Week 14 · Chapter

Chapter 14 — Gene Expression

Introduction to Biology Free sample edition

The module's primary reading · ~40–50 minutes · Objective 7 (explain the structure and semiconservative replication of DNA, apply Chargaff's rules and complementary base pairing, and trace gene expression through transcription and translation with the genetic code (the central dogma), decoding a sequence from template to protein), with Objective 3 (compare prokaryotic and eukaryotic cells, relate each eukaryotic organelle to its function, explain how substances cross the plasma membrane by passive and active transport (including osmosis and tonicity), and use the surface-area-to-volume ratio to explain why cells stay small) carried forward · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the lab all build on it.

In plain language

Every cell in your body carries the same DNA, but the DNA itself never leaves the nucleus. This week you follow one gene as the cell copies it into a working message and then reads that message to build a protein. The copying step is transcription (writing a gene's letters into a matching RNA message), and the reading step is translation (turning that message into a chain of amino acids). Francis Crick named this flow the central dogma in 1958, and its message is read three letters at a time, in a code nearly every living thing shares. By the end of the week you can take a short gene, write its message, and name the protein it makes.

Why you already care about this

Picture a library reference room with one rare cookbook on the shelf. The book cannot leave the room, so you copy one recipe onto a card and carry the card to the kitchen. The kitchen cooks from the card, and the book stays safe on the shelf. Your cells work the same way: the DNA stays in the nucleus, a copy of one gene travels out, and the cell builds from the copy.

The idea has a name and a date. Francis Crick (1958) gave the name central dogma to the flow of genetic information from DNA to RNA to protein. In this chapter you carry 2 model genes from DNA to protein, read 64 codons with a supplied table, and see 1 changed letter turn glutamic acid into valine in sickle cell disease.

You meet this flow in more places than a biology class. A nurse explaining a sickle cell test result, a lab technician reading a sequence at a bench, and a student checking a chatbot's homework all lean on the same few rules. This chapter turns those rules into steps you can follow.

By the end of this chapter you will be able to:

state the central dogma in order and name where transcription and translation happen;
tell gene expression apart from DNA replication;
transcribe a template strand into mRNA, with U in place of T;
read an mRNA in codons from AUG, using a codon table, to the first stop codon;
explain why the code's redundancy can leave a protein unchanged;
describe how the ribosome, tRNA, and anticodons build a protein;
decode a short gene from template to protein with every step shown;
explain how one base change, GAG to GUG, leads to sickle cell disease;
catch a stray T or a wrong reading frame in someone else's work.

Carry this sentence with you: the DNA stays home; the message travels.

Where we are

Already covered: Week 3 introduced DNA and RNA as nucleic acids and proteins as chains of amino acids. Week 4 placed the nucleus, the cytoplasm, and the ribosome in the cell. Week 13 built DNA's double helix and showed how a cell copies all of it before dividing.
This week: the central dogma, transcription, the genetic code, translation, two decoded genes, and the sickle-cell codon.
How they connect: the pairing rule you used to copy DNA last week is the same rule RNA polymerase uses to write a message this week, with one swap: U for T.
Where we're going next: Week 15 asks why cells switch genes on and off, sorts the kinds of mutations, and closes the course with the tools of biotechnology.

Week 13 is the chapter this one leans on hardest. You learned there that adenine pairs with thymine and guanine pairs with cytosine, and that each strand has a 5′ end and a 3′ end. You also learned that the two strands are complementary, not identical, so one strand tells you the other. Hold on to both facts. This week a cell uses them to write RNA instead of DNA.

1. Name the flow: the central dogma

Think about the recipe card again. The cookbook holds the instructions, the card carries one recipe to the kitchen, and the kitchen makes the dish. A cell has the same three parts: DNA holds the instructions, a message carries one gene's instructions, and the protein is the finished dish.

