Medicine College
Medical Biology
Dr. Bushra jabbar
Date:27-1-2016
Basic transcription
 Transcription is the synthesis of an RNA strand from a DNA template.
o A gene's protein building instructions are transcribed to messenger RNA (mRNA).
o mRNA carries the code from DNA to the ribosomes where translation into a protein
occurs.
 Transcription occurs in three stages:
1. Initiation:
 RNA
polymerase binds to DNA at a specific sequence of nucleotides called the
promoter.
 The promoter contains an initiation site where transcription of the gene begins.
 RNA polymerase than unwinds DNA at the beginning of the gene.
2.Elongation:
 Only one of the unmound DNA strands acts as a template for the RNA synthesis.
 RNA polymerase can only add nucleotids to the 3' end of the strand so like DNA,
RNA
must be synthesized in the 5' to 3' direction.
 Free ribonucleotides triphosphates from the cytoplasm are paired up with their
commplementary base on the exposed DNA template.
 RNA polymerase joins the ribonucleoside triphosphates to form an mRNA strand.
 As RNA polymerase advances, the process continues.
 The DNA that has been transcribed, re-winds to form a double helix.
3.Termination:
 RNA
polymerase continues to elongate until it reaches the terminator, a specific
sequence of nucleotides that signals the end of transcription.
 Transcription stops and mRNA polymerase and the new mRNA transcript are released
from DNA.
 The DNA double helix reforms.
 The termination sequence usually consists of a series of adjancent adenines preceded by
a nucleotide palindrome.
 This gives an RNA molecule that assumes a stem-and loop configuration.
 This configuration stops RNA polymerase from transcribing any further.
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Medicine College
Medical Biology
Dr. Bushra jabbar
Date:27-1-2016
New nucleotides are added to the growing RNA chain at the 3' end. The next nucleotides
to be added in the example here would contain the bases G and then C. The new G in
the RNA would complement the C below it in the template strand. Next after that in the
template strand is a G. That would be complemented by a C in the growing RNA.
.
After the end of the gene ("downstream" of the gene), there will be a termination
sequence of bases. Once the enzyme gets to those, it stops adding new nucleotides to
the chain and detaches the RNA molecule completely
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