DNA and RNA are polymers of nucleotides. Each nucleotide is a phosphate, a pentose sugar and a base; join them with phosphodiester bonds and pair two antiparallel strands, and you have a molecule stable enough to carry the instructions for a whole organism.
A nucleotide is a phosphate group, a pentose sugar (deoxyribose in DNA, ribose in RNA) and a nitrogen-containing base. Adenine and guanine are purines with a double ring; cytosine, thymine and uracil are pyrimidines with a single ring. ATP is a phosphorylated nucleotide: adenine, ribose and three phosphates. Nucleotides are joined into a polynucleotide by phosphodiester bonds (condensation reactions) between the phosphate of one and the sugar of the next. DNA is a double helix of two antiparallel strands, one running 5′ to 3′ and the other 3′ to 5′, held together by hydrogen bonds between complementary bases: A with T (two hydrogen bonds) and C with G (three). mRNA is a single, short, unpaired polynucleotide with ribose and uracil in place of deoxyribose and thymine.
Click through the four chips in the simulator: DNA nucleotide, RNA nucleotide, purines and pyrimidines, and ATP. Compare the sugar labels and count the phosphate groups on ATP.
| Feature | DNA nucleotide | RNA nucleotide |
|---|---|---|
| pentose sugar | deoxyribose | ribose |
| bases | adenine, thymine, cytosine, guanine | adenine, uracil, cytosine, guanine |
| phosphate groups | one | one |
Do not say thymine is a purine because it pairs with adenine. Pairing tells you nothing about the class: every pair is one of each. Learn the ring count instead (memory aid: pure As Gold): adenine and guanine are the double-ringed purines; C, T and U are single-ringed pyrimidines.
Click any base pair in the 20-pair helix and read off the bases and the number of hydrogen bonds. Then press Unwind to a ladder and check the 5′ and 3′ labels at the ends of each strand.
| Feature | DNA | mRNA |
|---|---|---|
| number of strands | two, as a double helix | one, single-stranded |
| sugar | deoxyribose | ribose |
| bases | A, T, C, G | A, U, C, G |
| length | very long; many genes | short; a copy of one gene |
| where found | nucleus (and mitochondria, chloroplasts) | made in the nucleus, works in the cytoplasm |
Antiparallel does not just mean the strands are side by side. It means they run in opposite directions: where one strand has its 5′ end, the other has its 3′ end.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the simulator, what is the difference between the DNA nucleotide and the RNA nucleotide shown?
The ATP chip shows adenine, ribose and how many phosphate groups?
You click a C-G pair in the helix simulator. How many hydrogen bonds does the readout show?
After pressing Unwind to a ladder, the left strand reads 5′ at one end and 3′ at the other. What does the right strand show?
RNA uses the same backbone chemistry as DNA, but its molecules are built for a different job: a short-lived copy of one gene that can leave the nucleus.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which list contains only pyrimidines?
A DNA sample contains 18% adenine. What percentage of its bases is guanine?
Which bond joins adjacent nucleotides in the same strand of DNA?
A section of DNA has 60 base pairs, of which 24 are A-T pairs. How many hydrogen bonds hold this section together?
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