Homeobox genes

All vertebrates looks very similar in the early stages of their embryonic development. All animals have the same body plan.

Drosophila melanogaster (fruitfly)

• The early steps in fruitfly developments are influenced by chemical gradients within the zygote.
• the gradients are generated by the synthesis of proteins in the eggs before fertilisation
• this uses information from the maternal effect genes
• a (bicoid) protein is most intense at the future head end of the fruitfly embryo
• at this stage, the fruitfly embryo is different from other embryos in that
• as the zygote divides, no cell membrane form
• so the nuclei are free in the cytoplasm rather than being contained within distinct cells forming syncytium
• however, soon the plasma membrane fold inwards around the nuclei in forming single layer of out cells
• After the initial steps in pattern formation have been taken under the guidance of the gradients, the homeotic genes take over the next more detailed stage (stage of development)

• homeotic genes that are amazingly similar in structure and function in all animals = genes that direct the building of bodies according to definite body plans
• homeotic genes code for proteins known as transcription factors which then contol the activity of other genes nearby 
• all of these genes contain identical sequence of 18 nucleotides called the HOMEOBOX which codes for 60 amino acids
• the proteins they code for are able to bind with DNa
• all the genes containing this sequence are called homeoox genes
• a peculiarity of these is that they are arranged along the chromosomes in the same linear order as the region of the embryo they are controlling
• since the discovery f Drosophila homeobox genes have been found in many other species incl humans
• it seems that homeobox genes arose early in evolutionary history and have been guiding embryonic development in animals and plants ever since.

HOX CLUSTERS

• homeobox genes are arranged in clusters called HOX clusters
• Nematodes have 1 HOX cluster, Drosophila have 2 HOX clusters and Vertebrates have 4 HOX clusters
• Increase in number of HOX clusters were probably due to mutations
• this allowed more complex body plans to develop
• by controlling the transcription in the cells, homeobox HOX genes act as aster switches determining cell growth and devlopment

Retinpoic acid and birth defects

• Vitamin A > Retanoic acid > Activates homeobox genes in vertebrates
• Retanoic acid is a MORPHOGEN - a substance that controls the pattern of development
• too much Vitamin A in the 1st month of pregnancy interferes with normal expression of genes and can result in birth defect including cranial deformities.