body made up of hundreds of cells
cellular diversity gives tissues and organs functions and distinct shapes
despite their differences, there is a similarity= exact DNA
-individual genes turning on and off inside each cell during development
makes protein that can affect how the cell behaves
making hands
Edgar Zwilling and John Saunders
experiment on chicken eggs
-how the pattern of the skeleton forms
discovered: two patches of tissue control the development of the pattern of bones inside limbs
strip of tissue at end of limb bud is essential- without it, the development stops
experiment on chicken egg
-took patch of tissue from pinky and put it on first finger
result: new fingers mirror images of normal set
-gene inside tissue able to control development of pattern of fingers
(mirror image duplication)
patch of tissue: zone of polarizing activity (ZPA)
-tissue that causes pinky side to be different from thumb side
-control formation of fingers and toes
how?
concentration-dependent idea- high concentration of ZPA
Denis Summerbell
foil between ZPA patch and rest of limb
-use barrier to prevent any molecule from diffusing to other side
results: cells on ZPA side formed digits
cells on opposite side barely formed digits (malformed)
something spread out of ZPA that controlled how digits formed (hedgehog gene)
the DNA recipe
one molecule started whole thing
fly experiment
-pattern of gene activity that makes body of fly from a single celled egg
hedgehog gene- made one end of body segment look different from the other
(similar to ZPA)
hedgehog gene has a specific DNA sequence
-scientists found sequence in other animals
animal version of gene= sonic hedgehog
(mimics activity of ZPA)
active in ZPA tissue
vitamin A cause s.h to turn on in the opposite side of limb
-make fingers distinct from one another-depends on how close finger is to gene
-when things go wrong with sonic hedgehog- hands messed up
not turned on properly during 8th week of human development=extra fingers
giving sharks a hand
sharks: earliest creatures that have fins with skeleton inside
mermaid’s purses- skate embryo
(cartilage)
-apply same experiments as they did to chickens
sonic hedgehog’s role in its limb?: YES
similar effect to fin (duplicate)
experiment
a skate embryo with a bead inside that leaks mouse s.h protein
result: develop different from one anther (responded to gene, like chickens did)
-s.h does something similar in skates and in humans
-genetic recipe similar
recipe that builds our hands has deep roots in other creatures
“inner fish”- biological tools that build fins
-all appendages are built by similar genes
-connections among creatures
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