AP Biology Protein Synthesis or from Gene to Protein
Big Idea 3: Living systems store, retrieve, transmit and respond to information essential to life
processes.
Essential knowledge
Chapters/sections
Illustrative examples covered
3.A.1: DNA, and in some cases 5.5
• Addition of a poly-A tail
RNA, is the primary
27.1
• Addition of a GTP cap
source of heritable information. 16.1, 16.2
• Excision of introns
17.1, 17.2, 17.3, 17.4
• Enzymatic reactions
19.2
• Transport by proteins
20.1, 20.2
• Synthesis
• Degradation
• Electrophoresis
• Plasmid-based transformation
• Restriction enzyme analysis of
DNA
• Polymerase Chain Reaction
(PCR)
• Genetically modified foods
• Transgenic animals
• Cloned animals
• Pharmaceuticals, such as
human insulin or factor X
3.C.1 Changes in genotype can
15.4
• Antibiotic resistance mutations
result in changes in phenotype.
16.2
• Pesticide resistance mutations
17.5
• Sickle cell disorder and
23.4
heterozygote advantage
From gene to protein
Page 1
How will the
albino’s
survival be
affected
differently to
the others
because of its
gene
expression?
Read ‘The products of gene expression: A developing story’ on page 326.
From gene to protein
Page 2
How did scientists discover that genes and proteins were interrelated?

Beadle and Edward Tatum were able to establish the link between genes and enzymes by
exploring the metabolism of a bread mold, Neurospora crassa.
Beadle and Tatum’s experiment (1940s) that provided evidence that one gene
controlled the production of one enzyme: Today – “one gene, one polypeptide.”

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Beadle and Tatum bombarded Neurospora with X-rays and screened the survivors for mutants
that differed in their nutritional needs from wild-type mold.
o Wild-type Neurospora can grow on a minimal medium of agar, inorganic salts, glucose, and
the vitamin biotin.
○
o

Beadle and Tatum identified mutants that could not survive on the minimal medium
because they were unable to synthesize certain essential molecules from the minimal
ingredients.
However, most of these nutritional mutants can survive on a complete growth medium that
includes all 20 amino acids and a few other nutrients.
One type of mutant required only the addition of the amino acid arginine to the minimal growth
medium.
o Beadle and Tatum concluded that this mutant was defective somewhere in the biochemical
pathway that normally synthesizes arginine.
From gene to protein
Page 3
o
o
o
They identified three classes of arginine-deficient mutants, each apparently lacking a key
enzyme at a different step in the synthesis of arginine.
They demonstrated this by growing these mutant strains in media that provided different
intermediate molecules.
Their results provided strong evidence for the one gene–one enzyme hypothesis.

Later research refined the one gene–one enzyme hypothesis.

It was found that not all proteins are enzymes.
o Keratin, the structural protein of hair, and insulin, a hormone, are proteins and gene products.

This tweaked the hypothesis to one gene–one protein.

Later research demonstrated that many proteins are composed of several polypeptides, each of which
has its own gene.

Beadle and Tatum’s idea has been restated as the one gene–one polypeptide hypothesis.

This hypothesis is not entirely accurate, however.
○
Many eukaryotic genes code for a set of closely related polypeptides in a process called
alternative splicing.
○
Some genes code for RNA molecules that play important roles in cells, although they are
never translated into protein.
From gene to protein
Page 4
Demonstrate a knowledge of the basic steps of protein synthesis, identifying the roles of DNA,
mRNA, tRNA, and ribosomes in the processes of transcription and translation.
See Campbells Activity 17A
What is transcription?

During transcription, a DNA strand provides a
template for the synthesis of a complementary RNA
strand.
o Just as a DNA strand provides a template for the
synthesis of each new complementary strand
during DNA replication, it provides a template
for assembling a sequence of RNA nucleotides.

Transcription of many genes produces a messenger
RNA (mRNA) molecule.
What is RNA processing?
The transcription of a protein-coding eukaryotic gene
results in pre-mRNA.
 The initial RNA transcript of any gene is called a
primary transcript.
 RNA processing yields the finished mRNA.
What is translation?
 During translation, the sequence of
codons along an mRNA molecule is
translated into a sequence of amino acids
making up the polypeptide chain.
_
BioFlix: Protein Synthesis
From gene to protein
Page 5
How are genes and proteins related?
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From gene to protein
Page 6
The Genetic Code: How is a sequence of bases in a DNA molecule used to determine the
sequence of amino acids in a protein?
1.
The sequence of bases on one side of the DNA molecule (the template strand) is a code
for the sequence of amino acids in a polypeptide (protein).
2.
There are 20 different amino acids, so the code needs at least 20 different ‘code words’.
3.
If the code is read in ‘ones’, there will be only 4 ‘code words’ A, T, C, and G, so we
could only specify four different amino acids.
4.
If the code is read in ‘twos’, there will be 16 ‘code words’:
A
T
G
C
A
T
G
C
5.
There are still not enough, because we need 20 ‘code words’ to specify the 20 different
amino acids.
6.
If the code is read in ‘threes’, there will be 64 different ‘code words’ - MORE than
enough!
A
T
G
C
AA
AT
AG
AC
TA
TT
TG
TC
GA
GT
GG
GC
CA
CT
CG
CC
From gene to protein
Page 7
E2.
Determine the sequence of amino acids coded for by a specific DNA sequence, given
a table of mRNA codons.
1.
If the DNA code on the template strand is TAC/GTT/AGC/GAT/CTG/ATC, what will be
the sequence of bases on the transcribed mRNA molecule?
_________AUG/CAA/UCG/CUA/GAC/UAG_
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2.
What will be the sequence of amino acids in the resulting polypeptide?.
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From gene to protein
Page 8
See Campbells Activity chapter 17
http://www.johnkyrk.com/er.html
In protein synthesis, what is the role of the DNA?
To provide the correct code for the codons of mRNA. TRANSCRIPTION
What is the role of the mRNA?
To translate this message in the formation of polypeptides =TRANSLATION
________________________________________________________________________
What is the role of the ribosomes?
__To facilitate the formation of polypeptides by tRNA and mRNA.
________________________
Do concept check 17.1
# 3.
From gene to protein
Page 9
From gene to protein
Page 10
From gene to protein
Page 11
TRANSCRIPTION http://www.johnkyrk.com/DNAtranscription.html
Study the diagram below to see the basic process of transcription.
What happens during the three stages of transcription?
 RNA polymerase separates the DNA strands at the appropriate point and joins
the RNA nucleotides as they base-pair along the DNA template.
o Like DNA polymerases, RNA polymerases can assemble a polynucleotide
only in its 53 direction.
o Unlike DNA polymerases, RNA polymerases are able to start a chain from
scratch; they don’t need a primer.
 Specific sequences of nucleotides along the DNA mark where gene transcription
begins and ends.
o RNA polymerase attaches and initiates transcription at the promoter.

