Information Transfer and Protein
Synthesis

“Central Dogma”
DNA
mRNA
Protein
(polypeptide)

Molecular Code
• DNA stores/transmits a
“code”
• 4 nitrogen bases A, T, C, G
• 20 essential amino acids
A colored scanning electron micrograph of
a group of human chromosomes (x6,875)

Transcription
Transcription is the process by which one
strand of the DNA is copied to form a
complimentary strand of messenger RNA
(mRNA)

Transcription

Transcription
initiation
elongation
termination

initiation
• RNA polymerase binds to the DNA at
the promoter region
• DNA unwinds exposing the coding
strand

elongation
• DNA acts as a template (pattern) for the
mRNA
• RNA polymerase moves along DNA
elongating (making longer) the mRNA
strand

termination
• RNA polymerase reaches the terminator
region on the DNA
• RNA is release RNA polymerase falls
off

A transmission electron micrograph
of an unidentified operon of the
bacterium Escherichia coli, x72,600.
Ribosomes attach to mRNA, and
protein synthesis begins even before
transcription is complete.

RNA Processing
• the primary mRNA transcript may be
200,000 nucleotides in length
• mRNA in the cytoplasm is only a few
1000 nucleotides in length
• mRNA is modified (processed) before
leaving the nucleus

Enzymes attach a
cap of chemically
modified guanine
nucleotides (methyl-
guanine, or mG) to
the starting end of
the mRNA molecule

Other enzymes then
replace part of the
opposite end with a tail
of 100–200 adenine
nucleotides called a poly-
A tail

The final step in mRNA
processing involves
removal of some internal
segments of the RNA that
do not code for protein
called introns

The parts of the
transcript that remain
(and code for protein)
are called exons. They
are joined together in a
process called splicing.