---
title: "From Gene to Protein"
book: "High School Biology"
subject: biology
language: en
chapter: 14
exercises: 15
source: https://one-course.com/books/biology/2/en/chapter/14-from-gene-to-protein
---

# Chapter 14 — From Gene to Protein

In 1961 a biochemist fed a cell-free extract of bacteria with an artificial [RNA](#def-g11-gene-expression-transcription) made of nothing but the letter U, repeated. The extract made a [protein](#def-g11-gene-expression-protein) made of nothing but 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), phenylalanine, repeated. With that single experiment the first word of the [genetic code](#def-g11-gene-expression-code) was read: UUU means phenylalanine. Within five years all sixty-four words were known, and they turned out to be the same words in a bacterium, a wheat plant and a human. This chapter follows the path from a [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene)’s sequence of [nucleotides](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide) to a [protein](#def-g11-gene-expression-protein)’s sequence of [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) — the path every trait of [Chapter 13](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#ch-g11-mutations) runs along.

## 14.1 Genes make proteins

**Proposition 14.1 (One gene, one protein).**

A [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) is expressed when the [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) uses its sequence to make the [protein](#def-g11-gene-expression-protein) it encodes. Most traits depend on [proteins](#def-g11-gene-expression-protein) — [enzymes](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-metabolism), structural fibres, carriers, receptors — and a [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) changes a trait by changing the [protein](#def-g11-gene-expression-protein) its [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) produces.

**Evidence.** Beadle and Tatum (1941) irradiated spores of a bread mould and collected mutants that could no longer grow on minimal medium unless one specific substance, such as the [amino acid](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) arginine, was added. Each mutant lacked one [enzyme](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-metabolism) of the chain of reactions that makes that substance; each defect was inherited as a single [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene); and mutants blocked at different steps of the same chain carried [mutations](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) in different [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene). One [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene), one [enzyme](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-metabolism) — and, as later work generalised, one [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene), one polypeptide chain. ∎

**Definition 14.2 (Protein, amino acid sequence).**

A *protein* is a chain of [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families), from a few dozen to several thousand long, drawn from a set of twenty kinds. The *sequence* — the order of the [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) — is what the [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) specifies; once made, the chain folds into a definite three-dimensional shape determined by that sequence, and the shape determines the function. Change 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 the fold, and the function, may change.

## 14.2 Transcription: the gene copied into RNA

**Definition 14.3 (Messenger RNA and transcription).**

*RNA* (ribonucleic acid) is a single-stranded chain of [nucleotides](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide) like those of [DNA](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information), except that its sugar is ribose and the base uracil (U) replaces thymine (T). *Transcription* is the copying of a [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene)’s sequence into a molecule of *messenger RNA* (mRNA): the [enzyme](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-metabolism) *RNA polymerase* opens the [double helix](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#prop-g10-universal-dna-helix) along the [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene), reads one strand (the *template strand*) and assembles the complementary RNA, U facing A, A facing T, G facing C and C facing G. The mRNA has the sequence of the [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene)’s other strand, with U for T; it leaves the [nucleus](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) for the [cytoplasm](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell).

![Transcription. RNA polymerase opens the helix, reads the template strand and builds a messenger RNA complementary to it (U in place of T). Behind the enzyme the helix closes again; the RNA peels off.](https://one-course.com/images/onecourse/chapters/biology-2/g11-gene-expression/fig-eccbbfb0cc56.svg)

*[Transcription](#def-g11-gene-expression-transcription). [RNA](#def-g11-gene-expression-transcription) polymerase opens the helix, reads the template strand and builds a [messenger RNA](#def-g11-gene-expression-transcription) complementary to it (U in place of T). Behind the [enzyme](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-metabolism) the helix closes again; the [RNA](#def-g11-gene-expression-transcription) peels off.*

**Example 14.4 (A transcript).**

Template strand `3’-TAC GGA CTT ATC-5’` gives the mRNA `5’-AUG CCU GAA UAG-3’`. The [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene)’s other strand reads `5’-ATG CCT GAA TAG-3’`: the mRNA is that strand’s sequence with U for T, which is why [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) sequences are conventionally written as that strand, the *coding strand*.

## 14.3 The genetic code

**Definition 14.5 (Codon and the genetic code).**

The mRNA is read in consecutive groups of three [nucleotides](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide), the *codons*, from a defined starting point. The *genetic code* is the correspondence between the 64 possible codons and the 20 [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families): 61 codons specify 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), three (UAA, UAG, UGA) are *stop* signals ending the chain, and AUG, which specifies methionine, also serves as the *start* codon. The code is *degenerate* — most [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) have several codons — and *universal*: the same table serves every known organism, with rare minor variants.

