---
title: "DNA: A Universal Genetic Molecule"
book: "High School Biology"
subject: biology
language: en
chapter: 3
exercises: 15
source: https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule
---

# Chapter 3 — DNA: A Universal Genetic Molecule

In a darkened laboratory, under a blue lamp, one of two white mice glows green from nose to tail. It carries, in every one of its [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), a single [gene](#def-g10-universal-dna-gene) taken from a jellyfish that lights up in the Pacific — and the mouse’s [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) read that jellyfish [gene](#def-g10-universal-dna-gene) exactly as the jellyfish’s [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) do. The experiment works because every living thing writes its [genes](#def-g10-universal-dna-gene) in the same molecule, with the same four letters, and reads them by the same rules. This chapter is about that molecule, [DNA](#def-g10-universal-dna-information): how it is built, what its structure explains, and what the glowing mouse proves.

## 3.1 The genetic material

**Definition 3.1 (Genetic information).**

The *genetic information* of a [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) is the set of instructions, transmitted from a [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) to its daughter [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) and from parents to offspring, that determines the traits of the organism and the [proteins](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) its [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) can make. In every living thing it is carried by molecules of *DNA* (deoxyribonucleic acid), packed with [proteins](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) into the *chromosomes* — in the [nucleus](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) of a [eukaryotic cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-prokaryote), free in the [cytoplasm](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) of a bacterium.

**Proposition 3.2 (DNA is the molecule of heredity).**

The hereditary material of a [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) is its [DNA](#def-g10-universal-dna-information), not its [proteins](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) or any other constituent: transferring purified [DNA](#def-g10-universal-dna-information) from one strain of bacteria to another transfers the first strain’s traits.

**Evidence.** Griffith (1928) found that harmless pneumonia bacteria, mixed with killed [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) of a deadly strain, became deadly and stayed so through generations: something from the dead [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) had "transformed" them. Avery, MacLeod and McCarty (1944) separated the extract of the dead [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) into its families of molecules and tested each one: only the [DNA](#def-g10-universal-dna-information) fraction transformed; treating the extract with an enzyme that destroys [DNA](#def-g10-universal-dna-information) abolished the effect, while enzymes destroying [proteins](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) or RNA did not. The transforming substance, and hence the carrier of the heritable trait, was [DNA](#def-g10-universal-dna-information). ∎

**Example 3.3 (Where the DNA is).**

A dye specific for [DNA](#def-g10-universal-dna-information) stains the [nucleus](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) of a cheek [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) and nothing else in the [cytoplasm](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell); in a dividing [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) it stains the compact [chromosomes](#def-g10-universal-dna-information). A bacterium takes up the dye in a central region without an envelope. In both, the amount of [DNA](#def-g10-universal-dna-information) doubles before each division and is halved between the two daughter [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) — the behaviour expected of information that must be copied and shared.

## 3.2 The structure of DNA

**Definition 3.4 (Nucleotide).**

[DNA](#def-g10-universal-dna-information) is a chain of *nucleotides*. Each nucleotide is made of three parts: a phosphate group, a sugar (deoxyribose) and one of four nitrogen-containing *bases*: adenine (A), thymine (T), guanine (G) and cytosine (C). Sugars and phosphates alternate to form the backbone of the chain; the bases stick out from it, one per nucleotide. The four nucleotides differ only by their base, so a strand of [DNA](#def-g10-universal-dna-information) is described by the sequence of its bases, written as a string of letters: `ATGGCTTAC…`

![The two strands of DNA, drawn flat. Each strand is a chain of nucleotides (phosphate, sugar, base). The strands run in opposite directions and are held together by base pairs: A always faces T, G always faces C. One base pair follows the next every 0.34\, nm.](https://one-course.com/images/onecourse/chapters/biology-2/g10-universal-dna/fig-9efc63ec251a.svg)

*The two strands of [DNA](#def-g10-universal-dna-information), drawn flat. Each strand is a chain of [nucleotides](#def-g10-universal-dna-nucleotide) (phosphate, sugar, base). The strands run in opposite directions and are held together by base pairs: A always faces T, G always faces C. One base pair follows the next every $0.34\,\mathrm{nm}$.*

**Proposition 3.5 (The double helix).**

A [DNA](#def-g10-universal-dna-information) molecule is made of two strands wound around each other in a right-handed *double helix* about $2\,\mathrm{nm}$ wide, with one turn every ten base pairs ($3.4\,\mathrm{nm}$). The strands run in opposite directions and face each other by their bases, which pair according to a strict rule of *complementarity*: A pairs only with T, G only with C. The sequence of one strand therefore fixes the sequence of the other.

