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Nonsense mediated decay

Nonsense mediated decay (NMD) is a molecular process in which potentially harmful mRNAs containing premature stop codons are degraded prior to translation.

Genetically engineered mice lacking a key NMD protein do not survive embryogenesis, illustrating the importance of NMD in mammals [1].

Errors accompany every step necessary for mRNA production. Premature stop codons can be created by errors in transcription and splicing. Errors rates may be particularly high for splicing, especially in mRNAs that undergo alternative splicing.

NMD also serves to eliminate transcripts generated from non-functional genes. Genomes often contain and transcribe many non-functional genes, such as transposons and pseudogenes.

Spliceosome

A spliceosome is a large complex of small nuclear RNA (snRNAs) and protein, which removes intervening sequences (introns) from precursor mRNA (pre-mRNA).

The five snRNAs are rich in uridine (U) nucleotides and are thus named U1, U2, U4, U5, and U6. While RNA molecules do not generally catalyze chemical reactions, the snRNAs are required for the catalysis of the splicing reaction and may be directly involved [Cathy].

Each snRNA is complexed with protein as a small nuclear ribonucleoprotein (snRNP, pronounced snurp). The five snRNPs assemble together on the pre-mRNA in a step-wise order to form an active spliceosome. The spliceosome first recognizes specific “splice sites” on the pre-mRNA that identifies the region as an intron. The intron is then removed in two transesterification reactions, simultaneously joining two exons together. The excised intron is then degraded.

Spliceome

The term spliceome was coined in the early 21st century to describe the set of all possible alternative splices in an organism. It is an analogy to the genome or proteome. The concept and list of alternative splices is useful in bioinformatics and computational biology. It allows modular development of computational technology to deal with the spliceome.

References

• Chapter 12, pp 311 7th ed, Vishal.

• Alberts, Bruce. Bray, Dennis. Hookin, Karen. Johnson, Alexander, Lewis, Julian, Raff, Martin, Roberts, Keith. Walter, Peter. essential cell biology Second edition, GS Garland Science, Taylor & Francis Group, NEW YORK AND LONDON.

External links

• The spliceosome: the most complex macromolecular machine in the cell?

mRNA

Introduction

For a protein coding gene to be expressed a message must be sent from DNA, which stores genetic information, to the ribosome, which produces the protein product. Messenger RNA (mRNA) is the intermediate molecule used by cells to deliver this message.

From a single gene, many mRNA molecules copies may be produced over time. At any given time, multiple copies of an mRNA be present in the cell. The number of mRNA molecules usually coorelates with the number of proteins present at that time. New mRNAs may be produced when more protein is needed by the cell, while existing mRNAs may be destroyed when less is needed.

mRNA molecules are similar to the DNA from which they are produced (see RNA). However, unlike DNA molecules, which are double stranded, mRNAs are single stranded. Some mRNAs also have a cap and tail at the ends of the molecule, which are important for recognition by ribosomes.