*The [genetic code](#def-g11-gene-expression-code), read on the mRNA. Each [codon](#def-g11-gene-expression-code) is found by its first letter (row), second letter (column) and third letter (line within the block). AUG is both methionine and the start signal; three [codons](#def-g11-gene-expression-code) are stops.*

**Proposition 14.6 (How the code was read).**

Every [codon](#def-g11-gene-expression-code)’s meaning was established experimentally.

**Evidence.** Nirenberg and Matthaei (1961) added synthetic [RNA](#def-g11-gene-expression-transcription) of a single repeated [nucleotide](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide) to a cell-free extract containing [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle), [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) and the [enzymes](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-metabolism) of [protein](#def-g11-gene-expression-protein) synthesis: poly-U yielded a chain of phenylalanines, poly-C a chain of prolines, poly-A of lysines. Synthetic [RNAs](#def-g11-gene-expression-transcription) of repeating pairs and triplets, and later the binding of single triplets to [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle), assigned the remaining [codons](#def-g11-gene-expression-code) by 1966. The three-letter length had been inferred from the arithmetic (two letters give only 16 combinations, too few for 20 [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families); three give 64) and confirmed by [mutations](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation): inserting one or two [nucleotides](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide) into a [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) destroys its [protein](#def-g11-gene-expression-protein), inserting three restores a nearly normal one. ∎

**Method 14.7 (Translating a sequence).**

1. Write the coding strand (or transcribe the template strand): replace T by U to get the mRNA.
2. Find the first AUG: it is the start, and the reading frame is fixed from it.
3. Cut the mRNA into consecutive triplets from the AUG and read each in the table.
4. Stop at the first stop [codon](#def-g11-gene-expression-code) ; the [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) listed, in order, are the [protein](#def-g11-gene-expression-protein) ’s sequence.
5. For a mutant, repeat and compare: same [protein](#def-g11-gene-expression-protein) (silent), 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) changed (missense), premature stop (nonsense), everything changed after the [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) (frameshift).

**Example 14.8 (Four mutations, four outcomes).**

Coding strand `ATG CCT GAA TTC TAG`: mRNA `AUG CCU GAA UUC UAG`, [protein](#def-g11-gene-expression-protein) Met–Pro–Glu–Phe.

- `ATG CC**C** GAA TTC TAG` : CCC is still Pro — *silent* .
- `ATG CCT G**C**A TTC TAG` : GCA is Ala — Met–Pro–Ala–Phe, *missense* .
- `ATG CCT **T**AA TTC TAG` : UAA is stop — Met–Pro, *nonsense* , a truncated [protein](#def-g11-gene-expression-protein) .
- `ATG CC**A**T GAA TTC TAG` : an inserted A shifts the frame: `AUG CCA UGA` … Met–Pro–stop — *frameshift* .

## 14.4 Translation: the message read

**Proposition 14.9 (Translation).**

*Translation* takes place on the *[ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle)*, 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). A [ribosome](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) binds the mRNA at its start [codon](#def-g11-gene-expression-code) and moves along it [codon](#def-g11-gene-expression-code) by [codon](#def-g11-gene-expression-code). Each [codon](#def-g11-gene-expression-code) is matched by a *transfer [RNA](#def-g11-gene-expression-transcription)* (tRNA), a small [RNA](#def-g11-gene-expression-transcription) carrying, at one end, the [three-nucleotide](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide) *anticodon* complementary to the [codon](#def-g11-gene-expression-code) and, at the other, the corresponding [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 [ribosome](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) joins each [amino acid](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) brought in to the growing chain and releases the empty tRNA; at a stop [codon](#def-g11-gene-expression-code) it releases the finished chain, which folds. Several [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) read one mRNA in succession, and one mRNA yields hundreds of copies of the [protein](#def-g11-gene-expression-protein) before it is degraded.

