Lecture: Gene Regulation, Mutation & Biotechnology
Lecture script · about 15–20 minutes spoken · Week 15
Opening
Hold up your arm for a second. The muscle cells in that arm carry every one of your genes. The nerve cells that tell those muscles when to move carry the very same genes. Same DNA, same cookbook, and yet two cells that look and act nothing alike. By the end of this lecture, you'll know why, and you'll know what happens when a recipe in that cookbook has a typo.
These ideas reach past this course: to a nursing student meeting gene therapies, a lab technician reading DNA bands, and anyone weighing whether embryos should be edited.
Here's where we are. In Week 13 you saw how DNA is built and copied. In Week 14 you watched a cell copy one gene onto an mRNA card and read it, codon by codon, into a protein. In this lecture I'll take that machinery further: how a cell decides which genes to read, what a typo does to a protein, and the tools that copy, sort, cut, and edit DNA. This is the last lecture of new material in the course.
1. Switching genes on and off
Picture one huge cookbook copied into every kitchen in a city. A bakery opens it to the bread pages. A soup kitchen opens it to the soup pages. Nobody cooks every recipe every day. Each kitchen reads the pages that fit its job.
Your cells work the same way. The genome is the complete set of DNA in an organism. Nearly every cell in your body carries the whole genome, even though each cell uses only part of it. A muscle cell switches on the genes for the proteins that let it contract. A neuron switches on the genes for the proteins that carry signals. The other genes stay on the shelf, closed.
That control has a name. Gene regulation is a cell's control over which genes are switched on and how much product each one makes. It pays off twice. It saves energy, because a cell doesn't build proteins it doesn't need. And it makes specialization possible, because one genome can build muscle, nerve, and skin when each cell type reads a different set of recipes.
Here's the trap: every cell carries every gene, but carrying isn't using.
Pause and try this. A muscle cell and a neuron come from the same person. Is the difference between them in their DNA, or in which genes they switch on?
Picture the two kitchens with one cookbook before you answer.
Here's the answer. The difference is in which genes they switch on, because both cells carry the same whole genome.
2. The lac operon: an on/off model
Now let's look at a switch up close, in a bacterium living in a gut. Some days a meal brings milk, with its sugar, lactose. Other days there's no lactose at all. Making lactose-digesting proteins on a day without lactose would waste the cell's energy. So the bacterium keeps those genes off until lactose shows up.
An operon is a group of bacterial genes that share one on/off switch on the DNA, so they're turned on or off together. The lac operon is the classroom model of gene regulation: the operon holding the genes for digesting lactose. Jacob and Monod described it in 1961.
Two more terms run the switch. A repressor is a protein that sits on the DNA at an operon and blocks its genes from being transcribed. Allolactose is a molecule the bacterium makes from lactose. It binds the repressor and pulls it off the DNA.
So run the two conditions. No lactose: the repressor sits on the DNA, RNA polymerase can't get through, and the genes stay off. Lactose arrives: the cell makes allolactose, allolactose pulls the repressor away, and the genes turn on. Full expression also needs glucose to be low, and that's a second condition you only need to name.
Think of the repressor as a brake. Lactose, working through allolactose, takes the brake off.
One honest note: the lac operon is a simple model. Your own cells regulate genes at many levels, from how tightly the DNA is packed to what happens to the finished protein.
Pause and try this. In the lac operon model, is the repressor the part that turns the genes on, or the part that keeps them off?
Say what the word "repressor" means before you decide.
Here's the answer. It keeps them off, because a repressor sitting on the DNA blocks transcription until allolactose pulls it away.
3. Typos that swap a letter
Copy a recipe by hand and you might slip a letter. The slip can change nothing, change an ingredient, or end the recipe early. DNA typos fall into the same three groups.
A mutation is a change in the DNA sequence. It's the ultimate source of new genetic variation. A point mutation changes a single base, and a substitution is a point mutation that swaps one base for another.
Let's run three substitutions on Gene 1 from last week. Its mRNA reads AUG GCU UAU UGA, which is 12 bases and 4 codons, and the codon table turns it into methionine, alanine, tyrosine, and then stop.
First change: GCU becomes GCC. That's 1 base. The codon table says GCC still means alanine, so the protein doesn't change at all. That's a silent mutation: the codon changes, the amino acid doesn't. Remember the cushion in the code from last week? This is it at work.
Second change: GCU becomes GAU. Again 1 base. But GAU means aspartic acid, so the protein becomes methionine, aspartic acid, tyrosine. One amino acid is different. That's a missense mutation: the codon now calls for a different amino acid.
