Genes and Body plans

How does an organism become a zygote?

How do the different parts of an organism know which way is “up”?

Learning OutcomesExplain that the genes that control development of body plans are similar in plants, animals and fungi, with reference to homeobox sequences (HSW1);

Key words…

•Homeobox genes•Transcription factor•Homeosis•Polarity genes•Body plan•Morphogen •Maternal-effect genes•Segmentation genes

What do homeobox genes do?

• Homeobox genes code for the production of transcription factors

• These proteins can bind to a particular region of DNA and cause it to be transcribed– A single homeobox gene can switch on a whole

collection of other genes, regulating gene expression

Homeobox genes• Homeobox genes determine how an organism’s body

develops as it grows from a zygote into a complete organism.

• They determine the organism’s body plan

• These sequences are highly conserved

• Homeobox genes have been discovered in animals, plants and fungi

Homologous homeobox genes

• These are the sequences of 60 amino acids in the proteins coded for by the homeobox genes Antp in a fruit fly and HoxB7 in a mouse.

• All animals have homologous homeobox genes – they are recognisably similar. Why is this significant?

Key words – homeobox, maternal effect and polarity genes

What are they? What do they mean?

• Homeobox genes control the development of the body plan of an organism, including the polarity (head and tail ends) and positioning of the organs

• Maternal effect genes determine the embryo’s polarity

• Polarity genes Help to define the anterior and posterior polarities within each embryonic segment of an organism, such as in the fruit fly

Homeosis - when one part of a developing embryo becomes anomalously transformed into another (mutation)

Some proteins are expressed and concentrated in a gradient, highest

in the head

Others are expressed in the head, or in both the head and the tail of

the body

Another set of proteins is expressed in a concentration

gradient from ventral to dorsal

Key words…

•Homeobox genes•Transcription factor•Homeosis•Polarity genes•Body plan•Morphogen •Maternal-effect genes•Segmentation genes

Write a summary to describe what is happening in this diagram as the drosophila fly develops from an embryo.

Use the key words to help you (you may not need all of them)

Name the type of gene, if mutated, gives rise to dramatic changes in body plan

Homeobox gene

Drosophila melanogaster – the fruit fly

A model animal for the study of genetics and gene regulation

The body of insects is segmented

This is obvious in larvae (e.g. maggots)

In adults these become specialised -many develop special appendages

Head Segments- labial palps

(mouth appendages)

- antennae

Thoracic Segments - legs

(balancing organs)

- wings- halteres

Abdominal Segments - no appendages

Identity of each segment is established in the embryo

Mutations can destroy the identity of a segment (more on this later)

Homeosis in Drosophila

These changes are the results of mutations in a set of genes (homeosis)

In this instance, are mutations a good thing?

Legs for antennae

Wings for halteres

Halteres for wings

Homeobox genes have been found in:

worms snails starfish fish mice humans

Summary• These genes are homeotic (regulatory) containing

_______base pairs forming the ____________sequence that codes for a gene product which binds to DNA and initiates ______________. In this way they switch genes that control the development of a _________ ______on or off.

• These genes show little mutation because they are very _____________. Mutation would have big effects and alter the body plan causing many other genes to be affected. Any mutation is likely to be ___________and so selected against

Key words…

•Homeobox genes•Transcription factor•Homeosis•Polarity genes•Body plan•Morphogen •Maternal-effect genes•Segmentation genes

Write a summary to describe what is happening in this diagram as the drosophila fly develops from an embryo.

Use the key words to help you (you may not need all of them)

Model answerThe body plan of the drosophila fly is controlled by the homebox genes. These contain transcription factors that regulate other genes so that the body plan develops normally. Transcription factors act like on/off switches for gene expression.

Some of these genes are maternal-effect genes. They will determine the polarity of the embryo. This means which end is the front and which is the back.

Segmentation genes are another group of genes that determine the dorsal and ventral ends of specific segments of the fly’s body.

Effect of thalidomide in embryo development

– Homeobox genes HoxA11 and HoxD11 switch on genes that cause forelimb development.

– The drug thalidomide affected the behaviour of these homeobox genes at a critical stage in embryonic development.

Retinoic acid and birth defects– Retinoic acid

• is a derivative of vitamin A• activates homeobox genes in

vertebrates• Is a morphogen (substance governing

pattern of tissue development).

– If a pregnant woman takes too much Vitamin A, it can interfere with the expression of homeobox genes causing birth defects in the central nervous system and axial skeleton.

• Write a short note to summarise the effects of retinoic acid and vitamin A on homebox genes and development.

Homeobox genes show astonishing similarity across widely different species of animal, from fruit flies, which are insects, to mice and humans, which are mammals. The sequences of these genes have remained relatively unchanged throughout evolutionary history and the same genes control embryonic development in flies and mammals.

(a) State what is meant by a homeobox gene. [2]

(b) Homeobox genes show ‘astonishing similarity across widely different species of animal’.

Explain why there has been very little change by mutation in these genes.[2]

(c) Frogs reproduce by laying eggs in water. Each egg develops into a tadpole, which has external gills to extract oxygen from the water, and a tail to help it swim. The tadpole gradually changes into an adult frog as it grows. During this time its gills and tail disappear.

List two cellular processes that must occur during the development of a tadpole into a frog. [2]

(d) Name another kingdom of organisms, other than animals, that have similar homeotic genes.[1]

Homeobox animation

• http://www.dnaftb.org/dnaftb/37/concept/index.html– If you have time, sit and watch this animation, as

well as investigating other aspects of this web site.