Lecture: Gene Expression
Lecture script · about 15–20 minutes spoken · Week 14
Opening
Picture a library reference room with one rare cookbook on the shelf. The book can't leave the room. So you copy one recipe onto a card, carry the card to the kitchen, and cook from the card. The book stays safe on the shelf, and the kitchen still gets its dish.
Your cells run that same system every minute. A nursing student explaining a sickle cell result to a family is talking about it. So is a lab technician reading a sequence at a bench, and so is anyone checking a chatbot's homework on genetics. Each one is following a message from a gene to a protein.
Here's where we are. In Week 3 you met DNA and RNA as nucleic acids and proteins as chains of amino acids. In Week 4 you placed the nucleus, the cytoplasm, and the ribosome in the cell. In Week 13 you built the double helix and watched a cell copy all of it. In this lecture I'll show you how a cell copies one gene into a message and reads that message to build a protein. We'll name the flow, keep it apart from copying, write a message, learn the code, see its cushion, build a protein, decode two genes, meet one famous typo, check our own work, and walk into the lab. Next week, in Week 15, we ask why cells switch genes on and off.
1. The flow: the central dogma
Go back to the library. The cookbook holds the instructions. The card carries one recipe. The kitchen makes the dish. A cell has the same three parts, and they come in the same order.
Here are the terms. 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. Francis Crick gave it that name in 1958.
Now the two steps, each with its place. Transcription is the first step, in which RNA polymerase reads a gene's template strand in the nucleus and builds a complementary messenger RNA. Messenger RNA, or mRNA, is a single-stranded RNA copy of one gene that carries the gene's message to a ribosome in the cytoplasm. Translation is the second step, in which a ribosome in the cytoplasm reads the mRNA one codon at a time and joins the matching amino acids into a chain.
So the order is DNA, then mRNA, then protein. The message is written in the nucleus and read in the cytoplasm. That order is a fact, not a suggestion.
Pause and try this. Which step happens in the nucleus, and which happens at a ribosome in the cytoplasm?
Say "write" and "read" to yourself first.
Here's the answer. Transcription happens in the nucleus and translation happens at a ribosome in the cytoplasm, because the message is written at home and read out in the kitchen.
2. Expression is not replication
Picture a print shop with two jobs. In one job, it copies a whole book, every page, because a second library needs its own. In the other job, it copies one recipe card, as often as a cook asks. The machines look alike, but the jobs don't.
That's the difference between replication and transcription. Replication copies the whole archive, every gene, once per cell division, and it makes DNA from DNA. Transcription copies one gene whenever the cell needs that gene's product, and it makes RNA from DNA. In both jobs the DNA archive stays in the nucleus. Only the mRNA card travels out.
Students often blur these two, because both start by reading DNA. The cure is to name the product. If the new strand is DNA, it's replication. If the new strand is RNA, it's transcription.
Pause and try this. A busy gene is copied into RNA many times a day. Is that replication or transcription?
Ask yourself what the product is.
Here's the answer. It's transcription, because the product is RNA, and the cell copies one gene as often as it needs that gene's product.
3. Writing the message: transcription
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 fits the pattern; it isn't a copy of it. Transcription works like that.
RNA polymerase is the enzyme that opens the DNA at a gene, reads the template strand, and builds a complementary RNA. The template strand is the DNA strand that RNA polymerase reads. The other strand is the coding strand, the strand that is not read; its sequence matches the mRNA except that it has thymine where the RNA has uracil. Uracil, written U, is the RNA base that takes the place of thymine.
The pairing rules are short. A DNA A becomes an mRNA U. A T becomes an A. A G becomes a C. A C becomes a G. RNA uses uracil, never thymine.
Here's Gene 1, a model gene built for this course. Its template triplets, read from the three-prime end, are TAC, CGA, ATA, and ACT. Pair each letter, and TAC becomes AUG, CGA becomes GCU, ATA becomes UAU, and ACT becomes UGA. So the mRNA is AUG, GCU, UAU, UGA.
Pause and try this. Scan that message, AUG, GCU, UAU, UGA. How many T's does it contain?
Look for one letter only.
