Transcript 1. Give early experimental evidence that implicated proteins as the

Chapter 17 RQ
1.
2.
3.
4.
5.
Why do dwarf peas fail to make their own
gibberellins?
What did the “one gene – one polypeptide”
hypothesis used to be called?
What is the “bridge” between DNA and protein
synthesis?
What is the process by which one gene is copied
into an mRNA strand?
How many mRNA nucleotide bases equal a
“codon”?
1. Give early experimental evidence that implicated
proteins as the links between genotype and
phenotype.
Inherited instructions in DNA direct
protein synthesis, thus proteins are the
links between genotype and phenotype
Garrod suggested that genes dictate
phenotypes through enzymes that catalyze
reactions 
2. Describe Beadle and Tatum’s experiments with
Neurospora, and explain the contribution they made
to our understanding of how genes control
metabolism.
Relationship between genes and enzymes
Wild type (bread mold) can survive on minimal
medium  looked for mutants or auxotrophs that
could not live because they can’t synthesize
molecules
Results  one gene – one enzyme hypothesis 
the function of a gene is to dictate the production
of a specific enzyme 
3. Distinguish between “one gene-one enzyme”
hypothesis and “one gene-one polypeptide”, and
explain why the original hypothesis was changed.
One gene – one enzyme
Gene codes for a
specific enzyme
One gene – one
polypeptide
Most enzymes are proteins
Many proteins are not
enzymes
Proteins that are not
enzymes are still gene
products
Many proteins are
comprised of 2 or more
polypeptide chains, each
chain specified by a
different gene 
4. Explain how RNA differs from DNA.
Both are nucleic acids; polymers of
nucleotides
RNA is different from DNA:
- the 5 carbon sugar is ribose not
deoxyribose
- the nitrogen base is uracil not
thymine 
5. Briefly overview, in your own words, how
information flows from gene to protein.
Two processes:
transcription and
translation
Transcription  the
synthesis of RNA
using DNA as a
template
Translation  the
synthesis of a
polypeptide which is
directed by mRNA
DNA  RNA  protein
6. Distinguish between transcription and translation.
Transcription  is the
synthesis of RNA
under the direction of
DNA
Translation  the
actual synthesis of a
polypeptide, which
occurs under the
direction of mRNA 
7. Describe where transcription and translation
occur in prokaryotes and in eukaryotes; explain why
it is significant that in eukaryotes, transcription and
translation are separated in space and time.
Prokaryotes  lack nuclei so DNA is not
segregated from ribosomes or the protein –
synthesizing machinery (occurs in rapid
succession)
Eukaryotes  have nuclear envelopes that
segregate transcription in the nucleus from
translation in the cytoplasm; mRNA (the
intermediary) is modified before it moves from
the nucleus to the cytoplasm where translation
occurs 
8. Define codon, and explain what relationship exists
between the linear sequence of codons on mRNA and
the linear sequence of amino acids in a polypeptide.
Codon  a 3-nucleotide sequence in
mRNA that specifies which amino acid will
be added to a growing polypeptide or that
signals termination  the basic unit of the
genetic code
Genes are not directly translated into
amino acids but are first transcribed as
codons into mRNA 
9. List the three stop codons and the one start
codon.
StartAUG
StopUAA
UAG
UGA
10. Explain in what way the genetic code is redundant
and unambiguous.
Redundant  two or more codons
differing only in their 3rd base can
code for the same amino acids
(UUU & UUC = phenylalanine)
Unambiguous  codons code for only
ONE amino acid
(UUU ONLY codes for phenylalanine)

11. Explain the evolutionary significance of a nearly
universal genetic code.
It indicates that the code was
established very early in life’s history
12. Explain the process of transcription including the
three major steps of initiation, elongation, and
termination.
Initiation  a RNA polymerase attaches at a
specific region of DNA called the
promoter, and begins transcription (often
called the TATA box)
Elongation  as RNA polymerase moves
along the DNA, 10 – 20 bases are exposed
at a time for pairing with RNA nucleotides
Termination  transcription proceeds until
RNA polymerase transcribes the
termination sequence 
13. Describe the general role of RNA polymerase in
transcription and explain how it recognizes where to
begin.
RNA polymerases bind at the
promoter and in eukaryotes they need
transcription factors to recognize
them
The enzyme separates the 2 DNA
strands at the initiation site and
transcription begins 
14. Specifically, describe the primary functions of
RNA polymerase II.
1. It untwists and opens a short segment of
DNA exposing about 10 nucleotide bases
 one of the exposed DNA strands is the
template for base-pairing with RNA
nucleotides
2. It links incoming RNA nucleotides to the
3’ end of the elongating strand, thus,
RNA grows one nucleotide at a time in
the 5’ to 3’ direction 
15. Distinguish among mRNA, tRNA, and rRNA.
mRNA  messenger RNA; what the DNA
nucleotide sequence is transcribed into
tRNA  transfer RNA
rRNA  ribosomal RNA; translation occurs
on ribosomes, complex particles composed
of rRNA and protein that facilitate the
orderly linking of amino acids into
polypeptide chains 
16. Describe the structure of tRNA and explain how
the structure is related to the function.
tRNA is transcribed from DNA templates,
made in the nucleus and travels out into
the cytoplasm
Used repeatedly – picks up its designated
amino acid in the cytosol, deposits it at the
ribosome, and leaves to pick up another
Consists of a single RNA strand that is only
about 80 nucleotides long, has a protruding
end which serves as the attachment site
for the amino acid 
17. Given the sequence of bases in DNA, predict the
corresponding codons transcribed on mRNA and the
corresponding anticodons of tRNA.
DNA
CTAGGATGCAAATGC
mRNA
GAUCCUACGUUUACG
tRNA
CUAGGAUGCAAAUGC