Third change: UAU becomes UAA. Still 1 base. But UAA is a stop codon. Translation ends after methionine and alanine, so the protein has 2 amino acids instead of 3. That's a nonsense mutation: an amino-acid codon turns into a stop.
You already know one missense mutation. In the gene for hemoglobin's beta chain, the codon GAG, glutamic acid, becomes GUG, valine. One base, one amino acid, and sickle cell disease.
Pause and try this. GCU becomes GAU in Gene 1. Is that silent, missense, or nonsense?
Read the new codon in the table before you name it.
Here's the answer. It's missense, because GAU codes for aspartic acid, a different amino acid from the alanine GCU coded for.
4. Typos that slip the frame
Try reading a sentence in three-letter words: the cat saw the dog. Now delete the c from cat. Regroup in threes and you get nonsense from that point on. One missing letter scrambles every word after it.
A cell reads mRNA the same way, three bases at a time from AUG. That's the reading frame from last week. A frameshift mutation is an insertion or deletion of bases, not in a multiple of three, that shifts the reading frame so every codon after it changes.
Let's do it to Gene 1. Delete the first base after AUG, the G at the front of GCU. The gene had 12 bases, so 11 remain. Regroup from AUG and you read AUG, then CUU, then AUU, and then 2 bases left over, GA. That's 3 whole codons with 2 bases hanging off the end.
Now compare. After AUG, every codon has changed: CUU where GCU used to be, AUU where UAU used to be. And the UGA stop is gone from the frame. A substitution changes one codon. A deletion like this changes every codon after it, and it usually leaves a protein that doesn't work.
Pause and try this. One base is deleted right after AUG in Gene 1. How many codons after AUG change: one, or all of them?
Regroup the letters in threes in your head.
Here's the answer. All of them change, because the deletion moves every codon boundary after it.
5. Neutral, harmful, or helpful
Most people hear "mutation" and picture something going wrong. But the word only means a change in the DNA sequence.
A mutation's effect ranges from neutral to harmful to beneficial. Our silent change on Gene 1 left the protein the same, so it's neutral for that protein. The nonsense change cut the protein short, and the frameshift scrambled it, so those are likely harmful. And once in a while a change suits an organism better to its environment. That rare change is the raw material adaptation is built from.
Some things push the rate up. A mutagen is anything that damages DNA and raises the rate of mutation.
Here's where Week 1 comes back one last time. Mutation is the raw material of variation, and natural selection acts on that variation. Meiosis shuffles the versions of genes that exist; mutation makes new ones.
Pause and try this. Someone tells you that every mutation is harmful. Which of our Gene 1 changes shows that claim is wrong?
Think back to the change that left the protein alone.
Here's the answer. The silent change, GCU to GCC, shows it, because the protein stayed methionine, alanine, tyrosine, so that mutation was neutral.
6. Copy, then sort: PCR and the gel
Picture a tiny trace of blood left at a crime scene. It holds far too little DNA to study. You need a way to make many copies and a way to sort them.
Biotechnology is the use of living things and their molecules, such as DNA and enzymes, to make products or solve problems. The copier is PCR, the polymerase chain reaction, a method that copies a chosen stretch of DNA over and over. Each round runs in order. Denature: heat separates the two strands. Anneal: short starter pieces of DNA bind to the strands. Extend: a polymerase builds the new partner strands. That polymerase comes from the bacterium Thermus aquaticus, and because it's heat-stable it survives the heating step every round.
The sorter is gel electrophoresis, a method that sorts DNA fragments by size. Picture a slab of gel with a row of small wells along one edge. DNA is negatively charged, so when the current runs, the DNA moves away from the wells toward the positive end. The gel acts like a crowded room. Small fragments slip through quickly and travel farther. Big ones push through slowly and stay near the wells.
Beside the samples runs a size ladder, a lane of fragments of known sizes. On this week's virtual gel, which is illustrative, the ladder has bands at 500, 1,000, 2,000, and 4,000 base pairs. Each band is twice the one before. The 500 band travels farthest, and the 4,000 band stays nearest the wells. Since 4,000 divided by 500 is 8, the band nearest the wells is 8 times the size of the farthest one.
So PCR copies, and a gel sorts. Two tools, used in that order.
Pause and try this. A 200 base-pair fragment and a 3,000 base-pair fragment run on the same gel. Which one travels farther?
Picture the crowded room before you answer.
Here's the answer. The 200 base-pair fragment travels farther, because smaller fragments slip through the gel faster; the 3,000 fragment is 15 times larger.
7. Cut, paste, and edit
Insulin is a human protein, made by human cells. One way to get a lot of it is to hand the human insulin gene to bacteria and let them make it. That takes cutting and pasting DNA.
