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Nucleic Acids and Protein Synthesis
Section Review
1. What are the main functions of DNA? (2 marks)
DNA stores and transmits genetic information and codes for making proteins.
2. Identify the types and locations of covalent bonds and hydrogen bonds in a
DNA molecule. (3 marks)
Covalent bonds exist between the deoxyribose sugar and phosphate molecule
and between the nitrogen base and deoxyribose sugar. This maintains the
structure of the double helix. Hydrogen bonds join the nitrogen bases. This
allows for simple unzipping for replication and transcription.
3. List the base-pairing rules. (2 marks)
Guanine and cytosine pair.
Thymine and adenine pair.
4. What roles do enzymes play in DNA replication? (5 marks)
Helicase attaches to DNA and breaks hydrogen bonds between the
complementary bases and the chains separate. DNA polymerases bind to the
separated chains of DNA. DNA polymerase identifies an unpaired base on a
template chains, locates the nucleotide that has the complementary base in the
surrounding medium, and attaches the free nucleotide to the new nucleotide
chain that is being formed. Other enzymes are responsible for identifying and
repairing errors in replicated DNA.
5. How would the deoxyribose sugar – phosphate backbone of nucleotide chains
look if purines paired only with purines and pyrimidines paired only with
pyrimidines? (2 marks)
The backbone would appear uneven, similar to the appearance of a roller
coaster.
DNA correct
DNA if purines and pyrimidines
connected together
6. Define transcription. List the main steps involved in this process. (5 marks)
Transcription is the process of producing RNA from DNA
a) RNA polymerases synthesizes RNA copies of specific sequences of DNA
b) RNA polymerase initiates RNA transcription by binding to specific
regions of DNA called promoters.
c) When RNA polymerase binds to a promoter, the DNA molecule in the
region separates.
d) RNA polymerase attaches to the first DNA nucleotide of the template
chain. Then it begins adding complementary RNA nucleotides to the
newly forming RNA molecule.
In what ways does the structure of RNA differ from that of DNA? (10 marks)
RNA consists of a single chain of nucleotides, has a ribose sugar instead of a
deoxyribose, has uracil rather than thymine, made by transcription rather than
replication, has three types rather than one type, is found in the nucleolus and
cytoplasm
7.
8. Describe the structure and function of each of the three types of RNA.
(6 marks)
a) Messenger RNA (mRNA) – is an uncoiled chain of RNA that carries
the message from the nucleus to the ribosome.
b) Transfer RNA (tRNA) – cloverleaf shaped RNA that transfers the
amino acid from the cytoplasm to the ribosome where the protein is
being made.
c) Ribosomal RNA (rRNA) – globular shaped that acts as the site for the
making of a protein molecule.
9. List three roles of RNA polymerase in transcription. (3 marks)
a) RNA polymerase initiates transcription by binding to promoters.
When RNA polymerase binds to a promoter, the DNA molecule in that
region separates.
b) RNA polymerase attaches to the first DNA nucleotide of the
template chain. Then it begins adding complementary RNA
nucleotides to the newly forming RNA molecule.
c) At the termination signal, RNA polymerase releases DNA and RNA.
10. What basic principle ensures that the transcribed RNA molecule is carrying
the right genetic message? (2 marks)
Accuracy of the genetic message is assured by complementary base pairing.
11. Compare transcription and translation. (4 marks)
Transcription is the process of producing RNA from DNA. Translation is the
process by which polypeptides are assembled from the information encoded in
mRNA.
12. Distinguish a codon from an anticodon, and explain the significance of each.
(2 marks)
A codon is a series of three nucleotides in mRNA that code for a specific amino
acid. An anticodon on tRNA is a set of three bases complementary to the codon.
13. How does the structure of tRNA relate to its function in translation?
(2 marks)
The structure of tRNA allows an amino acid at one end to be positioned in a
polypeptide when the anticodon at the other end of the tRNA pairs with a
codon.
14. Using the information in the mRNA codon table, list the amino acids that are
coded for by the codons AGU, GGG, CCU, and GUG. (4 marks)
Serine – glycine – proline – valine
Explain the significance of the start codon and the stop codons. Do all
polypeptides begin with the amino acid coded for by the start codon? (3 marks)
The start codon (AUG) engages a ribosome to start translating an mRNA
molecule. Stop codons (UAA, UAG, UGA) cause the ribosome to stop translating
mRNA. All polypeptides do not begin with the amino acid coded for by the start
codon.
15.
16. What protein sequence would result form the following DNA code: ATT GTT
CCT CGA AGG?
No protein would be produced because the mRNA begins with a stop codon. (5
marks)
If you did the answer below then it is only 3 marks
mRNA – UAA CAA GGA GCU UCC  stop – glutamine – glycine – alanine – serine
17. What are some ways that the structure of a DNA molecule is related to its
function? (2 marks)
The complementary structure of DNA allows it to replicate itself, passing
identical copies of itself and the information it encodes to offspring cells. The
sequence of three nucleotides allows DNA to encode genetic information.
18. Is there an advantage to multiple sites of DNA replication? Explain your
answer. (2 marks)
Yes. It decreases the time necessary for DNA replication.
19. What functions are carried out by those few codons that do not code for
amino acids? (2 marks)
They signal start and stop.
20. What is the role of ribosomes in protein synthesis? (2 marks)
The ribosome holds mRNA and is the site where anticodons are matched with
codons so that an enzyme can form the peptide bonds between amino acids.
21. What is the evolutionary significance of a near-universal genetic code? (2
marks)
The near-universality of the genetic code is evidence that all organisms are
evolutionarily related. The genetic code was probably operational in the earliest
cells.
22. Complete the following chart: (12 marks – ½ mark per answer)
DNA
mRNA
tRNA
Amino Acid
TAT
AUA
UAU
isolecine
ATG
UAC
AUG
tyrosine
ACG
UGC
ACG
cysteine
ACC
UGG
ACC
Tryptophan
GAT
CUA
GAU
leucine
CGG
GCC
CGG
alanine
ATA
UAU
AUA
tyrosine
TAC
AUG
UAC
Methionine
CAT
GUA
CAU
valine
CAG
GUC
CAG
valine
23. What would happen to the translation process if one nucleotide were not
transcribed correctly? Present a minimum of three possibilities. (3 marks)
Possibilities include the following: (a) the mRNA is not translated because the
start codon was affected, (b) one amino acid is incorrectly translated, (c) too
much of the mRNA is translated if the error occurred in the stop codon, and (d)
there is no effect if the nucleotide affected was not between the start codon
and the stop codon.
24. How is a system composed of three bases per codon better suited to code
for 20 amino acids than a system composed of two bases per codon? (4
marks)
With only 2 base codes there is a possibility of 16 codes only, so there would be
some amino acids that would not have codes or there would be some replication
of a code for two different amino acids which would mess up the code and there
would have to be some other mechanism available to distinguish between the
codes.
25. Genetic engineering involves inserting segment of DNA taken from one
organism into the DNA of another organism. What would be the likely result
of an experiment in which a scientist replaces a faulty stop codon in the
DNA of mouse cells with the codon UAG taken from the DNA of a frog, a
pine tree, or a clam? What do the results of this experiment suggest about
the evolutionary ancestry of these organisms? (4 marks)
There would not be any affect to the code; it would stop the production of
proteins. The results indicate that the same nitrogen bases are in every
organism so all organisms originated from the same organism.