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regions orintrons). Intron–exon boundaries are generally determined by the sequence

GU–AG and need to be removed or spliced before the mature mRNA is formed

(Fig. 5.18). The process of intron splicing is mediated by small nuclear RNAs (snRNAs)

which exist in the nucleus as ribonuclear protein particles. These are often found in

Regulatory

gene -Galactosidase

Permease

Transacetylase

Promoter Operator

mRNA RNA polymerase blocked by

repressor protein binding to

operator sequence

Repressor protein

(a)

(b)

ROY

Cannot bind operator

Inducer–repressor complex

Inducer (lactose)

-Galactosidase

Permease

Transacetylase

P Z A

ROP Z YA

Fig. 5.16Lactose operon (a) in a state of repression (no lactose present) and (b) following induction by lactose.

158 Molecular biology, bioinformatics and basic techniques