Download The timing of gene expression

The timing of gene expression
Summarise the role of gene cascades determining
limb formation in birds and mammals
Gene cascade: a sequence in which genes are turned on
and expressed.
Genes are turned on and off in a particular order and
only in the correct cells
Protein produced by one gene acts as a transcription
factor to turn on the next gene, and so on
As a result genes are switched on in the right sequence
and in the right places to form working parts
Limb Formation:
During embryonic development, tiny bulges bud out –
the beginning of limbs. Position of limb buds is
determined by HOX genes that start the gene cascade.
Genes at the start of the cascade are switched on and as
each gene is expressed, the next is turned on.
In humans, limbs develop in weeks 5 and 6 and in a
precise pattern: shoulders to fingers, thumb to little
finger, back of the hand to the palm.
In chick embryos, buds become recognizable wings and
legs by the 10th day.
Homeotic genes:
Any of a group of genes that control the pattern of body
formation during early embryonic development of
organisms
Hox genes:
Homeobox genes (Hox) are master genes that produce
proteins that activate or express a large number of genes
for the formation of body structures (e.g. begin the gene
cascade for limb formation)
Hox 9-13 control limb development in birds and
mammals
Homeotic genes were first discovered through mutations
which caused limbs or organs to develop in the wrong
parts of the body
Drosophila with one pair of wings (a) and a
mutant fly with two pairs of wings (b)
Describe the evidence which indicates the presence of
ancestral vertebrate gene homologues in lower animal classes
Gene homologue: Similar DNA sequences in different organisms.
Homeotic genes often are homologous, coding for the same
function in many different organisms
similar homologue genes have been found in every eukaryote
studied including: invertebrates (e.g. fruit flies and worms),
vertebrates (e.g. frogs, chickens and humans), yeasts and plants
they control the development of similar body parts in the
developing embryos of many different species
presence of these similar DNA sequences in organisms suggests a
common ancestry of all eukaryotic organisms
Orgnization
and
expression of
Hox genes in
Drosophilla
and in the
mouse
Identify data sources gather process and analyse information from
secondary sources and use available evidence to assess the evidence
that analysis of genes provides for evolutionary relationships
Globin protein
Globin proteins carry oxygen, e.g. haemoglobin
Studies of DNA sequences of globin proteins across
species such as insects and primates reveal similar amino
acid sequences that suggest an evolutionary relationship
Differences can help map the evolutionary relationship,
e.g. myoglobin in insects has alpha chains whereas
haemoglobin in mammals has alpha and beta chains.
Identify data sources gather process and analyse information from
secondary sources and use available evidence to assess the evidence
that analysis of genes provides for evolutionary relationships
DNA repair
DNA repair genes are very similar in all organisms from
yeasts to humans.
Shows the repair process evolved early and has remained
unchanged since then