Homeobox genes and Apoptosis Apoptosis Progammed cell death

Homeobox genes and Apoptosis

Apoptosis • Progammed cell death • Normal cells can only divide certain number of times • Balance between cell death and mitosis controlled by cell signalling – cytokines • Cancer cells immortal- damaged DNA – cell doesn’t respond to signals

Apoptosis in development

What is a Homeobox gene? • Homeo = same • A gene whose activity switches a whole set of genes on or off • Strictly speaking ‘homeobox’ refers to a DNA sequence of 180 nucleotides ( ‘Homeodomain is protein coded for by homeobox) • Controls organisms body plan • Discovered in 1983

All animals have homeobox genes that are similar • A mouse eye homeobox gene produces an eye in Drosophila – even in wrong part of body!

• homeobox and evolution

Homeobox genes are highly conserved • Homeobox genes are so fundamental any mutation will mean that the organism will not survive. • There are homeobox genes in fungi and plants too • Plant homeobox genes are similar to those in other plants but different to those in animals • Each kingdom has its own set of homeobox genes.

Insect body plan • 3 parts • Head – 3 segments fuse • Thorax – 3 segments • Abdomen – 8 segments • Pair of legs from each segment

Drosophila • Pair of wings from T 2 • Halteres – for balance from T 3 • Homeobox gene Ubx stops wing formation in T 3 • If mutation in both copies of Ubx wings grow in T 3 instead of halteres • Can’t fly!

Antp homeobox • • Turned on in thorax Makes leg develop Turned off in head Mutation of Antp causes it to be switched on in the head • Legs develop instead of antennae • Mice have similar homeobox – if put in fruit fly head same effect as Antp

• Homeotic genes control specialisation of segments of body. Homeotic describes process by which one body part becomes like another by assuming its identitity • Mammals have homeotic genes called ‘hox’ genes • Hox genes are special types of homeobox in which genes are additionally linked (or lined up) in a sequence which relates to the pattern the genes are expressed in.

How do homeobox genes work? • Homeobox genes code for proteins that are transcription factors. • These bind to region of DNA and cause it to be transcribed • Switches on whole collection of other genes • Normal Antp genes for leg