Molecular biologists refer to the direction of transcription as “downstream” and the other direction as
“upstream.”
From gene to protein
Page 12
This diagram illustrates the process of initiation of transcription in eukaryotes:

The stretch of DNA that is transcribed into an RNA molecule is called a transcription unit.

Bacteria have a single type of RNA polymerase that synthesizes all RNA molecules.

In contrast, eukaryotes have three RNA polymerases (I, II, and III) in their nuclei.
RNA polymerase II is used for mRNA synthesis.
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From gene to protein
Page 13
In eukaryotes, what happens during RNA processing?
Concept 17.3: Eukaryotic cells modify RNA after transcription
Activity: RNA Processing
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From gene to protein
Page 14
How are introns spliced OUT of the
pre-mRNA?
How are exons related to protein domains?
Pyruvate kinase has 3 domains.
From gene to protein
Page 15
TRANSLATION
In translation, what is the role of the tRNA? To bring the appropriate amino acid to the ribosome
where the complementary mRNA strand is.
In translation, what is the role of the mRNA?
To provide the codon for he matching tRNA anticodon
In translation, what is the role of the ribosome? _______________________________________
To facilitate the connecting of the codon and anticodon so that the amino acids can bond by
covalent peptide bonds to form polypeptides.
From gene to protein
Page 16
Transfer RNA brings the amino acids to the ribosome:
The enzyme tRNA synthetase attaches amino acids to the
tRNA
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From gene to protein
Page 17
A few more details about the ribosome:
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From gene to protein
Page 18
Translation can be broken down into three stages: initiation, elongation and termination:
http://www.johnkyrk.com/DNAtranslation.html
Initiation ________________
________________________
________________________
________________________
Elongation
________________
________________
________________
________________
________________
________________
Termination _____________
___________________________
From gene to protein
Page 19
Polyribosomes increase the efficiency of translation:
Polyribosomes are free ribosomes
Certain proteins are targeted for the ER: These would be proteins that need to be in a vesicle to
be transported out of the cell.
From gene to protein
Page 20
Here is a review of the various types of RNA molecules:
From gene to protein
Page 21
Compare transcription and translation in eukaryotes and prokaryotes:
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From gene to protein
Page 22
In this diagram you can see the complete process of protein synthesis, starting in the nucleus –
transcription making mRNA – and finishing at the ribosome – translation making the protein.
From gene to protein
Page 23
E3: Give examples of two environmental mutagens that can cause mutations in humans.
http://www.hhmi.org/biointeractive/poster-genetic-mutations-and-disease
1.
First, you need to be able to answer the question: What is a mutation?
____A change in the sequence of bases in DNA. ___________
2.
Next, you need to know the difference between a gene mutation (also referred to as a
point mutation) and a chromosomal mutation?
Here are some sample gene (point) mutations:
DNA
T A C T T C A A A C C G A T T
DNA
T A C T T C A A A C C G A T T
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From gene to protein
Page 24
Here are some sample chromosomal mutations:
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3.
What do we mean by a spontaneous mutation?
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4.
What is a mutagen?
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5.
Finally, what are some example mutagens?
____________________________________________________________________________________
________Radiation: UV, radioactive gamma, beta), _________chemical carcinogens such as cigarette
chemicals, alcohol, pesticides, heavy metals, fire retardants, PCBs
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From gene to protein
Page 25
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From gene to protein
Page 26
E4: Use examples to explain how mutations in DNA affect protein synthesis and may lead to
genetic disorders.
A genetic disorder is a disorder which is inherited because it is caused by a mutation that is passed on in
the sperm or egg.
The mutation that leads to abnormal (sickling) hemoglobin and can cause the genetic disorder called
Sickle Cell Anemia is an excellent example:
The substitution mutation of A for T in the DNA molecule results in the substitution of U for A in the
mRNA. This results in the substitution of Valine for Glutamic Acid in the hemoglobin molecule.
If a person inherits 2 copies of the mutated gene, they will have sickle cell anemia:
From gene to protein
Page 27
From gene to protein
Page 28
Concept 17.5 #4
From gene to protein
Page 29
17.5 # 3
From gene to protein
Page 30
Test understanding #8
From gene to protein
Page 31