**Evidence.** Chargaff (1950) measured the base composition of [DNA](#def-g10-universal-dna-information) from many species and found, in every one, as much A as T and as much G as C, while the proportion of A+T to G+C varied from species to species — a pairing rule, not a chemical accident. Franklin’s X-ray photographs of [DNA](#def-g10-universal-dna-information) fibres (1952) showed the signature of a helix of $2\,\mathrm{nm}$ diameter and $3.4\,\mathrm{nm}$ pitch. Watson and Crick (1953) built the only model that fits both: two backbones outside, paired bases inside, A–T and G–C pairs having the same width so that the helix is regular. ∎

**Example 3.6 (Chargaff’s rule in numbers).**

Human [DNA](#def-g10-universal-dna-information): A 30.9%, T 29.4%, G 19.9%, C 19.8% — A$\,\approx\,$T and G$\,\approx\,$C to within the precision of the measurement, and A+T $= 60\%$. *E. coli*: A 24.7%, T 23.6%, G 26.0%, C 25.7%, A+T $= 48\%$. Yeast: A+T $= 64\%$. If a [DNA](#def-g10-universal-dna-information) sample contains 32% of A, [complementarity](#prop-g10-universal-dna-helix) predicts 32% of T and $(100 - 64)/2 = 18\%$ each of G and C.

![Base composition of DNA in four species. In each, the A and T bars match and the G and C bars match — the signature of base pairing — while the A+T share differs from one species to another.](https://one-course.com/images/onecourse/chapters/biology-2/g10-universal-dna/fig-80649903953c.svg)

*Base composition of [DNA](#def-g10-universal-dna-information) in four species. In each, the A and T bars match and the G and C bars match — the signature of base pairing — while the A+T share differs from one species to another.*

**Method 3.7 (Writing the complementary strand).**

Given one strand, write under each base its partner (A$\leftrightarrow$T, G$\leftrightarrow$C), then read the result in the opposite direction if the convention of the exercise requires it. For `5’-ATGGCTTAC-3’` the partner strand, read in its own direction, is `5’-GTAAGCCAT-3’`. Counting: a strand with $n_\mathrm{A}$ adenines faces a strand with $n_\mathrm{A}$ thymines, so in the whole molecule $n_\mathrm{A} = n_\mathrm{T}$ and $n_\mathrm{G} = n_\mathrm{C}$.

**Proposition 3.8 (What the structure explains).**

Two properties of the genetic material follow from the structure.

- *Information* : the order of the bases along a strand is not constrained by the chemistry — any sequence is possible — so the sequence can carry a message, as the order of letters carries a text. A human [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) holds $3.2 \times 10^{9}$ base pairs in each set of [chromosomes](#def-g10-universal-dna-information) .
- *Copying* : since each strand determines the other, the molecule can be duplicated by separating the strands and building a new partner on each. The mechanism is the subject of [Chapter 11](https://one-course.com/books/biology/2/en/chapter/11-dna-replication#ch-g11-dna-replication) .

**Proof.** *Admitted at this level.* ∎

## 3.3 Genes, alleles, chromosomes

**Definition 3.9 (Gene and allele).**

A *gene* is a segment of a [DNA](#def-g10-universal-dna-information) molecule — typically some thousands of base pairs — whose sequence carries the instructions for making one [protein](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) (or, sometimes, one working molecule of RNA). It occupies a fixed position on a given [chromosome](#def-g10-universal-dna-information). The versions of a gene that differ by a few bases are its *alleles*: same position, same function, slightly different sequence, hence often a slightly different [protein](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families).

