Directionality (molecular biology)



  Directionality, in molecular biology, refers to the end-to-end chemical orientation of a single strand of polymerases are usually noted as being either upstream (towards the 5' end) or downstream (towards the 3' end).

The importance of having this naming convention lies in the fact that nucleic acids can only be synthesized in vivo in a 5' to 3' direction, as the RNA sequences are written in 5' to 3' direction.


5' end

The 5' (pronounced "five prime") end is named as the strand terminates at the chemical group attached to the fifth carbon in the sugar-ring. If a phosphatase.

The 5' end is also the site at which translation, providing resistance to the degradative effects of exonucleases.[citation needed] It consists of a methylated nucleotide (methylguanosine) attached to the messenger RNA in a rare 5' to 5' triphosphate linkage.

The 5' flanking region of a gene often denotes a region of DNA which is not transcribed into RNA. The 5'-flanking region contains the gene promoter, and may also contain enhancers or other protein binding sites.

The ribosome binding site and Kozak sequence which determine the translation efficiency of the mRNA, or which may affect the stability of the mRNA.

3' end

  The 3' (pronounced "three prime") end of a strand is so named due to it terminating at the hydroxyl (-OH) group of the third carbon in the sugar-ring, and is known as the tail end. The 3'-hydroxyl is necessary in the synthesis of new nucleic acid molecules as it is ligated (joined) to the 5'-phosphate of a separate nucleotide, allowing the formation of strands of linked nucleotides.

Molecular biologists can use DNA. This technique is known as both the dideoxy termination method and the Sanger method, and was used to determine the order of nucleotides in DNA.

The 3' end is also the site of post-translational polyadenylation, which attaches a chain of 50 to 250 translation. This chain helps in determining how long the messenger RNA lasts in the cell, and therefore how much protein is produced from it.

The 3' flanking region is a region of DNA that is not copied into the mature mRNA, but which is present adjacent to 3' end of the gene. It was originally thought that the 3' flanking DNA was not transcribed at all, but it was discovered to be transcribed into RNA and quickly removed during processing of the primary transcript to form the mature mRNA. The 3' flanking region often contains sequences that affect the formation of the 3' end of the message. It may also contain enhancers or other sites to which proteins may bind.

The 3' untranslated region is a region of the DNA which IS transcribed into mRNA and becomes the 3' end or the message, but which does not contain protein coding sequence. Everything between the stop codon and the polyA tail is considered to be 3' untranslated (see Figure 4). The 3' untranslated region may affect the translation efficiency of the mRNA or the stability of the mRNA. It also has sequences which are required for the addition of the poly(A) tail to the message (including one known as the "hexanucleotide", AAUAAA).

References

  • Lodish et al., Molecular Cell Biology, 5th edn., 2004, W.H. Freeman and Company, New York. ISBN 0-7167-4366-3
 
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