Biology 40S
Unit 3 – Mechanisms of Inheritance
Unit 3 – Mechanisms of Inheritance
Unit Review: ANSWER KEY
Review Questions and Main Ideas:
1. What molecules make up DNA (and RNA)? What are the major components of these
molecules?
DNA and RNA are made up of nucleotides. The major components of nucleotides
are: a phosphate group, a sugar (either ribose or deoxyribose), and a
nitrogenous base.
2. What are the four nitrogenous bases used by DNA?
DNA will use adenine, guanine, cytosine, and thymine.
3. What is the difference between purines and pyrimidines? Which bases are classified
as purines, and which are classified as pyrimidines?
Purines are nitrogenous bases made up of a two-ring structure, while
pyrimidines are nitrogenous bases that consist of a single ring. The purines are
adenine and guanine, while cytosine and thymine (and uracil!) are pyrimidines.
4. Using a ladder as an analogy, describe the structure of a double-stranded DNA
molecule. Be sure to include the locations of the nitrogenous bases, sugar molecules,
and phosphate groups in your description.
In the ladder analogy, the sides of the ladder are made up of alternating
phosphates and deoxyribose sugars. These two sides are connected by pairs of
nitrogenous bases that are linked to their respective sides of the ladder. These
base pairs make up the rungs (or steps) of the ladder, and are held together
right in the middle by hydrogen bonding.
5. What are the base-pairing rules for DNA?
For DNA, adenine will pair with thymine, and guanine will pair with cytosine.
6. Why do we say that the two strands of a DNA molecule are anti-parallel to each
other?
Because they run in opposite directions to each other without crossing (ie: one
points from 3’  5’, and the other points from 5’  3’).
7. Describe the process of DNA replication in a cell. Include all enzymes and
components involved, and be sure to indicate what is produced at the end of this
process. You may consider drawing a simple sketch in your explanation.
DNA replication begins with the enzyme helicase binding to double-stranded
DNA. This separates (unzips) the two strands, exposing the individual bases on
each strand.
Once exposed, these bases will start to pair up with free nucleotides that are
present in the nucleus. The enzyme DNA polymerase will move along each of the
DNA strands and make sure that the correct bases are pairing. It will help form
the hydrogen bonds between the old DNA strand and the new nucleotides.
Once this is done, the enzyme ligase will close any gaps between the sugars and
phosphates of the new nucleotides, forming a new, complete strand of DNA that
is complementary to the original strand it is paired with.
Biology 40S
Unit 3 – Mechanisms of Inheritance
At the end, two double-stranded DNA molecules are present, and each is made
up of one old strand and one newly-created strand.
8. Why is DNA replication a semi-conservative process?
Because each new double-stranded DNA molecule is made up of one new strand
of DNA and one old strand of DNA.
9. Why do cells bother creating mRNA when DNA already has all of the instructions we
need?
DNA is not usually permitted to leave the nucleus, but translate (which creates
proteins) happens outside of the nucleus. To make sure the needed instructions
to create proteins can get out, we have to make a copy of those instructions in
mRNA, which CAN leave the nucleus!
10. Describe the structural and chemical differences between DNA and RNA.
DNA uses deoxyribose, while RNA uses ribose. DNA is double-stranded, RNA is
single-stranded. DNA is helical (spiraled), while RNA generally isn’t. DNA’s bases
include thymine but not uracil, while RNA’s bases include uracil, but not
thymine.
11. What are the roles of the three main types of RNA?
mRNA – carries instructions (based off of DNA) from the nucleus and out to the
ribosomes.
tRNA – delivers specific amino acids to the ribosomes during translation.
rRNA – a major component of ribosomes themselves, helps to synthesize new
proteins.
12. Describe, briefly, the process of transcription. Include the product(s), the steps
involved, and any enzymes involved.
Transcription is the process of taking the information in DNA and copying it out
as a strand of mRNA.
It begins with RNA polymerase unzipping DNA and reading a template strand
(the 3’  5’ strand). As it reads the DNA template, RNA polymerase helps to
ensure that RNA nucleotides are properly pairing with the exposed DNA bases,
and it joins the RNA nucleotides into a complete, single strand of mRNA.
13. Describe the modifications made to mRNA as a result of it being “processed” once it
has been synthesized.
Sections of “junk” genetic material called introns are cut out of the mRNA, and
the remaining, useful pieces (exons) are spliced together.
As well, a poly-adenine (poly-A) tail is added to the 3’ end of the mRNA as a
protective measure.
