strand terminologies

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Names given to distinguish the two strands of a DNA molecule. Each strand of a DNA molecule has a 5′ end and a 3′ end. The 5′ end has a PO4 molecule connected to the number 5 carbon of the first sugar. The 3′ end has an OH group connected to the number 3 carbon of the last sugar. The two strands of each DNA molecule are aligned in an antiparallel configuration, that is, they point in opposite directions. Terminologies for these strands depend on conventions adopted for messenger RNA. Since it represents a set of instructions, mRNA is considered to be a “sense” molecule, and therefore a synthetic RNA molecule with a complementary nucleic acid sequence has been named an antisense RNA (q.v.). Here the prefix anti signifies opposite to or lying alongside. When the nucleotide sequence of an mRNA is printed in a scientific publication, the 5′ end is always drawn above (as in the diagram above) or to the left. The direction of transcription is then down or from left to right. When mRNA is translated in a ribosome, the amino end of the new protein is the first and the carboxyl end the last to be formed. The DNA strand that serves as the template for mRNA is called the template strand. The other strand will contain segments that are identical in sequence to the codons in the mRNA, assuming one substitutes Ts for Us. For this reason, the DNA strand complementary to the template strand is labeled “sense.” It is the sense strand that is drawn when a “gene sequence” is presented in the literature. Upstream refers to the 5′ direction and downstream to the 3′ direction on the sense strand. For example, the promotor sequence will be upstream (to the left) of the first exon, and the polyadenylation site will be downstream (to the right) of the last exon. Other terms such as coding strand, anticoding strand, and antisense strand are found in the literature, but since they are used inconsistently, they should be avoided in the future. See deoxyribonucleic acid, leader sequence, plus (+) and minus (−) viral strands, polyadenylation, posttranscriptional processing, trailer sequence, transcription unit.

Subjects: Genetics and Genomics.

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