The fourth kind of large molecule holds the instructions for the other three. Nucleic acids are polymers too, built by the same dehydration synthesis from Monomers and Polymers.
Key idea
DNA and RNA are chains of nucleotides. Each nucleotide has a sugar, a phosphate and a nitrogen base. DNA stores genetic information; RNA helps use it to build proteins.
| DNA | RNA | |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C (uracil instead of thymine) |
| Strands | Usually two, wound into a double helix | Usually one |
| Job | Stores genes long term | Carries and reads the instructions (mRNA, tRNA, rRNA) |
Why it works
In the chain, the sugar of one nucleotide bonds to the phosphate of the next, making a sugar-phosphate backbone. The bases stick out to the side. In DNA, the bases of the two strands pair by hydrogen bonds: A with T, G with C. The information is in the order of the bases, the way the meaning of a word is in the order of its letters. You will see this in detail in The Structure of DNA.
Worked example: Spot the molecule
A nucleic acid has ribose and contains the base U. Is it DNA or RNA?
- Ribose is RNA's sugar (DNA has deoxyribose).
- Uracil (U) is found in RNA, not DNA.
- Both clues say RNA.
Answer: RNA
Worked example: Count the parts
A piece of double-stranded DNA is 40 base pairs long. How many nucleotides does it have, and how many phosphate groups?
- Each base pair is two nucleotides, one on each strand: $40 \times 2 = 80$ nucleotides.
- Every nucleotide has one phosphate: 80 phosphates.
Answer: $80$ nucleotides and $80$ phosphates
Common mistake
Thinking DNA and RNA are made of entirely different building blocks. Both are chains of nucleotides with the same plan (sugar, phosphate, base) and three of the same four bases. They differ in three ways only: the sugar (deoxyribose or ribose), one base (thymine or uracil), and usually the number of strands (two or one).