Not every bond in a protein or DNA is in the chain
- The idea
- Predict
- See it
- Why
- Check
The idea
“If two parts of a protein or DNA are bonded, that bond must be part of the chain.”
A disulfide bridge is covalent, just like a peptide bond. So it's easy to count it as one more link in the chain.
The short version
The mistake: If two parts of a protein or DNA are bonded, that bond must be part of the chain.
What's actually true: Backbone bonds join each unit to the next. Cross-links like disulfide bridges and base pairs join parts that aren't neighbors.
- A backbone is a row of covalent bonds joining each monomer to the next. In proteins they're peptide bonds. In DNA and RNA they're phosphodiester bonds.
- A chain grows only when a new backbone bond forms at one end, as when a ribosome adds an amino acid.
- R groups can link parts of a chain that aren't neighbors. Two cysteines can form a covalent disulfide bridge. Others form hydrogen or ionic bonds.
- Only hydrogen bonds between paired bases hold DNA's two strands together. So the strands can separate while each backbone stays whole.
On the exam: Ask whether a bond joins neighbors in the chain. Only backbone bonds make a chain longer.