Gene Regulation: Hox Genes | A-level Biology | OCR, AQA, Edexcel

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The key points covered in this video include:

1. Homeotic Genes
2. Hox Genes
3. Hox Genes and Animal Development

Homeotic Genes

One of the roles of DNA is to contain the instructions necessary for building the bodies of organisms. DNA contains genes that regulate morphogenesis, which is the process that causes an organism to form its shape. These genes are called homeotic genes. A subset of homeotic genes are called homeobox genes which contain a 180 base pair length of DNA called a homeobox. The homeobox sequence is highly conserved in plants, animals and fungi. Conserved genes are genes that have remained unchanged throughout the evolution of different descendent species. The homeobox sequence codes for a specific sequence of 60 amino acids within the synthesised protein called homeodomain. The homeodomain sequence folds into a specific shape consisting of three a-helices. The second and third helix create a helix-turn helix which consists of two a-helices connected by a short loop of amino acids. The helix-turn helix shape allows the protein to bind to DNA and regulate the transcription of nearby genes. The proteins that contain a homeodomain are therefore transcription factors.

Hox Genes

A subset of homeobox genes are called genes. Hox genes are homeobox genes that are only found in animals. Hox genes are involved in the correct positioning of body parts in an organism. Hox genes are found in all bilaterian animals suggesting that Hox genes must have existed in the common ancestor of all bilaterians. Several Hox genes are found next to each other on a chromosome - they are arranged in clusters. In some animal lineages, including vertebrates, Hox genes have been duplicated, resulting in multiple Hox clusters. When a Hox gene is mutated, body parts end up developing in the wrong place on the body - these are called homeotic mutations.

Hox Genes and Animal Development

Hox genes are expressed in early embryonic development along the anterior-posterior (head-tail) axis of the organism. Interestingly, the order of the genes on the chromosomes matches the expression patterns along the embryo, showing spatial linearity. As well as spatial linearity, the expression of the genes also occurs in temporal order, starting with expression of the anterior Hox genes. This phenomenon is known as colinearity and scientists do not yet understand the importance of this for Hox genes. When each Hox gene is expressed, it encodes a specific Hox protein that acts as transcription factor. This transcription factor binds to specific regions of DNA and switches on a specific set of genes in each segment. These activated genes help promote the correct development of each body segment by regulating mitosis, apoptosis and cell differentiation.

Summary

Homeotic genes regulate morphogenesis - the process in which an organism forms its shape during development
A subset of homeotic genes are homeobox genes
Homeobox genes contain a 180 base pair homeobox which is a conserved sequence of DNA found in plants, fungi and animals
A subset of homeobox genes are Hox genes which are only found in animals
Hox genes regulate the correct positioning of body parts in an organism
Hox genes are expressed along the anterior-posterior axis in spatial and temporal order
Each Hox gene encodes a different Hox transcription factor which activates a specific set of genes in each segment
These activated genes promote the correct development of each body segment

Видео Gene Regulation: Hox Genes | A-level Biology | OCR, AQA, Edexcel канала SnapRevise