**Example 3.10 (Numbers).**

The human genome — one complete set of [chromosomes](#def-g10-universal-dna-information) — carries $3.2 \times 10^{9}$ base pairs and about $20\,000$ [genes](#def-g10-universal-dna-gene); [genes](#def-g10-universal-dna-gene) occupy only a few per cent of the sequence. A [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) of *E. coli* carries one circular [DNA](#def-g10-universal-dna-information) molecule of $4.6 \times 10^{6}$ base pairs and about 4300 [genes](#def-g10-universal-dna-gene). Two unrelated humans differ at roughly one base in a thousand: some three million positions, most of them outside [genes](#def-g10-universal-dna-gene) and of no consequence, a few of them [alleles](#def-g10-universal-dna-gene) that change a trait.

**Proposition 3.11 (Chromosomes).**

A [chromosome](#def-g10-universal-dna-information) is one [DNA](#def-g10-universal-dna-information) molecule, wound around packing [proteins](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) and coiled on itself. Stretched out, the [DNA](#def-g10-universal-dna-information) of one human [chromosome](#def-g10-universal-dna-information) would be from $1.5\,\mathrm{cm}$ to $8.5\,\mathrm{cm}$ long; the 46 [chromosomes](#def-g10-universal-dna-information) of a [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) together hold about $2\,\mathrm{m}$ of [DNA](#def-g10-universal-dna-information) in a [nucleus](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) $6\,\text{µ}\mathrm{m}$ across. [Chromosomes](#def-g10-universal-dna-information) are visible as separate bodies only during [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) division, when they are coiled most tightly; between divisions they are unwound and fill the [nucleus](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) as a tangle.

**Proof.** *Admitted at this level.* ∎

## 3.4 One molecule for all living things

**Proposition 3.12 (Universality of DNA).**

All living things — bacteria, fungi, plants, animals — carry their [genetic information](#def-g10-universal-dna-information) as [DNA](#def-g10-universal-dna-information) built from the same four [nucleotides](#def-g10-universal-dna-nucleotide), and read a [gene](#def-g10-universal-dna-gene)’s sequence by the same rules. Consequently a [gene](#def-g10-universal-dna-gene) taken from one species and inserted into the [DNA](#def-g10-universal-dna-information) of another is read by the recipient’s [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), which make the [protein](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) it codes for: *transgenesis* is possible, and the transgenic organism passes the foreign [gene](#def-g10-universal-dna-gene) to its descendants like any other.

**Evidence.** The [gene](#def-g10-universal-dna-gene) for the green fluorescent [protein](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) (GFP) of the jellyfish *Aequorea victoria* has been inserted into bacteria, yeast, plants, fish, mice and rabbits: every one of them makes the [protein](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) and glows green under blue light, and the trait is inherited. The human [gene](#def-g10-universal-dna-gene) for insulin, inserted into *E. coli* in 1978, makes the bacterium produce human insulin, which has been the source of the hormone for diabetics ever since. No case is known of a species whose [genes](#def-g10-universal-dna-gene) are written in another chemistry. ∎

![Transgenesis: a gene isolated from one species is inserted into the DNA of a fertilised egg of another. Every cell of the resulting organism carries it, reads it, and passes it on.](https://one-course.com/images/onecourse/chapters/biology-2/g10-universal-dna/fig-cb825c854083.svg)

*[Transgenesis](#prop-g10-universal-dna-universal): a [gene](#def-g10-universal-dna-gene) isolated from one species is inserted into the [DNA](#def-g10-universal-dna-information) of a fertilised egg of another. Every [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) of the resulting organism carries it, reads it, and passes it on.*

![Under blue light, a mouse carrying the jellyfish gene for the green fluorescent protein glows beside an ordinary mouse. Same molecule, same reading rules: the jellyfish instruction runs in the mouse.](https://one-course.com/images/onecourse/chapters/biology-2/g10-universal-dna/img-5719870411a3.jpg)

*Under blue light, a mouse carrying the jellyfish [gene](#def-g10-universal-dna-gene) for the green fluorescent [protein](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) glows beside an ordinary mouse. Same molecule, same reading rules: the jellyfish instruction runs in the mouse.*

**Example 3.13 (Uses of transgenesis).**

Human insulin and growth hormone from bacteria; a [gene](#def-g10-universal-dna-gene) for resistance to an insect pest inserted into maize or cotton; rice carrying [genes](#def-g10-universal-dna-gene) for making a precursor of vitamin A; the GFP [gene](#def-g10-universal-dna-gene) attached to another [gene](#def-g10-universal-dna-gene) so that the [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) where that [gene](#def-g10-universal-dna-gene) is active light up under the microscope — the last use, a research tool, is the commonest of all. Each use raises its own questions of safety and of consequences for the environment; each rests on the same fact, the universality of the genetic molecule.

