Que 1. Hormones
are particulary pervasive in insect systems, affecting a wide variety of
physiological processes including, embryogenesis, postembryonic development,
behavior, water, balance, metabolism, caste determination, polymorphisms,
mating, reproduction and diapauses. Discuss the types of hormones in insects.
Include in your essay, the various types of insect hormones, factors that
affect their activity, their synthesis, release sites and their modes of action.
Ans:
Hormones are the
chemical messengers of multicellular organisms that allow the cells to
communicate and engage in coordinated responses.
PROTHORACIOTROPIC
HORMONE
PTTH is produced
in the lateral neurosecretory cells of the brain and is released in the corpus
cardiacum that terminates in the wall of the aorta or, in some insects, is
released by the corpus allatum. PTTH acts on the prothoracic gland to regulate
the synthesis of ecdysteroid
Mode of Action
The Corpus Cardium is the major neurohemal
organ in insects and releases a large number of neuropeptides. It is the
release of PTTH that determines the occurrence of the molt by activating the
prothoracic glands to produce the ecdysteroid molting hormone. Because PTTH is
a peptide hormone, it is unable to enter the cells of the prothoracic gland and
must exert its influence from the outside through a G protein coupled receptor.
This G-protein coupled receptor activation increases intracellular Ca2+ as a
second messenger, which then activates protein kinases that can phosphorylate
and activate enzymes in the biosynthetic pathways that lead to a cellular
response.
ECDYSTEROIDS
Ø Ecdysone
is a steroid hormone belonging to the class of substances known as terpenoids.
Ø The
precursors for ecdysteroid synthesis by the prothoracic gland of insects are
sterols, such as cholesterol.
Ø The
primary site of ecdysteroid synthesis is the prothoracic gland.
Ø In
adults, the site of ecdysteroid synthesis has been shifted to the ovaries and
the testes.
Ø In
many female insects, ecdysteroids are produced by the follicle cells of the
ovaries.
Mode of Action
of Ecdysteroids
The ecdysteroid
receptor (EcR) and the product of the ultraspiracle gene (USP) bind to the
hormone. The ecdysteroid receptor complex binds to early genes, stimulating
their transcription but inhibiting the transcription of the late genes. The
early gene product that is produced subsequently inhibits the early genes but
stimulates the late genes, demonstrating the cascade of gene activity that is
involved in salivary gland morphogenesis.
THE
JUVENILE HORMONES
The corpus
allatum (CA) is the major organ of JH synthesis and release.
Mode
of action
Ø The
major role of JH in insects is to modify the action of ecdysteroids and prevent
the switch in the commitment of epidermal cells.
Ø In
the presence of ecdysteroids, JH preserves the current program of gene
expression.
Ø JH
both influences the stage-specific expression of the genome that is initiated
by ecdysteroids and also acts by itself to modulate the expression of certain
specific genes.
Factors
affecting hormonal activity
Hormonal
activity in the circulatory system is regulated by
Ø its
rate of synthesis by the endocrine glands,
Ø the
rate of release into the blood,
Ø its
degradation in the blood,
Ø development
and presence of hormone receptors on target cell.
Que.2. Outline the synthesis and processing of
insect peptide hormones
Ans:
Ø Peptide
hormones are usually synthesized as larger precursor preprohormones and prohormones
and then processed by proteolytic enzymes into the smaller final hormone.
Ø The
peptide must be inserted into the cisterna of the endoplasmic reticulum, and a
signal peptide portion must be attached in order for this to occur.
Ø The
pre- and pro-portions are cleaved, and the peptide hormone is then released
from the cell by exocytosis.
DNA
Transcription
mRNA
Translation
Preprohormone
Signal
transduction
Proteolysis
Prohormone
Proteolysis
Inactive
Fragment Glycosylation
Phosphorylation
Hormone
The synthesis and
processing of peptide hormones
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