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Transcription:
The whole truth
(… I didn’t reveal the whole
story……)
• The rate of amino acid synthesis is 200 per minute
• There are 10 million ribosomes in an average
mammalian cell
• The LARGEST protein is called “Titin”. It is 34,000
amino acids long (muscle fiber protein)
• The smallest human protein is “Sarcolipin”. It is only
31 amino acids long (another muscle protein)
• The smallest is “TRP Cage” which is a saliva protein
found in Gila Monsters ( 20 amino acids long )
Why Proteins?
Proteins are keys to almost everything that
living cells do.
*They do most of the work in cells and are required
for the structure, function, and regulation of the
body’s tissues and organs.
•Genes code for pigments (eye and skin color)
•Surface antigens are produced from proteins
•Regulate growth rates and patterns
•Proteins regulate chemical reactions (enzymes)
Titin - youtube
(the longest word in
english pronounced)
Which organelles in a cell are involved in protein
production and distribution?
*
* nucleus
rough
*
mitochondria
*
Rough Endoplasmic Reticulum
1) The ER is the compartment where newly-synthesized polypeptides
fold
2) where many protein complexes are assembled (2 or more proteins
= quaternary structure)
3) and where glycoproteins acquire their “glycans” (polysaccharides)
ex. Some hormones, anti-freeze protein, components of mucus
4) Exocytosis of ER delivers the “almost complete” proteins to the
Golgi Apparatus by means of a vesicle
Golgi Apparatus
1) They attach various sugar monomers to proteins as the
proteins move through the apparatus (finishes them!).
2) The Golgi apparatus packages proteins into membranebound vesicles inside the cell before the vesicles are sent
to their destination. (determines final destination)
THE CENTRAL DOGMA
“The central dogma of molecular biology is “the flow of
genetic information within a biological system”……….. deals
with the detailed residue-by-residue transfer of sequential
information. It states that information cannot be transferred
back from protein to either protein or nucleic acid.”
In other words, once information gets into protein, it can’t
flow back to nucleic acid.
DNA  mRNA (Transcription)
 Protein (Translation)
A closer look at
Transcription
Making mRNA from DNA
(not quite as easy as previously shown)
(2 new things)
Nuclear Membrane
A Problem…..
DNA is too big to leave the nucleus!
Nuclear Pore
The Solution!!!
Make something that can leave the nucleus
Solution = Ribonucleic Acid (RNA) – smaller
(thinner)
Three types:
1. mRNA (messenger RNA)
2. tRNA (transfer RNA)
3. rRNA (ribosomal RNA)
Transcription - RNA production
•Message on the DNA molecule is “transcribed” onto
RNA To do this:
1. DNA is unzipped by DNA Helicase
2. RNA Polymerase binds free floating RNA
nucleotides to one exposed strand of DNA.
3. New strand of RNA breaks away and is free to
leave the nucleus.
a. Introns removed, Exons stay
b. Methyl cap added to 5’
c. Poly-A tail added 3’
4. DNA reattaches to reform the double helix
5. mRNA is made at the rate of (on avg.)
40 bases/second.
Dictionary:
•Scribe - somebody who copied manuscripts in medieval times
•Transcribe - to rewrite something
1) Gene Expression - One
gene can actually code for
more than one protein.
What? How?
…….. the end that leaves the
nuclear pore is protected by
something called a methyl cap
THE FAT COW CAT ATE THE HAY RAT
EITHER Green OR Red
represent…..
Black represents…..
(and color not chosen as
an intron)
Introns
Exons
Prokaryotes don’t
have introns and
exons. Why?
Over time, introns were lost from prokaryotes as a way to
make proteins more efficiently. ... The mixing and
matching of exons from the same gene can lead to proteins
with different functions. Eukaryotes might need this
diversity in proteins because they have many types of cells
all with the same set of genes
The enzyme that removes the
introns is called a “spliceosome”
There are about 20,000 genes in
your genome (all your DNA) but
you can make 100,000+ proteins
* The promoter (aka TATA Box) is an area that lets other molecules
know where transcription starts – found in eukaryotes & archae
Poly A tail is a string of A’s (AAAAAAAAA) - What
reason would it have this?
Exonucleases – their job is to
“cut” the A’s off……. The more
it cuts or the longer the tail the
more protein that is made.
