24.3 Biomedical Progress
Key Takeaways
Barrier methods block sperm physically, while estrogen and progestin suppress ovulation by negative feedback that keeps FSH and LH low.
An intrauterine device is a separate category of contraception, and in vitro fertilization joins sperm and egg outside the body before an embryo is transferred.
A restriction enzyme cuts DNA, a plasmid can carry a gene into a bacterium, and recombinant DNA is DNA joined from more than one source.
CRISPR-Cas9 uses a guide RNA to direct a cut, and the cell's repair then changes the sequence.
Gene therapy treats a person's cells, which is not the same as editing an embryo that can pass a change to children, and these tools raise questions of consent and access.
24.3 Biomedical Progress
Biomedical progress, on this outline, means control of human reproduction and genetic engineering. Both rest on mechanisms from earlier chapters: hormones and gametes on one side, DNA cutting and joining on the other. Scientific work also has social consequences. Those consequences sit beside the mechanism. They do not replace it.
Control of human reproduction
Chapter 14 followed the reproductive hormones. Gonadotropin-releasing hormone from the hypothalamus causes the anterior pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH supports growth of an ovarian follicle. A surge of LH triggers ovulation. Estrogen and progesterone usually restrain that axis by negative feedback. The methods below use that physiology. None of them is a dosing schedule.
Barrier methods block sperm physically. A condom or a diaphragm keeps sperm from reaching the egg. The mechanism is separation. The barrier does not lower FSH or LH, and it does not cancel the LH surge. If the barrier is missing or fails, the ovarian cycle continues.
Hormonal contraception that supplies synthetic estrogen and progestin works by negative feedback. The added hormones act on the hypothalamus and pituitary in the same restraining direction as the body's own sex steroids. FSH and LH stay low. Without the usual FSH support, the follicle does not mature in the ordinary way. Without the LH surge, ovulation is suppressed. In this introductory account the method prevents ovulation. It does not work by killing a fetus. A numerical dose is not part of the mechanism.
An intrauterine device is a separate category. It is placed in the uterus. It is not a barrier used at the time of intercourse, and it is not the systemic estrogen-progestin feedback just described. This account leaves its cellular action unspecified. Keep the device separate from barriers and from systemic hormonal feedback.
In vitro fertilization places a sperm and an egg together outside the body and then transfers an embryo to the uterus. Fertilization is still one sperm joining one egg. The difference is where that joining happens and the later transfer. That definition is the exam point, not a sequence of laboratory steps.
Genetic engineering
Genetic engineering changes DNA by design and puts that DNA to work in a cell. The introductory story has three pieces, and then a newer tool at the same level of detail.
A restriction enzyme cuts DNA. Bacteria use these enzymes to cut at short, particular sequences. In this account, the enzyme opens DNA so a gene-sized piece can be moved. The fact to retain is the cut.
A plasmid is a small DNA molecule common in bacteria, separate from the bacterial chromosome. A plasmid can carry a gene into a bacterium, and the bacterium can copy that plasmid as it grows. A plasmid is not a human chromosome. Putting a gene on a plasmid does not, by itself, edit a human embryo.
Recombinant DNA is DNA joined from more than one source. A cut can open a plasmid, and a gene taken from another molecule can be joined into that opening. The new molecule was not a single stretch in either source. Recognizing the cut, the plasmid carrier, and the joined molecule is the introductory task.
CRISPR-Cas9 uses a guide RNA matched to a chosen DNA sequence. The guide directs the Cas9 protein to that site, and Cas9 cuts there. The cell's own repair then changes the sequence. Remember the guide, the directed cut, and repair that changes the DNA.
Uses, and the social consequences
These tools have medical and agricultural uses. A bacterium that carries a human insulin gene on a plasmid can make insulin used as a medicine. Plant breeding can use a DNA change of this kind to alter a crop trait. The uses are still the biology above.
The same tools raise social questions, because science has social consequences. Consent asks whether the person who will live with a change agreed, which is especially sharp when that person cannot yet agree. Access asks who can obtain a therapy or an improved crop and who cannot. Edited human embryos add a further question. A change made in an embryo can enter every cell of the person who develops and can be passed to children. A successful edit does not settle consent, access, or inheritance.
Gene therapy aims to treat cells of a person, often a child or an adult with a disease, by adding or editing a gene in those cells. The aim is to change that person's illness. It is not the same project as editing an embryo so that children inherit the change. Treatment of body cells in one person and a heritable embryo edit are different biological targets.
| Method or tool | Mechanism in this account | Limit of the account |
|---|---|---|
| Barrier method | Physically blocks sperm | Does not keep FSH and LH low |
| Estrogen plus progestin | Negative feedback keeps FSH and LH low, so ovulation is suppressed | Does not work by killing a fetus |
| Intrauterine device | A device placed in the uterus, in its own category | No cellular mechanism is stated here |
| In vitro fertilization | Sperm and egg join outside the body, then an embryo is transferred | Not contraception, and not a laboratory protocol |
| Plasmid and recombinant DNA | A plasmid can carry a gene into a bacterium; recombinant DNA is joined from more than one source | A plasmid is not a human chromosome |
| Gene therapy | Treats cells of one person | Not an embryo edit passed on to children |
Warning
In this introductory account, hormonal contraception suppresses ovulation by negative feedback. It does not work by killing a fetus. A plasmid is a small DNA molecule that can carry a gene into a bacterium. It is not a human chromosome.
Barriers block sperm, and estrogen plus progestin keeps FSH and LH low so ovulation is suppressed. A restriction enzyme cuts, a plasmid can carry a gene into a bacterium, and CRISPR-Cas9 uses a guide RNA so the cell's repair can change a sequence. Consent, access, and edited embryos that pass a change to children are the social consequences that travel with those tools.
In the introductory account, how do combined estrogen and progestin contraceptives suppress ovulation?
They raise FSH and LH, which forces the release of several eggs.
They use negative feedback to keep FSH and LH low, so the signals that mature a follicle and trigger ovulation stay weak.
They kill a fetus after implantation by blocking the corpus luteum.
They cut sperm DNA with a restriction enzyme carried on a human chromosome.
Which description matches a plasmid in introductory recombinant-DNA work?
A plasmid is a small DNA molecule that can carry a gene into a bacterium, and it is not a human chromosome.
A plasmid is the guide RNA that CRISPR-Cas9 uses to choose where to cut.
A plasmid is the restriction enzyme that cuts DNA, and it is the same molecule as the cell's repair system.
A plasmid is a human chromosome edited so that a child inherits the change.
How is gene therapy different from editing an embryo that can pass a change to children?
Gene therapy is a barrier that physically blocks sperm from reaching the egg.
Gene therapy aims to treat cells of a person for that person's disease, and that aim is not the same as an embryo edit descendants can inherit.
Gene therapy keeps FSH and LH high so that ovulation is suppressed.
Gene therapy is defined as placing sperm and egg together outside the body and transferring an embryo.
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