F215 Module 1 – Cellular control and variation
Genes, body plans and apoptosis
Specification
 Explain that the genes that control development of body plans are similar in
plants, animals and fungi, with reference to homeobox sequences
 Outline how apoptosis (programmed cell death) can act as a mechanism to
change body plans
Body Plans
 The blueprint for the way the body of an organism is laid out
 The study of body plans has informed the study of evolution
 Body plan is the basis for phylum – there are 35 different basic animal body
plans
 Drosophila has been studied in detail and is used as an example
 (diagram)
 The study of body plans has helped to learn how genes control the
development of structural features through a cascade of processes in which
key genes produce chemicals (morphogens) which diffuse through the body
to produce a gradient that acts as a position indicator for cells, turning on
other genes some of which in turn produce other morphogens
Homeobox genes and body plans
 A key discovery (in 1983/4) was the existence of groups of homeobox genes
which are responsible for laying down the basic body plan in organisms
 A homeobox is about 180 base pairs long
 It encodes a polypeptide (small protein)
 Some of these polypeptides are transcription factors able to bind to DNA
further along, initiating transcription and regulating the expression of other
genes
 Homeobox genes are arranged in clusters – Hox clusters
 Nematodes (roundworms) have one Hox cluster, Drosophila have 2 and
vertebrates have approx 10, located on different chromosomes
 Homeobox genes are remarkably similar in all animals and have remained
unchanged for hundreds of millions of years (highly conserved)
 A mutation in these homeobox genes seems so catastrophic the organism
cannot survive
 Homeobox genes have also been discovered in plants and fungi and – theirs
are homologous (very similar) within kingdoms
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F215 Module 1 – Cellular control and variation
Much of the work on these genes has focussed on mutations and their effects, and
genetic manipulation
e.g. Mutation in Drosophila can make antennae grow in place of legs or vice versa
e.g. Insertion of a mouse eye development homeobox gene into a Drosophila wing
segment causes a Drosophila eye to develop (shows the gene is sufficiently
similar/homologous)
Retinoic acid and birth defects
 Retinoic acid is a derivative of Vitamin A
 It is a morphogen
 It activates homeobox genes in the same order that they are expressed in
developing systems e.g. the skeleton and central nervous system
 Concentration is crucial
 Too much causes birth defects
 Liver contains a high concentration of Vitamin A – so pregnant women are
advised not to eat much liver
Thalidomide and birth defects
The drug affects some homeobox genes leading to limb deformities
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F215 Module 1 – Cellular control and variation
Apoptosis (pronounced ‘apo tosis’)
 Is programmed cell death
 Occurs in multicellular organisms
 Cell undergoes a series of biochemical events leading to an orderly and tidy
cell death (compare necrosis – an untidy and damaging death which is
caused by the release of hydrolytic enzymes)
 Triggered by diverse range of cell signals, some of which come from inside
the cell, others from outside
 Normal process and in a normal adult 50-70 million cells per day undergo
apoptosis
Sequence of events
 Enzyme break down cytoskeleton
 Cytoplasm becomes dense, with organelles tightly packed
 The cell surface membrane changes and small bits called blebs form
 Chromatin condenses
 The nuclear envelope breaks
 DNA breaks into fragments
 Cell breaks into vesicles
 These are taken into other cells by phagocytises
 Process happens very quickly
 No other cell is harmed as any hydrolytic enzymes are always contained in
vesicles
Apoptosis is a natural process in development in plant and animal tissue
development. Extensive cell division is followed by programmed apoptosis. Each
tissue is regulated by different cell signals.
Cancer is unregulated cell division where the cells do not respond to the normal
apoptosis triggers. If cells break away from the tumour they can enter the blood or
lymph and travel to another part of the body where they can set up secondary
cancers. This is called metastasis and the cancerous tumour is malignant.
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