14. Genetic Control of Development

Developmental Patterning Genes

14. Genetic Control of Development

Developmental Patterning Genes: Videos & Practice Problems

Topic summary

Developmental patterning genes are crucial in determining the organism's body plan, with anterior (front), posterior (back), dorsal (top), and ventral (bottom) orientations. Maternal effect genes, like bicoid and nanos, establish initial gradients that activate zygotic gap genes, which further segment the embryo. This cascade continues with pair-rule and segment polarity genes, ultimately leading to the expression of Hox genes that dictate organ development. In plants, similar homeotic genes (Class A, B, C) control floral structures, demonstrating the conserved nature of developmental genetics across species.

concept

Segmentation Genes

Video duration:

11m

Segmentation Genes Video Summary

Developmental patterning genes play a crucial role in the early stages of organism development, determining the basic body plan, including the anterior (front), posterior (back), dorsal (top), and ventral (bottom) orientations. The initial phase of development is influenced by maternal effect genes, which are inherited from the mother and found within the egg. These genes, such as bicoid and nanos, establish concentration gradients that dictate the anterior-posterior axis of the developing embryo. Specifically, a high concentration of bicoid correlates with the anterior region, while a high concentration of nanos indicates the posterior region.

Once these maternal genes are activated, they trigger the expression of zygotic genes, starting with gap genes. These genes segment the embryo into distinct body regions, further activating pair-rule genes that refine these segments into pairs. Subsequently, segment polarity genes define the anterior and posterior ends of each segment, ensuring proper organization and development. This cascade of gene activation is essential, as any disruption can lead to severe developmental issues.

Following the establishment of segments, homeotic genes, commonly referred to as Hox genes, come into play. These genes are responsible for the development of specific structures within the defined segments, such as limbs and organs. Hox genes contain a conserved sequence known as a homeobox, approximately 180 base pairs long, which encodes a protein domain that binds to DNA. This binding suggests that Hox proteins function as transcription factors, regulating the expression of downstream genes necessary for the formation of body structures.

In organisms like fruit flies, there are two main clusters of Hox genes: the antennopedia and the bithorax. The antennopedia cluster influences head and thorax development, while the bithorax cluster affects the posterior thorax and abdomen. The precise expression of these genes is critical; mutations in any of these genes can lead to significant developmental defects, such as the absence of functional limbs. Interestingly, Hox genes are highly conserved across species, with some organisms possessing multiple clusters, which may allow for redundancy in function. This redundancy can provide a buffer against mutations, ensuring that essential structures can still develop even if one gene is compromised.

Overall, the interplay between maternal effect genes, zygotic genes, and Hox genes is fundamental to the proper segmentation and development of organisms, highlighting the intricate genetic regulation that underpins embryonic development.

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Plant HOX genes

Video duration:

Plant HOX genes Video Summary

Plant development is guided by a unique set of homeotic genes, known as Hox genes, which play a crucial role in determining the structure and formation of various plant organs. In the model organism Arabidopsis, these Hox genes are categorized into three classes: Class A, Class B, and Class C. Each class is responsible for the development of specific floral structures.

Class A genes are responsible for forming the sepals, which are the small, leaf-like structures at the base of the flower. When both Class A and Class B genes are expressed together, they lead to the formation of petals, the colorful parts of the flower that attract pollinators. The stamen, which is the male reproductive part of the flower, develops from the expression of both Class B and Class C genes. Finally, Class C genes alone are responsible for the formation of carpels, the female reproductive structures of the flower.

To visualize this, consider the following breakdown of floral structures based on gene expression:

Sepals: Formed by Class A gene expression.
Petals: Formed by the combined expression of Class A and Class B genes.
Stamen: Formed by the combined expression of Class B and Class C genes.
Carpels: Formed by Class C gene expression alone.

Understanding these gene classes and their roles in plant anatomy is essential for studying plant development. The interaction of these genes not only determines the physical structure of the flower but also plays a significant role in the reproductive success of the plant.

Problem

Which genes are the first genes that control patterning of the offspring during early development?

Anterior genes

Maternal effect genes

Zygotic genes

Dorsal genes

Problem

Areas with higher bicoid expression will develop into which body pattern?

Posterior

Dorsal

Ventral

Problem

Activation of the segmentation genes occurs in which of the following orders?

Maternal effect → gap → segment polarity → pair rule

Gap → maternal effect → pair rule → segment polarity

Maternal effect → gap → pair rule → segment polarity

Segment polarity → pair rule → gap → maternal effect

Problem

Which of the following HOX clusters are responsible for forming the abdominal in Drosophila development?

Segment polarity

Antennapedia

Bithorax

Pair rule

Problem

In Arabidopsis, which class of HOX genes are responsible for forming the plant carpels?

Class A

Class B

Class C

Class B and C

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Here’s what students ask on this topic:

What are the roles of maternal effect genes in developmental patterning?

Maternal effect genes are crucial in the earliest stages of development. These genes, such as bicoid and nanos, are derived from the mother and are present in the egg before fertilization. They establish initial concentration gradients that determine the anterior (front) and posterior (back) ends of the developing embryo. Bicoid is concentrated at the anterior end, while nanos is concentrated at the posterior end. These gradients activate the zygotic genes, starting with gap genes, which further segment the embryo. Thus, maternal effect genes set the foundational body plan of the organism.

How do Hox genes influence organ development in animals?

Hox genes, also known as homeotic genes, play a pivotal role in determining the identity and positioning of body segments and organs during development. These genes contain a specific DNA sequence called a homeobox, which encodes a protein domain that binds to DNA and regulates the expression of other genes. In fruit flies, for example, Hox genes are organized into two clusters: antennapedia and bithorax, which control the development of the head, thorax, and abdomen. By activating or repressing target genes, Hox genes ensure that each body segment develops the appropriate structures, such as legs, antennae, or wings.

What is the significance of concentration gradients in developmental patterning?

Concentration gradients are essential in developmental patterning as they provide spatial information that guides the formation of body axes and segments. For instance, maternal effect genes like bicoid and nanos create gradients in the early embryo, with high concentrations at specific ends. These gradients activate subsequent sets of genes, such as gap genes, pair-rule genes, and segment polarity genes, in a sequential manner. Each gene set is activated at specific concentration thresholds, ensuring precise spatial and temporal control over the development of body segments and structures. Disruptions in these gradients can lead to severe developmental abnormalities.

How do gap genes contribute to embryonic segmentation?

Gap genes are the first set of zygotic genes activated by maternal effect genes like bicoid and nanos. They play a critical role in dividing the embryo into broad regions or segments. Gap genes encode transcription factors that regulate the expression of pair-rule genes, which further refine the segmentation pattern. Examples of gap genes include hunchback, Kruppel, and knirps. These genes are expressed in specific regions of the embryo, and their expression patterns help establish the anterior-posterior axis and the initial segmentation of the developing organism. Mutations in gap genes can result in the loss of entire segments.

What are the differences between animal and plant homeotic genes?

While both animals and plants use homeotic genes to control development, there are key differences. In animals, Hox genes are responsible for segment identity and organ development, such as the formation of legs, antennae, and wings in fruit flies. These genes are organized into clusters and contain a homeobox sequence. In plants, homeotic genes are categorized into classes A, B, and C, which control the development of floral structures like sepals, petals, stamens, and carpels. Despite these differences, both animal and plant homeotic genes function as transcription factors that regulate the expression of other genes to ensure proper development.

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