TRANSCRIPTION
Transcription is the process by which an RNA molecule
is synthesized from a DNA template. It involves the enzyme RNA polymerase,
which catalyzes the synthesis of RNA using a DNA template. Transcription can be
broken down into several stages:
1. Initiation:
- Promoter
Recognition: RNA polymerase II (Pol II) recognizes
specific DNA sequences known as promoters. Promoters are usually located
upstream (towards the 5' end) of the transcription start site.
- Formation
of Transcription Initiation Complex: Transcription
factors bind to the promoter region, recruiting RNA polymerase II to form
the transcription initiation complex.
2. Formation of the Open Complex:
- RNA
polymerase II unwinds the DNA double helix near the transcription start
site, creating an open complex.
3. Elongation:
- RNA
polymerase II moves along the DNA template in the 3' to 5' direction,
synthesizing an RNA molecule in the 5' to 3' direction.
- As
RNA polymerase II advances, it continues to unwind the DNA double helix
ahead and synthesizes RNA.
4. Termination:
- Polyadenylation
(Poly-A) Signal: In eukaryotes, the termination signal
is often a polyadenylation signal (AAUAAA) in the pre-mRNA.
- Cleavage
and Polyadenylation: RNA polymerase II recognizes the
poly-A signal, and the pre-mRNA is cleaved downstream of this signal. A
poly-A tail (adenine nucleotides) is added to the 3' end of the mRNA.
- Termination
of Transcription: RNA polymerase II dissociates
from the DNA template, and the newly synthesized mRNA is released.
5. RNA Processing:
- 5'
Capping: A 7-methylguanosine cap is added to
the 5' end of the mRNA. This cap protects the mRNA from degradation and
facilitates its export from the nucleus.
- Splicing:
Introns (non-coding regions) are removed, and exons (coding regions) are
joined together through splicing. This process occurs in the nucleus and
involves small nuclear ribonucleoproteins (snRNPs).
- 3'
Polyadenylation: The mRNA receives a poly-A tail at
its 3' end, which aids in mRNA stability and translation.
6. RNA Polymerase I:
- RNA
polymerase I is responsible for transcribing ribosomal RNA (rRNA).
- The
rRNA genes are located in the nucleolus, and RNA polymerase I synthesizes
the precursor rRNA (pre-rRNA), which is later processed into the mature
forms of rRNA.
In summary, transcription involves the synthesis of RNA
from a DNA template. RNA polymerase II is responsible for transcribing
protein-coding genes, and the resulting pre-mRNA undergoes various processing
steps, including capping, splicing, and polyadenylation, to form mature mRNA.
RNA polymerase I transcribes rRNA genes, producing precursor rRNA molecules
that undergo further processing. The coordinated action of transcription
factors, RNA polymerases, and other associated proteins ensures the accurate
and regulated synthesis of RNA molecules in the cell.
LET’S EXPLORE THIS SIMPLY
let's break down the process of transcription in
simpler terms:
- Initiation:
- Imagine
you have a special machine called RNA polymerase II. This machine
recognizes a specific starting point on the DNA called a promoter.
- With
the help of some friends (transcription factors), the RNA polymerase II
machine attaches to the DNA at the promoter, like a key fitting into a
lock.
- Formation
of the Open Complex:
- Once
attached, the RNA polymerase II machine unwinds the DNA, like opening a
zipper, creating an open area.
- Elongation:
- The
RNA polymerase II machine starts moving along the DNA, reading the
information on one of the strands.
- As
it moves, it builds a new strand, which is an RNA copy of the DNA. It's
like making a copy of a recipe from a cookbook.
- Termination:
- After
copying a specific section of the DNA, the RNA polymerase II machine
recognizes a signal that tells it to stop.
- It
finishes making the RNA copy, and the new RNA strand is released.
- RNA
Processing:
- Imagine
the RNA strand is like a raw dough, and it needs some finishing touches.
- A
special cap is added to the beginning, like putting a nice cover on a
book.
- Unwanted
parts are cut out, and the remaining pieces are joined together, like
editing a film to remove unnecessary scenes.
- At
the end, a tail is added to the RNA, giving it stability and a signal
that it's ready to go.
- RNA
Polymerase I (Bonus Round):
- There's
another machine called RNA polymerase I that handles a different type of
copying. It deals with making the important parts for building cell
structures.
In simpler terms, transcription is like making a copy
of a recipe (DNA) to follow and build something (RNA). The RNA polymerase II
machine reads the recipe, creates a new version, and then adds some special
touches to make it complete and ready for use. RNA polymerase I works on a
different set of recipes for building cell structures.