6.1.6 Gene Control: Body Plans Flashcards
What do cells in developing organisms need to be able to do and how can they do this?
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)
What is a homeobox?
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
What is a homeobox gene?
A homeobox gene is any gene that contains a homeobox sequence
What is the link between homeobox genes in plants, animals and fungi?
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
What are homeobox genes responsible for?
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
What are 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
What groups are Hox genes organised into?
These Hox genes are organised into groups known as Hox clusters
How many hox clusters do vertebrates have?
Vertebrates have four Hox clusters (each containing 9-11 Hox genes), which are found on different chromosomes
What type of order is there to the hox genes in each hox cluster?
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
