Cloning, Stem Cells, Reproductive Technology and Forensics

1Overview

Some of the most striking applications of genetics are also the most debated: copying animals, growing replacement tissue from stem cells, helping people to have children, and identifying a person from a trace of DNA. This lesson covers the rest of the official applied-genetics content: cloning, stem cells, reproductive technology, forensics, and applications to species, populations and ecosystems.

2Cloning

Cloning refers to scientific methods used to make identical or nearly identical copies of an organism, a cell or a DNA sequence. The word covers several different things.

• Cellular cloning. Single-celled organisms such as bacteria clone themselves through binary fission.
• Molecular (gene or DNA) cloning. A specific DNA segment is isolated and duplicated so that it can be studied or amplified from a small sample, such as blood or saliva, for genetic tests. This is the plasmid method from the previous lesson.
• Reproductive cloning. A whole animal is produced that is genetically almost identical to a donor. Dolly the sheep, born in 1996, was the first cloned mammal. She lived seven years, and cloned animals often show abnormalities.
• Therapeutic cloning. The aim is to produce stem cells rather than whole organisms. Some people object to it on bioethical grounds.

3Stem cells

A stem cell is a cell with the potential to form many of the different cell types found in the body. When a stem cell divides, it can produce more stem cells or specialised cells. The earlier lesson on gene expression explained that specialisation (differentiation) happens because different genes are switched on.

• Embryonic stem cells come from early embryos (about 6 to 8 days after fertilization in people). They can form a complete individual and have more developmental potential than adult stem cells.
• Adult (somatic) stem cells are found in mature organisms and form only certain cell types. Bone marrow stem cells produce blood cells, and other stem cells are found in the liver, nervous tissue and muscle.

In symmetric division both daughter cells remain stem cells, which expands the pool, for example after a bone marrow transplant. In asymmetric division one daughter stays a stem cell and the other becomes specialised, which is the usual pattern in bone marrow and keeps stem-cell numbers stable. Stem cells keep dividing as long as the individual lives. Their medical value lies in replacing damaged tissue and in studying how diseases develop; the source of embryonic stem cells is the main reason they are ethically debated.

4Reproductive technology

In vitro fertilisation (IVF) is a fertility treatment in which eggs are fertilised with sperm in a laboratory and the resulting embryo is placed in the womb. It is usually considered when pregnancy is difficult and other treatments have not worked. A full cycle takes about three to six weeks:

1. Daily medication for two to three weeks stops natural egg release.
2. Daily injections for about two weeks boost the egg supply, with monitoring.
3. Eggs are collected with a needle, under sedation if wanted.
4. A sperm sample is provided (or donor sperm prepared).
5. Eggs are fertilised in the laboratory.
6. An embryo is transferred to the womb a few days later. A pregnancy test follows about 16 days afterwards, and spare suitable embryos may be frozen.

Chances of success depend on the cause of fertility problems, age, body mass index and lifestyle. Risks include ovarian hyperstimulation syndrome, multiple pregnancy and ectopic pregnancy. Access varies: in the UK, public funding depends on age, number of cycles and local rules, while private clinics set their own limits, and clinics abroad may not be regulated to the same standard.

Genetic testing is closely linked to reproduction. Tests for carrier status can be done before pregnancy, and during pregnancy options include early blood tests, chorionic villus sampling (CVS), amniocentesis and noninvasive prenatal testing (NIPT). Genetic testing is voluntary, and a genetic counsellor can explain what a test can and cannot show.

5Forensics and DNA fingerprinting

DNA fingerprinting (DNA profiling) is a laboratory technique for estimating a person’s probable identity from DNA sequences in regions that differ between individuals. It compares two profiles, for example one from a known person and one from an unknown sample, to see whether they match. Uses include criminal investigations, other forensic purposes and paternity testing.

Two laboratory tools make this possible:

• PCR (polymerase chain reaction) copies a chosen region of DNA in three repeated steps (denaturation, annealing and synthesis), using a heat-stable polymerase from the bacterium Thermus aquaticus. It can produce enough DNA from a tiny sample.
• Gel electrophoresis separates nucleic acids by size. DNA is negatively charged, so it moves toward the positive electrode, and smaller fragments travel farther.

Other forensic uses include mitochondrial DNA, which is inherited from the mother and is used to trace genealogy, and microbial genomics, which in 2001 showed that the anthrax mailings came from one specific strain.

