19.3 Body Plans

Question 1

Define body plan

Answer 1

The general structure of an organism

Question 2

What are hox genes?

Answer 2

Proteins that control body plan development are coded for by Hox genes
These proteins help set up the basic body plan so that everything is in the right place e.g. legs grow where legs should grow

Hox genes have regions called home box sequences, which are highly conserved — these sequences change very little (very similar in plants, animals and fungi)

Hox genes are one group of homeobox genes only present in animals.

Question 3

How do Hox genes control development?

Answer 3

Homeobox sequences code for a part of the protein called the homeodomain
The homeodomain binds to specific sites on the DNA, enabling the protein to work as a transcription factor.
The proteins bind to DNA at the start of developmental genes, activating or repressing transcription and so altering the production of proteins involved in the development of the body plan.

Question 4

What two processes are involved in the development of body plans?

Answer 4

Mitosis and apoptosis

Question 5

Role of mitosis and apoptosis in development

Answer 5

Hox genes regulate both mitosis and apoptosis
- mitosis and differentiation create the bulk of the body parts and then apoptosis refines the parts by removing unwanted structures
E.g. when hands and feet first develop in humans, the fingers and toes are connected, they’re only separated when cells in the connecting tissue undergo apoptosis
As tadpoles develop into frogs, their tail cells are removed by apoptosis

• during development, genes that control mitosis and genes that control apoptosis are switched on and off in appropriate cells. Cells undergoing apoptosis can also release chemical signals which stimulate mitosis and cell proliferation leading to remodelling of tissues.

Question 6

Symmetry of body shapes in animals

Answer 6

• radical symmetry (no left or right sides just a top and bottoms - e.g. jellyfish (diploblastic animals)
• bilateral symmetry (left and right sides and a head and tail e.g. humans)
• asymmetry (no lines of symmetry)

Question 7

what is apoptosis and how does it work?

Answer 7

• programmed cell death, that removes unwanted cells and tissues. Important for shaping organisms.

Once apoptosis has been triggered the cell is broken down in a series of steps:
1) digestive/ hydrologic enzymes inside the cells breaks down cells components e.g. proteins, DNA
2) the cell shrinks and begins to fragments, nucleus breaks down
3) phagocytes engulf and digest the cell fragments

Question 8

What factors could affect the expression of regulatory genes?

Answer 8

The genes that regulate the progression through the cell cycle (i.e mitosis) and apoptosis can respond to both internal and external stimuli
E.g.
- an internal stimulus could be DNA damage. If DNA damage is detected during the cell cycle this can result in the expression of gens which cause the cell cycle to be paused and can even trigger apoptosis
- an external stimulus, such as stress an be cause by a lack of nutrient availability, could result in gene expression that prevents cells from undergoing mitosis
Gene expression which leads to apoptosis being triggered can also be caused by an external stimulus such as attack by a pathogen