Voltage-gated Na+ channels are highly concentrated in the nodes of Ranvier and the initial segment in myelinated neurons. 

TLS Online TPP Program

#Id: 3657

#Unit 3. Fundamental Processes

Human proteome contains 25 selenoproteins. Selenocysteine, sometimes called the “twenty-first amino acid,”

TLS Online TPP Program

#Id: 3658

#Unit 3. Fundamental Processes

Selenocysteine is incorporated into proteins with the aid of a tRNA that interprets the UGA Stop codon as a Sec codon. tRNASec is initially charged with serine in a reaction catalyzed by the same SerRS that charges tRNASer.

TLS Online TPP Program

#Id: 3659

#Unit 3. Fundamental Processes

Instead, a dedicated protein (a special elongation factor) named SELB in complex with GTP is required to deliver Sec–tRNASec to the ribosome.

TLS Online TPP Program

#Id: 3660

#Unit 3. Fundamental Processes

SELB ∙ GTP ∙ Sec–tRNASec reads the UGA codon as Sec rather than “Stop,” provided that the ribosomally bound mRNA has a hairpin loop on the 3′ side of the UGA specifying Sec.

TLS Online TPP Program

#Id: 3661

#Unit 3. Fundamental Processes

The gene can be restored to normal function if a second mutation either (1)converts the misplaced termination codon to a codon specifying an amino acid or (2) suppresses the effects of the termination codon, such restorative mutations are called nonsense suppressors.

TLS Online TPP Program

#Id: 3662

#Unit 3. Fundamental Processes

Pyl is specified by the codon UAG (normally a Stop codon).