A scientist analyses double-stranded DNA from a bacterium…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A scientist analyses double-stranded DNA from a bacterium. Guanine makes up per cent of its bases. The piece has base pairs. (a) What per cent of the bases is cytosine? (b) What per cent is adenine? (c) How many of the base pairs are guanine-cytosine pairs? (d) How many hydrogen bonds hold the two strands together?
Need a reminder? The key ideaShow
Nucleic acids are polynucleotides. A nucleotide is a nitrogenous base, a pentose sugar and a phosphate group; a base plus a sugar is a nucleoside. Purines: adenine and guanine. Pyrimidines: cytosine, thymine and uracil. DNA has deoxyribose and thymine; RNA has ribose and uracil. Double helix of Watson and Crick: antiparallel strands, A with T by two hydrogen bonds and G with C by three, a turn of 3.4 nm with 10 base pairs, so 0.34 nm between base pairs. Chargaff: A equals T and G equals C.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Cytosine. Cytosine equals guanine, so it is per cent.
Step 2. Adenine. Guanine and cytosine take per cent. The rest is per cent, shared equally by adenine and thymine. So adenine is per cent.
Step 3. Guanine-cytosine pairs. per cent of is pairs.
Step 4. Hydrogen bonds. The adenine-thymine pairs number . The bonds are .
Answer: (a) per cent; (b) per cent; (c) pairs; (d) hydrogen bonds.
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