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HORMONES: Chemical messengers, released directly into the blood from endocrine glands (which do not have ducts).
The glands that make hormones are not themselves affected by their products; therefore most hormones are produced within secretory
vesicles by the Golgi apparatus.
The vesicles fuse with the cell surface membrane, releasing their contents by exocytosis.
Each hormone is SPECIFIC to target cells, modifying their activity.
Hormones are carried around in the bloodstream.
HOW HORMONES AFFECT CELLS
Peptide hormones are protein chains e.g. insulin. They are not able to move through cell membranes easily, because they are charged.
Instead they bind to a receptor on the cell membrane; this then activates another molecule in the cytoplasm a second messenger. The
functional second messenger brings about chemical changes within the cell directly or indirectly by affecting gene transcription.
Steroid hormones are formed from lipids and have complex ring structures. They pass
through the cell membrane and binds directly to a receptor molecule within the cytoplasm. Once
activated, the hormone-receptor complex brings about characteristic responses, resulting
from its effect on transcription.
HOW TRANSCRIPTION FACTORS WORK
Transcription is initiated by an enzyme called RNA polymerase and a cluster of associated protein
transcription factors binding to the DNA the transcription initiation complex.
The complex binds to a section of DNA adjacent to the gene to be transcribed (the promoter region).
Most transcription factors are created in an inactive form, and are then converted into the active form by the action of hormones, growth
factors or other regulatory factors. The gene remains switched off until all the required transcription factors are present in their active form at
the promoter region.
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Transcription of a gene can be prevented by protein repressor molecules attaching to the DNA of the promoter region this blocks the
attachment sites for transcription factors. Protein repressor molecules can also attach to the transcription factors themselves, preventing
them from forming the complex.
The control of transcription initiation is an important mechanism that determines whether a gene is expressed, and therefore what proteins
are synthesised.…read more