Gene expression is the process of turning the instructions in a gene into a working product, usually a protein, by transcription and then translation.
The central dogma is the direction genetic information flows in a cell: from DNA to messenger RNA by transcription, then from messenger RNA to protein by translation.

The flow is written as an ordered chain with its places: DNA → mRNA (transcription, in the nucleus) → protein (translation, in the cytoplasm at a ribosome).

Transcription is the first step of gene expression, in which RNA polymerase reads a gene's template strand in the nucleus and builds a complementary messenger RNA.
Translation is the second step of gene expression, in which a ribosome in the cytoplasm reads messenger RNA one codon at a time and joins the matching amino acids into a chain.
Messenger RNA (mRNA) is a single-stranded RNA copy of one gene that carries the gene's message from the DNA in the nucleus to a ribosome in the cytoplasm.

Worked example — say the order, name the place. You state each step of the central dogma with its compartment.

Step 1: transcription. Input: a gene in the DNA. Output: an mRNA. Place: the nucleus.
Step 2: the mRNA leaves the nucleus for the cytoplasm.
Step 3: translation. Input: the mRNA. Output: a protein. Place: a ribosome in the cytoplasm.

In words: the message is written in the nucleus and read in the cytoplasm. The order is a fact, not a suggestion.

Misconception check. Students often blur transcription and translation, because both words start with "trans" and both happen in gene expression. The cure is the pair of verbs. Transcription writes the message, in the nucleus; translation reads it, in the cytoplasm. A second slip is to place translation in the nucleus. The ribosomes that read mRNA sit in the cytoplasm, so translation happens there.

2. Keep expression apart from replication

Picture a print shop asked for two different jobs. In one job, the shop copies an entire book, every page, because a second library needs its own book. In the other job, the shop copies one recipe card, as often as a cook asks for it. The machines are similar, but the jobs are not.

Replication and transcription differ in the same way:

Replication copies the whole archive, every gene, once per cell division, and makes DNA from DNA.
Transcription copies one gene whenever the cell needs that gene's product, and makes RNA from DNA.

The DNA archive stays in the nucleus during both. Only the mRNA card travels out to a ribosome.

Worked example — sort four statements. You decide whether each statement describes replication or transcription.

"Makes a full second copy of every chromosome before the cell divides." The whole archive, DNA from DNA, so replication.
"Makes a short RNA copy of one gene." One gene, RNA from DNA, so transcription.
"Uses thymine in the new strand." The new strand is DNA, so replication; an RNA copy never uses thymine.
"Can happen many times a day for one busy gene." One recipe on demand, so transcription.

In words: replication copies everything once per division; transcription copies one gene as often as the cell needs it.

Misconception check. Students often treat replication and protein synthesis as the same process, because both start by reading DNA. The cure is to name the product. Replication makes DNA from DNA. Gene expression makes RNA from DNA and then protein from RNA.

3. Transcribe a gene

Think about tracing a stencil. You lay a blank page over a cut-out pattern and fill in the shape the pattern allows. The new page matches the pattern's openings, not the pattern itself. Transcription works like that: one DNA strand is the pattern, and the new RNA is built to fit it.

RNA polymerase is the enzyme that opens the DNA at a gene, reads the template strand from its 3′ end to its 5′ end, and builds a complementary RNA from its 5′ end to its 3′ end.
The template strand is the DNA strand that RNA polymerase reads during transcription; the messenger RNA is built complementary to it, base by base.
The coding strand is the DNA strand that is not read during transcription; its sequence matches the messenger RNA except that it has thymine where the RNA has uracil.
Uracil is the RNA base, written U, that takes the place of thymine; it pairs with adenine, so an adenine on the template strand becomes a uracil in the messenger RNA.

The pairing rules for transcription are:

DNA A → mRNA U
DNA T → mRNA A
DNA G → mRNA C
DNA C → mRNA G

Worked example — transcribe Gene 1. The two decoded genes are model sequences built for this course, and every codon call comes from the standard genetic code table. Gene 1's template strand is 3′-TAC CGA ATA ACT-5′, read by RNA polymerase from its 3′ end.