Recombinant DNA is DNA spliced together from two different sources. Here's the order. A restriction enzyme, an enzyme that cuts DNA at a specific base sequence, cuts out the insulin gene and cuts open a plasmid, a small ring of DNA in a bacterium. DNA ligase, the sealing enzyme from Week 13, joins the gene into the plasmid. The plasmid carries the gene into a bacterium, which is now a transgenic organism, an organism carrying a gene from another species. Then the bacterium transcribes and translates the gene like any other and makes human insulin. Cut, join, carry, express.
Cutting and pasting moves a whole gene. Editing changes a gene right where it sits. CRISPR-Cas9 is a gene-editing tool adapted from a bacterial immune defense. A guide RNA is made to match the target DNA sequence, and it leads the Cas9 protein to that spot, where Cas9 cuts. Think of the guide RNA as the address and Cas9 as the scissors. Charpentier and Doudna shared the 2020 Nobel Prize in Chemistry for developing this method of genome editing.
Pause and try this. In CRISPR-Cas9, which part decides where the cut happens: the guide RNA or Cas9?
Separate the address from the scissors.
Here's the answer. The guide RNA decides, because it's matched to the target sequence and leads Cas9 there to cut.
8. Reading a gel like a crime lab
Back to the trace of blood. PCR has copied its DNA. Now the lab runs the copies beside samples from people it wants to compare. DNA profiling is comparing the band patterns of DNA samples on a gel to see whether they could come from the same person.
Here's this week's virtual gel, which is illustrative. The crime-scene lane has 3 bands, at 500, 2,000, and 4,000 base pairs. Suspect 1 has bands at 1,000, 2,000, and 4,000. Two of the 3 bands line up, but the smallest one doesn't: 1,000 where the crime scene has 500. Suspect 1 is excluded. Suspect 2 has bands at 500, 2,000, and 4,000. All 3 line up, so Suspect 2 matches.
Now the subtle part. One band that differs is enough to rule a person out. A match only fails to rule a person out, since someone else could share the pattern. So an exclusion is firmer than a stand-alone match. That's the heart of this week's lab, Whose DNA Is It?
Pause and try this. On this gel, which call is firmer: excluding Suspect 1, or matching Suspect 2?
Ask what each result rules out.
Here's the answer. Excluding Suspect 1 is firmer, because one mismatched band settles it, while a match only leaves Suspect 2 in.
9. Gene editing as open questions
Suppose a tool could remove the typo behind a serious inherited disease. Should doctors use it in a sick person? In an embryo? This course leaves those questions open.
Start with a line. Somatic editing is gene editing of body cells, which changes only the person treated and isn't passed to that person's children. Germline editing is gene editing of reproductive cells, such as eggs and sperm, or of an embryo, so the change can pass to future generations.
Two real events sit on either side of that line. In 2023 a somatic CRISPR therapy for sickle cell disease was approved in the United States. In 2018 an announcement that human embryos had been edited drew international condemnation from scientists.
Now hear both cases at full strength. The case for editing: some heritable diseases are devastating, and editing could prevent them. The case for caution: Cas9 can cut in the wrong place, and future generations can't consent to a change made before they exist. A tool built for therapy could slide into enhancement. And a costly therapy raises the question of who gets access.
The science tells you what each edit does and whom it affects. It doesn't tell you which risks are worth taking. That's a question of values, and thoughtful people land in different places.
Pause and try this. A described edit changes a person's skin cells. Is that somatic or germline editing?
Ask whether the change could reach that person's children.
Here's the answer. It's somatic, because skin cells are body cells, so the change stays with the person treated.
Closing
Let me pull it together in the order we built it. Every cell carries the whole genome, and gene regulation decides which genes each cell reads. In the lac operon, the repressor is the brake, and allolactose takes it off. A substitution can be silent, missense, or nonsense, and the codon table tells you which. A frameshift moves the reading frame, so every codon after it changes. Effects range from neutral to harmful to beneficial, and mutation is the raw material of variation. PCR copies, and a gel sorts, with the smallest fragments traveling farthest. Recombinant DNA cuts, joins, and carries a gene, and CRISPR-Cas9 edits where a guide RNA leads. One mismatched band excludes a person, and an exclusion is firmer than a match. And the gene-editing questions stay questions, with both cases weighed.
Back to the cookbook. Cells already choose which recipes to read. Now people can copy, sort, and rewrite the recipes. What we should do with that is yours to think through.
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 Whose DNA Is It? lab. After that comes finals week, with the Final Practice Exam and the final, and nothing new to learn there, only the habits you've built across the whole course.
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