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

![Translation. The ribosome holds two codons; a tRNA whose anticodon matches each codon brings its amino acid, the chain is transferred onto the newcomer, and the ribosome steps one codon along. At a stop codon the chain is released.](https://one-course.com/images/onecourse/chapters/biology-2/g11-gene-expression/fig-0e3b4374dcf6.svg)

*[Translation](#prop-g11-gene-expression-translation). The [ribosome](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) holds two [codons](#def-g11-gene-expression-code); a tRNA whose anticodon matches each [codon](#def-g11-gene-expression-code) brings its [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 chain is transferred onto the newcomer, and the [ribosome](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) steps one [codon](#def-g11-gene-expression-code) along. At a stop [codon](#def-g11-gene-expression-code) the chain is released.*

**Example 14.10 (Speed and yield).**

A [ribosome](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) adds 5 to 20 [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) per second: a [protein](#def-g11-gene-expression-protein) of 300 [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) takes under a minute. [Ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) follow each other on an mRNA about 80 [nucleotides](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide) apart, so a message of 900 [nucleotides](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide) carries a dozen at once; over its life of a few hours a single mRNA produces a thousand [protein](#def-g11-gene-expression-protein) molecules. 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* makes some $20\,000$ [protein](#def-g11-gene-expression-protein) molecules per second; a red blood [cell](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell)’s precursor, some $5 \times 10^{8}$ haemoglobin molecules in its lifetime.

## 14.5 One gene, several messages

**Proposition 14.11 (Maturation of the message in eukaryotes).**

In [eukaryotic cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-prokaryote) the sequence of a [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) is interrupted by *introns*, segments that are transcribed but then cut out of the [RNA](#def-g11-gene-expression-transcription) before it leaves the [nucleus](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle); the segments kept and joined end to end, the *exons*, form the mature mRNA. Many [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) can be cut in more than one way, keeping different sets of exons (*alternative splicing*), so that one [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) yields several distinct mRNAs and several related [proteins](#def-g11-gene-expression-protein): the $20\,000$ human [genes](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) encode well over $100\,000$ [proteins](#def-g11-gene-expression-protein).

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

![A eukaryotic gene is transcribed whole, then the introns are removed. Keeping all the exons, or leaving one out, gives two mRNAs and two proteins from the same gene.](https://one-course.com/images/onecourse/chapters/biology-2/g11-gene-expression/fig-749b205009a5.svg)

*A eukaryotic [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) is transcribed whole, then the [introns](#prop-g11-gene-expression-splicing) are removed. Keeping all the exons, or leaving one out, gives two mRNAs and two [proteins](#def-g11-gene-expression-protein) from the same [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene).*

**Remark 14.12 (The flow of information).**

[DNA](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) is transcribed into [RNA](#def-g11-gene-expression-transcription), [RNA](#def-g11-gene-expression-transcription) is translated into [protein](#def-g11-gene-expression-protein), and [protein](#def-g11-gene-expression-protein) makes the trait: the information flows one way. A changed [protein](#def-g11-gene-expression-protein) never rewrites the [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene), which is why a lifetime of training or sunburn is not inherited, and why only [mutations](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) of the [DNA](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) ([Chapter 13](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#ch-g11-mutations)) change what the next generation receives. The same three steps run in 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 every organism, in the same code: the universality of [Chapter 3](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#ch-g10-universal-dna), now down to the last word.

## 14.6 Exercises

**Exercise 14.1 ★.**

Give three differences between [DNA](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-information) and [RNA](#def-g11-gene-expression-transcription).

**Solution of Exercise 14.1.**

[RNA](#def-g11-gene-expression-transcription) is single-stranded, its sugar is ribose, and it uses uracil in place of thymine (it is also short-lived and leaves 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 14.2 ★.**

Transcribe the template strand `3’-TAC CGA AAA ATT-5’` into mRNA.

**Solution of Exercise 14.2.**

`5’-AUG GCU UUU UAA-3’`.

**Exercise 14.3 ★.**

Translate the mRNA `AUG GGC AAA UGU UAA` with the code table.

**Solution of Exercise 14.3.**

Met–Gly–Lys–Cys, then stop.

**Exercise 14.4 ★.**

What are the roles of the [ribosome](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) and of the transfer [RNAs](#def-g11-gene-expression-transcription) in translation?

**Solution of Exercise 14.4.**

The [ribosome](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) reads the mRNA [codon](#def-g11-gene-expression-code) by [codon](#def-g11-gene-expression-code) and joins the [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) into a chain; each transfer [RNA](#def-g11-gene-expression-transcription) matches one [codon](#def-g11-gene-expression-code) with its anticodon and brings the corresponding [amino acid](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families).

**Exercise 14.5 ★.**

Why must the code use at least three [nucleotides](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide) 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)?

