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 T2 Halteres – for balance from T3 Homeobox gene Ubx stops wing formation in T3 If mutation in both copies of Ubx wings grow in T3 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 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

Homeobox genes in humans In Humans Hox A11,D11  limbs Drug Thalidomide affected 1 or more homeobox genes Vitamin A can affect homeobox genes

http://www.dnalc.org/view/16760-Animation-37-Master-genes-control-basic-body-plans-.html

fruit fly development