14. Describe how the genetic code in mRNA is read.
The code is read as triplets of bases called codons. Each codon designates a
certain amino acid that will be part of a chain that will form a protein.
15. Why are there so many repetitions (redundancies) in the number of codons that
code for a given amino acid?
Biology 40S
Unit 3 – Mechanisms of Inheritance
This is to help prevent changes to the amino acid sequence in a protein in the
event that a codon becomes mutated. For example, if AAA mutates into AAG, the
protein will still have lysine in the proper place in the chain because both AAA
and AAG code for lysine.
16. What codon ALWAYS starts translation off?
AUG (which codes for the amino acid methionine).
17. How does a tRNA molecule know exactly where to deliver the amino acid it carries?
Why is there not simply chaos in the cell, with rampant tRNA’s flying all over the
place at random?
Each tRNA has an anticodon on it – a triplet of bases that perfect complements
one, AND ONLY ONE, codon on mRNA. For example, the tRNA with the anticodon
UAC will perfectly match a segment of mRNA with the codon AUG. It will only
bind there and deliver its amino acid just to that spot.
18. Describe the three stages of translation, including the roles of the ribosome (small
subunit, A site, P site) and of tRNA in the process. Include, as well, the effect of a stop
codon on the process.
Chain initiation – when translation first starts, an mRNA strand will move out
into the cytoplasm and will attach to the small subunit of a ribosome. The small
subunit attaches to the start codon (AUG). A tRNA (with the anticodon UAC) will
attach to the start codon, carrying methionine. The large subunit of the
ribosome will then attach to the small subunit, so that the tRNA is sitting in the P
site.
Chain elongation – the A site will be empty now, and it will contain the codon
next to AUG. The codon will attract another tRNA that will carry a new amino
acid. This tRNA will sit in the A site. The amino acid on the tRNA in the P site will
be passed to the tRNA in the A site, and then the now empty tRNA in the P site
will leave, causing the P site to be empty. The ribosome will slide over so that the
P site is now occupied by the tRNA that used to be in the A site, and this will
cause the A site to be empty. A new codon will be exposed, attracting a new tRNA
that will bring a new amino acid; this tRNA will sit in the A site. The amino acids
on the tRNA in the P site will be passed to the one in the A site. The empty tRNA in
the P site will leave, the ribosome will slide over again, causing the A site to be
empty. The process will continue over and over again until…
Chain termination – when a STOP CODON reaches the A site, instead of a tRNA
being attracted, a protein called a release factor will arrive at the A site. This
factor causes the ribosome to disassemble, causes the mRNA to be released, and
causes the chain of amino acids to be released from the tRNA.
19. What constitutes a gene mutation?
A gene mutation is any change to the sequence of bases in DNA.
20. What are three distinct causes of gene mutations?
Errors in DNA replication, mutagens, and transposons.
21. What are point mutations, and into what categories are they further classified?
A point mutation is a change in a single base in a given gene. They are further
classified as silent mutations (where the change still results in the same amino
acid being coded for), nonsense mutations (where the change causes the
Biology 40S
Unit 3 – Mechanisms of Inheritance
creation of a premature stop codon), and missense mutations (where the
changes causes a different amino acid to end up in the protein that was
intended).
22. What effect does a frameshift mutation have on the process of translation?
It causes codons to be read in the wrong order. In other words, it causes the
reading frame to move over.
23. Which type(s) of mutation cause the least effect on us?
Generally, it is the silent mutations that have the least effect on us. Generally.
24. Describe the mutations that can lead to the onset of cancer (ie: what are the genes
involved, and what happens to them if they mutate?).
For cancer to develop, we generally need to accumulate mutations in oncogenes
and tumor suppressor genes. Normally, oncogenes accelerate the cell cycle,
leading to more rapid cell division. If these genes mutate, they may accelerate
the cell cycle to an uncontrollable level that will push past any stops that the cell
tries to put into place. Tumor suppressor genes are supposed to slow or stop the
cell cycle and, if necessary, cause cells to self-destruct. If they mutate, they may
become non-functioning, preventing cancerous cells from being destroyed or
from stopping them from dividing.
 Be able to create strands of DNA complementary to a given DNA strand.
 Be able to create strands of mRNA complementary to a template strand of DNA.
 Be able to identify codons and use the chart of mRNA codons to write a sequence of
amino acids.
 Be able to interpret the effects of point mutations and frameshift mutations on a
sequence of codons and the amino acids they code for.