Consider the even-skipped regulatory sequences in Figure 18.9.

Consider the binding sites for gap proteins and Bicoid in the stripe 2 enhancer module. What sites are occupied in parasegments 2, 3, and 4, and how does this result in expression or no expression?

Verified step by step guidance

1. <Identify the role of gap proteins and Bicoid in the regulation of gene expression. Gap proteins are transcription factors that can either activate or repress gene expression, while Bicoid is a morphogen that acts as a transcriptional activator.>
2. <Examine the stripe 2 enhancer module, which contains binding sites for both gap proteins and Bicoid. Determine which proteins bind to these sites in parasegments 2, 3, and 4.>
3. <In parasegment 2, analyze the concentration of Bicoid and gap proteins. High Bicoid concentration and low repressive gap protein concentration lead to the activation of the even-skipped gene.>
4. <In parasegment 3, consider the balance between Bicoid and gap proteins. If repressive gap proteins are present in higher concentrations, they may outcompete Bicoid, leading to no expression of the even-skipped gene.>
5. <In parasegment 4, evaluate the presence of Bicoid and gap proteins. If Bicoid is absent or present in low concentrations and repressive gap proteins are dominant, the even-skipped gene will not be expressed.>

Key Concepts

Regulatory Sequences

Regulatory sequences are regions of DNA that control the expression of genes. They include enhancers and silencers, which can increase or decrease the likelihood of transcription. In the context of developmental biology, these sequences are crucial for spatial and temporal gene expression, influencing how genes are activated in specific tissues or stages of development.

Gap Proteins

Gap proteins are transcription factors that play a key role in early embryonic development by establishing broad regions of gene expression. They are produced in response to maternal gradients and help define the anterior-posterior axis of the embryo. Their binding to specific regulatory sequences, such as enhancers, determines which genes are activated in particular parasegments.

Bicoid Protein

Bicoid is a maternal effect protein that acts as a transcription factor and is essential for anterior development in Drosophila. It forms a concentration gradient in the embryo, influencing the expression of target genes in a spatially dependent manner. In the stripe 2 enhancer module, Bicoid's binding can activate or repress gene expression based on its concentration and interaction with other regulatory proteins.