A sequence of 3 dna nucleotides that stands for 1

a sequence of 3 DNA nucleotides that stands for 1 amino acid is a base
triplet
 a 3-base sequence in mRNA is called a codon
 stop codons signal the “end of the message” & enable the cell’s
protein-synthesizing machinery to sense that it has reached the end of
instruction for a particular protein; these include: UAG, UGA, and UAA
 the codon AUG plays two roles:
 it serves as a code for methionine
 it serves as a start codon
Protein Synthesis (book)
 DNA & RNA collaborate together to produce proteins
 genetic code in DNA specifies which protein a cell can make
 all body cells except the sex cells & some immune cells contain
identical genes; however, different genes are activated in different
cells (example: genes for digestive enzymes are active in stomach cells
but not muscle cells)

 any given cell only uses one-third to two-thirds of its genes; rest
remain dormant in that cell, but may be functional in other types of
cells
 when gene is active, messenger RNA (mRNA) is made – a mirror image
of the gene
 mRNA migrates from the nucleus to cytoplasm where it serves as a
code for assembling amino acids in the right order to make a particular
protein
 think of the process as DNA  mRNA  protein (each arrow reading as
“codes for the production of)
 the step from DNA to mRNA is transcription
 occurs in the nucleus
 the step from mRNA to a protein is translation
 occurs in the cytoplasm
Transcription
 DNA is to large to leave the nucleus & participate directly in
cytoplasmic protein synthesis, so it is necessary to therefore make a
small mRNA copy that can migrate through a nuclear pore into the
cytoplasm
 just as we may transcribe (copy) a document, transcription , means the
process of copying genetic instructions from DNA to RNA
 an enzyme called RNA polymerase binds to the DNA & assembles the
RNA
 RNA polymerase opens up the DNA helix, reading the bases from one
strand of the DNA & making a corresponding RNA; then rewinds the
DNA helix behind it
 the RNA produced by transcription is an “immature” form called pre-
mRNA containing “sense” portions called introns that must be removed
before translation
 enzymes remove & degrade introns & splice the exons together into a
functional mRNA molecule, which then leaves the nucleus
 introns are removed while still in the nucleus & the exons being
ex ported from the nucleus to undergo translation in the
cytoplasm
 through a mechanism called alternate splicing , 1 gene can code for
more than 1 protein
Translation
 just as we might translate Spanish into English, genetic translation
converts the language of nucleotides into the language of amino acids
 this job is carried out by the following participants:
 messenger RNA (mRNA): carries the genetic code from the nucleus to
the cytoplasm

 during its synthesis in the nucleus, mRNA acquires a protein cap
that acts like a passport, permitting it to pass through a nuclear