**Solution of Exercise 14.5.**

With 4 letters, words of two give $4^2 = 16$ combinations, fewer than 20 [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families); words of three give 64, enough.

**Exercise 14.6 ★★.**

A [protein](#def-g11-gene-expression-protein) has 412 [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families). What is the minimum length of its mRNA’s coding part, stop [codon](#def-g11-gene-expression-code) included, and of the corresponding [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) in base pairs?

**Solution of Exercise 14.6.**

$412 \times 3 + 3 = 1239$ [nucleotides](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide); a [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) of at least 1239 base pairs (more in a eukaryote, with [introns](#prop-g11-gene-expression-splicing)).

**Exercise 14.7 ★★.**

The coding strand `ATG AAA GGC TGG TAA` is mutated to `ATG AAA GGC TGA TAA`. Give both [proteins](#def-g11-gene-expression-protein) and classify the [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation).

**Solution of Exercise 14.7.**

Original: Met–Lys–Gly–Trp. Mutant: UGA is a stop, so Met–Lys–Gly: a nonsense [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation), truncating the [protein](#def-g11-gene-expression-protein).

**Exercise 14.8 ★★.**

Same starting sequence, mutated to `ATG AAG GGC TGG TAA` and to `ATG AAA GGG CTG GTA A`. Classify each and give the [proteins](#def-g11-gene-expression-protein).

**Solution of Exercise 14.8.**

`AAG` is still Lys: silent, Met–Lys–Gly–Trp unchanged. The second is an insertion of G shifting the frame: `AUG AAA GGG CUG GUA A`… Met–Lys–Gly–Leu–Val…, a frameshift with no stop in the fragment.

**Exercise 14.9 ★★.**

Explain why a substitution in the third position of a [codon](#def-g11-gene-expression-code) is often silent, using the table.

**Solution of Exercise 14.9.**

In most blocks of the table the four [codons](#def-g11-gene-expression-code) sharing the first two letters specify 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) (Pro, Thr, Ala, Gly, Val, Ser, Leu…): changing the third letter then changes nothing. The degeneracy of the code sits mainly in the third position.

**Exercise 14.10 ★★.**

Poly-UC [RNA](#def-g11-gene-expression-transcription) (`UCUCUCUC`…) gives a [protein](#def-g11-gene-expression-protein) alternating serine and leucine. Show that this is consistent with a three-letter code and use it to assign two [codons](#def-g11-gene-expression-code).

**Solution of Exercise 14.10.**

Read in threes, `UCU CUC UCU CUC`… alternates two [codons](#def-g11-gene-expression-code), UCU and CUC, so the [protein](#def-g11-gene-expression-protein) alternates two [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) — as observed. With a two-letter code the same repeat would give a single [codon](#def-g11-gene-expression-code) and a single [amino acid](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families). Since poly-U gives Phe and the table’s UCU is Ser and CUC Leu, the experiment assigns UCU to Ser and CUC to Leu (or the reverse, settled by other experiments).

**Exercise 14.11 ★★.**

A human [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) of $30\,000$ base pairs produces a [protein](#def-g11-gene-expression-protein) of 500 [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families). Explain the discrepancy.

**Solution of Exercise 14.11.**

Only 1503 base pairs are needed for the coding sequence; the rest are [introns](#prop-g11-gene-expression-splicing), transcribed and then cut out of the pre-mRNA, plus regulatory sequences at the [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene)’s ends.

**Exercise 14.12 ★★★.**

A drug blocks bacterial [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) but not human ones. Explain why it can be an antibiotic, and what its existence implies about [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) across the two groups.

**Solution of Exercise 14.12.**

Blocking translation kills the bacterium while leaving the patient’s [cells](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-cell) working. The drug’s selectivity implies that bacterial and human [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle), though they do the same job with the same code, differ in structure enough for a molecule to bind one and not the other — a difference accumulated since their separation.

**Exercise 14.13 ★★★.**

Explain how the jellyfish [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) of [Chapter 3](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#ch-g10-universal-dna) could be read by a mouse, in the vocabulary of this chapter, and what would happen if the mouse used a different code.

**Solution of Exercise 14.13.**

The mouse’s [RNA](#def-g11-gene-expression-transcription) polymerase transcribed the jellyfish [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene), and its [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) and tRNAs translated the mRNA with the same [codon](#def-g11-gene-expression-code) table, so 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) sequence, hence the same fluorescent [protein](#def-g11-gene-expression-protein), was made. With a different code the [codons](#def-g11-gene-expression-code) would be read as other [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) and the [protein](#def-g11-gene-expression-protein) would be a meaningless, non-fluorescent chain.

