How DNA Fingerprinting Can Pick One Person Out of a Crowd

Not every gene is active all the time — bacteria switch genes on only when they are needed. At a much larger scale, reading the entire human genome and comparing DNA between people has transformed medicine and forensic science.

This part covers the lac operon, the Human Genome Project, its applications and issues, and DNA fingerprinting.

An operon is a set of genes controlled together by a common promoter and operator; in the lac operon of E. coli, a repressor blocks the operator when lactose is absent, but lactose acts as the inducer, inactivating the repressor so that the genes for lactose use are transcribed.

Parts of the lac operon:

- **Regulatory gene i — codes for the repressor, which is made all the time
- Promoter — where RNA polymerase binds
- Operator — where the repressor binds
- Structural genes z, y and a — code for $\beta$-galactosidase, permease and transacetylase

Without lactose:

- The repressor binds the operator
- RNA polymerase cannot transcribe the structural genes, so the operon is switched off

With lactose:

- Lactose, the inducer, binds the repressor and inactivates** it
- RNA polymerase transcribes z, y and a, so the operon is switched on
- $\beta$-galactosidase splits lactose into glucose and galactose, and permease helps lactose enter the cell

Because the repressor switches the operon off, this control is called negative regulation.

An everyday example. A motion-sensor light in a building corridor stays off until someone walks past — the lac operon stays off until lactose arrives.

The substance. The cell saves energy — making lactose-digesting enzymes with no lactose around would waste resources.

The Human Genome Project aimed to identify all human genes and sequence the roughly 3 billion base pairs of human DNA, using expressed sequence tags to identify expressed genes and sequence annotation to sequence the whole genome, and it found that less than 2 percent of the genome codes for proteins.

Goals:

- Identify all the genes in human DNA
- Determine the sequence of about 3 billion base pairs
- Address the ethical, legal and social issues that arise

Methodologies:

- Expressed Sequence Tags (ESTs) — identifying genes that are expressed as RNA
- Sequence annotation — sequencing the whole genome, coding and non-coding, and later assigning functions

Salient findings:

- The genome has about 3164.7 million base pairs and roughly 30 000 genes
- 99.9 percent of bases are identical in all people
- Less than 2 percent of the genome codes for proteins, and repeated sequences make up a large part
- Chromosome 1 has the most genes and the Y chromosome the fewest
- About 1.4 million sites with single-base differences, SNPs, were identified

An everyday example. A detailed road atlas of India shows where everything lies before we know what every building is used for — much like a genome sequence.

The substance. Knowing a sequence is not the same as knowing its function — many sequenced genes still have unknown roles.

Genome knowledge helps diagnose and treat genetic disorders, trace human evolution and personalise medicine, but analysing such vast data created the field of bioinformatics and raised ethical concerns about privacy and discrimination; SNPs, single-base differences between people, help locate disease genes and trace ancestry.

Applications:

- Understanding, diagnosing and treating genetic diseases
- Studying the genomes of bacteria, yeast, nematodes, fruit flies and rice
- Tracing human evolution and relationships between populations
- Developing personalised medicine

Bioinformatics. Storing, retrieving and analysing sequence data needs powerful computers and software, giving rise to bioinformatics, which combines biology, computer science and statistics.

SNPs:

- Sites where individuals differ by a single base
- Help find chromosomal locations of disease-associated sequences
- Help trace human history and migration

Challenges and ethical issues:

- Privacy — who may see a person's genetic information
- Discrimination — possible misuse by employers or insurers
- Consent and the psychological impact of learning about disease risks

An everyday example. Some hospitals test a cancer patient's genes to choose the medicine most likely to work — personalised medicine in practice.

The substance. Most SNPs have no effect on health, but those close to disease genes act as useful signposts.

DNA fingerprinting compares repetitive DNA sequences called VNTRs, whose number of repeats differs from person to person; DNA is isolated, cut with restriction enzymes, separated by electrophoresis, blotted onto a membrane, hybridised with a labelled VNTR probe and detected by autoradiography, giving a band pattern unique to each individual except identical twins.

The basis:

- Repetitive DNA separates out as satellite DNA in density-gradient centrifugation
- VNTRs (variable number of tandem repeats) are satellite sequences whose copy number varies between individuals
- This inherited variation is a form of DNA polymorphism

The steps:

- Isolation of DNA from blood, hair roots, skin or saliva
- Digestion with restriction endonucleases
- Separation of fragments by gel electrophoresis
- Blotting onto a nitrocellulose or nylon membrane — Southern blotting
- Hybridisation with a labelled VNTR probe
- Detection by autoradiography

Applications:

- Forensic science — linking suspects to blood, hair or skin found at a crime scene
- Parentage disputes — each band of a child must match a band of the mother or the father
- Studying genetic diversity and evolution in populations

An everyday example. Courts in India accept DNA evidence in paternity cases and criminal trials.

The substance. Identical twins have the same DNA fingerprint, because they develop from a single zygote.