Question

The floral homeotic genes of Arabidopsis belong to the MADSbox gene family, while in Drosophila, homeotic genes belong to the homeobox gene family. In both Arabidopsis and Drosophila, members of the Polycomb gene family control expression of these divergent homeotic genes. How do Polycomb genes control expression of two very different sets of homeotic genes?

Short answer

Short Answer: Polycomb gene family controls the expression of MADSbox and homeobox gene families (found in Arabidopsis and Drosophila, respectively) by targeting and modifying chromatin features associated with these homeotic genes, such as histone modifications and chromatin accessibility. This mechanism operates irrespective of the DNA-binding domain differences between MADSbox and homeobox genes, allowing Polycomb genes to regulate their expression and coordinate developmental processes in both plants and animals.

Step-by-step solution

Understanding Homeotic Genes

Homeotic genes are involved in determining the overall body pattern and the formation of structures during development. These genes are responsible for organizing the identity of different body segments in both plants and animals.

Differences between MADSbox and Homeobox Gene Families

MADSbox and homeobox gene families are two different families of homeotic genes. MADSbox gene family, found mostly in plants like Arabidopsis, consists of genes that code for a specific DNA-binding domain called the MADS domain. On the other hand, homeobox gene family, found in animals like Drosophila, consists of genes that code for a similar DNA-binding domain called the homeodomain.

Role of Polycomb Gene Family in Gene Expression

Polycomb group (PcG) genes are a set of conserved genes that are responsible for controlling the expression of homeotic genes by modifying the chromatin structure. PcG proteins are known to form complexes that can specifically bind to chromatin, altering its structure, and thereby, repress target gene expression. Polycomb gene family operates based on epigenetic regulation to maintain the desired pattern of gene expression.

Answering the Main Question

Despite the differences in the DNA-binding domains of MADSbox and homeobox gene families, Polycomb gene family can control their expression because it doesn't rely on the sequence-specific DNA-binding domains. Instead, Polycomb proteins target specific chromatin features associated with these homeotic genes, such as histone modifications and chromatin accessibility. By targeting and modifying these chromatin features, Polycomb proteins can inhibit or promote the expression of both MADSbox and homeobox homeotic genes, regardless of their DNA-binding domain differences. This allows Polycomb genes to control the expression of two very different sets of homeotic genes in Arabidopsis and Drosophila.

Key concepts

Homeotic Genes

Homeotic genes are like the blueprint architects of living organisms. They decide which part of the organism will develop into what specific structure, ensuring that everything is in its proper place. These genes are critical during early development stages. Imagine them as the instructions telling a builder where to place doors, windows, and roofs in a house. If something goes wrong with these genes, it could lead to structures appearing in the wrong locations or not forming correctly at all. In plants and animals alike, homeotic genes ensure that each organism follows its developmental plan in order to function and survive properly.

MADSbox Gene Family

The MADSbox gene family is primarily found in plants. These genes have a special mission. They help regulate the flowering process and developmental processes in plants like Arabidopsis, a common model organism. The name 'MADS' comes from four gene families: MCM1, AGAMOUS, DEFICIENS, and SRF — all of which contribute to this plant-specific group. These genes code for a specific DNA-binding domain that allows them to activate or deactivate genes at precise stages of a plant's life cycle.

• Control flowering time and patterns
• Regulate fruit development
• Influence root and shoot development

Without the precise action of MADSbox genes, plants might not develop flowers properly, affecting their ability to reproduce.

Homeobox Gene Family

The homeobox gene family appears predominantly in animals, including creatures like Drosophila (fruit flies) and humans. These genes contain a specific sequence called a homeodomain, allowing them to play their role of directing body plan development. The homeodomain is a DNA-binding region that the genes use to control the activity of other genes, turning them "on" or "off" at just the right times.

• Set up the body’s axis (front, back, left, right)
• Dictate limb formation and placement
• Ensure the proper development of tissues and organs

This family of genes is crucial because it ensures that animals develop with all parts in the right places, allowing them to function efficiently and survive.

Chromatin Structure

Chromatin is a dynamic structure made up of DNA and proteins, mainly histones. It serves as the scaffold that holds the DNA in organized loops and coils within the nucleus. Its structure plays a critical role in gene expression because it regulates how DNA is accessed for transcription. During times when a gene is active, the chromatin loosens to allow transcription factors to access the DNA. Conversely, chromatin tightens to prevent gene transcription.

• Can be tight (heterochromatin) — gene expression turned down or off
• Can be loose (euchromatin) — gene expression turned on

Mechanisms, such as histone modification, take place to control these shifts, which can lead to long-term changes in gene expression called epigenetic changes.

Gene Expression Regulation

Gene expression regulation is a fundamental concept ensuring that proteins are produced at the right place and time in a cell. It happens through various mechanisms at multiple stages: transcription, RNA processing, translation, and post-translation modifications. Polycomb proteins play a pivotal role in this by acting at the chromatin level to repress or activate homeotic genes. They do not look for specific sequences but instead modify the structure of chromatin to dictate which genes should be expressed.

• Regulates how much of a protein is produced
• Decides when a gene is expressed in the life cycle
• Determines in which cell or tissue a gene is active

In conclusion, the regulation of gene expression is the orchestra conductor ensuring each performer plays at the right moment, maintaining harmony within the organism’s development and function.