Basic Processes

Gene expression is important because it converts the information stored in DNA into functional products—mainly proteins—which produce an organism’s traits. Thus, proteins link genotype and phenotype. Gene expression also allows cells to make the right proteins at the right time and in the right amount.

The steps involved are:
1. Transcription – the genetic information is copied from DNA to mRNA.
2. Translation – the information in mRNA is decoded on ribosomes to form a polypeptide chain.

So, gene expression follows the central dogma: DNA → RNA → Protein.

In bacteria, genes with related functions are often arranged together in an operon, so they can be controlled by a single switch.

The lac operon is an inducible operon that controls lactose metabolism in *E. coli*.

### Structure
- lacZ → codes for β-galactosidase
- lacY → codes for β-galactoside permease
- lacA → codes for β-galactoside transacetylase
- lacI → regulator gene that makes the repressor
- Promoter → site where RNA polymerase binds
- Operator → site where repressor binds

### In absence of lactose
- The repressor produced by lacI is active.
- It binds to the operator.
- This blocks RNA polymerase from transcribing lacZ, lacY, lacA.
- So, the structural genes remain switched off.

### In presence of lactose
- Some lactose is converted into allolactose.
- Allolactose acts as an inducer.
- It binds to the repressor and makes it inactive.
- The repressor can no longer bind the operator.
- RNA polymerase transcribes the structural genes into a polycistronic mRNA.
- The enzymes for lactose breakdown are produced.

Thus, the lac operon shows negative control in the absence of lactose and is induced when lactose is present.

If a cell loses all proteins, DNA replication would stop.

Reason:
- DNA replication needs many enzymes and proteins such as DNA polymerase, primase, helicase, topoisomerase, SSB proteins, and DNA ligase.
- Helicase unwinds the DNA.
- Primase makes the RNA primer.
- DNA polymerase synthesises the new DNA strand.
- Topoisomerase relieves strain.
- SSB proteins stabilise single strands.
- DNA ligase joins fragments.

Without proteins, the DNA cannot unwind, cannot be copied, and cannot be joined properly. So replication cannot occur.

UV rays damage DNA by causing the formation of pyrimidine dimers, especially thymine-thymine (T-T) dimers.

### Effect on DNA structure
- The adjacent pyrimidine bases join together.
- This distorts the DNA helix.
- The damaged DNA no longer has its normal structure.

### Molecular basis of the mutation
- UV light can cause deletion or substitution mutations.
- The distorted DNA may lead to incorrect copying during replication.
- This can alter the sequence of bases in the DNA.

### Repair mechanism
Cells remove such damage by nucleotide excision repair (NER):
1. A protein complex scans DNA and detects the T-T dimers.
2. The damaged section is cut out by cleaving the DNA a few nucleotides away from the lesion.
3. The removed fragment is taken away.
4. DNA polymerase I fills the gap with correct nucleotides.
5. DNA ligase seals the remaining nick.

So, UV rays cause pyrimidine dimers, lead to DNA helix distortion, and the damage is corrected mainly by nucleotide excision repair.