Nucleic acids and chromatin
Nucleic acids and chromatin

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Nucleic acids and chromatin

Antisense regulation of gene expression

The term antisense refers to the use of a nucleic acid that is complementary to the coding (i.e. ‘sense’) base sequence of a target gene. When nucleic acids that are antisense in nature are introduced into cells, they can hybridise to the complementary ‘sense’ mRNA through normal Watson-Crick base pairing. Synthetic antisense DNA chains as short as 15–17 nucleotides in length have been used to block specific gene expression by either physically blocking translation of the target mRNA or causing its degradation. The latter mechanism relies on induction of the ribonuclease RNAase H, which specifically cleaves the RNA–DNA duplex reg;on, thereby triggering further degradation of the now damaged mRNA.

SAQ 23

What properties of the target mRNA might inhibit this process?

Answer

For this process to be successful, the target region must be available for base pairing with the incoming DNA chain. The RNA secondary structure may prevent the antisense DNA from hybridizing.

In practice, to ensure success, antisense sequences are tested against several different target sequences in the mRNA. Antisense sequences (oligonucleotides) have been designed that can specifically inhibit a single gene transcript and can even inhibit an allele or an mRNA carrying a specific combination of spliced exons, while avoiding other members of the same family. Various modifications of the oligonucleotides have been made that counteract their rapid degradation by cellular nucleases. Antisense therapeutic agents have been made that successfully target HIV, hepatitis B, herpes simplex and papillomavirus infections as well as various cancers. As well as using exogenous antisense DNA oligonucleotides, which have only transient effects, continuous expression of antisense RNAs can be achieved by introducing transgenes into whole organisms and cell lines.

You may note a similarity between antisense technology and aspects of the properties of intrinsic MiRNAs.

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