Pairing: each template base pairs with an RNA base, A → U, T → A, G → C, C → G, so TAC → AUG, CGA → GCU, ATA → UAU, and ACT → UGA.
mRNA: the mRNA is 5′-AUG GCU UAU UGA-3′; it carries U, never T.
U-not-T scan: the mRNA has 0 T's; every A on the template became a U.
Coding strand: the coding strand is 5′-ATG GCT TAT TGA-3′, the same as the mRNA with T in place of U.

In words: the template set every letter of the message, and the message is the coding strand rewritten in RNA's alphabet.

Misconception check. Students often write a T into an mRNA, because DNA has trained the hand to write T beside A. The cure is a scan. After you write any mRNA, read it once looking only for T. RNA uses uracil, never thymine, so a single T means a slip somewhere.

4. Read the genetic code

Picture a three-letter postal code on an envelope. Each group of three letters points to one exact place. The cell reads an mRNA the same way, three bases at a time, and each group points to one amino acid.

The genetic code is the set of rules a cell uses to read messenger RNA three bases at a time, in which each of the 64 codons stands for one amino acid or for stop.
A codon is a group of three bases on messenger RNA that stands for one amino acid or for stop; for example, GCU stands for alanine.
The start codon is AUG, which marks where translation begins, sets the reading frame, and also codes for the amino acid methionine.
A stop codon is one of the three codons UAA, UAG, and UGA, which code for no amino acid and tell the ribosome to end the chain.
The reading frame is the way a messenger RNA is split into groups of three bases, set by the start codon; reading from the wrong base splits every codon differently.

Worked example — count the codons. You find how many codons exist and how many code for amino acids.

Each position in a codon can hold 1 of 4 bases: A, U, G, or C.
Code size: 4 bases taken 3 at a time gives 4 × 4 × 4 = 64 codons.
Stops: 3 of those codons are stop codons.
Sense codons: 64 − 3 stop codons = 61 codons for amino acids.

In words: 61 of the 64 codons name an amino acid, and 3 say stop.

You never need to memorize the codon table. In this course a codon table is always supplied, and the skill is reading it correctly. The codons you meet this week are:

Codons used this week and what each codes for in the standard genetic code
Codon	Codes for
AUG	Met (methionine), also the start codon
GCU, GCC	Ala (alanine)
UAU	Tyr (tyrosine)
UUU	Phe (phenylalanine)
CCC, CCU	Pro (proline)
GGA	Gly (glycine)
GAG	Glu (glutamic acid)
GUG	Val (valine)
GAU	Asp (aspartic acid)
UAA, UAG, UGA	stop

Misconception check. Students sometimes think a codon can be read from anywhere along the mRNA. The idea is tempting because the letters run on without spaces. The cure is the start codon: reading begins at AUG and moves in steps of three. Students also forget the stops. UAA, UAG, and UGA end the chain; AUG starts it and is methionine.

5. Use the code's cushion: redundancy

Think of a school with more locker numbers than students. Several lockers can be assigned to the same student, and a slip of one digit sometimes still opens a locker that belongs to that same student. The genetic code has the same kind of slack.

There are 61 codons for amino acids but only 20 common amino acids. So most amino acids have more than one codon.

Redundancy of the genetic code is the feature of the genetic code that most amino acids have more than one codon, because 61 codons stand for only 20 common amino acids; GCU and GCC both stand for alanine.

Worked example — two cushions from the table. You read two pairs of codons from the supplied table.

GCU → Ala and GCC → Ala. The codons differ only in the third base, and the amino acid is the same.
CCC → Pro and CCU → Pro. Again the third base differs, and the amino acid is the same.
Count: 2 codons each for those 2 amino acids.

In words: a change from GCU to GCC would leave the protein unchanged. Next week you will give that kind of change its name.