Here's the answer. It contains no T at all, because every A on the template became a U, and RNA never carries thymine.
4. Reading the code
Picture a three-letter code stamped on a parcel. Each group of three letters points to one exact shelf in a warehouse. The cell reads an mRNA the same way, three bases at a time.
The genetic code is the set of rules a cell uses to read messenger RNA three bases at a time. A codon is a group of three bases on mRNA that stands for one amino acid or for stop. Four bases taken three at a time give 4 times 4 times 4, which is 64 codons. Of those, 3 are stop codons, so 64 minus 3 leaves 61 codons for amino acids.
Two kinds of codons run the show. The start codon is AUG; it marks where translation begins, sets the reading frame, and codes for methionine. A stop codon is one of UAA, UAG, and UGA; it codes for no amino acid and tells the ribosome to end the chain. The reading frame is the way an mRNA is split into groups of three, set by the start codon.
And here's some good news. Nobody in this course asks you to memorize the codon table. It's always supplied, and the skill is reading it carefully.
Pause and try this. Out of the 64 codons, how many tell the ribosome to stop, and what are they?
Count them on your fingers.
Here's the answer. Three codons say stop, UAA, UAG, and UGA, because they code for no amino acid.
5. The cushion: redundancy
Think of a building with more locker numbers than people. Some people get two lockers, so a one-digit slip sometimes still opens a locker that belongs to the same person. The genetic code has that 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. That's called redundancy of the genetic code: most amino acids have more than one codon.
Here's the cushion in action, straight from the table. GCU codes for alanine, and so does GCC. CCC codes for proline, and so does CCU. In each pair, only the third base differs, and the amino acid stays the same.
So a change from GCU to GCC leaves the protein exactly as it was. That's a cushion, not a shield, though. Plenty of single-letter changes do swap the amino acid, and you'll meet one soon.
Pause and try this. If a gene's GCU changed to GCC, would the protein change?
Check both codons against the table.
Here's the answer. No, the protein stays the same, because GCU and GCC both code for alanine.
6. Reading the message: translation
Picture a factory line. A worker reads an order slip one line at a time. A runner brings the matching part for each line, the part is added, and when the slip says "end," the finished product rolls off. That's translation.
The ribosome is the worker; it holds the mRNA and reads it one codon at a time, starting at AUG. Transfer RNA, or tRNA, is the runner: a small RNA that carries one amino acid on one end and an anticodon on the other. The anticodon is three bases that pair with a matching codon. Keep the two straight: the codon is on the mRNA, and the anticodon is on the tRNA.
Each time a tRNA's anticodon pairs with the next codon, its amino acid joins the chain. When the ribosome reaches a stop codon, no tRNA fits. A release factor, a protein that recognizes a stop codon, lets the finished chain go. That chain is a polypeptide, a chain of amino acids joined in the order the codons set, and it folds into a working protein.
For Gene 1, the codon AUG pairs with the anticodon UAC, which brings methionine.
Pause and try this. Which molecule carries the anticodon, the mRNA or the tRNA?
Think about who does the carrying.
Here's the answer. The tRNA carries the anticodon, because the tRNA is the runner that brings the amino acid, while the codon sits on the mRNA.
7. Decoding two genes
Now let's put every step together, like following a recipe card all the way to a finished plate.
Gene 1's mRNA is AUG, GCU, UAU, UGA. It has 12 bases, and 12 divided by 3 is 4 codons. The first codon is AUG, so we start there. AUG is methionine, GCU is alanine, UAU is tyrosine, and UGA is stop. So the protein is Met, Ala, Tyr. Four codons minus one stop codon leaves 3 amino acids.
Gene 2's template triplets are TAC, AAA, GGG, and ATC. Pair them, and you get the mRNA AUG, UUU, CCC, UAG. AUG is methionine, UUU is phenylalanine, CCC is proline, and UAG is stop. So Gene 2's protein is Met, Phe, Pro.
Notice what fixed what. The template fixed the mRNA. The mRNA, read in frame from AUG, fixed the protein.
Pause and try this. Why does Gene 2's protein have 3 amino acids and not 4?