**Remark 3.14 (Unity and diversity, at the molecular level).**

The universality of [DNA](#def-g10-universal-dna-information) is the molecular face of the unity of life met in [Chapter 1](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#ch-g10-chemistry-of-life): one chemistry, one information molecule, one reading system for every organism on Earth, which is what a common origin predicts ([Chapter 6](https://one-course.com/books/biology/2/en/chapter/6-body-plans-and-common-ancestry#ch-g10-common-ancestry)). The diversity of life is written in the same molecule too, as differences of sequence: between two [alleles](#def-g10-universal-dna-gene) a few bases, between two species a few per cent of the genome, between a bacterium and a human the whole organisation of the text.

## 3.5 Exercises

**Exercise 3.1 ★.**

Name the three parts of a [nucleotide](#def-g10-universal-dna-nucleotide) and the four bases of [DNA](#def-g10-universal-dna-information).

**Solution of Exercise 3.1.**

A phosphate group, a sugar (deoxyribose) and a base. Bases: adenine, thymine, guanine, cytosine.

**Exercise 3.2 ★.**

Write the strand complementary to `TACGGATTCA`.

**Solution of Exercise 3.2.**

`ATGCCTAAGT`, written base under base.

**Exercise 3.3 ★.**

A [DNA](#def-g10-universal-dna-information) sample contains 21% guanine. Give the percentages of C, A and T.

**Solution of Exercise 3.3.**

C $= 21\%$; A $=$ T $= (100 - 42)/2 = 29\%$ each.

**Exercise 3.4 ★.**

Define [gene](#def-g10-universal-dna-gene) and [allele](#def-g10-universal-dna-gene) in one sentence each, using the word "sequence".

**Solution of Exercise 3.4.**

A [gene](#def-g10-universal-dna-gene) is a segment of [DNA](#def-g10-universal-dna-information) whose sequence carries the instructions for one [protein](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families), at a fixed position on a [chromosome](#def-g10-universal-dna-information). An [allele](#def-g10-universal-dna-gene) is one of the versions of a [gene](#def-g10-universal-dna-gene), differing from the others by a few bases of its sequence.

**Exercise 3.5 ★.**

What is a transgenic organism? Give two examples from the chapter.

**Solution of Exercise 3.5.**

An organism into whose [DNA](#def-g10-universal-dna-information) a [gene](#def-g10-universal-dna-gene) from another species has been inserted, and which reads it and transmits it. Examples: mice carrying the jellyfish GFP [gene](#def-g10-universal-dna-gene); *E. coli* carrying the human insulin [gene](#def-g10-universal-dna-gene).

**Exercise 3.6 ★★.**

A [gene](#def-g10-universal-dna-gene) is 1500 base pairs long. What is its length in nanometres, and how many turns of the helix does it make?

**Solution of Exercise 3.6.**

$1500 \times 0.34 = 510\,\mathrm{nm}$; $1500/10 = 150$ turns.

**Exercise 3.7 ★★.**

Compute the total length of the [DNA](#def-g10-universal-dna-information) in one human [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), given $3.2 \times 10^{9}$ base pairs per set of [chromosomes](#def-g10-universal-dna-information) and two sets per [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell). Compare with the diameter of the [nucleus](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle), $6\,\text{µ}\mathrm{m}$.

**Solution of Exercise 3.7.**

$2 \times 3.2 \times 10^{9} \times 0.34\,\mathrm{nm} \approx 2.2\,\mathrm{m}$, about $3.6 \times 10^{5}$ times the diameter of the [nucleus](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle).

**Exercise 3.8 ★★.**

In Avery’s experiment, why was it necessary to test the extract treated with a DNA-destroying enzyme, and not only the purified [DNA](#def-g10-universal-dna-information) fraction?

