OCR A Level Biology

Revision Notes

6.1.6 Gene Control: Body Plans

Body Plans & Hox Genes

  • Cells in developing organisms need to be able to differentiate and specialise for different roles
    • In order to do this, they must be able to control which genes are functioning at a particular time
    • This is achieved by ‘switching on‘ and ‘switching off‘ genes
    • This must occur in a specific, tightly controlled sequence
    • This sequence is determined by transcription factors (proteins that bind to specific DNA sequences in order to control the rate at which particular genes are transcribed into mRNA)

Homeobox genes

  • A homeobox is a DNA sequence that codes for a protein transcription factor
    • The transcription factors (that homeobox sequences code for) attach to DNA at specific locations and regulate the transcription of genes (e.g. genes that control the early development of eukaryotic organisms) by turning various different genes on and off in the correct order
  • A homeobox gene is any gene that contains a homeobox sequence
  • Homeobox gene sequences in plants, animals and fungi are similar and highly conserved (meaning they have been maintained by natural selection i.e. they remain relatively unchanged when travelling back in evolutionary time)
    • The sequences are all similar as they all code for amino acid sequences that will form transcription factors, the DNA-binding regions of which must all have the same shape
    • Mutations that cause changes in these homeobox sequences can lead to organisms that are not viable (not properly developed) so they are not favoured by natural selection. This strong negative selection pressure explains why the sequences are highly conserved
  • Homeobox genes are responsible for the genetic control of the development of body plans in different organisms
    • This means they help to form the basic pattern of the body
    • For example, they control the polarity of the organism (which end will develop into the head and which end will develop into the tail)
    • They also control the segmentation of organisms such as insects and mammals into distinct body parts and they control the development of body parts such as wings and limbs, as well as what organs are present in each section of the body
  • In this way, homeobox genes can be seen as ‘master genes‘ that control which genes function at different stages of development

Hox genes

  • Hox genes are a very important subset of homeobox genes
  • They determine the identity of embryonic body regions along the anterior-posterior axis (i.e. the head-tail axis)
  • These Hox genes are organised into groups known as Hox clusters
  • Vertebrates have four Hox clusters (each containing 9-11 Hox genes), which are found on different chromosomes
  • There is a linear order to the Hox genes in each Hox cluster and this order is directly related to the order of the regions of the body that they affect
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