body plan

homeobox genes

a group of regulatory genes that control the development of body plans

roles of homeobox genes

-control the development of body plans

-controls positioning of body structures

-Switch genes on and off in different tissues

-regulates mitosis and apoptosis

homeobox genes are..

highly conserved in plants, animals and fungi [are similar]

why are homeobox genes similar in different organisms?

The homeobox genes are highly conserved. Their homeobox sequences have changed very little during evolution. Therefore, the development of body plans in different organisms are controlled in a similar way.

name of sequence and what does it contain

homeobox genes contains homeobox sequence

→contains 180 base pairs

what does homeobox sequence code for

The homeobox sequence codes for a homeodomain (transcription factors)

what does homeodomain act as and its role

acts as a transcription factor

→ it switches genes on and off→ controls the transcription of genes→ helps control body plan

subset of homeobox genes

hox genes

hox genes

homeobox genes found in animals

examples of developments that hox genes control

-position and development of limbs, eyes etc

-number of body layers

-segmentation of the body

the processes that control the development of body plans

-mitosis

-apoptosis

apoptosis

programmed cell death

importance of mitosis and apoptosis in the development of body plans

mitosis increases the number of cells for growth

apoptosis removes damaged cells and shapes the structures

what does apoptosis prevent

prevents uncontrolled cell division→ stops damaged cells from dividing→ helps prevent tumour formation

process of apoptosis

-enzymes inside the cell break down cell components (DNA, proteins)

-the cell shrinks and breaks into small fragments [apoptotic bodies]

-phagocytes engulf and digest the cell fragments

What do genes that regulate the cell cycle and apoptosis respond to?

internal and external stimuli

examples:

internal stimuli

external stimuli

internal

-DNA damage→ When DNA is damaged, regulatory genes cause the cell cycle to stop and trigger apoptosis

external

-cytokines

-hormones

-pathogens (bacteria and viruses)→ triggers apoptosis

-pollution [can damage DNA] → trigger apoptosis

-stress→ can be caused by cells with a lack of nutrients→ triggers apoptosis to remove unhealthy cells

what regulates apoptosis and mitosis?

homeobox genes

role of homeobox genes in brain development and heart development

-help determine the head and tail (anterior–posterior) regions, so the brain and spinal cord form in the correct positions

-Switch genes on or off in the brain to form specialised neurones

-regulates mitosis and apoptosis of neurones to ensure brain develops properly

-help determine the position where the heart develops in the body

-Switch genes on or off to form specialised cardiac cells

-regulate mitosis and apoptosis of cells so the heart develops to the correct size and shape

explain the importance of mitosis and apoptosis as mechanisms controlling the development of body form.

mitosis

→produces genetically identical cells

→increases number of cells for growth

→allows cell differentiation

e.g. development of neurones

Formation of limbs (arms and legs) during embryonic growth

apoptosis

→is programmed cell death

→removes damaged cells

→prevents tumour formation

e.g. -removes tissues between fingers

-tail in tadpoles disappear as it develops into a frog→ tail cells are removed by apoptosis

-hox genes does not produce transcription factor

-apoptosis does not occur so fingers are not separated

how can the control of development by homeobox genes be seen in stage A? (2)

Describe how the process of apoptosis can be seen between stage:

A and B

B and C

how can the control of development by homeobox genes be seen in stage A?

limbs are developed

eyes are developed

A and B

-tail disappears

B and C

-tissues [webbing] removed between fingers

Imagine an image of a double headed cow. Suggest why this happened?

-mutation in homeobox genes occurs

-mutation alters body plan

-mitosis and apoptosis is not regulated

-structures develop in incorrect positions