From Gene to Protein: Chapter 17
What we know already: We know DNA is responsible for phenotype.
But how is DNA responsible? It codes for proteins responsible for phenotype.
Example: Pea plants can be tall or short. Short pea plants don’t make gibberellin which is a plant growth hormone. They
lack the enzyme for gibberellin synthesis. The recipe for that enzyme is lacking in their DNA
Connection between Genes and Proteins
1909 - Garrod first suggested this connection with what he called “inborn errors in metabolism.”
Alkaptonuria – urine contains dark chemical alkapton because body lacks enzyme to break down alkapton.
He theorized that this lack of enzymes was inherited.
One Gene – One Enzyme
George Beadle and Edward Tatum
Worked with bread mold (Neurospora)
First proposed one gene-one enzyme hypothesis – every gene gives rise to an enzyme.
However, since not all proteins are enzymes, more accurately: one gene gives rise to one protein.
However, since some proteins are made of more than one polypeptide chain the most current and accurate way to say this is
one gene gives rise to one
polypeptide.
It’s termed the one gene-one
polypeptide hypothesis and is currently
being shot down because of
exceptions. It’s a good general rule
though.
How DNA becomes a protein
DNA is transcribed and translated to
involved are collectively called the
Biology”
make a protein. The general steps
“Central Dogma of Molecular
Protein Synthesis varies between prokaryotes and eukaryotes
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Prokaryotes have no nucleus so the 2 steps, transcription and translation, both occur in the cytoplasm
Eukaryotes have 3 steps, transcription and RNA processing occurs in the nucleus, and translation occurs in the
cytoplasm
RNA
Like DNA – made of nucleotides
Unlike DNA –
Uses ribose sugar instead deoxyribose sugar
Uses uracil instead of thymine
Is single stranded instead of double stranded
RNA can be found in 4 forms
4 kinds of RNA
1.
2.
3.
4.
Messenger RNA (mRNA) – DNA is copied into mRNA during transcription.
Transfer RNA (tRNA) used during translation to carry the correct amino acid.
Ribosomal RNA (rRNA) – folded strands of RNA used to make a ribosome. Site of protein synthesis.
Small Nuclear RNA(snRNA)- Combined with proteins to make a structure called a spliceosome which helps with
editing the RNA molecule.
From DNA to Protein
Remember that our ultimate goal is a protein. We’ll cover the three steps (transcription, processing and translation) in detail
later.
How does mRNA code for proteins?
There are 20 amino acids. How do you code for 20 amino acids with only 4 nucleotide bases?
Codons
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In order to get at least 20 different combinations, we have to use at least 3 nucleotides.
Codons are blocks of 3 mRNA nucleotides that code for an amino acid.
Scientists have determined which codons code for which amino acids.
Chains of mRNA
Chains of mRNA come about during transcription as a copy of the DNA is made.
DNA = TTAGACTAG
mRNA = AAUCUGAUC
In that chain of mRNA there are 3 codons: AAU, CUG and AUC
Each codon can be matched up to an amino acid.
Breaking the Genetic code
Nirenberg and Matthei determined that the 1st codon – amino acid match
UUU codes for phenylalanine
They did this by taking a chain of uracil and adding it to a test tube of ribosomes and nucleotides…they got a
chain of phenylalanine
The genetic code
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Same for all life…called universal for that reason.
Code is one of the strongest supporting arguments for a common origin for all life

The genetic code is redundant but not ambiguous.
A reading frame is a set of codons read correctly. The reading frame can be shifted and the results are almost always
devastating.
Transcription
mRNA copy of the DNA is made in the nucleus.
RNA polymerase attaches at a region upstream from the gene called the promoter and unwinds and copies the
template side of the DNA.
Animation I
Animation II
RNA processing
Before leaving the nucleus, the newly formed mRNA strand is modified by:
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Adding a 5’ cap
Adding a poly A tail on the 3’ end
Cutting out the introns
Splicing exons together
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Why is there alteration of mRNA ends
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A modified guanine is added to the 5’ end. It will help protect the molecule from degradation and it will signal the
attachment to a ribosome in the cytoplasm
A series of 50 to 200 adenines are added to the 3’ end. This is called a poly A tail. Helps mRNA to leave the
nucleus, prevents degradation and signal attachment of a ribosome
Exons and Introns
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Sections of DNA that code for a protein will include interspersed sections that interrupt. In mRNA they are
removed. These are called “intervening sequences” or introns. (Also called “junk DNA”)
When introns are cut out the remaining mRNA is eventually expressed and are called exons.
mRNA splicing
 Introns are cut out and exons are pasted together.
 snRNA forms a complex with other proteins to make a spliceosome which recognizes introns and
cuts them out.