Look at the last codon.
Here's the answer. The last codon, UAG, is a stop codon, and a stop codes for no amino acid, so only the first 3 codons add to the chain.
8. One letter that matters: the sickle-cell codon
Picture a text message with one letter wrong. "Meet at the dock" becomes "meet at the duck." One letter changed, and the meaning changed with it.
A gene can make 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. In sickle cell disease, that codon is GUG, which codes for valine. Compare them letter by letter: G matches G, the middle A became U, and G matches G. One base changed, and one amino acid changed.
That single swap sits at position 6 of the 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. Those fibers bend red blood cells into a rigid crescent shape that can block small blood vessels. Ingram identified that chemical difference in 1957. It's the structure-determines-function idea from Week 3, traced back to one letter.
Pause and try this. GAG becomes GUG. How many bases changed, and which amino acid replaces glutamic acid?
Compare the three letters one at a time.
Here's the answer. One base changed, the middle A to U, and valine replaces glutamic acid, because GUG codes for valine.
9. Checking the frame and the letters
Picture a sentence with every space moved one letter to the right. "The dog ran far" turns into nonsense. Every letter is still there, and the meaning is gone. That's what a wrong reading frame does.
Read Gene 1's mRNA starting one base too late. You get UGG, CUU, AUU, and two leftover letters, with no AUG anywhere in that frame. Not one of those codons matches the real ones. Start at AUG and count in threes, and the message comes back.
The second check is the letter scan. Suppose a chatbot transcribes the critique template, TAC, GGA, CCT, ACT, and writes AUG, CCT, GGA, UGA. Scan for T, and the second codon fails. Re-pair that triplet: the template's GGA becomes CCU. Now the message reads AUG, CCU, GGA, UGA, which is methionine, proline, glycine, and stop.
A chatbot is often right about the big picture and wrong on one letter. You're the check.
Pause and try this. A message reads AUG, CCT, GGA, UGA. What's wrong with it?
Scan for one forbidden letter.
Here's the answer. The second codon, CCT, contains a T, and an mRNA never carries thymine, so that codon should be CCU.
10. Carrying a gene through the lab
In this week's Biology Lab & Inquiry, you do the whole process yourself. You can use the Learn.Genetics Transcribe and Translate a Gene interactive, or work by hand with a supplied codon table.
The lab has a checklist, and it's the same one you've practiced here. First, transcribe each template into mRNA. Second, scan the mRNA for any T. Third, check the frame: start at AUG and count in threes. Fourth, translate each codon with the table, stopping at the stop codon. Fifth, name where each step happens: transcription in the nucleus, translation at a ribosome in the cytoplasm.
Then comes the step a chatbot can't do for you. You give a chatbot the critique template and ask it to transcribe and translate. Then you check its work letter by letter and frame by frame, and you correct any slip by hand.
Pause and try this. In the lab, after you write an mRNA, what are the two quickest checks before you translate?
Name one check about letters and one about counting.
Here's the answer. Scan the mRNA for any T, and confirm it starts at AUG and splits evenly into threes, because those two slips are the most common ones.
Closing
Let me pull it together in the order we built it. The central dogma runs from DNA to mRNA to protein. Transcription writes the message in the nucleus, and translation reads it at a ribosome in the cytoplasm. Replication copies every gene once per division; transcription copies one gene when it's needed. RNA polymerase reads the template strand and writes an mRNA with U, never T. The code is read in threes: 64 codons, 61 for amino acids, and 3 stops, with AUG as the start and methionine. Redundancy means GCU and GCC both give alanine. The codon is on the mRNA, and the anticodon is on the tRNA. Gene 1 gives Met, Ala, Tyr, and Gene 2 gives Met, Phe, Pro. One letter, GAG to GUG, swaps glutamic acid for valine in sickle cell disease. And every message gets two checks: no T, and start at AUG.
Next in this module come the slide deck, the Readings & Resources page, the lecture tutorial, the practice set, and the quiz, then the discussion, the assignment, and the Transcribe and Translate a Gene lab. Next week, in Week 15, we ask why cells switch genes on and off, and what happens when a gene's letters change.
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