Difficulty: Intermediate | Prerequisites: Basic DNA structure (Ch. 16), amino acids and protein structure (Ch. 5)
This chapter traces the path from gene to functional protein, the central question of molecular biology. You need to understand how DNA's nucleotide sequence is first copied into RNA (transcription) and then decoded into a sequence of amino acids (translation). The material builds directly on DNA structure from Chapter 16 and connects forward to gene regulation in Chapter 18. If you missed DNA replication, review that first; the base-pairing rules are the same engine driving transcription.
Genes are expressed in two stages: transcription (DNA to mRNA) and translation (mRNA to polypeptide). The genetic code uses three-letter codons to specify amino acids, with 61 sense codons and 3 stop codons. The code is redundant but never ambiguous, and it is nearly universal across all life.
Gene expression
The process by which DNA directs the synthesis of proteins (or, in some cases, functional RNA molecules). In simple terms, it is how the information stored in a gene gets turned into something the cell can use.
One gene-one enzyme hypothesis
The original proposal (Garrod, later Beadle and Tatum) that each gene codes for one enzyme. Think of it as the first attempt to connect genes to proteins, before we knew that not all proteins are enzymes.
One gene-one polypeptide hypothesis
The refined version: each gene codes for one polypeptide chain. This is more accurate because many proteins are not enzymes (e.g. keratin, insulin) and some proteins contain multiple polypeptide chains coded by different genes.
Transcription
The synthesis of RNA using a DNA template. In simple terms, the cell copies the gene's message from DNA into messenger RNA. This is the first stage of gene expression.
Translation
The synthesis of a polypeptide using the information in mRNA. The ribosome reads the mRNA's codons and assembles amino acids in the correct order. This is the second stage of gene expression.
Messenger RNA (mRNA)
The RNA molecule that carries the genetic message from DNA in the nucleus to the ribosomes in the cytoplasm. Think of it as the molecular courier between the blueprint (DNA) and the construction site (ribosome).
Central dogma
The principle, articulated by Francis Crick, that genetic information flows in one direction: DNA to RNA to protein. This is the organising framework for everything in this chapter.
Codon
A three-nucleotide sequence on mRNA (read 5' to 3') that specifies a particular amino acid or a stop signal. There are 64 possible codons: 61 code for amino acids and 3 are stop signals.
Template strand
The DNA strand that is read by RNA polymerase during transcription. It runs 3' to 5' and provides the pattern for the mRNA sequence. The template strand is always the same for a given gene but can differ between genes.
Coding strand (nontemplate strand)
The DNA strand that is not transcribed. Its sequence matches the mRNA (with T in place of U), so it is sometimes called the coding strand. Think of it as the DNA strand that "looks like" the mRNA.
Reading frame
The way the nucleotide sequence is grouped into codons, starting from a fixed point and reading in the 5' to 3' direction. A shift of even one nucleotide changes every downstream codon.
Initiation codon (start codon)
AUG, the codon that signals the ribosome to begin translation. It also codes for the amino acid methionine, so every newly made polypeptide begins with methionine.
Termination codons (stop codons)
UAA, UAG, and UGA. These codons do not code for any amino acid; instead, they signal the ribosome to stop translation and release the polypeptide.
Alternative splicing
A process by which a single eukaryotic gene can produce two or more different polypeptides, depending on which exons are included in the final mRNA. This is why an organism can make far more proteins than it has genes.
Archibald Garrod first proposed that genes dictate phenotypes through enzymes, calling inherited diseases "inborn errors of metabolism"
Example: people with alkaptonuria have black urine because they lack the enzyme that breaks down alkapton
Beadle and Ephrussi extended this, showing that each mutation affecting fruit fly eye colour blocks pigment synthesis at a specific enzymatic step
They used the bread mould Neurospora crassa (haploid, so mutations show immediately)
Wild-type mould grows on minimal medium (inorganic salts, glucose, biotin)
After X-ray mutagenesis, they identified nutritional mutants that could only grow on complete medium (minimal medium plus all 20 amino acids and other nutrients)
By testing which single nutrient restored growth, they linked each mutation to a single missing enzyme
This confirmed the one gene-one enzyme hypothesis
Not all gene products are enzymes; keratin and insulin are non-enzyme proteins
Many proteins have multiple polypeptide chains, each specified by its own gene (e.g. haemoglobin has two types of polypeptide, so two genes code for it)
The hypothesis was refined to the one gene-one polypeptide hypothesis
Even this is not perfectly accurate, because:
Alternative splicing lets one eukaryotic gene produce multiple related polypeptides
Some genes code for functional RNA molecules that are never translated
DNA does not build protein directly; RNA is the bridge
RNA differs from DNA in three ways: it contains ribose instead of deoxyribose, uses uracil (U) instead of thymine (T), and is usually single-stranded
DNA bases: A, G, C, T. RNA bases: A, G, C, U
Transcription: DNA is rewritten into RNA. This is the general term for the synthesis of any kind of RNA on a DNA template
mRNA: the specific RNA that carries the protein-building instructions from the gene to the ribosome
Translation: the ribosome reads the mRNA codon by codon and assembles a polypeptide
In bacteria (no nucleus), transcription and translation happen simultaneously in the cytoplasm
In eukaryotes, transcription occurs in the nucleus; the pre-mRNA is processed and then exported to the cytoplasm for translation
The initial RNA transcript in eukaryotes is called pre-mRNA; the general term for any initial transcript (including non-coding RNA) is the primary transcript
The central dogma: DNA to RNA to protein
Four nucleotide bases must specify 20 amino acids
Two-letter combinations give only 16 possibilities (4^2), which is not enough
Three-letter combinations give 64 possibilities (4^3), more than enough
The code is therefore a triplet code: each set of three consecutive, non-overlapping nucleotides specifies one amino acid
During transcription, the template strand (3' to 5') is read to produce mRNA (5' to 3')
The mRNA triplets are called codons, written 5' to 3'
The nontemplate (coding) strand of DNA has the same sequence as the mRNA, except with T instead of U
Example: DNA template 3'-ACC-5' produces RNA 5'-UGG-3'
All 64 codons were deciphered by the mid-1960s
61 codons specify amino acids; 3 are stop codons (UAA, UAG, UGA)
AUG serves double duty: it codes for methionine and acts as the start codon
The code is redundant (multiple codons can specify the same amino acid) but never ambiguous (a given codon always specifies the same amino acid)
Synonymous codons usually differ only in the third (3') nucleotide position
The reading frame is set by the start codon and reads 5' to 3'; a shift of even one base changes every downstream codon
The genetic code is nearly universal across all domains of life
Example: the codon CCG specifies proline in every organism
This universality is what makes biotechnology possible: bacteria can be given human genes and will produce human proteins (e.g. insulin)
Students often think "one gene = one protein." Many proteins contain multiple polypeptide chains coded by different genes (e.g. haemoglobin), and one gene can produce multiple polypeptides through alternative splicing.
