Human Physiology

Concept: Protein synthesis (translation) begins with a specific initiator amino acid that is incorporated into the first position of the growing polypeptide chain. The initiator amino acid is delivered by a specialized initiator tRNA that recognizes the start codon on mRNA.
Step 1:Start codon in translation.
The process of translation typically begins at the start codon: [ AUG ] This codon signals the ribosome to begin protein synthesis.
Step 2:Initiator amino acid in eukaryotes.
In eukaryotic cells, the codon (AUG) codes for the amino acid methionine. A special initiator tRNA carrying methionine recognizes this start codon and binds to the ribosome to initiate translation.
Step 3:Difference between prokaryotes and eukaryotes.
Although methionine is also the starting amino acid in prokaryotes, it is modified into N-formylmethionine (fMet) in bacteria. Thus, the universal initiator amino acid for protein synthesis in eukaryotes is methionine.

Concept: The "fight or flight" response is a physiological reaction that occurs when the body encounters a stressful or threatening situation. This response prepares the body for rapid action.
Step 1:Role of the sympathetic nervous system.
When the brain perceives danger, the sympathetic nervous system stimulates the adrenal medulla to release hormones into the bloodstream.
Step 2:Release of adrenaline.
The adrenal medulla secretes the hormone adrenaline (epinephrine), which rapidly prepares the body to respond to stress.
Step 3:Physiological effects of adrenaline.
Adrenaline produces several immediate effects:

• Increases heart rate and blood pressure
• Dilates airways in the lungs
• Increases blood glucose levels
• Redirects blood flow to muscles

These changes enable the body to either confront the threat or escape from it. Therefore, the hormone primarily responsible for the "fight or flight" response is adrenaline (epinephrine).