Four letters, two strands, every living thing

A1.2.1 DNA as the genetic material

Every living thing keeps its instructions in DNA

An organism inherits instructions from its parents: how to build its cells, which proteins to make, when to grow. The molecule that holds those instructions is its genetic material.

In every living organism studied, the genetic material is DNA, short for deoxyribonucleic acid. That is true of bacteria, archaea, protists, fungi, plants and animals. In a cell with a nucleus, most of the DNA is in the nucleus, with a little in the mitochondria and, in plants, the chloroplasts. In a bacterium it lies loose in the cytoplasm.

DNA belongs to the nucleic acids. The other one, RNA, does many jobs in the cell (chapter 6), and a few viruses use it to carry their genes: influenza, measles and the coronavirus that causes COVID-19 are RNA viruses. Other viruses carry DNA. How anyone found out that DNA, and not protein, carries genes is the story of chapter 11.

Both nucleic acids are polymers built from one kind of monomer (chapter 2), and both contain carbon, hydrogen, oxygen, nitrogen and phosphorus.

One more distinction. DNA is the whole molecule. A gene is one stretch of its base sequence, and a single DNA molecule carries many genes.

A1.2.3 A1.2.5 Sugar–phosphate bonding and RNA

Building a strand with a strong backbone

Nucleotides join by condensation, as in the sugars and lipids of B1.1. The phosphate of each new nucleotide bonds to the sugar of the last nucleotide in the chain, and a molecule of water is released. The reverse is hydrolysis: water is added back and the bond breaks, which is how your digestion takes the nucleic acids in food apart.

Repeat the join and you build a strand: sugar, phosphate, sugar, phosphate. Every link is a covalent bond, so the strand is one continuous chain of covalently bonded atoms. That is the sugar–phosphate backbone: strong, and it fixes the order of the nucleotides along it. (Chemists call the link a phosphodiester bond; the IB does not ask for the name.)

The bases are not part of the backbone. Each sticks out sideways from its sugar, all on the same side, free to pair with another base (chapter 4). The backbone is the same all along, whatever the bases are, and any base can follow any other. Chapter 7 builds on that.

RNA is one such strand, a polymer of ribonucleotides made by condensation. DNA strands are built the same way; the enzymes that do it come in D1.1.

To draw an RNA strand, stack the nucleotides, join the lower left corner of each pentagon to the circle of the next, and point every base the same way. To mark one nucleotide, ring a circle, a pentagon and a rectangle together, not a pentagon, a rectangle and the circle below it.

One nucleotide has no order. A strand does. Order is a property of the polymer that its monomers do not have.

A1.2.6 The double helix

Two strands, one helix

DNA is two strands held together by hydrogen bonds between complementary bases: A with T and G with C. The bases are on the inside and the two sugar–phosphate backbones are on the outside. A pair of bases facing each other is a base pair.

The strands are antiparallel: they run in opposite directions, like the two lanes of a road. In a drawing, the second strand's nucleotides are upside down, with the pentagons inverted and the phosphates on the other side. What direction means chemically is chapter 8.

The pair of strands winds into a double helix. In an exam you draw the two strands antiparallel and paired, with A opposite T and G opposite C. You do not have to draw the twist.

One hydrogen bond is weak, but millions of them hold the two strands firmly together. How that squares with copying and reading DNA, which need the strands parted, is the prediction below.

A1.2.9 A1.2.10 Information storage and the genetic code

Endless sequences, one code

Along a strand any base can follow any other: the backbone sets no rule. And a DNA molecule can be any length. So any sequence, of any length, is possible. With four choices at every position the possibilities multiply very fast, as the slider shows. The capacity is, for every practical purpose, limitless.

That information is kept at the scale of molecules. Each human cell with a nucleus holds about 6 billion base pairs, two sets of about 3 billion, which would stretch to about 2 m if laid end to end. They are packed into a nucleus a few micrometres across. Chapter 10 shows how. (Red blood cells have no nucleus, so no DNA.) In one 2017 study, researchers stored computer files in DNA at about 215 petabytes per gram, in the laboratory.

Within a species, DNA sequences are almost the same, with small differences that are the source of different alleles. Between species they differ more. Same molecule, endless messages.

The genetic code is the set of rules by which a sequence of bases in messenger RNA is translated into a sequence of amino acids, three bases at a time. Each group of three is a codon. Almost every organism uses the same code: a codon means the same amino acid in a bacterium, a fungus, a plant and a human, with a few small variations, for example in mitochondria.

The code is arbitrary: no chemical law makes a given codon stand for a given amino acid. If life had begun separately several times, the codes would most likely differ. The best explanation for one shared code is that all life inherited it from a universal common ancestor. A code shared like that is conserved across life, and it is evidence of that shared ancestry. A gene moved from one species into another is read correctly because of it. You do not need to memorise examples: the one in the second figure is only an illustration.

That answers the first guiding question. A chain of simple units can store hereditary information because any order is possible, so the order can carry the message.