Translation: a closer look
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The key RNA in translation is transfer RNA.
tRNA is a chain of about 80 or so bases. Three of bases are called the anticodon.
The are complementary to the codon in mRNA.
The top of the structure has an amino acid.
How the amino acid joins - The amino acids (remember that there’s 20 of them) are in abundance in the cytoplasm.
They are attached to their specific tRNA using an enzyme called aminoacyl-tRNA synthetase. (see fig. 17.14)
Ribosome
Made up two subunits – large and small.
It has 4 bonding sites:
1 mRNA site on the small subunit
3 for tRNA site on the large subunit. Those three are called the E, P and A sites
Three parts of translation
1. Initiation – Energy is required. Result of initiation: translation initiation complex: mRNA strand (always starts with
AUG), the initiator tRNA (carrying methionine) in the P site and small and large unit of the ribosome
2. Elongation – Amino Acids are added on at a time.
a. Codon recognition: Incoming tRNA binds to the A site.
b. Peptide bond formation - A peptide bond forms between the new amino acid and the end of a growing polypeptide
chain.
c. Translocation – the tRNA in the A site moves to the P site and the tRNA in the P site moves to E site and is
released
3. Termination – elongation continues until stop codon is reached (UAA, UGA or UAG). Stop codons have a release factor
that bonds to them rather than tRNA. This factor causes the chain to hydrolyse itself from the ribosomes.
Movie clip – translation
http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/translation.swf
http://www-class.unl.edu/bioch/gp2/m_biology/animation/gene/gene_a3.html
http://www.wisc-online.com/Objects/ViewObject.aspx?ID=AP1302
Some other ones I think might be helpful!!
http://www.youtube.com/watch?v=5bLEDd-PSTQ
http://www.youtube.com/watch?v=Ikq9AcBcohA
http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120077/micro06.swf::Protein%20Synth
esis
This one is just pretty!!
http://www.dnalc.org/view/15501-Translation-RNA-to-protein-3D-animation-with-basic-narration.html
1971 – Stanford University – really weird video of college kids on a football field acting out protein synthesis…fast
forward to the 3:13 mark. College = good times!!  College in the 70’s in California = REALLY good times!! 
http://www.youtube.com/watch?v=u9dhO0iCLww
Polyribosomes- An mRNA molecule translated simultaneously by many ribosomes in a row
Signal Mechanisms – There are portions of the gene that will produces a signal peptide (2 or more amino acids long that
attaches to a SRP (Signal Recognition Particle). The SRP will attach to a SRP receptor protein in the ER membrane. This
brings the ribosome to the ER so that the protein produced is translocated right into the ER. (Thus, rough ER)
Mutations
Mutations are changes in the genetic material of a cell or virus
Point mutations involve chemical changes in just one base pair
If the mutation occurs in a gamete or a cell that gives rise to a gamete, it can affect the entire offspring and be transmitted to
future generations. These are called genetic disorders
Types of Point Mutation: Substitution
A. Silent – makes no difference in the resulting a.a. strand
B. missense – changes one a.a. in the resulting a.a. strand.
C. Nonsense – inserts a stop codon into the sequence effectively ending the a.a. sequence
Insertions and Deletions
Insertion – addition of one or more nucleotides
Deletion – deletion of one or more nucleotides
Insertion and deletion mutations are almost always devastating because it will cause a frame-shift to occur.
Imagine if a sentence of 3-letter words lost a letter?
Original sentence: The cat and dog are fat.
Mutated sentence: Thc ata ndd oga ref at.
The same things happen to DNA when it mutates
Frameshift causing extensive missense.
Frame shift causing immediate nonsense
Insertion and deletion of 3 nucleotides, no frame shift; extra or missing amino acid.
An overview: