Body Plans and Homeobox Genes

How does a fertilised egg — a single cell — develop into an organism with a head, legs, organs and a nervous system all in the right places? The answer lies in a special class of "master" genes that switch on other genes at the right time and in the right place. The most famous of these are the homeobox (Hox) genes. Their discovery revolutionised developmental biology: it turned out that flies, mice and humans all use essentially the same ancient toolkit of master genes to pattern their bodies. OCR A-Level Biology A specification module 6.1.1(c) requires you to know about homeobox genes, their role in body plans, embryonic development and apoptosis.

Key Definitions:

• Homeobox — a conserved DNA sequence of approximately 180 base pairs found in homeotic genes.
• Homeodomain — the 60-amino-acid DNA-binding domain encoded by the homeobox.
• Hox genes — a subset of homeobox genes that control body-segment identity along the head-tail axis.
• Homeotic mutation — a mutation that transforms one body part into another (e.g. an antenna into a leg).
• Apoptosis — programmed cell death, an orderly form of cell suicide essential for development.
• Morphogen — a signalling molecule whose concentration gradient gives cells positional information.

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The Problem of Development

A multicellular organism must solve three connected problems:

1. Where am I? (positional information) — how does a cell "know" where it sits in the embryo?
2. What should I become? (cell fate) — how does it choose the right identity for that position?
3. When should I appear or disappear? (timing) — how are the stages of development coordinated?

The answers involve signalling molecules, transcription factors and — at the top of the hierarchy — the homeobox genes.

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Discovery of Homeobox Genes

Homeotic mutations in the fruit fly Drosophila melanogaster gave the first clues. In the 1970s geneticist Ed Lewis and others noticed that mutations in certain genes caused spectacular transformations: the Antennapedia mutation turned antennae into legs, and the bithorax mutations turned the third thoracic segment into a copy of the second, giving flies four wings instead of two. These genes were therefore controlling the identity of body segments.

When these genes were sequenced in 1983, they all contained a conserved 180 bp sequence dubbed the homeobox. The 60-amino-acid protein domain it encodes — the homeodomain — is a DNA-binding motif that allows the protein to act as a transcription factor. The same 180 bp sequence was then found in mice, humans, frogs and even yeast, always playing a role in development.

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Homeobox vs Hox Genes

It is easy to confuse these terms.

• Homeobox is the 180 bp DNA sequence itself. Many different genes contain it.
• Hox genes are a specific family of homeobox-containing genes that specify the body plan along the anterior-posterior (head-tail) axis.

Humans have 39 Hox genes in four clusters (HOXA, HOXB, HOXC, HOXD) on four different chromosomes. Flies have one cluster of 8.

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Colinearity

A striking feature of Hox gene clusters is colinearity: the order of the genes on the chromosome matches the order of the body regions they control. The gene at the 3' end of the cluster is expressed first, in the head region; the gene at the 5' end is expressed last, in the tail region.

flowchart LR
    A[Hox 1 - head] --> B[Hox 2 - neck]
    B --> C[Hox 3 - thorax]
    C --> D[Hox 4 - abdomen]
    D --> E[Hox 5 - tail]

This colinearity is conserved across animals from flies to mammals — strong evidence that all bilaterally symmetrical animals inherited Hox clusters from a common ancestor hundreds of millions of years ago.

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How Do Hox Genes Work?

Hox gene products are transcription factors. Each has:

• The homeodomain, which binds a specific short DNA sequence (the "homeobox binding site") in the promoters or enhancers of downstream target genes.
• A regulatory domain that either activates or represses transcription of those targets.