How a Cell Reads a Gene and Builds a Protein From It

A gene is only a stretch of DNA. To make a protein, the cell first copies that stretch into RNA, then reads the RNA three bases at a time to join amino acids in the right order — and it switches genes on or off depending on what it needs.

This lesson covers transcription and RNA processing, the genetic code and translation, the lac operon, and the Human Genome Project with DNA fingerprinting.

Transcription copies the template strand of a gene into RNA using RNA polymerase, and in eukaryotes the first transcript is processed by splicing out introns, adding a cap at the 5' end and adding a poly-A tail at the 3' end.

A transcription unit:

- Promoter — where RNA polymerase binds
- Structural gene — the stretch copied into RNA; the template strand runs 3' to 5', while the coding strand has the same sequence as the RNA, with T in place of U
- Terminator — marks where transcription stops

Steps:

- Initiation — RNA polymerase binds the promoter, helped by initiation factors
- Elongation — RNA is built in the 5' to 3' direction by base pairing
- Termination — a termination factor releases the RNA at the terminator

RNA polymerases in eukaryotes. RNA polymerase I makes rRNA, II makes the mRNA precursor called hnRNA, and III makes tRNA and small RNAs.

Processing of hnRNA:

- Splicing — non-coding introns are removed and coding exons are joined
- Capping — methyl guanosine triphosphate is added to the 5' end
- Tailing — a string of adenine nucleotides is added to the 3' end

An everyday example. An editor trimming unwanted footage from a wedding video before sharing it works like splicing, which removes introns before the message leaves the nucleus.

The substance. Bacteria do not need splicing — their genes lack introns, and translation can begin while transcription is still going on.

The genetic code is a triplet, nearly universal, unambiguous and degenerate code with 61 codons for amino acids and 3 stop codons, and translation joins amino acids on ribosomes in the order set by mRNA codons, using tRNA as the adapter.

Features of the genetic code:

- Triplet — three bases form a codon, giving 64 codons in all
- 61 codons code for amino acids, and 3 — UAA, UAG and UGA — are stop codons
- Unambiguous — one codon codes for only one amino acid
- Degenerate — some amino acids are coded by more than one codon
- Nearly universal — the same codons mean the same amino acids in almost all organisms
- AUG is the start codon and also codes for methionine

Translation in prokaryotes:

- Charging of tRNA — each amino acid is attached to its tRNA using energy from ATP
- Initiation — the ribosome binds mRNA at AUG, where the initiator tRNA carrying methionine pairs with it
- Elongation — tRNAs pair their anticodons with codons, and a ribozyme in the ribosome forms peptide bonds
- Termination — at a stop codon, a release factor frees the polypeptide

An everyday example. Human insulin made in bacteria for diabetic patients works because the genetic code is nearly universal — bacteria read the human gene correctly.

The substance. A single base deletion changes every codon after it — because the code is read continuously, the reading frame shifts.

The lac operon is a set of genes for using lactose that is switched off by a repressor when lactose is absent and switched on when lactose, acting as the inducer, binds the repressor and frees the operator.

Parts of the lac operon:

- Regulator gene (i) — makes the repressor protein
- Operator — the switch where the repressor binds
- Structural genes — z codes beta-galactosidase, which splits lactose into glucose and galactose; y codes permease, which lets lactose into the cell; a codes transacetylase

When lactose is absent:

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

When lactose is present:

- Lactose acts as an inducer and binds the repressor
- The inactive repressor cannot bind the operator
- RNA polymerase transcribes z, y and a, and the enzymes are made

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

The substance. The enzymes are made only when needed — a bacterium does not waste energy making lactose-digesting enzymes when there is no lactose.

The Human Genome Project aimed to identify all human genes and determine the sequence of base pairs in human DNA, and DNA fingerprinting compares highly variable repeated DNA sequences to identify individuals and relationships.

Goals of the Human Genome Project:

- Identify all the genes in human DNA
- Determine the sequence of the base pairs that make up human DNA

Findings and uses:

- Less than 2 per cent of the genome codes for proteins, and repeated sequences make up a large portion

DNA fingerprinting:

- Based on variable number tandem repeats (VNTRs), short repeated sequences whose number differs between people
- DNA is isolated, cut with restriction enzymes, separated by gel electrophoresis and transferred to a membrane by Southern blotting
- A labelled VNTR probe binds matching fragments, and autoradiography reveals a band pattern unique to each person

Applications. Forensic identification, settling parentage disputes, and studying genetic diversity in populations.

An everyday example. Forensic laboratories helping police in Indian cities match DNA from crime scenes with suspects using these band patterns.

The substance. Identical twins share the same DNA fingerprint — because they come from one zygote, their VNTR patterns match.