Download Lecture 3 - Transcription (student)

BUT MOM... I don’t want to be a
protein!
Too bad.
Once upon a time... In 1902...
• Archibald Garrod, a UK physician, noticed
certain illnesses recurring in families
• He came up with a hypothesis
– Enzymes are under the control of THE hereditary
molecule
– A defective enzyme causes an “inborn error of
metabolism”
Once upon a time... In 1902...
• He analyzed blood/urine samples of patients
with alkaptonuria
– A condition that turns urine black due to the
presence of alkapton
• He proposed that individuals with alkaptonuria
had a defective enzyme
– A alkapton-metabolizing enzyme!
33 years later...
• George Beadle and Edward Tatum confirmed
Garrold’s hypothesis studying red bread mold
– Neurospora Crassa (N. Crassa)
• Hoped to discover a relationship between genes
and enzymes
• Used multiple strains of
N. Crassa and grew them
on different media
33 years later...
Procedure:
• Grew one strain of N. Crassa on a nutrient medium
with simple inorganic salts, sugars, and vitamins
– Result: N. Crassa was able to synthesize all complex AAs
• Mutant strains were produced via x-rays
– Result: Descendents could NOT grow on medium
• they could no longer produce all essential compounds to
sustain life
33 years later...
• To find out which AA they could not produce
– They placed mutant strains in vial that had basic nutrients
plus ONE EXTRA AA
• The only growth occurred with ARGININE
– Thus, 1+ enzymes in the arginine pathway were defective
• Conclusion: a lack of a specific enzyme corresponded
to a mutation in specific gene
Final piece of the puzzle...
• In 1956, Vernon Ingram confirmed all previous
experiments using sickle cell anemia
– Condition where red blood cells are deformed
• He studied the AA sequence of hemoglobin for
individuals that had sickle cell anemia
• He found a single AA mutation
that completely altered the
shape of the red blood cell
Final piece of the puzzle...
• Ingram found that valine replaces glutamic acid in
individuals with sickle cell anemia
Conclusion: genes specify the type and location of
each amino acid in polypeptide chain
Other examples of simple AA mutations:
hemophilia and cystic fibrosis
What’s the deal with proteins?
• Are the REAL building blocks of life.
• Technically… they shouldn’t exist at all.
– There may be as many as a million types of proteins &
each one is a miracle!
• You need to assemble AA in a particular order
– Similar to assembling letters to spell a word
– Except…
• Words using the AA alphabet are INCREDIBLY LONG
Try this out…
• Collagen needs 1,055 AA in its sequence
– Get this! You don’t make it, it creates itself…
• AUTOMATICALLY
Picture a slot machine…
*That has 1,055 wheels (not 3)
*that’s 90ft long!
*20 symbols on each wheel
Try this out…
• Most proteins have 200 AA
– The odds of creating that is 1 in 10260
• That’s A LOT of zeros
• Hemoglobin has 146 AAs
– Max Perutz took 23 years to unravel the sequence
“Assembling a protein is like a whirlwind spinning
through a junkyard…
… and leaving behind a fully assembled
jumbo jet”
- Fred Hoyle
So how are proteins made?
• There is a problem:
• DNA makes proteins, but…
– DNA can’t leave the nucleus or it will be destroyed
– Proteins are constructed outside the nucleus
• mRNA acts as the middle man
– It can take the DNA message out of the nucleus to
synthesis polypeptides
RNA – RiboNucleic Acid
The world’s greatest translator
Different from DNA in 3 ways
1. DNA has deoxyribose sugar
RNA has ribose sugar (-OH on 2’ carbon)
2. DNA contains Thymine (A  T)
RNA contains Uracil (A  U)
3. DNA is double stranded
RNA – RiboNucleic Acid
3 types of RNA
1. mRNA – messenger RNA
*synthesizes proteins using DNA’s info
2. tRNA – transfer RNA
*transfer appropriate AAs to build proteins
3. rRNA – ribosomal RNA
*structural component of ribosome that is used
TRANSCRIPTION
• The process of converting DNA’s genetic
information into a more usable form (mRNA)
• There are 4 phases in transcription
1. Initiation
2. Elongation
3. Termination
4. Post-transcriptional Modifications
**VERY SPECIFIC ENZYMES ARE USED**
PHASE 1: INITIATION
• A promoter is used to signal where RNA
polymerase should bind to DNA strand
– A promoter is a segment of DNA that is usually high in
A&T
• They only have 2 H-bonds and will break open easily
• If RNA polymerase were to bind randomly,
– correct genes & proteins wouldn’t be produced
• RNA polymerase will unwind DNA and begin to
synthesize the complementary RNA strand
PHASE 2: ELONGATION
• RNA polymerase starts to build mRNA in the 5’ 
3’ direction
– Starts as soon as RNA polymerase hits promoter
• **The promoter DOES NOT get transcribed**
• RNA polymerase uses one strand as a template is
the TEMPLATE STRAND
• The strand not used for transcription is called the
CODING STRAND
PHASE 3: TERMINATION
• mRNA will continue elongation until RNA
polymerase hits the terminator sequence
– Causes dissociation of RNA from DNA
• RNA polymerase is then free to bind to another
promoter region
PHASE 4: Post-transcription mod’s
mRNA can’t leave nucleus immediately after
transcription
◦ Primary transcript
The following 3 steps must first occur:
1. 5’ cap of GTP is added to start of mRNA
*this protects mRNA from enzyme attack which is
inevitable in the cytoplasm
PHASE 4: Post-transcription mod’s
mRNA can’t leave nucleus immediately after
transcription
◦ Primary transcript
2. 3’ end obtains a poly-A tail (string of 200 As)
Introns vs. Exons
• The very interesting part of DNA is that 97% of it
does nothing
– Only 3% of our DNA code for genes that make proteins
• INTRONS = non-coding DNA
• EXONS = coding DNA
Introns vs. Exons
• If introns are not removed, proper protein folding
will not occur
– Leads to decreased function or death of proteins
• Introns are removed by spliceosomes
• Once removed, introns remain in nucleus
– Broken down by enzymes to recycle parts
Errors with Transcription
• DNA replication used DNA polymerase I & III to act
as proof readers to fix mistakes
– Chances of mistakes are less likely
• Transcription has no quality control mechanism
– Errors are more likely
• However, errors aren’t as detrimental because a
gene is transcribed hundreds of times
– If one copy of the gene has an error, it won’t have a
huge effect