Problem 7 - Campbell Biology

Question

Absence of bicoid mRNA from a Drosophila egg leads to the absence of anterior larval body parts and mirror-image duplication of posterior parts. This is evidence that the product of the bicoid gene \begin{equation} \begin{array}{l}{\text { (A) normally leads to formation of head structures. }} \\ {\text { (B) normally leads to formation of tail structures. }} \\\ {\text { (C) is transcribed in the early embryo. }} \\ {\text { (D) is a protein present in all head structures. }}\end{array} \end{equation}

Step-by-Step Solution

Answer

Option (A): normally leads to formation of head structures.

Step 1: Understand the Context

The problem describes what happens to a Drosophila egg when bicoid mRNA is missing. Specifically, it mentions the absence of anterior (head) parts and the duplication of posterior (tail) parts.

Step 2: Determine the Role of Bicoid

Since the absence of bicoid mRNA leads to a lack of anterior body parts and a duplication of posterior parts, the bicoid gene must be responsible for the formation of anterior (head) structures.

Step 3: Match With Provided Options

Next, match the correct role of the bicoid gene to the given choices. The correct answer must indicate that the bicoid gene leads to the formation of head structures.

Step 4: Select Correct Answer

Given the analysis, option (A) 'normally leads to formation of head structures' is correct.

Key Concepts

Drosophila embryogenesis

Drosophila embryogenesis is a critical phase in the early development of the fruit fly. It begins with a fertilized egg, leading to the formation of a multi-cellular embryo.
Key stages include:

• Cleavage: Rapid cell division without growth, forming a syncytial blastoderm.
• Gastrulation: Cells begin to move and form three germ layers (ectoderm, mesoderm, and endoderm).
• Segmentation: The embryo starts to form distinct segments along its body.

Drosophila embryogenesis is highly controlled by genetic factors, ensuring the correct formation of segmented body structures. Any disruption, like the absence of the bicoid gene, can lead to significant developmental issues.

anterior-posterior axis formation

The anterior-posterior axis formation in Drosophila defines the head-to-tail orientation of the embryo. This axis is established early in development and is crucial for proper body patterning.
Key points include:

• Bicoid Gene: The bicoid gene's mRNA is localized at the anterior end of the egg. It is essential for forming head structures.
• Gradient Formation: Bicoid protein forms a concentration gradient from the anterior to the posterior end of the embryo, providing positional information.
• Nanos Gene: The nanos gene, localized at the posterior end, helps define tail structures.

The interaction between these and other genes ensures the embryo's body plan is correctly formed, with head structures at the anterior and tail structures at the posterior.

genetic regulation of development

Genetic regulation of development in Drosophila involves precise control of genes that dictate the formation and differentiation of various body parts.
Key regulators include:

• Maternal Effect Genes: These genes, like bicoid and nanos, are provided by the mother and set up initial developmental cues in the embryo.
• Segmentation Genes: These genes define distinct segments in the developing embryo. They include gap genes, pair-rule genes, and segment polarity genes.
• Homeotic Genes: These genes determine the identity of each segment, ensuring that the correct body parts form in the right places.

Mutations or absence of these key genes can lead to developmental abnormalities, demonstrating the importance of genetic regulation for normal development.