**Exercise 14.14 ★★★.**

A [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) changes the tRNA whose anticodon reads UAG so that it carries 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) instead of stopping. Predict its effect on 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 [proteins](#def-g11-gene-expression-protein) in general, and on a mutant [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) carrying a premature UAG.

**Solution of Exercise 14.14.**

UAG would no longer stop translation: every [protein](#def-g11-gene-expression-protein) whose [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) ends in UAG would be extended past its normal end, often losing function — a widespread harm. But a [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) truncated by a premature UAG would now be read through, restoring a nearly full-length [protein](#def-g11-gene-expression-protein): the second [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) suppresses the first.

**Exercise 14.15 ★★★.**

Discuss why a frameshift near the start of a [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) is almost always worse than one near its end, and why a missense [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) can be anything from harmless to fatal.

**Solution of Exercise 14.15.**

A frameshift scrambles everything downstream and usually meets a stop within a few [codons](#def-g11-gene-expression-code): near the start, essentially no correct [protein](#def-g11-gene-expression-protein) is made; near the end, most of the [protein](#def-g11-gene-expression-protein) is intact and may still fold and work. A missense changes 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): harmless if that position tolerates the change, fatal if it is at the active site or breaks the fold — the effect depends on which [amino acid](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families), where.

## 14.7 Problem: A Gene Read Four Ways

**Problem 14.1.**

Weekend problem — one short gene transcribed, translated, mutated three times and timed: from the DNA to the amino acids, and the sixty seconds a protein takes

The coding strand of a short [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) reads:

`ATG GCT TGG AAA CCC GAA TAC GGT CAT TTC TGA`

**Part I — Reading the [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene).**

1. Write the template strand.
2. Write the mRNA transcribed from the [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) .
3. Translate it, [codon](#def-g11-gene-expression-code) by [codon](#def-g11-gene-expression-code) , and give the [protein](#def-g11-gene-expression-protein) ’s sequence of [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) .
4. How many [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) does the [protein](#def-g11-gene-expression-protein) contain, and how many [nucleotides](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide) of the mRNA are used to specify them, stop included?
5. Which [codon](#def-g11-gene-expression-code) started the reading, and what fixes the reading frame for all the others?

**Part II — Three mutants.**

6. Mutant 1: `ATG GCT TGG AAA CCG GAA TAC GGT CAT TTC TGA` . Give the [protein](#def-g11-gene-expression-protein) and classify the [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) .
7. Mutant 2: `ATG GCT TGG AAA CCC GAA TAC GGT CAT TTC TGA` with the ninth [codon](#def-g11-gene-expression-code) `CAT` replaced by `CGT` . Give the [protein](#def-g11-gene-expression-protein) and classify.
8. Mutant 3: `ATG GCT TGA AAA CCC GAA TAC GGT CAT TTC TGA` . Give the [protein](#def-g11-gene-expression-protein) and classify. Which single substitution produced it?
9. Mutant 4: a T is inserted after the sixth [nucleotide](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide) : `ATG GCT TTG GAA ACC CGA ATA CGG TCA TTT CTG A` . Give the [protein](#def-g11-gene-expression-protein) and classify.
10. Rank the four mutants from least to most disruptive for the [protein](#def-g11-gene-expression-protein) and justify.

**Part III — Why the code is what it is.**

11. How many [codons](#def-g11-gene-expression-code) specify leucine? Serine? Tryptophan? Which [amino acid](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) is most likely to be changed by a random substitution in its [codon](#def-g11-gene-expression-code) , and which least?
12. Compute the fraction of all substitutions at the third position of a leucine [codon](#def-g11-gene-expression-code) `CUx` that are silent.
13. Explain why a code of two letters could not work, and why a code of four is not needed.
14. Poly-U gives poly-phenylalanine and poly-A poly-lysine. Which entries of the table do these two experiments establish?
15. The code is the same in the bacterium and in a human. State the consequence for [transgenesis](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#prop-g10-universal-dna-universal) and the consequence for kinship.

**Part IV — Timing the factory.** A [ribosome](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) adds 10 [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) per second and [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) follow each other 80 [nucleotides](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide) apart along an mRNA; each mRNA lives 2 hours.

