidines and how does this relate to the structure of DNA?

Purines contain two nitrogen–carbon rings, while pyrimidines have one ring. The double helix of DNA has uniform width because a purine on one strand is always opposite a pyrimidine on the other strand.

Question 2

How can DNA molecules be very diverse, even though they appear to be structurally similar?

While DNA molecules are similar in diameter and configuration, their base sequences are different. Differences in base sequence provide the informational content of DNA.

Question 3

Single-stranded nucleic acids about 25 bases long are called aptamers. Because many sequences are possible, particular aptamers can be used to bind to specific targets, either as drugs to a protein target or for quantitating a small molecule in tissues or fluids. How can aptamers have such diversity and specificity for binding?

The number of possible 25-unit sequences of four nucleotides is 425, a large number indeed. Because of internal base pairing of the single strand (as in RNA), many folded configurations are possible, which allows specific binding to target molecules.

We have seen that the nucleic acids RNA and DNA carry the blueprint of life, and that the inheritance of these macromolecules reaches back to the beginning of evolutionary time. But when, where, and how did nucleic acids arise on Earth? How did the building blocks of life, such as amino acids and sugars, originally arise?