Lesson: Transcription
Definition for Transcription
Transcription is a process by which cells are able to express their genes.
DNA is used as a Template for complementary RNA strand synthesis.
mRNA may then direct the synthesis of various proteins.
Synthesis is catalysed by RNA Polymerases.
Stages of Transcription
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Initiation
Elongation
Termination
Initiation
RNA polymerase. It attaches to and moves along the DNA molecule until it recognises a promoter
sequence.
Prokaryotic Promoter contains characteristic consensus sequences.
Two are recognised by RNA polymerase
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-35 sequence TTGACA, present 35 bases upstream from start of Transcription
Prinbnow box TATAAT, present 10 bases upstream
It then unwinds a portion of the DNA double helix, exposing the bases on each of the singular DNA strands.
One of the strands will act as a template to create the new mRNA strand.
Eukaryotic RNA polymerase recognises TATA or Hogness box (-25)
Transcription Bubble
Elongation
This is the process by which ribonucleotides are added to the template strand, enabling growth of the
mRNA transcript. The mRNA transcript is made in a 5’ to 3’ direction.
Incoming ribonucleotides are used by RNA polymerase to form the mRNA strand.
The RNA polymerase then binds the ribonucleotides together covalent by phosphodiester bonds.
RNA poly uses nucleoside triphosphates as substrates and releases pyrophosphate each time a
nucleotide monophosphate is added to the chain
Termination
Elongation will continue until the RNA polymerase encounters a stop sequence. At this
point, transcription stops and the RNA polymerase releases the DNA template.
It is either intrinsic or dependant on a protein(rho factor)
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ρ Dependant Termination : A hexameric ATPase with helicase activity.Reaches
RNA poly at paused at termination site and seperates RNA-DNA hybrid helix.
ρ Independant Termination : Requires self complementary sequence in nascent
RNA that will fold back on itself forming a GC rich stem.
GC hairpin
Antiobiotics
Rifampin
Actinomycin D
Inhibits transcription by binding to B subunit
of prokaryotic RNA polymerase
Intercalatesbetween G and C pairs and
inhibits initiation
Treats Tuberculosis
Therapeutic application in chemotherapy
Xeroderma Pigmentosum
Causes
XP is an autosomal recessive disease
Exposure to unfiltered Sunlight make pyrimidine dimers
Those genes are mutated that code for UV-specific endonucleases
NER: Nucleotide Excision Repair is reduced or eliminated
Lack of DNA repair results in more mutations and cancer when exposed to
sunlight
Signs and symptoms
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sunburn when exposed to only small amounts of sunlight. These often occur during a child's first exposure to sunlight.
Development of many freckles at an early age
Rough-surfaced growths (solar keratoses), and skin cancers
Eyes that are painfully sensitive to the sun and may easily become irritated, bloodshot and clouded
Blistering or freckling on minimum sun exposure
Telangiectasia (spider veins)
Limited growth of hair on chest and legs
Scaly skin
Xeroderma (dry skin)
Irregular dark spots on the skin
Corneal ulcerations