Q.55 DNA sample collected from an unidentified bacterial species (Y) contains 13% of
adenine. The G+C content (in percentage) of Y is ___________.
DNA from bacterial species Y contains 13% adenine (A)
By Chargaff’s rules, thymine (T) equals A at 13%, making A+T 26%, so G+C is 74%.
Step-by-Step Solution
Chargaff’s rules state that in double-stranded DNA, A pairs with T and G pairs with C, so %A = %T and %G = %C.
Given %A = 13%, then %T = 13%.
Total A+T content = 13% + 13% = 26%.
G+C content = 100% – 26% = 74%.
Common Multiple-Choice Traps
- Mistaking A for total purines (A+G), yielding wrong G+C of 74% still but via error.
- Ignoring base pairing, assuming A=13% implies G=13% (incorrect; yields G+C=26%).
- Forgetting double-strand equivalence, treating as single strand (yields inconsistent totals).
Article Content
In DNA analysis for unidentified bacterial species Y with 13% adenine, determining G+C content relies on Chargaff’s base pairing rules. Adenine percentage directly reveals GC-content through simple subtraction after equating thymine. This calculation appears in competitive exams like GATE Biotechnology.
Key Concepts
- Chargaff’s rule: %A = %T; %G = %C in dsDNA.
- Total bases sum to 100%, so G+C = 100 – (A+T).
- Bacterial genomes vary in GC-content (13-75%), influencing stability.
Practice Tip
For IIT JAM or GATE, memorize: If %A given, G+C = 100 – 2×(%A). Here, 100 – 2(13) = 74.