Students often confuse the template strand with the coding strand. The template strand is the one RNA polymerase reads (3' to 5'). The coding (nontemplate) strand has the same sequence as the mRNA (with T instead of U) and is not transcribed.
Students often think "redundancy" means "ambiguity." Redundancy means several codons can code for the same amino acid. Ambiguity would mean one codon codes for more than one amino acid, and that does not happen.
Students often assume that every codon codes for a unique amino acid. In fact, 61 codons code for only 20 amino acids, so most amino acids are specified by more than one codon.
⚠️ Be able to trace the path from a DNA template strand sequence to the mRNA codon to the amino acid. This is the single most common exam exercise for this chapter.
⚠️ Know the start codon (AUG) and the three stop codons (UAA, UAG, UGA) cold.
⚠️ Understand why the code is called redundant but not ambiguous, and be ready to explain the difference in one sentence.
⚠️ Know the key difference between bacteria and eukaryotes: in bacteria, transcription and translation are coupled; in eukaryotes, they are separated by the nuclear envelope and mRNA processing occurs in between.
⚠️ The Beadle and Tatum Neurospora experiment is a classic exam question. Be able to describe the logic: X-ray mutagenesis, growth on minimal vs. complete medium, identification of the missing nutrient.
True or False: The template strand of DNA has the same base sequence as the mRNA. (False. The coding/nontemplate strand matches the mRNA, with T in place of U.)
Fill in the blank: The three stop codons are ______, ______, and ______. (UAA, UAG, UGA)
True or False: The genetic code is ambiguous because multiple codons can code for the same amino acid. (False. That property is called redundancy, not ambiguity. The code has no ambiguity.)
Fill in the blank: The start codon ______ codes for the amino acid ______. (AUG; methionine)
True or False: In bacteria, translation can begin before transcription of the mRNA is complete. (True. There is no nuclear envelope separating the two processes.)
Q: If a DNA template strand reads 3'-TACAAGGCC-5', what is the mRNA sequence and what amino acids does it encode?
A: The mRNA would be 5'-AUGUUCCGG-3'. Reading in codons: AUG (Met), UUC (Phe), CGG (Arg). The polypeptide is Met-Phe-Arg.
Q: Describe the logic of Beadle and Tatum's Neurospora experiment and explain what it demonstrated.
A: They irradiated Neurospora spores with X-rays to induce mutations, then tested whether mutant strains could grow on minimal medium. Mutants that could not grow on minimal medium but could grow on complete medium were nutritional mutants, each lacking the enzyme needed to synthesise a particular nutrient. By adding individual nutrients back to minimal medium, they identified which enzyme was missing. This showed that one gene specifies one enzyme.
Q: Why was the one gene-one enzyme hypothesis revised to the one gene-one polypeptide hypothesis?
A: Two key reasons. First, not all proteins are enzymes (e.g. keratin, insulin), yet they are still gene products. Second, many proteins consist of multiple polypeptide chains, each coded by a separate gene (e.g. haemoglobin). The revision acknowledged that a gene specifies a polypeptide, not necessarily a whole functional protein.
Q: Explain the difference between redundancy and ambiguity in the genetic code, and give an example of redundancy.
A: Redundancy means more than one codon specifies the same amino acid (e.g. both GGC and GGU code for glycine). Ambiguity would mean one codon could specify more than one amino acid. The genetic code is redundant but never ambiguous.
Q: A single gene in a eukaryote can give rise to multiple different polypeptides. What process makes this possible?
A: Alternative RNA splicing. During RNA processing, different combinations of exons can be joined together, producing different mature mRNAs from the same pre-mRNA. Each variant mRNA is then translated into a different polypeptide.
This material connects directly to DNA replication (Ch. 16), since the same base-pairing rules that drive replication also drive transcription. It leads into gene regulation (Ch. 18), which explains how and when genes are turned on or off. The universality of the genetic code is also the foundation for biotechnology and genetic engineering (Ch. 20), where genes from one species are expressed in another.
gene expression, transcription, translation, genetic code, codon, codon table, mRNA, messenger RNA, central dogma, template strand, coding strand, nontemplate strand, reading frame, start codon, stop codon, AUG, UAA, UAG, UGA, one gene one enzyme, one gene one polypeptide, Beadle and Tatum, Neurospora, Garrod, inborn errors of metabolism, alkaptonuria, alternative splicing, triplet code, redundancy vs ambiguity, DNA to RNA to protein