5.3 TRANSCRIPTION
Modified from: myclass.peelschools.org
Central Dogma of Molecular
Biology
Proposed by Crick
 DNA → RNA → Protein

Transcription Stages
•
DNA  mRNA
•
This happens in the
nucleus
•
Has three stages:
(1)Initiation
(2)Elongation
(3)Termination
Messenger RNA (mRNA)

Made in the nucleus based on DNA sequences of specific
genes

mRNA is an individual "recipe" or copy from the "recipe
book" (DNA)

Each mRNA is copied from ONE of the 2 strands of DNA
by the same base-pairing principle as DNA replication

Each mRNA has a 5' and 3' end, - ribonucleotides are
added to the 3' –OH of the growing chain

RNA polymerases adds nucleotides with synthesis in the 5'
 3' direction

Process is called transcription
Transcription: Initiation
RNA polymerase binds to the DNA
at promoter region upstream from
beginning of gene to be transcribed
Promoter region = TATA box
high in A's and T's
takes
less energy to break the 2 H
bonds between A and T than the 3
H-bonds between C & G
DNA
unwinds to expose the
template strand
Transcription: Elongation

No primer required

Promoter is not transcribed

Build mRNA 5'3' (just like in DNA replication)

What is the direction of the template strand? 3'  5'

Template Strand = strand used as template; aka “non
coding strand”

Coding Strand = DNA strand that is not used

Once DNA is transcribed it gets rewound immediately
(polymerase moves)

Note: A – U and G – C
Transcription: Termination
Terminator sequence:
A
sequence of bases at the end of a gene that
signals the RNA polymerase to stop transcribing
This
sequence differs between prokaryotes and
eukaryotes
Here,
mRNA disassociates with template and
transcription stops
Now
RNA polymerase is free to bind with other
promoter regions of other genes
Transcription vs. Replication

Transcription and Replication are similar,
but different!

Transcription = creation of mRNA, uses
RNA Polymerase

DNA Replication = creation of new DNA
strand, uses DNA Polymerase

Both involve synthesis of a polynucleotide
Post-Transcription Modifications
(PTM)
• After termination, there are some changes
to the mRNA before it leaves the nucleus
1. 5' cap added
– Modified guanine nucleoside triphosphate (GTP)
• "7-methyl guanonsine"
– Protects mRNA from digestion from nucleases and
phosphatases in the cytoplasm
– Plays role in initiating translation
2.
3' end added
– ~ 200 adenine ribonucleotides added with the
assistance of enzyme poly-A polymerase
– Known as “poly-A tail”
3. Splicing = Remove introns
– DNA of eukaryotic genes have exons (coding regions)
and introns (non-coding regions)
– Introns removed by spliceosomes and exons joined
together (introns are degraded and nucleotides are
recycled)
Intron’s nucleotides
get recycled
End up with mRNA transcript ready to
leave nucleus to be translated by a
ribosome into a protein.
•No proof-reading
•More errors in transcription than in DNA
replication
•Multiple transcription of same gene so
errors not as bad
Transcription Animations
1.
Virtual Cell:
http://vcell.ndsu.edu/animations/transcription/movie-flash.htm
2.
McGraw Hill:
http://highered.mcgraw-hill.com/sites/0072995246/student_view0/
chapter6/mrna_synthesis__transcription_.html
3. University of Nebraska:
http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/
gene_a2.html
H – Human – This particular virus can only infect human
beings.
I – Immunodeficiency – HIV weakens your immune system by
destroying T-cells that fight disease
and infection. A "deficient" immune
system can't protect you.
V – Virus – A virus can only reproduce itself by taking over a
cell in the body of its host.
Viruses are NOT living organisms!
Homework

Read “HIV Case Study” – pg. 245 &
answer # 1 – 6 on pg. 248 (“Application
questions”)

Review “Summary” on pg. 248-249

Do questions pg # 249 Q # 1 – 5, 8 & 9