**Solution of Exercise 3.8.**

A "purified" fraction might still contain traces of other molecules, so its activity alone does not prove that [DNA](#def-g10-universal-dna-information) is responsible. Destroying [DNA](#def-g10-universal-dna-information) specifically, in the complete extract, and seeing the activity vanish while protein- and RNA-destroying enzymes leave it intact, pins the effect on [DNA](#def-g10-universal-dna-information) and on nothing else.

**Exercise 3.9 ★★.**

Explain why Chargaff’s data ruled out a model in which the bases were stacked in a regular repeating order such as `ATGCATGC…`

**Solution of Exercise 3.9.**

A fixed repeat would give the same composition (25% of each base) in every species. Chargaff found A+T ranging widely between species: the order of bases is not a fixed pattern, so the sequence is free to vary — which is what makes it capable of carrying information.

**Exercise 3.10 ★★.**

Two [alleles](#def-g10-universal-dna-gene) of a [gene](#def-g10-universal-dna-gene) differ by a single base among 2000. Explain how such a small difference can change a [protein](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families), and why it may also change nothing.

**Solution of Exercise 3.10.**

The sequence is read in order to build the [protein](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families); a changed base at a position that specifies an [amino acid](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) can substitute one [amino acid](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) for another and alter the [protein](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families)’s shape or activity. If the changed base lies in a part of the [gene](#def-g10-universal-dna-gene) that is not read, or specifies the same [amino acid](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families), the [protein](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) is unchanged.

**Exercise 3.11 ★★.**

The GFP [gene](#def-g10-universal-dna-gene) inserted into a mouse’s fertilised egg is found in its skin [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell), its liver [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) and its sperm [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell). Explain why, using the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) theory and [Proposition 3.8](#prop-g10-universal-dna-explains).

**Solution of Exercise 3.11.**

All the [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) of the mouse descend by division from the fertilised egg; at each division the [DNA](#def-g10-universal-dna-information), transgene included, is copied ([complementarity](#prop-g10-universal-dna-helix)) and shared. Skin, liver and sperm [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) therefore all carry the [gene](#def-g10-universal-dna-gene) — and the sperm passes it to the next generation.

**Exercise 3.12 ★★★.**

A bacterium’s [DNA](#def-g10-universal-dna-information) molecule is $4.6 \times 10^{6}$ base pairs long. Compute its length; compare with the $2\,\text{µ}\mathrm{m}$ of the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell); then explain why a bacterium can nevertheless have room for it.

**Solution of Exercise 3.12.**

$4.6 \times 10^{6} \times 0.34\,\mathrm{nm} \approx 1.6\,\mathrm{mm}$, some 800 times the length of the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell). But the molecule is only $2\,\mathrm{nm}$ wide: its volume, about $\pi \times 1^2 \times 1.6 \times 10^{6}\,
\mathrm{nm}^{3} \approx 5 \times 10^{-3}\,\text{µ}\mathrm{m}^{3}$, is about 1% of the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell)’s; folded and coiled, it fits with room to spare.

**Exercise 3.13 ★★★.**

A friend claims that human [DNA](#def-g10-universal-dna-information) must be "more complex" than a bacterium’s because it has more bases. Compare $3.2 \times 10^{9}$ and $4.6 \times 10^{6}$ base pairs with $20\,000$ and 4300 [genes](#def-g10-universal-dna-gene), and discuss what the extra [DNA](#def-g10-universal-dna-information) is and is not.

**Solution of Exercise 3.13.**

Human [DNA](#def-g10-universal-dna-information) is 700 times longer but carries only about 5 times more [genes](#def-g10-universal-dna-gene): most of it is not [genes](#def-g10-universal-dna-gene). Part of the extra [DNA](#def-g10-universal-dna-information) regulates when [genes](#def-g10-universal-dna-gene) are used, part is repeated sequence with no known function. Base count measures the size of the text, not the complexity of the organism; the number of [genes](#def-g10-universal-dna-gene) and, above all, how they are combined and regulated matter more.

**Exercise 3.14 ★★★.**

Suppose a species were discovered whose genetic material used a different molecule and a different code. Which of the chapter’s propositions would it contradict, and what would it imply for the common origin of that species and the rest of life?

