Consider the even-skipped regulatory sequences in the following figure:
How are the sharp boundaries of expression of Eve Stripe 2 formed?

Verified step by step guidance

1. Understand that the even-skipped (Eve) gene is expressed in distinct stripes during Drosophila embryogenesis, and each stripe's sharp boundary is controlled by specific regulatory sequences.
2. Recognize that the sharp boundaries of Eve Stripe 2 expression are formed through the combined action of activator and repressor transcription factors binding to the stripe 2 enhancer region.
3. Identify the key activators such as Bicoid and Hunchback proteins that bind to the enhancer to promote expression in the central region of Stripe 2.
4. Note the role of repressors like Giant and Kruppel proteins, which bind to sites flanking the activator binding sites, restricting expression and creating sharp anterior and posterior boundaries.
5. Conclude that the interplay between these spatially distributed activators and repressors, through their binding to the regulatory sequences, results in the precise and sharp expression pattern of Eve Stripe 2.

Key Concepts

Gene Regulatory Sequences

Gene regulatory sequences are DNA regions that control the timing, location, and level of gene expression. In the case of even-skipped (Eve), specific enhancers respond to transcription factors to activate or repress expression in precise spatial patterns, such as the formation of stripes in Drosophila embryos.

Transcription Factor Gradients and Combinatorial Control

Sharp expression boundaries arise from the combined effects of activator and repressor transcription factors distributed in gradients. For Eve Stripe 2, activators like Bicoid and Hunchback promote expression, while repressors like Giant and Kruppel inhibit it, creating a precise spatial pattern through their overlapping domains.

Enhancer-Mediated Spatial Patterning

Enhancers integrate multiple transcription factor inputs to generate distinct expression domains. The Eve Stripe 2 enhancer contains binding sites for both activators and repressors, enabling it to interpret positional information and produce a sharp, well-defined stripe of gene expression in the embryo.