Central Dogma Part 2
Another complication: the central dogma states
that information flows in one direction,
beginning with DNA. There is an exception.
Sometimes RNA can be read by an enzyme
called reverse transcriptase; this produces DNA.
This enzyme was first found in retroviruses
(such as HIV). It is also found in stem cells, and
it allows these cell to divide without limit
because it allows repair of chromosome ends
(telomeres) shortened by replication.
Transcription begins when the enzyme RNA
polymerase finds a special start sequence (called a
promoter) on the DNA, pulls apart the DNA, and
copies the information of one strand by making a
complementary strand of mRNA. This continues
until it reaches a sequence called the terminator.
[Maybe we could call it Arnold for short.] This
first transcript contains some “junk” – non-coding
sections called introns. Before the mRNA can
leave the nucleus to make protein it must be
processed. Spliceosomes (consisting of special
nuclear RNA and proteins) cut out the introns and
splice the mRNA together.
RNA gets “edited.” The
so-called junk DNA
(introns) gets removed
to make the final mRNA
[As we learn more
about these processes
we find that the junk
often has a role to play.]
Diagrams from Campbell and Reece (2005)
Then a CAP and a Poly-A tail are put on the mRNA;
it leaves the nucleus through a nuclear pore.
The 5’ CAP keeps the mRNA from being attacked
by enzymes and signals ribosomes where to start.
The Poly(A) tail also protects the RNA and helps
ribosomes attach; it also seems to help in exporting
the mRNA from the nucleus.
can see the