**Solution of Exercise 3.14.**

It would contradict the universality of [DNA](#def-g10-universal-dna-information) ([Proposition 3.12](#prop-g10-universal-dna-universal)). Since a shared molecule and code are inherited from a common ancestor, such a species would have to descend from a separate origin of life: its [genes](#def-g10-universal-dna-gene) could not be read by our [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) and [transgenesis](#prop-g10-universal-dna-universal) between the two lines would be impossible.

**Exercise 3.15 ★★★.**

Human insulin produced by bacteria replaced insulin extracted from pig pancreases. List two biological reasons why the bacterial product was an improvement, and one question a regulator would still have to ask.

**Solution of Exercise 3.15.**

The bacterial product is the exact human [protein](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) (pig insulin differs by one [amino acid](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) and can provoke immune reactions), and it is made in unlimited quantity, free of pig pathogens. A regulator would still ask whether the purified [protein](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) is free of bacterial contaminants and whether it is correctly folded and active.

## 3.6 Problem: The Jellyfish Gene

**Problem 3.1.**

Weekend problem — one gene from a jellyfish followed into a mouse: its length, its letters, why the mouse can read it, and the two metres of DNA that carry it in every cell

The green fluorescent [protein](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) of the jellyfish *Aequorea victoria* is a chain of 238 [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families). Its [gene](#def-g10-universal-dna-gene), as used in the laboratory, is a [DNA](#def-g10-universal-dna-information) segment of 720 base pairs. Take $0.34\,\mathrm{nm}$ per base pair, ten base pairs per turn, and a mouse genome of $2.7 \times 10^{9}$ base pairs per set of [chromosomes](#def-g10-universal-dna-information), two sets per [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell).

**Part I — The molecule.**

1. Compute the length of the GFP [gene](#def-g10-universal-dna-gene) in nanometres, and the number of turns of helix it makes.
2. One strand of the [gene](#def-g10-universal-dna-gene) begins `ATGAGTAAAGGAGAAGAAC` . Write the complementary strand.
3. In the whole [gene](#def-g10-universal-dna-gene) , adenine represents 27% of the bases. Give the percentages of the other three bases.
4. The [gene](#def-g10-universal-dna-gene) has 720 base pairs and codes for 238 [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) . Compute the ratio, and propose what it suggests about how many bases specify one [amino acid](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) (a hypothesis that [Chapter 14](https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein#ch-g11-gene-expression) will confirm).
5. How many different sequences of 720 bases are possible in principle? Write the answer as a power of 4, then explain in one sentence why the sequence can be called information.

**Part II — Why the mouse can read it.**

6. State the property of [DNA](#def-g10-universal-dna-information) that makes it possible for a mouse [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) to use a jellyfish [gene](#def-g10-universal-dna-gene) .
7. The [gene](#def-g10-universal-dna-gene) is inserted into a fertilised egg rather than into the skin of an adult mouse. Using the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) theory, explain what difference this makes to where the [gene](#def-g10-universal-dna-gene) ends up.
8. The adult mouse glows in every tissue. What does this show about the [DNA](#def-g10-universal-dna-information) content of its different [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) types?
9. The glowing mouse is mated with an ordinary mouse. Predict, with a reason, whether some of the offspring glow.
10. A control mouse is injected with purified GFP [protein](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) instead of the [gene](#def-g10-universal-dna-gene) : it glows faintly for a few days, then stops. Explain the difference between the two experiments.

**Part III — The evidence behind the story.**

11. In Avery’s experiment, name the fraction that transformed the bacteria and the treatment that abolished transformation.
12. Explain why the transformation experiment is a natural version of [transgenesis](#prop-g10-universal-dna-universal) .
13. Chargaff found A $\,=\,$ T and G $\,=\,$ C in every species but A+T varying from 25% to 75%. Which of the two observations supports base pairing, and which shows that the sequence is free?
14. Franklin’s photographs gave the diameter of the helix as $2\,\mathrm{nm}$ . Explain why pairing a large base with a small one (A with T, G with C) rather than two large ones makes the helix regular.
15. Why did the discovery of the structure immediately suggest how [DNA](#def-g10-universal-dna-information) is copied?

