FoS II · Midterm 2

Lecture 7 — Agricultural GMOs

Every method in this lecture changes a crop's DNA; they differ in how precisely, how fast, and where the new DNA comes from. The trait only appears if the changed gene is expressed into a working protein, and even then the environment shapes the result.

Four ways to produce genetic change

MethodHow the change happensPrecisionWhere the DNA comes from
Selective breedingChoose parents with the trait you want; repeat over generationsLow: whole genomes mix, many genes change at onceExisting variation in the species
Mutation breeding (mutagenesis)Radiation or chemicals cause random DNA changes; screen for useful onesLow: random, many unknown changesThe plant's own DNA, altered at random
Recombinant DNA (transgenesis)A specific gene is cut out, joined to other DNA, and inserted into the plant genomeHigh for which gene; insertion site often not controlledOften a different species (transgenic)
CRISPR genome editingA guide RNA leads a Cas protein to one DNA sequence, which it cuts; the cell's repair makes the editHigh for where the change happensUsually the plant's own gene, edited in place

Key distinction: breeding and mutagenesis are old and imprecise but are not usually labelled GMO. Recombinant DNA and CRISPR are deliberate biotechnology. CRISPR can make a small edit that looks like a natural mutation; recombinant DNA adds a new sequence.

Four ways to change a crop's DNA, ordered by targeting precision From low to high precision: selective breeding and mutagenesis are both low precision; recombinant DNA is high for which gene but the insertion site is often not controlled; CRISPR is high for where the change happens. DNA origin is shown separately: only recombinant DNA often uses a gene from another species. targeting precision: low → high CRISPR genome editing High for where the change happens DNA from: usually its own gene, edited in place Recombinant DNA (transgenesis) High for which gene; insertion site often not controlled DNA from: often a different species (transgenic) ↑ deliberate biotechnology ↓ old methods, not usually labelled GMO Mutation breeding (mutagenesis) Low: random, many unknown changes DNA from: the plant's own DNA, altered at random Selective breeding Low: whole genomes mix, many genes change at once DNA from: existing variation in the species same species another species
Precision and DNA origin are separate questions. Precision climbs from breeding to CRISPR, but only recombinant DNA typically brings in another species' gene; CRISPR usually edits the plant's own gene in place, so its result can look like a natural mutation.

Transgenic vs cisgenic

  • Transgenic: the gene comes from a species that could never cross with the crop. Bt corn carries a gene from the bacterium Bacillus thuringiensis.
  • Cisgenic: the gene comes from the same species or a sexually compatible relative, such as a disease-resistance gene moved from a wild potato into a cultivated potato. Breeding could in principle produce the same result, just far more slowly.
  • The distinction is about the origin of the gene, not the method used to insert it.

From gene to trait

Gene (DNA) → transcribed into mRNA → translated into protein → protein does something → trait appears.

Example: the Bt gene is expressed into a Cry protein; the protein damages the gut of specific insect larvae that eat the plant; the visible trait is pest resistance. If the gene is present but poorly expressed, the trait is weak. Every exam answer about a GMO trait should walk this chain.

RNA interference (RNAi)

  • The plant makes a double-stranded RNA that matches part of a target gene's mRNA.
  • Cell machinery chops it up and uses the pieces to find and destroy (or block) the matching mRNA.
  • Less mRNA → less protein → the trait linked to that protein is reduced. This is gene silencing.
  • The DNA of the target gene is not changed. RNAi acts on RNA; CRISPR acts on DNA.
  • RNAi usually reduces protein (knockdown). A CRISPR knockout can eliminate it.
  • Common example: non-browning apples, where the enzyme that causes browning is silenced.
Central dogma, with where CRISPR and RNAi act DNA is transcribed into mRNA, mRNA is translated into protein, and the protein produces the trait. CRISPR points at the DNA step: it cuts and edits the gene. RNAi points at the mRNA step: it destroys the matching mRNA and leaves the DNA intact. DNA (gene) e.g. the Bt gene transcription mRNA working copy of the gene translation Protein e.g. Cry protein protein does its job Trait e.g. pest resistance CRISPR cuts and edits the DNA. The gene itself changes. Knockout: protein gone. RNAi destroys matching mRNA. The DNA is left intact. Knockdown: less protein. Either route can give the same trait, while the DNA differs.
CRISPR changes the gene; RNAi intercepts the message. Both can reduce the same protein and produce the same trait, but only CRISPR alters the DNA. If sequencing shows the gene is intact, the change was made at the mRNA step.

Genotype × environment

  • Phenotype = genotype + environment + how they interact.
  • The same seed can perform very differently with different water, temperature, soil or pests.
  • A drought-tolerance gene may show no advantage in a wet year. A trait measured in one test plot is evidence for that environment only.
Genotype by environment The same seed, carrying a drought-tolerance gene, is planted in two seasons. In a dry season the engineered crop yields more than the control. In a wet season the two yield the same. Phenotype equals genotype plus environment plus their interaction. Same seed, same genotype carries a drought-tolerance gene Dry season the gene's advantage shows control engineered Wet season no advantage at all control engineered Phenotype = genotype + environment + how they interact
One genotype, two results. The same drought-tolerance seed can win in a dry season and show nothing in a wet one. A trait measured in one environment is evidence for that environment only.

Monogenic vs polygenic traits

  • Monogenic traits (herbicide tolerance, Bt pest resistance) depend mainly on one gene, so adding or editing that gene gives a predictable result.
  • Polygenic traits (yield, drought tolerance) depend on many genes with small effects, each interacting with the environment. Changing one gene rarely moves the trait much, and the result is hard to predict.
  • This is why the biggest biotech successes are single-gene traits.

Goals of crop biotechnology

  • Pest and disease resistance: Bt crops; virus-resistant papaya.
  • Food quality: non-browning apples and potatoes.
  • Nutrient content: Golden Rice, engineered to make beta-carotene (provitamin A) in the grain.

Check yourself

  • Why can a transgenic gene from a bacterium work in a plant at all? (Hint: the genetic code is shared; see Lecture 12.)
  • Given a new crop trait, can you say whether it is likely monogenic or polygenic and what that predicts?
  • Can you explain why an RNAi crop and a CRISPR crop might show the same trait while having different DNA?

Further reading

Outside sources, opening in a new tab. The exam is on the course material; use these to check your understanding, not to learn new facts.

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