18. Describe the wobble effect.
It is a relaxation of the base-pairing
rules
If one tRNA variety existed for each
of the mRNA codons that specifies an
amino acid, there would be 61 tRNAs,
there are only 45
- this is because some tRNAs can
recognize two or more codons 
19. Explain how an aminoacyl-tRNA synthetase
matches a specific amino acid to its appropriate
tRNA; describe the energy source that drives this
endergonic process.
There are 20 types of these enzymes in a
cell, each specific for an amino acid
The active site of each enzyme fits only a
specific combination of amino acid and
tRNA
The synthetase catalyzes the covalent
attachment of the amino acid to its tRNA
in a process driven by the hydrolysis of
ATP (which loses 2 phosphates!)
20. Describe the structure of a ribosome and explain
how this structure relates to function.
A ribosome is made up of 2 subunits (large
and small) – these are constructed of
proteins and ribosomal RNA molecules, and
are made in the nucleolus
Function is to bring mRNA together with
the amino acid-bearing tRNAs, therefore
they have binding sites for mRNA and
tRNA 
21. Describe the process of translation including
initiation, elongation, and termination and explain
what enzymes, protein factors, and energy sources
are needed for each stage.
Initiation  when
mRNA, tRNA and the
first amino acid come
together with the
ribosome
- protein initiation
factors bring
everything together
to begin (GTP
provides energy) 
Elongation…
Elongation  amino acids are added one by one, helped by
protein elongation factors
1. Codon recognition – mRNA codon makes a hydrogen bond
with the tRNA anticodon (requires GTP hydrolysis)
2. Peptide bond formation – a ribozyme catalyzes the
peptide bond creating a polypeptide which then separates
from it’s tRNA
3. Translocation – the tRNA moves to another part of the
ribosome, and the next codon to be translated steps up;
finally the tRNA leaves (requires hydrolysis of GTP) 
Termination…
Termination  elongation continues until there is a
stop codon; a protein release factor binds and
adds a water to finish the polypeptide 
22. Explain what determines the primary structure
of a protein and describe how a polypeptide must be
modified before it becomes fully functional.
A gene determines the protein’s primary
structure (it’s amino acid sequence)
Primary structure then determines
conformation changes
Posttranslational modifications  chemical
modification by adding sugars, lipids,
phosphates, or others 
23. Describe what determines whether a ribosome
will be free in the cytosol or attached to rough ER.
Free  suspended in cytosol and mostly
synthesize proteins that dissolve in the
cytosol and function there
Bound  attached to the cytosol side of
the ER and make proteins which are
secreted from the cell (ex: insulin)
- occurs if the growing polypeptide ITSELF cues
the ribosome to attach to the ER – marked by a
signal peptide, which targets the protein to the
ER 
24. Explain how proteins can be targeted for specific
sites within the cell.
A signal peptide is
recognized by the SRP
(signal-recognition
particle), and this dictates
where that particular
protein will be headed for
work.
- as the polypeptide is
being synthesized, it
begins to snake around to
where it will be located
within the cell (ER,
mitochondria, chloroplast,
etc.) 
25. Describe the difference between prokaryotic
and eukaryotic mRNA.
Prokaryotic
A transcription unit
can contain several
genes, so the
resulting mRNA
code may code for
different, but
functionally
related, proteins
Eukaryotic
A transcription unit
contains a single
gene, so the
resulting mRNA
codes for synthesis
of only one
polypeptide 
26. Explain how eukaryotic mRNA is processed
before it leaves the nucleus.
In eukaryotes, RNA transcripts are
modified before leaving the nucleus to
make functional mRNA
This can happen in two ways:
1. Covalent alteration of both the 3’ and 5’
ends
2. Removal of intervening sequences
“Pre-mRNA” is what the molecule is called
prior to this alteration 
27. Describe some biological functions of introns and
gene splicing.
Introns – the noncoding segments of
nucleic acid that lie between coding regions
Exons – segments which are eventually
expressed through amino acid sequences
Splicing – occurs when the introns are cut
out of the initial length to transcribe and
translate the portion that will code for
used information 
28. Explain why base-pair insertions or deletions
usually have a greater effect than base-pair
substitutions.
Substitutions  the replacement of
one nucleotide and its partner in the
complementary DNA strand with
another pair of nucleotides
Insertions and deletions  the
additions or losses of one or more
nucleotide pairs in a gene 
29. Describe how mutagenesis can occur.
Mutagenesis  the creation of
mutations
- due to: errors in DNA replication,
repair, or recombinations that result
in base-pair substitutions, insertions,
or deletions 
The End!