Caveat: redundancy is a cushion, not a shield. Many single-base changes do switch the amino acid, as Section 8 shows.

Misconception check. Students often think every change to a gene must change the protein. The idea is tempting because a gene is a code, and codes seem fragile. The cure is the table: GCU and GCC both give alanine, so some changes leave the protein the same.

6. Translate the message

Picture a factory line where a worker reads an order slip one line at a time. A runner brings the matching part for each line, the part is added to the product, and when the slip says "end," the finished product rolls off. Translation runs the same way.

A ribosome holds the mRNA and reads it one codon at a time, starting at AUG.
Transfer RNA (tRNA) is a small RNA that carries one amino acid on one end and a three-base anticodon on the other, and brings that amino acid to the ribosome when its anticodon pairs with a codon.
An anticodon is the three bases on a transfer RNA that pair with a matching codon on the messenger RNA; the codon is on the messenger RNA, and the anticodon is on the transfer RNA.
A release factor is a protein that recognizes a stop codon at the ribosome and lets the finished chain of amino acids go.
A polypeptide is a chain of amino acids joined in the order the codons set; it folds into a working protein.

Translation in order:

Start: the ribosome finds AUG, and a tRNA carrying methionine pairs with it.
Elongation: for each next codon, the matching tRNA pairs by its anticodon, and its amino acid joins the growing chain.
Stop: at UAA, UAG, or UGA no tRNA fits; a release factor binds instead, and the finished polypeptide is let go.

Worked example — the anticodons for Gene 1. You pair each codon of Gene 1's mRNA, 5′-AUG GCU UAU UGA-3′, with its anticodon.

AUG pairs with the anticodon 3′-UAC-5′, which brings Met.
GCU pairs with 3′-CGA-5′, which brings Ala.
UAU pairs with 3′-AUA-5′, which brings Tyr.
UGA is a stop codon, so it has no tRNA; a release factor ends the chain.

In words: each anticodon is the codon's complement, written in RNA's letters, and the stop codon is read by a protein instead of a tRNA.

Misconception check. Students often put the anticodon on the mRNA. The two words differ by one prefix, so they blur. The cure is to tie each to its carrier: the codon is on the mRNA, the message; the anticodon is on the tRNA, the adapter that carries an amino acid.

7. Decode two genes from template to protein

Now put every step together. Think of it like following a recipe card all the way to a finished plate: copy, check, split, look up, stop.

Worked example — Gene 1, every step. You carry Gene 1 from template to protein.

Template: Gene 1's template strand is 3′-TAC CGA ATA ACT-5′.
Transcribe: TAC → AUG, CGA → GCU, ATA → UAU, and ACT → UGA, so the mRNA is 5′-AUG GCU UAU UGA-3′.
Scan: 0 T's in the mRNA.
Count: 12 bases ÷ 3 bases per codon = 4 codons.
Frame: the first codon is AUG, the start codon, so reading begins there in groups of three.
Translate: AUG → Met, GCU → Ala, UAU → Tyr, UGA → stop.
Protein: Met–Ala–Tyr; 4 codons − 1 stop codon = 3 amino acids.

Worked example — Gene 2, the check. You run the same steps on Gene 2.

Template: 3′-TAC AAA GGG ATC-5′.
Transcribe: the mRNA is 5′-AUG UUU CCC UAG-3′.
Translate: AUG → Met, UUU → Phe, CCC → Pro, UAG → stop.
Protein: Met–Phe–Pro.

The two decoded genes (model sequences built for this course): template, mRNA, and protein
Gene	DNA template (3′ to 5′)	mRNA (5′ to 3′)	Protein
Gene 1	TAC CGA ATA ACT	AUG GCU UAU UGA	Met–Ala–Tyr, then stop
Gene 2	TAC AAA GGG ATC	AUG UUU CCC UAG	Met–Phe–Pro, then stop

In words: each gene has 12 bases, which make 4 codons, and the last codon is a stop, so each protein has 3 amino acids. The template fixed the mRNA, and the mRNA, read in frame from AUG, fixed the protein.

