Free Controlling the Development of Body Form OCR A Level Biology revision notes – covering specification points 6.1.1(c) and 6.1.1(d).
In order for an organism to assume its particular shape and size (its morphology), mitosis, differentiation and apoptosis must be coordinated between different cells to ensure these processes proceed at the correct rate, and at the right time.
The coordination of mitosis, differentiation and apoptosis ensures that:
- Mitosis produces sufficient cells during periods of growth, and replaces cells at a sufficient rate without tissues atrophying or tumours forming.
- Differentiation occurs at the appropriate time in an organism’s development, so that specialised cells arise in the correct location.
- Apoptosis allows for tissues to be sculpted (such as separating toes), and safely removes unneeded or defective cells (such as defective T-lymphocytes, or in the formation of a lumen) during organ system development.
It is important to note that OCR A Level Biology highlights the importance of apoptosis in shaping the morphology of a developing organism.
The image below shows the separation of digits in the paw of a developing kitten foetus over time due to apoptosis:
The coordination of mitosis, differentiation and apoptosis is controlled by genes.
Hox Genes
Hox genes control the development of the body plan along the anterior–posterior axis (front to back), which goes in the direction of the head to the tail in the embryo of animals, and determine where anatomical structures develop.
Hox genes are arranged in clusters (of up to 10) on the chromosomes, and their order along the cluster reflects the relative location in the embryo in which they are expressed.
Homeobox Sequence
The homeobox sequence is a 180-base pair-long* section of DNA found in many (but not all) genes which control the morphological development of most organised multicellular organisms, including animals, plants, and fungi.
*Once any introns have been removed.
The homeobox sequence codes for an amino acid sequence that folds into a DNA-binding domain that allows it to act as a transcription factor.
The homeodomain enables the proteins it is a part of to bind to DNA, allowing them to act as transcription factors that can bind to genes, allowing for the control of their gene expression.
The homeobox gene’s conservation in animals, plants, and fungi is molecular evidence for the common ancestry between these types of organisms.