6Species, populations and ecosystems

Genetic tools are used well beyond human medicine.

• Conservation genetics. Genetic diversity is central to the long-term survival of any species. Researchers use molecular-genetic methods to study rare and endangered species, to measure how roads affect the movement of populations, and to guide protection planning.
• Environmental DNA (eDNA). DNA from air, water and soil samples is used to monitor habitats and detect rare species.
• Metagenomics. Studying the combined genomes of many species in a habitat helps identify new species and the effects of pollutants.
• Agriculture and ecosystems. Genomic data guide crop breeding. Engineered genes can also escape into wild populations, so effects on non-target organisms and biodiversity are monitored.

Each of these tools raises questions about who benefits, who bears the risks and who decides. The last lesson takes up those ethical considerations.

7Practice problems with solutions

Problem 1: read a DNA fingerprint

A gel shows three bands for a crime-scene sample at positions 2, 5 and 8 (counted from the top). Suspect 1 has bands at 2, 5 and 8. Suspect 2 has bands at 2, 4 and 8. Suspect 3 has bands at 3, 5 and 7. Suspect 1 matches and cannot be excluded. Suspects 2 and 3 differ in at least one band and are excluded. A match is not proof by itself, because investigators also consider other evidence and the possibility that other people share a pattern. Because smaller fragments travel farther, bands lower on the gel are smaller pieces of DNA.

Problem 2: choose the stem cell

A researcher wants cells that could form any tissue in a developing embryo. Embryonic stem cells have the greatest developmental potential. A doctor treating a blood disorder with a bone-marrow transplant uses adult stem cells, because bone-marrow stem cells give rise to blood cells. Each type suits a different purpose, and the source of embryonic cells is part of the ethical debate.

Problem 3: what does cloning copy?

A sheep is cloned from the nucleus of an adult sheep’s cell. The clone has almost the same DNA as the donor, but it will not be identical in every respect, because the environment affects development, and cloned animals often show abnormalities. Cloning copies a genotype, not an individual’s life.

Problem 4: IVF timeline

If an IVF cycle takes about three to six weeks, and a pregnancy test comes about 16 days after embryo transfer, then the results arrive well after the egg collection, which is why the cycle involves several appointments and monitoring. Success depends on age, cause of infertility and lifestyle, so counsellors discuss expectations carefully.

Tools used in the lab: PCR in more detail

PCR repeats three steps. In denaturation, heat separates the two DNA strands. In annealing, short primers bind to the target region on each strand. In synthesis, a heat-stable polymerase (Taq polymerase, from Thermus aquaticus) builds new strands. Each cycle doubles the number of copies of the target, so a few cycles are enough to produce a visible amount. The heat-stable enzyme matters because it survives the heating step in every cycle. This is why a trace sample from a scene can be analysed.

Genomics for species and ecosystems

Environmental DNA lets researchers detect a rare fish from a water sample without catching it. Metagenomics can reveal which microbes live in soil and how pollutants change them. Genetic data on the connectivity of populations can show whether a road is separating animals into isolated groups, which would reduce gene flow and genetic diversity. All of these applications use the same core idea: the DNA sequence is a record that can be read, compared and interpreted.

Summary

• Cloning can mean copying a gene, a cell or an organism.
• Stem cells can form many cell types, and embryonic and adult stem cells differ in potential.
• IVF fertilises eggs in a laboratory and transfers an embryo, with eligibility and cost varying by location.
• DNA fingerprinting compares band patterns made by PCR and gel electrophoresis and can exclude or support, but not prove, a link.
• Genetic methods help monitor and protect species, populations and ecosystems.

Study guide: key distinctions

Applying the ideas

A student asked to design a game or activity about heredity could build a card game that uses Punnett squares, a detective activity that matches DNA band patterns, or a board game about stem-cell differentiation. Whichever is chosen, the activity should state the rules of inheritance accurately, include chance, and show how the environment influences outcomes.

Reading a news story critically

Stories about cloning, stem cells or DNA evidence often simplify. A useful checklist is to ask what kind of cloning or stem cell is meant, whether the claim describes an animal study or a human trial, how large the sample was, whether the source is a research institution or a company, and whether the claim says “prove” where the evidence only supports. Applying this checklist is part of evaluating evidence, a core skill of the course.

?Quick check

Try each question first, then reveal the answer.