Three genes identified in the diagram as A, B, and C are transcribed from a region of DNA. The 5'-to-3' transcription of genes A and C elongates mRNA in the right-to-left direction, and transcription of gene B elongates mRNA in the left-to-right direction. For each gene, identify the coding strand by designating it as an 'upper strand' or 'lower strand' in the diagram. <>

Key Concepts

Transcription Directionality

Transcription directionality refers to the orientation in which RNA polymerase synthesizes mRNA from a DNA template. In eukaryotic cells, transcription occurs in the 5'-to-3' direction, meaning that the RNA strand grows by adding nucleotides to the 3' end. This directionality is crucial for understanding which DNA strand serves as the template and how the resulting mRNA aligns with the coding sequence.

Coding and Template Strands

In DNA, the coding strand (also known as the sense strand) has the same sequence as the mRNA produced, except that thymine (T) is replaced by uracil (U) in RNA. The template strand (or antisense strand) is complementary to the mRNA and is the strand that RNA polymerase uses to synthesize the mRNA. Identifying which strand is the coding strand is essential for determining the correct sequence of the mRNA.

Gene Orientation in a DNA Diagram

Gene orientation in a DNA diagram indicates the direction in which each gene is transcribed and helps in identifying the coding and template strands. In the given scenario, genes A and C are transcribed in one direction, while gene B is transcribed in the opposite direction. Understanding this orientation is key to designating the upper and lower strands correctly, as it affects how the mRNA is synthesized and its corresponding coding sequence.