16. How long does one [ribosome](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) take to translate this [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) ’s [protein](#def-g11-gene-expression-protein) ? And a [protein](#def-g11-gene-expression-protein) of 600 [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) ?
17. How many [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) can read an mRNA of 1800 [nucleotides](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide) at once?
18. If a new [ribosome](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) starts every 8 seconds, how many copies of the [protein](#def-g11-gene-expression-protein) does one mRNA yield in its lifetime?
19. A bacterium needs $2000$ copies of an [enzyme](https://one-course.com/books/biology/2/en/chapter/4-cell-metabolism#def-g10-cell-metabolism-metabolism) in 10 minutes. How many mRNAs of the [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) , transcribed at once, are needed?
20. State the result: the [protein](#def-g11-gene-expression-protein) encoded by the [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) , the [mutation](https://one-course.com/books/biology/2/en/chapter/13-mutations-and-genetic-variation#def-g11-mutations-mutation) among the four that abolished it, and the time one [ribosome](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) needs to build it.

**Solution of Problem 14.1.**

**1.** `3’-TAC CGA ACC TTT GGG CTT ATG CCA GTA AAG ACT-5’`.

**2.** `AUG GCU UGG AAA CCC GAA UAC GGU CAU UUC UGA`.

**3.** Met–Ala–Trp–Lys–Pro–Glu–Tyr–Gly–His–Phe, stop.

**4.** 10 [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families); 33 [nucleotides](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-nucleotide) (11 [codons](#def-g11-gene-expression-code) including the stop).

**5.** AUG; the position of that first AUG fixes the frame, and every following [codon](#def-g11-gene-expression-code) is read in step from it.

**6.** CCG is still Pro: silent, [protein](#def-g11-gene-expression-protein) unchanged.

**7.** CGU is Arg in place of His: missense, Met–Ala–Trp–Lys–Pro–Glu–Tyr–Gly–Arg–Phe.

**8.** UGA is stop: Met–Ala, nonsense. A single substitution G to A in the third [codon](#def-g11-gene-expression-code) (TGG to TGA).

**9.** mRNA `AUG GCU UUG GAA ACC CGA AUA CGG UCA UUU CUG A`: Met–Ala–Leu–Glu–Thr–Arg–Ile–Arg–Ser–Phe–Leu…, no stop in the fragment: frameshift, every [amino acid](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) after the second is changed.

**10.** Mutant 1 (silent, no change) $<$ mutant 2 (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) changed, function possibly kept) $<$ mutant 4 (frameshift, all but two [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) wrong) $\approx$ mutant 3 (nonsense, a two-amino-acid fragment: the [protein](#def-g11-gene-expression-protein) is gone).

**11.** Leu 6, Ser 6, Trp 1. Trp, with a single [codon](#def-g11-gene-expression-code), is changed by any substitution; Leu or Ser, with six, is most often unchanged.

**12.** CUU, CUC, CUA, CUG are all Leu: every one of the 3 possible substitutions at the third position is silent — 100%.

**13.** Two letters give 16 [codons](#def-g11-gene-expression-code), too few for 20 [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) plus a stop; three give 64, already more than enough, so four (256) would be waste.

**14.** UUU $=$ Phe and AAA $=$ Lys.

**15.** A [gene](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#def-g10-universal-dna-gene) transferred between [species](https://one-course.com/books/biology/2/en/chapter/5-biodiversity-at-every-scale#def-g10-biodiversity-scales-species) is read identically, so [transgenesis](https://one-course.com/books/biology/2/en/chapter/3-dna-a-universal-genetic-molecule#prop-g10-universal-dna-universal) works; and a shared code, inherited unchanged, is evidence that all organisms descend from a common ancestor that already used it.

**16.** 10 [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families): $1\,\mathrm{s}$. 600 [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families): $60\,\mathrm{s}$.

**17.** $1800/80 \approx 22$ [ribosomes](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) at once.

**18.** $7200/8 = 900$ copies.

**19.** One mRNA gives $600/8 = 75$ copies in 10 minutes; about 27 mRNAs are needed.

**20.** The [protein](#def-g11-gene-expression-protein) Met–Ala–Trp–Lys–Pro–Glu–Tyr–Gly–His–Phe; mutant 3, the single G-to-A substitution creating a stop at the third [codon](#def-g11-gene-expression-code), abolished it; one [ribosome](https://one-course.com/books/biology/2/en/chapter/2-cells-the-common-unit-of-life#def-g10-cells-common-unit-organelle) builds the ten [amino acids](https://one-course.com/books/biology/2/en/chapter/1-the-chemical-makeup-of-living-things#def-g10-chemistry-of-life-families) in about one second.