Misconception check. Students often count the stop codon as an amino acid and write a four-part protein such as Met–Ala–Tyr–stop. The idea is tempting because the stop sits in the same row of the table as everything else. The cure is the rule: a stop codon codes for no amino acid, so it is written "stop" and never joins the chain.

8. See why one letter matters: the sickle-cell codon

Picture a one-letter typo in a text message: "meet at the dock" becomes "meet at the duck." One letter changed, and the meaning changed with it. A gene can suffer the same kind of slip.

Hemoglobin is a protein packed inside red blood cells. In the gene for its beta chain, one codon is GAG, which codes for glutamic acid (Glu). In sickle cell disease, that codon is GUG, which codes for valine (Val).

Worked example — the sickle-cell codon. You compare the two codons with the supplied table.

Normal codon: GAG → Glu.
Sickle-cell codon: GUG → Val.
Compare base by base: G and G match, A and U differ, G and G match.
Count: 1 base changed (the middle A became U), so 1 amino acid changed.

In words: one base out of three changed, and the amino acid changed from glutamic acid to valine. That single swap sits at position 6 of hemoglobin's beta chain, one change among roughly 600 amino acids counting all four chains of the protein. The altered hemoglobin links into long fibers when oxygen is low, bending red blood cells into a rigid crescent shape that can block small blood vessels.

This is the structure-determines-function idea from Week 3, now traced back to one letter of DNA. Ingram (1957) identified the chemical difference between normal and sickle-cell hemoglobin.

Misconception check. Students often think a single base change cannot matter in a gene with hundreds of bases. The idea is tempting because one letter sounds tiny. The cure is the sickle-cell codon: GAG to GUG is one base, and it changes Glu to Val, the protein's shape, and its job.

Pause and reflect. Choose one of these two prompts and think it through, or jot a few sentences in your notes.

(a) The sickle-cell change is one base out of many. In your own words, explain to a friend why a change that small can affect a whole red blood cell.
(b) The GCU-to-GCC change leaves the protein the same, while GAG-to-GUG does not. Explain what the codon table shows about why one change is quiet and the other is not.

9. Check the frame and scan for T

Think about reading a sentence with every space moved one letter to the right: "the dog ran far" becomes "t hed ogr anf ar." Every letter is still there, and the meaning is gone. The reading frame works the same way.

Worked example — the wrong frame. You read Gene 1's mRNA, 5′-AUG GCU UAU UGA-3′, starting one base too late.

Drop the first base, A, and read from the second base, U.
Split into threes: UGG CUU AUU, then 2 leftover bases.
Look for a start: there is no AUG in that frame.
Compare: not one of those codons matches the real codons AUG, GCU, UAU, or UGA.

In words: Gene 1's mRNA read from its second base gives UGG CUU AUU plus 2 leftover bases, and no AUG start, so every codon is wrong.

Worked example — check a chatbot's mRNA. Suppose a chatbot transcribes the critique template, 3′-TAC GGA CCT ACT-5′, and writes 5′-AUG CCT GGA UGA-3′.

Scan for T: the second codon, CCT, contains a T, so it cannot be RNA.
Re-pair that triplet: the template GGA → CCU.
Corrected mRNA: 5′-AUG CCU GGA UGA-3′.
Translate: AUG → Met, CCU → Pro, GGA → Gly, UGA → stop, so the protein is Met–Pro–Gly.

In words: one stray T would send you hunting in a codon table that has no T at all. The scan catches it in seconds.

Honesty note: a chatbot is often right about the big picture and wrong on a single letter. You are the check, and the U-not-T scan and the start-at-AUG rule are your two fastest tools.

Misconception check. Students often trust any sequence that looks tidy. A tidy row of triplets feels finished. The cure is to run both checks every time: scan the mRNA for T, then confirm the first codon is AUG and count in steps of three.

