Regulation Human Genome

6.7 Regulation of Gene Expression

Cells don't translate ALL genes ALL the time. They turn genes ON or OFF in response to environmental cues — a process called gene regulation. The most studied example is the lac operon in E. coli.

6.7.1 The Lac Operon — An Inducible System (Jacob & Monod, 1961)

The lac operon controls how E. coli digests lactose. The operon has these components:

6.7.2 How the Lac Operon Works

When lactose is ABSENT (operon OFF):

• The repressor protein (made by gene i) binds to the operator (o).
• RNA polymerase cannot move past — transcription is blocked.
• The cell does NOT waste energy making enzymes it doesn't need.

When lactose is PRESENT (operon ON):

• A small amount of lactose enters the cell and is converted to allolactose (the inducer).
• Allolactose binds the repressor, changing its shape — repressor falls OFF the operator.
• RNA polymerase transcribes the structural genes z, y, a as a single polycistronic mRNA.
• The cell makes enzymes to metabolise lactose. Lactose induces its own breakdown.

6.7.3 The Trp Operon — A Repressible System

The trp operon works in reverse. It produces enzymes that synthesize the amino acid tryptophan. When tryptophan is plentiful in the environment, the cell doesn't need to make it. So:

• Trp absent: The trp repressor is INACTIVE → operon is ON (enzymes made, trp synthesized).
• Trp present: Trp binds repressor and ACTIVATES it → repressor binds operator → operon is OFF (no enzymes; no need).

Trp itself is a corepressor. The trp operon is a repressible operon — repressed when its end-product is plentiful.

6.8 The Human Genome Project (HGP)

The Human Genome Project (1990–2003) was a massive international effort to map and sequence the entire human genome.

6.8.1 Goals of the HGP

1. Identify all the genes in human DNA (~25,000 genes estimated).
2. Determine the sequence of all 3.2 billion DNA base pairs.
3. Store this information in databases.
4. Improve tools for data analysis.
5. Address the ethical, legal, and social issues (ELSI) raised by the project.

6.8.2 Salient Features of the Human Genome

• The human genome contains 3.2 × 10⁹ base pairs.
• The average gene is ~3000 bases. The largest gene is dystrophin (2.4 million bp).
• About 30,000 genes were initially predicted; current estimate is ~20,000–25,000.
• Less than 2% of the genome codes for proteins. The rest includes regulatory regions, introns, repetitive sequences.
• Repeated sequences make up large fractions; their function is mostly unknown ("junk DNA" — but increasingly we find some are functional!).
• Chromosome 1 has the most genes (~2968), Y has the fewest (~231).
• Humans share more than 99.9% of DNA with each other; the differences (~0.1%) account for variations between individuals.
• Identified ~1.4 million SNPs (Single Nucleotide Polymorphisms).

6.8.3 Methodology

Two approaches were used:

• Hierarchical sequencing (BAC): Genome cut into pieces, cloned in BAC vectors, mapped, then sequenced — public consortium method.
• Shotgun sequencing: Random fragments sequenced and computationally assembled — Celera Genomics' approach.

6.9 DNA Fingerprinting

DNA fingerprinting (DNA profiling) is a technique to identify individuals from a DNA sample. Developed by Alec Jeffreys in 1984. The technique is based on:

6.9.1 Variable Number Tandem Repeats (VNTRs)

The human genome contains regions where short DNA sequences are repeated multiple times in tandem (VNTRs / minisatellites). The number of repeats varies dramatically between individuals:

• Person A might have 12 repeats of "CAGT" at a particular location.
• Person B might have 25 repeats at the same location.
• Many such loci exist; combining them gives a unique pattern for each person.

6.9.2 Steps in DNA Fingerprinting

1. Isolate DNA from sample (blood, hair, saliva).
2. Digest DNA with restriction enzyme — cuts at specific sequences.
3. Separate fragments by gel electrophoresis (small fragments move faster).
4. Transfer to nylon membrane (Southern blotting).
5. Hybridise with radioactive VNTR-specific probe.
6. Expose to X-ray film — VNTR-containing fragments appear as bands of different sizes (the "fingerprint").
7. Compare fingerprints between samples.

6.9.3 Applications of DNA Fingerprinting

• Forensics: identify criminals from blood/hair at crime scenes; exonerate the innocent.
• Paternity / maternity testing.
• Identification of human remains in disasters, war.
• Tracing genealogy — finding biological relatives.
• Conservation: identifying poached endangered species DNA.
• Disease diagnosis: some genetic disorders are detected via specific DNA patterns.

Worked Examples