Difference Between Homeobox Homeotic and Hox Genes

The main difference between homeobox homeotic and hox genes is that homeobox is a specific DNA sequence found within homeotic genes while homeotic genes are the genes responsible for the regulation of the patterns of anatomical development in animals, plants, fungi, and some unicellular eukaryotes, and Hox genes are a subset of homeotic genes that only occurs in bilateral animals.

Homeobox, homeotic genes, and Hox genes are three types of DNA sequences found in the genome of eukaryotes. They are responsible for morphogenesis. Furthermore, homeobox is around 180 base pairs long while homeotic genes mainly encode for transcription factors, which direct cells to form a particular body part. Moreover, homeotic genes with a homeobox are known as homeobox genes. Generally, humans have over 200 homeobox genes; out of that, around 39 genes are Hox genes. 

Key Areas Covered 

1. What is Homeobox
     – Definition, Structure, Importance
2. What are Homeotic Genes
     – Definition, Structure, Importance
3. What are Hox Genes
     – Definition, Structure, Importance
4. What are the Similarities Between Homeobox Homeotic and Hox Genes
     – Outline of Common Features
5. What is the Difference Between Homeobox Homeotic and Hox Genes
     – Comparison of Key Differences

Key Terms 

Homeobox, Homeodomain, Homeotic Genes, Hox Genes, Morphogenesis, Transcription Factors

What is Homeobox 

Homeobox is a DNA sequence that occurs in homeotic genes. It was first discovered in Drosophila. Generally, it is 180 base pairs long. Also, it occurs within homeotic genes of all eukaryotes, including animals, plants, and fungi. On the other hand, homeobox synthesizes a protein domain called homeodomain, which is 60 amino acids long. 

Figure 1: Homeodomain Binds to DNA

Furthermore, this domain contains three alpha-helices of which the second and third alpha-helices connect through a small loop, forming a motif. Moreover, the third alpha-helix directly interacts with DNA by forming hydrogen bonds and hydrophobic interactions. It forms indirect interactions with DNA through water molecules. Therefore, the main function of homeodomain is to recognize DNA, allowing the regulation of gene expression. 

What are Homeotic Genes 

Homeotic genes are a type of genes that occur in eukaryotes, regulating morphogenesis. They regulate the development of anatomical structures in various eukaryotic organisms by means of regulation of gene expression. Therefore, they serve as transcription factors which affect the genes through regulatory genetic pathways. Furthermore, mutations in homeotic genes cause displacement of body parts, which can be lethal. 

Figure 2: ABC Model of Flower Development

The two main types of homeotic genes are Hox genes and ParaHox genes. Here, Hox genes only occur in bilateral animals, including humans. Also, ParaHox genes are known as Hox-Like (HoxL) genes. Moreover,  MADS-box-containing genes are a type of homeotic genes found in flowering plants. However, they occur in other organisms, including mammals, yeasts, and insects. Also, these genes are responsible for the ABC model of flower development, proto-oncogene transcription, and gene regulation in specific cells based on the type of organism. 

What are Hox Genes 

Hox genes are a subset of homeotic genes. Significantly, they only occur in animals with bilateral symmetry. Therefore, they specify the development of organs of the embryo along the head-tail axis. Also, they ensure the formation of correct structures in the correct position in the body. Typically, humans have over 200 homeotic genes, and out of that, 39 are Hox genes. 

Figure 3: Hox Genes in Drosophila

Moreover, Hox genes are often conserved across species. Therefore, some of the Hox genes in humans are homologous to that in Drosophila. On the other hand, the protein products of Hox genes are transcription factors. Since Hox genes are a type of homeotic genes, they contain those DNA sequences called homeobox. However, the activation of Hox genes is regulated by gap genes and pair-rule genes in many animals. Furthermore, the transcription factors produced by Hox genes bind to the corresponding genes responsible for the development of body parts. Therefore, they regulate their gene expression. 

Similarities Between Homeobox Homeotic and Hox Genes 

• Homeobox, homeotic, and Hox genes are three types of DNA sequences found in the genome of eukaryotes. 
• They are responsible for the regulation of morphogenesis, the development of different structures of the body. 
• Moreover, they encode for transcription factors which can regulate morphogenesis by means of regulating gene expression. 

Difference Between Homeobox Homeotic and Hox Genes 

Definition 

A homeobox refers to a slightly conserved DNA sequence contained in some genes, regulating gene expression, and homeotic genes refer to the genes that regulate the development of anatomical structures in eukaryotes, while Hox genes refer to a subset of homeobox genes, which specify regions of the body plan of an embryo along the head-tail axis. Thus, this is the main difference between homeobox homeotic and hox gene.

Significance 

Furthermore, homeobox is a specific DNA sequence found within homeotic genes, homeotic genes are the genes responsible for the regulation of the patterns of anatomical development in animals, plants, fungi, and some unicellular eukaryotes, while hox genes are a subset of homeotic genes, which only occurs in bilateral animals. 

Occurrence 

Another difference between homeobox homeotic and hox gene is their occurrence. Homeobox occurs in all eukaryotes, including animals, plants, and fungi, and homeotic genes also occur in all eukaryotes including animals, plants, and fungi, while Hox genes only occur in bilateral animals. 

Structural Significance 

Homeobox is around 180 base pairs long DNA sequence; humans have over 200 homeotic genes, while humans have around 39 Hox genes. 

Functional Significance

Furthermore, homeobox produces homeodomain, which binds to DNA, while both homeotic and hox genes serve as transcription factors, which regulate the development of body parts.

Conclusion 

Homeobox is a DNA sequence which occurs in homeotic genes. Moreover, it is around 180 base pairs long. It produces a homeodomain around 60 amino acids in size. And, this domain binds to DNA. Meanwhile, homeotic genes are the genes that regulate morphogenesis in eukaryotes. Also, they encode for transcription factors which can regulate gene expression. On the other hand, the hox genes are a subset of homeotic genes only found in animals. Therefore, the main difference between homeobox homeotic and hox genes is their structure and occurrence. 

References:

1. “Homeotic Genes and Body Patterns.” Learn.genetics – GENETIC SCIENCE LEARNING CENTER, University of Utah, Available Here.

Image Courtesy:

1. “Homeodomain-dna-1ahd” By Opabinia regalis – Self created from PDB entry 1AHD using the freely available visualization and analysis package VMD (CC BY-SA 3.0) via Commons Wikimedia   
2. “ABC Model” By Ian Alexander – Own work (CC BY-SA 4.0) via Commons Wikimedia   
3. “Hoxgenesoffruitfly” By PhiLiP – self-made, base on Hoxgenesoffruitfly.png (Public Domain) via Commons Wikimedia