10. Carry a gene through the lab

This week's Biology Lab & Inquiry asks you to do the whole process yourself. You carry each gene from template to protein, check your work, and name where each step happens.

In practice (your tools). Open the Learn.Genetics Transcribe and Translate a Gene interactive in your browser, or work the same steps by hand with a printed codon table. In the interactive, follow its on-screen steps to carry a DNA sequence through transcription and then translation. By hand, write the template, pair each base under it, split the mRNA into codons from AUG, and look each codon up in the table. Record each gene in a table with four columns: template, mRNA, codons, and protein.

Worked example — the lab's checklist on Gene 2. You run the five checks the lab asks for.

Transcribe: 3′-TAC AAA GGG ATC-5′ → 5′-AUG UUU CCC UAG-3′.
U-not-T scan: 0 T's in the mRNA.
Frame check: the first codon is AUG, and 12 bases ÷ 3 = 4 codons.
Translate: Met–Phe–Pro, then stop at UAG.
Locations: transcription in the nucleus; translation at a ribosome in the cytoplasm.

In words: every check passes, so the protein Met–Phe–Pro stands.

Back to the library: the cookbook stayed on the shelf, the recipe card went to the kitchen, and the kitchen made the dish. In your lab, the template is the cookbook, your mRNA is the card, and your protein is the dish.

Limitation: the interactive and the hand method both use short model genes. Real genes are far longer, and their starts and stops are found the same way, codon by codon.

Misconception check. Students sometimes think the lab is over once a protein is written down. The idea is tempting because the protein feels like the finish line. The cure is the checklist: scan for T, check the frame, and name both locations. A correct protein with a missing location is still an unfinished answer.

Key terms

Gene expression — The process of turning the instructions in a gene into a working product, usually a protein, by transcription and then translation. (first used: 1. Name the flow: the central dogma)
Central dogma — The direction genetic information flows in a cell: from DNA to messenger RNA by transcription, then from messenger RNA to protein by translation. (first used: 1. Name the flow: the central dogma)
Transcription — The first step of gene expression, in which RNA polymerase reads a gene's template strand in the nucleus and builds a complementary messenger RNA. (first used: 1. Name the flow: the central dogma)
Translation — The second step of gene expression, in which a ribosome in the cytoplasm reads messenger RNA one codon at a time and joins the matching amino acids into a chain. (first used: 1. Name the flow: the central dogma)
Messenger RNA (mRNA) — A single-stranded RNA copy of one gene that carries the gene's message from the DNA in the nucleus to a ribosome in the cytoplasm. (first used: 1. Name the flow: the central dogma)
RNA polymerase — The enzyme that opens the DNA at a gene, reads the template strand from its 3′ end to its 5′ end, and builds a complementary RNA from its 5′ end to its 3′ end. (first used: 3. Transcribe a gene)
Template strand — The DNA strand that RNA polymerase reads during transcription; the messenger RNA is built complementary to it, base by base. (first used: 3. Transcribe a gene)
Coding strand — The DNA strand that is not read during transcription; its sequence matches the messenger RNA except that it has thymine where the RNA has uracil. (first used: 3. Transcribe a gene)
Uracil — The RNA base, written U, that takes the place of thymine; it pairs with adenine, so an adenine on the template strand becomes a uracil in the messenger RNA. (first used: 3. Transcribe a gene)
Genetic code — The set of rules a cell uses to read messenger RNA three bases at a time, in which each of the 64 codons stands for one amino acid or for stop. (first used: 4. Read the genetic code)
Codon — A group of three bases on messenger RNA that stands for one amino acid or for stop; for example, GCU stands for alanine. (first used: 4. Read the genetic code)
Start codon — The codon AUG, which marks where translation begins, sets the reading frame, and also codes for the amino acid methionine. (first used: 4. Read the genetic code)
Stop codon — One of the three codons UAA, UAG, and UGA, which code for no amino acid and tell the ribosome to end the chain. (first used: 4. Read the genetic code)
Reading frame — The way a messenger RNA is split into groups of three bases, set by the start codon; reading from the wrong base splits every codon differently. (first used: 4. Read the genetic code)
Redundancy of the genetic code — The feature of the genetic code that most amino acids have more than one codon, because 61 codons stand for only 20 common amino acids. (first used: 5. Use the code's cushion: redundancy)
Transfer RNA (tRNA) — A small RNA that carries one amino acid on one end and a three-base anticodon on the other, and brings that amino acid to the ribosome when its anticodon pairs with a codon. (first used: 6. Translate the message)
Anticodon — The three bases on a transfer RNA that pair with a matching codon on the messenger RNA; the codon is on the messenger RNA, and the anticodon is on the transfer RNA. (first used: 6. Translate the message)
Release factor — A protein that recognizes a stop codon at the ribosome and lets the finished chain of amino acids go. (first used: 6. Translate the message)
Polypeptide — A chain of amino acids joined in the order the codons set; it folds into a working protein. (first used: 6. Translate the message)

