Oligonucleotides are short sequences of nucleotides, which are the building blocks of DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Oligonucleotides typically consist of 10 to 30 nucleotides, though their size can vary. They are widely used in molecular biology, genetics, and biotechnology due to their versatility and ability to interact specifically with DNA or RNA sequences.

Oligonucleotides can be synthesized artificially and have various applications, including:

1. Primers for polymerase chain reaction (PCR): PCR is a common technique used to amplify specific DNA sequences. Oligonucleotides serve as primers that bind to complementary DNA sequences, allowing the enzyme Taq polymerase to initiate the replication process.
2. Gene synthesis: Oligonucleotides can be used as building blocks to create longer DNA sequences for the synthesis of specific genes.
3. Antisense oligonucleotides: These are single-stranded oligonucleotides designed to bind to specific mRNA sequences. By doing so, they can block protein translation or promote mRNA degradation, effectively regulating gene expression.
4. RNA interference (RNAi): Small interfering RNAs (siRNAs) are a type of oligonucleotide that can be used to silence specific genes by inducing the degradation of target mRNA molecules.
5. CRISPR/Cas9 gene editing: Can be used as guide RNAs to direct the Cas9 nuclease to specific DNA sequences, enabling targeted gene editing.
6. Molecular diagnostics: Can be used as probes to detect the presence of specific DNA or RNA sequences in samples, such as in identifying pathogens or genetic mutations.

Overall, oligonucleotides are critical tools in molecular biology and biotechnology, with a wide range of applications in research, diagnostics, and therapeutics.