**Part IV — Two metres in a [nucleus](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle).**

16. Compute the total length of [DNA](#def-g10-universal-dna-information) in one mouse [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) .
17. The [nucleus](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) is $6\,\text{µ}\mathrm{m}$ across. By what factor must the [DNA](#def-g10-universal-dna-information) be shortened by coiling to fit?
18. What fraction of the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) ’s [DNA](#def-g10-universal-dna-information) is the GFP [gene](#def-g10-universal-dna-gene) ?
19. A mouse has about $2 \times 10^{4}$ [genes](#def-g10-universal-dna-gene) averaging $3 \times 10^{4}$ base pairs including their non-coding parts. What fraction of the genome do [genes](#def-g10-universal-dna-gene) occupy?
20. State the result in one sentence: how much [DNA](#def-g10-universal-dna-information) does a mouse [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) carry, and what single fact about that [DNA](#def-g10-universal-dna-information) allows one jellyfish [gene](#def-g10-universal-dna-gene) among its thousands to be read?

**Solution of Problem 3.1.**

**1.** $720 \times 0.34 \approx 245\,\mathrm{nm}$; $720/10 = 72$ turns.

**2.** `TACTCATTTCCTCTTCTTG`.

**3.** T $= 27\%$; G $=$ C $= (100 - 54)/2 = 23\%$.

**4.** $720/238 \approx 3.0$: three bases per [amino acid](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) (the last three signal the end of the chain).

**5.** $4^{720}$ sequences, about $10^{433}$. The chemistry allows any order, so the particular order chosen among all these possibilities is a message — information.

**6.** Universality: every living thing uses the same four [nucleotides](#def-g10-universal-dna-nucleotide) and reads a sequence by the same rules.

**7.** All the mouse’s [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) descend from the egg by division, so a [gene](#def-g10-universal-dna-gene) inserted in the egg is copied into every [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell). Inserted into adult skin, it would be present only in the treated skin [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) and their descendants.

**8.** Every [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) type contains the whole [DNA](#def-g10-universal-dna-information), transgene included: specialisation does not discard [genes](#def-g10-universal-dna-gene).

**9.** Yes: the germ [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) carry the [gene](#def-g10-universal-dna-gene). If the mouse has one copy on one [chromosome](#def-g10-universal-dna-information) of a pair, about half its offspring inherit it and glow.

**10.** The [protein](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) is not information: it is used up and never renewed, so the glow fades. The [gene](#def-g10-universal-dna-gene) is copied at every division and read continuously, so the glow is permanent and inherited.

**11.** The [DNA](#def-g10-universal-dna-information) fraction; treatment with an enzyme that destroys [DNA](#def-g10-universal-dna-information).

**12.** Bacteria took up [DNA](#def-g10-universal-dna-information) from another strain, integrated it and expressed and transmitted the trait: a [gene](#def-g10-universal-dna-gene) transferred from one organism to another, which is what [transgenesis](#prop-g10-universal-dna-universal) does deliberately.

**13.** A$\,=\,$T and G$\,=\,$C support pairing; the variable A+T share shows that the order of the pairs is free.

**14.** A and G are large two-ring bases, T and C small one-ring bases: a large paired with a small one always spans the same distance, so the two backbones stay $2\,\mathrm{nm}$ apart along the whole molecule. Two large bases would bulge, two small ones leave a gap.

**15.** Because each strand is the complement of the other: separate them, build a partner on each by the pairing rule, and two identical molecules result.

**16.** $2 \times 2.7 \times 10^{9} \times 0.34\,\mathrm{nm} \approx
1.8\,\mathrm{m}$.

**17.** $1.8\,\mathrm{m} / 6\,\text{µ}\mathrm{m} = 3 \times 10^{5}$: a three-hundred-thousand-fold compaction.

**18.** $720 / 5.4 \times 10^{9} \approx 1.3 \times 10^{-7}$: about one ten-millionth of the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell)’s [DNA](#def-g10-universal-dna-information).

**19.** $2 \times 10^{4} \times 3 \times 10^{4} = 6 \times 10^{8}$ base pairs, about 22% of the genome; the rest lies between [genes](#def-g10-universal-dna-gene).

**20.** A mouse [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) carries about $1.8\,\mathrm{m}$ of [DNA](#def-g10-universal-dna-information); because that [DNA](#def-g10-universal-dna-information) is built and read exactly as the jellyfish’s, the one foreign [gene](#def-g10-universal-dna-gene) among twenty thousand is read like the others.