Metamorphosis

Question 1

What triggers metamorphosis?

Answer 1

THYROID HORMONES
main product of thyroid gland
thyroid produces thyroxine (t4), but triiodothyronine t3 (more active form) made by target tissues from t4 with deiodinase enzymes

Question 2

What are the 2 types of deiodinase hormones and their functions?

Answer 2

D2 - CONVERTS T4 - T3 (HIGHLY ACTIVE)
D3 - CONVERTS T4 - RT3 (INACTIVE)
Most thyroid hormone present in blood is form of T4
If D2:D3 ratio high, most of T4 that enters a cell converted to T3

If D2:D3 low most goes to rT3

Question 3

Describe signal transduction by thyroid hormones

Answer 3

Bind to thyoid receptors (TRS) in the nucleus.
TRs bind to DNA sequences called THYROID RESPONSE ELEMENTS (TREs)
TRs bind to TREs whether hormone present or not

If not present receptor represses target gen.

When hormone binds to TR’s stimulates expression of target gene by exchaning co repressors for co activators

Question 4

What is Neotony?

Answer 4

Organisms that become sexually mature whilst in their larval state, usually staying in that state throughout life

Question 5

why is the timing of metamorphosis important?

Answer 5

limb formation triggered by low levels of thyroid hormone
tail RE ABSORPTION triggered by high levels of thyroid hormone

thyroid levels accumulate as metamorphosis progresses

SO… important as tail must recieve high amounts of thyroid to reabsorb, which is good otherwise would reabsorb before limbs develop and it would die!

tail has very low ratios of D2:D3

Question 6

what is the definition of metamorphosis?

Answer 6

complex post embryonic developmental event that transforms larval form of an animal into radically different juvenile
dramatic change of morphology

Question 7

what are the three types of changes that occur during metamorphosis?

Answer 7

cell death and resorption
growth of new structures
remodelling of existing structures

Question 8

when are genes are regulated by thyroid hormones during metamorphosis?

Answer 8

thyroid hormone treatment of tadpole induces 2 waves of gene activation or repression
1st wave genes directly regulated couple of hours after treatment
2nd wave about 48 hours after - genes indirectly regulated by thyroid hormones

Question 9

describe thyroid hormone regulation of genes in the tail

Answer 9

approx 1500 up regulated in response to T3
approx 600 down regulated.
genes up regulated are genes involved in cell death and tissue degredation
down regulated genes are those involved in energy generation and biosynthesis