A nucleotide is three parts stuck together. Chain them up, pair them off, and you have a molecule that can copy itself: and store more information than any language ever written.
Every nucleotide is a phosphate group, a pentose sugar and a nitrogenous base. Draw them as a circle, a pentagon and a rectangle.
Condensation reactions join the sugar of one nucleotide to the phosphate of the next, making a continuous covalently bonded sugar–phosphate backbone. Strong, and the same all the way along.
DNA is the genetic material of all living organisms. Some viruses use RNA instead: but viruses are not considered to be living.
Two antiparallel strands, three base pairs. Tap a component to isolate it: this is the diagram convention the IB expects you to draw.
Circle = phosphate, pentagon = pentose sugar, rectangle = nitrogenous base. Draw the two strands antiparallel; you are not expected to draw the helical twist.
Three differences, and you should be able to sketch the first one from memory.
Purines (two rings): adenine, guanine · Pyrimidines (one ring): cytosine, thymine, uracil
Tap bases to build a template strand. Because pairing is fixed, the other DNA strand and the mRNA transcript follow automatically: which is exactly why genetic information can be copied at all.
Adenine pairs with thymine, cytosine pairs with guanine, and both pairs are held by hydrogen bonds. Add or remove bases and watch every other row update itself.
A DNA molecule can be any length and carry any sequence of bases. A stretch of just 10 bases already has 410 ≈ a million possible sequences: and it is stored with extraordinary economy.
The same base sequences specify the same amino acids in bacteria, bananas and blue whales. A code this arbitrary being universal is powerful evidence that all life shares a common ancestor.
The carbons of the pentose are numbered. The phosphate sits on carbon 5′ and a free OH on carbon 3′, so each new nucleotide can only be added to the 3′ end. Replication, transcription and translation all run 5′ → 3′.
The two DNA strands run in opposite directions: one 5′ → 3′, the other 3′ → 5′. This is why the two strands of a replication fork are handled differently.
A two-ring purine always pairs with a one-ring pyrimidine, so every base pair is the same length and the helix keeps the same three-dimensional shape whatever the sequence.
DNA is far too long to sit loose in a nucleus. It is wound onto protein spools. Tap each component.
A length of DNA wrapped around a core of eight histone proteins, with one additional histone holding it in place where the DNA enters and leaves. Repeat this along the molecule and you get the "beads on a string" arrangement.
Two classic results. One proved DNA was the genetic material; the other destroyed the leading hypothesis about its structure. Tap through the experiment.
Bacteriophages are made of only two things: protein and DNA. If you can label each one separately and see which enters the bacterium, you have your answer.
Percentages of each base measured in different organisms. Read down the columns before you read across.
%A ≈ %T and %G ≈ %C, every time. Chargaff had no structural explanation for this: but it is exactly what you would expect if A always pairs with T and G always pairs with C.
The A+T to G+C ratio differs from organism to organism. That killed the tetranucleotide hypothesis, which claimed DNA was a monotonous repeat of the four bases in equal amounts: if it were, every row here would read 25, 25, 25, 25.
Nature of science: no amount of data can prove a hypothesis, but a single set of contrary measurements can falsify one
Tap a concept to light up how it connects.
One monomer builds both nucleic acids. The pairing rule that holds the double helix together is the same rule that lets the information be copied: structure and function are the same fact seen twice.
Tap any concept to trace its connections · tap the background to reset
Drag each term into the gap it belongs in. Two terms are traps.
Single best answer, Paper 1 style. HL-only questions are marked.