1. DNA Learning Center RNA Splicing
2) Gene Regulation
A gene coding for a
protein can regulate
when a protein is
made and when to stop
producing it! How?
Example: lac operon system – in prokaryotic cells (p 235)
THIS IS
DNA
In Eukaryotic cells (like ours)…
We have about 19,000 to 20,000 genes
in the human body…..
The rest is what they used to call
“Junk DNA”
We have about 19,000 to 20,000
Genes in the human body…..
The rest is what they used to call
“Junk DNA”
We have about 19,000 to 20,000
Genes in the human body…..
The rest is what they used to call
“Junk DNA”.....
...... But now they
know its importance!!
THIS IS
DNA
THIS IS
DNA
THIS IS
DNA
Gene Regulation in Eukaryotic Cells
Basal Factors
Binding
Protein
(DNA Strand)
RNA
Polymerase
3’AAGTATAACGCCTTACGCCGCTACCCCGCATGCTAGAG 5’
Pre-mRNA
Coding region (protein)
mRNA 5’ AUGCGGCGAUGGGGCGUACGAUCUC 3’
(no introns taken out in this example)
*Where does the Binding Protein go?
*Where does the RNA Polymerase attach?
*How does the Binding Protein keep the RNA Polymerase
from moving until a protein is “signaled” from the activators?
*Where is the Coding Region? (protein produced)
The DNA “folds over” like a switch
to activate (like a current of
electricity) the RNA primase to
produce the mRNA strand
Actual DNA in
a eukaryote
undergoing
transcription
Our most chronic diseases, including various cancers, diabetes, heart
disease and hypertension, probably result from a complex recipe of
dysfunction, not just in the genes, but in a variety of hormones,
enzymes and other metabolic factors (including these “switches” that
affect them. Rare diseases may be caused by mutations in the
genetic code while the more common, complicated diseases may be
traced to genetic changes in the switches.
1. HHMI.org – click on “BioInteractive”
search box – DNA Transcription (advanced detail)
activators
Students: This is
where we stopped
this powerpoint
New info:
• …. The code of TTATTT (on the DNA) is
what the mRNA Polymerase recognizes as a
stopping point to finish making the mRNA.
• Then more bases ... #? .... And then the
poly-A tail is added.... #? (use AP text to
find these numbers)
• Teacher note: Should this info be added in
2017-18?
Translation
Using mRNA to make a protein!
Ribosomal RNA - rRNA
•Where tRNA & mRNA come together
Ribosome subunits
How a ribosome works…..
Start here
Protein Synthesis
Section 12-3
Nucleus
Messenger RNA
Messenger RNA is transcribed in the nucleus.
Phenylalanine
tRNA
The mRNA then enters the cytoplasm and
attaches to a ribosome. Translation begins at
AUG, the start codon. Each transfer RNA has
an anticodon whose bases are complementary
to a codon on the mRNA strand. The ribosome
positions the start codon to attract its
anticodon, which is part of the tRNA that binds
methionine. The ribosome also binds the next
codon and its anticodon.
Ribosome
Go to
Section:
mRNA
Transfer RNA
Methionine
mRNA
Lysine
Start codon
Translation (Continued)
Section 12-3
The Polypeptide “Assembly Line”
The ribosome joins the two amino acids—
methionine and phenylalanine—and breaks
the bond between methionine and its tRNA.
The tRNA floats away, allowing the ribosome
to bind to another tRNA. The ribosome moves
along the mRNA, binding new tRNA molecules
and amino acids.
Lysine
Growing polypeptide chain
Ribosome
tRNA
tRNA
mRNA
Completing the Polypeptide
mRNA
Ribosome
Go to
Section:
Translation direction
The process continues until the ribosome reaches
one of the three stop codons. The result is a
growing polypeptide chain.
Structure of tRNA
tRNA
Amino Acid (Lysine)
This tRNA molecule carries the anticodon
UUU. It will be called in by the mRNA codon
AAA.
Since there are 4 bases, and they work in groups of 3, there are 64
possible combinations possible for codons (4 x 4 x 4 = 64). There
are 20 different amino acids, so more than one codon can code for
each amino acid. The codon AUG is called the start codon and
three separate codons are called stop codons.