A bacterium carries a small ring of DNA, called a plasmid, with 4800 base pairs…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A bacterium carries a small ring of DNA, called a plasmid, with base pairs. (a) How long would the plasmid be if it were cut open and stretched straight? Give the answer in nanometres. (b) How many full turns of the helix does it have? (c) How many nucleotides does it hold in all?
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. Length. nm.
Step 2. Turns. One turn holds base pairs, so turns.
Step 3. Nucleotides. Each base pair has one nucleotide on each strand, so nucleotides.
Answer: (a) nm; (b) turns; (c) nucleotides.
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