How Hox genes control development of body plans in animals

There are some key terms you need to know regarding how certain genes control the development of body plans in animals.

The mechanism of how Hox genes control development:

1. Homeobox sequences encode the homeodomain, the part of a protein that binds to DNA.
2. The homeodomain operates as a transcription factor.
3. It binds to DNA, switching developmental genes on or off.
4. This modifies the transcription of proteins necessary for the development of body plans.

Hox genes can control the development of the following:

• The number of body layers.
• The symmetry of the body.
• The anterior-posterior axis, ensuring a head is at one end and any tail is at the other.
• The segmentation of the body into head, thorax, abdomen, and so on.
• The position and development of limbs, eyes, and other important structures.

Why homeobox genes are so conserved:

• A mutation would have large effects by altering the organism's body plan.
• Many other genes would also be affected by a mutation in a homeobox gene.
• Mutations are likely to be lethal and selected against.

Fruit flies in studies on genes that control body plan

Fruit flies are often used in studies on the activity of genes that control body plan.

This is because:

1. There are fewer ethical concerns than other animals.
2. They are low cost.
3. Their genetics and development are well understood.
4. They have a rapid reproduction rate.
5. They have a simple body plan.
6. Their mutations can be studied with a low-powered microscope.

How apoptosis and mitosis shape the body plan

Body morphology is refined through a combination of cell death (apoptosis) and cell division (mitosis), alongside cell specialisation via differentiation. For instance, mitosis causes embryonic fingers to grow as more cells are added lengthways, and the connective tissue that forms webbing between these fingers is removed through apoptosis.

Hox genes control the rate and location of cell division during growth and tissue formation, and they also control programmed cell death during development.

The genes controlling apoptosis and mitosis receive various signals, ensuring a balance between cell elimination and renewal.

How developmental genes respond to stimuli

Developmental genes can correct deviations and adapt to environmental conditions by responding to a range of signals. These regulatory genes usually achieve this by regulating the cell cycle and apoptosis.

Internal stimuli:

• Regulatory genes are able to respond to internal stimuli like stress, drugs, or hormones, which can cause DNA damage.
• When DNA is damaged, regulatory genes trigger the cell cycle to stop and initiate apoptosis.
• This prevents the DNA damage from being replicated into new cells.

External stimuli:

• Regulatory genes can respond to external stimuli like changes in temperature or light intensity.
• For instance, a lack of nutrients may trigger cell division.