Summary

The central dogma: DNA → mRNA → protein.
Transcription writes the message in the nucleus; translation reads it at a ribosome in the cytoplasm.
Replication copies every gene once per division; transcription copies one gene when needed.
RNA polymerase reads the template strand and builds a complementary mRNA with U, never T.
The code is read in codons of three: 64 codons, 61 for amino acids, 3 stops.
AUG starts translation, codes for methionine, and sets the reading frame.
Redundancy: GCU and GCC both give alanine.
The codon is on the mRNA; the anticodon is on the tRNA.
Gene 1 gives Met–Ala–Tyr; Gene 2 gives Met–Phe–Pro.
GAG to GUG swaps glutamic acid for valine in sickle cell disease.
Scan every mRNA for T, and read every message from AUG in steps of three.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

An illustrative gene from a houseplant has the template strand 3′-TAC CTC CGG ATT-5′. Write its mRNA, check it for T, and use the codon table in Section 4 to name the protein.
A tRNA at the ribosome carries the anticodon 3′-CCU-5′. Which codon does it pair with, and which amino acid does it bring?
An illustrative bakery-yeast gene is 18 bases long, starts with AUG, and ends with a stop codon. How many codons does it have, and how many amino acids are in its protein?
A friend says a skin cell and a liver cell must be using different DNA, because they make different proteins. Using what you know about transcription, explain what is wrong with that claim.

Answers (worked reasoning — read after attempting)

Pair each template base: TAC → AUG, CTC → GAG, CGG → GCC, and ATT → UAA. The mRNA is 5′-AUG GAG GCC UAA-3′, and a scan finds 0 T's. Translate from AUG: AUG → Met, GAG → Glu, GCC → Ala, UAA → stop. The protein is Met–Glu–Ala, three amino acids, because the stop codon adds none.
The anticodon pairs base by base with the codon, so 3′-CCU-5′ pairs with the codon 5′-GGA-3′: C with G, C with G, and U with A. The table gives GGA → Gly, so the tRNA brings glycine. The codon sits on the mRNA; the anticodon sits on the tRNA.
Codons: 18 bases ÷ 3 bases per codon = 6 codons. The last one is a stop, which codes for no amino acid, so 6 − 1 = 5 amino acids. In words, the protein is a chain of 5 amino acids, beginning with methionine.
Every body cell carries the same DNA. What differs is which genes each cell transcribes. A skin cell copies some genes into mRNA, a liver cell copies others, and each cell's ribosomes make proteins only from the messages that cell wrote. Same archive, different recipe cards. Next week gives this idea its name: gene regulation.

Next up in this module: the lecture, the slide deck, the Readings & Resources page, the lecture tutorial, the practice set, and the quiz; next week, the lac operon model shows how a cell switches a gene on and off.

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