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BIOLOGY SEMESTER ONE
UNIT 17
CHECKLIST UNIT 17: FROM GENE TO PROTEIN
In this module you will investigate protein synthesis, a process in which cells build proteins.
Translation is the term used to describe this process, as the sequences of DNA nucleotides are
transcribed and translated by a various forms of RNA into the specific protein coded for by that
gene sequence. By copying the DNA and using the copy to make proteins, it reduces the risk of
the original DNA being altered or damaged, thus reducing the risk of mutations. Three forms of
RNA are involved in transferring the DNA information from the nucleus into the cytoplasm to
waiting ribosomes, where protein synthesis occurs. Messenger RNA (mRNA) are the
“transcribed,” coded copies of the DNA sequence, and move from the nucleus to the
cytoplasm. Ribosomal RNA (rRNA) is the major component of the ribosomes that decodes
mRNA; transfer RNA (tRNA) assist in polypeptide (protein) construction by bringing in the
specific amino acids that string together to create the protein.
Protein synthesis begins with the “unzipping” of DNA by the enzyme helicase in the nucleus. As
the DNA nucleotides unbind from their partner pairs, mRNA nucleotides attach to the DNA
strand along the gene (the section of DNA that has the information for a particular protein).
This process (making a complementary RNA copy of a DNA strand) is called transcription. The
DNA strand that mRNA complements is called the template strand. The process of transcription
can be divided into three stages: Initiation, Elongation and Termination, each regulated by a
large number of proteins ,such as transcription factors and co-activators, that ensure the
correct gene is properly transcribed. The DNA strand is read in the 3' to 5' direction, and the
mRNA is transcribed in the opposite (5' to 3') direction by the RNA polymerase. The structure
of RNA is very similar to the DNA structure, with the exception that, in RNA, the nucleotide
uracil takes the place that thymine occupies in DNA. Once the gene has been transcribed, the
single strand of mRNA releases from the DNA and leaves the nucleus through nuclear pores,
migrating into the cytoplasm.
The second part of protein synthesis is known as translation. Translation occurs in the
cytoplasm, where the ribosomes are located. Ribosomes are mainly rRNA, and a single
ribosomal complex consists of both a small and a large subunit that surround the incoming
mRNA. In translation, the mRNA sequence is used as a template to guide the synthesis of a
chain of amino acids that together form a specific protein. A third type of RNA, called transfer
RNA, sequentially pairs with the mRNA to link the correct amino acid sequences together to
make the protein. The grouping and order of bases on an RNA strand have significance: The
bases can be thought of as individual letters of three letter words, each word having its own
meaning; these “words” are referred to as codons. Most codons are translated into specific
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BIOLOGY SEMESTER ONE
UNIT 17
amino acids (i.e. CUU is Leucine, ACU is Threonine). However, some of the codons are
specialized for a different purpose: for example, UAA, UAG, and UGA are “stop codons,”
signalling the end of the gene sequence and instructing the ribosome to stop translating.
For synthesis of protein, a succession of tRNA molecules charged with appropriate amino acids
are brought together with an mRNA molecule and matched up by base-pairing through their
anti-codons. The amino acids are linked together to extend the growing protein chain, and the
tRNAs are released. This whole complex of processes is carried out by the ribosomal complex,
formed from two main chains of RNA, called ribosomal RNA (rRNA), and more than 50 different
proteins.
Translation proceeds in four phases: Activation, Initiation, Elongation, and Termination (all
describing the growth of the amino acid chain, or polypeptide, that is the product of
translation).
1. In activation, the first step in translation, the correct amino acid is joined to the correct
transfer RNA (each tRNA molecule can only carry the specific amino acid that correlates
with its “anticodon”—anticodons are tRNA sequences that may complement mRNA
codons). When the tRNA has an amino acid linked to it, it is referred to as "charged."
2. Initiation involves the binding of the small ribosomal subunit to the 5' end of mRNA. “
Initiation factors” are a group of proteins that assist in the binding process.
3. Elongation occurs when the next charged tRNA in line binds to the ribosome to pair with
the next mRNA codon.
4. Termination of the polypeptide chain occurs when the ribosome reaches a stop codon
(UAA, UAG, or UGA). When this happens, no tRNA can recognize it and bind, but a
“releasing factor” can recognize one of these three codons and cause the release of the
polypeptide chain from the ribosome.
Once the amino acid chain has been established, the resulting protein undergoes a series of
primary, secondary and tertiary folding. This is a result of the numerous molecular interactions
between amino acid groups within the protein chain that create bonds between groups along
the chain. Once the protein is in its final form, it often requires further activation by enzymes.
Only then is it ready to be released to undertake its biological function.
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BIOLOGY SEMESTER ONE
UNIT 17
LEARNING OBJECTIVES
At the end of this module you should be able to do the following:
1. Distinguish between the “one gene-one enzyme” hypothesis and the “one gene-one
polypeptide” hypothesis and explain why the original hypothesis was changed.
2. Explain how RNA differs from DNA.
3. Briefly explain how information flows from gene to protein.
4. Describe the events and locations associated with transcription and translation, and
compare between eukaryotes and bacteria.
5. Using a chart, be able to identify what amino acids are specified by various codons.
6. Define and discuss redundancy and ambiguity in the genetic code.
7. Explain the significance of the reading frame during translation.
8. Explain the evolutionary significance of a nearly universal genetic code.
9. Explain how RNA polymerase recognizes where transcription should begin. Describe the
role of the promoter, the terminator, and the transcription unit.
10. Describe the functional and evolutionary significance of introns.
11. Explain why, due to alternative RNA splicing, the number of different protein products
an organism can produce is much greater than its number of genes.
12. Describe the significance of polyribosomes.
13. Explain what determines the primary structure of a protein and describe how a
polypeptide must be modified before it becomes fully functional.
14. Define “point mutations”. Distinguish between base-pair substitutions and base-pair
insertions. Give an example of each and note the significance of such changes.
15. Distinguish between a mis-sense and a nonsense mutation.
CHECK LIST
 Read Chapter 17: From Gene to Protein, of Campbell and Reece’s Biology, 9th Ed.
 As you are reading, address each of the learning objectives listed above.
 M flash cards for the terminology list provided. This will be beneficial for studying for
the midterm and final exams later in the semester.
 You may be able to review the PowerPoint Lecture and other resources for this unit.
Refer to your instructor’s notes for more details.
 Review the video clip “Protein Synthesis,” and answer the questions in the “Protein
Synthesis Video Worksheet.”
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BIOLOGY SEMESTER ONE
UNIT 17
 Discussion Post:
o Using the diagram here as
reference, describe the events of
transcription and translation, in
your own words, as it occurs in
the eukaryotic cell. Be sure to
include all of the names of the
structures and molecules
involved, as well as the location
of each in the cell. These
descriptions will be posted on the
discussion forum. You will be
responsible for reading and
commenting on two of your
classmates’ posts. Include any
corrections you deem needed,
and also point out anything they
have summarized in a way that
you find helpful in understanding
protein synthesis.
 For extra practice, try the Self Quiz or Practice Test on the Mastering Biology Website.
To log onto the website, use the access code provided in your textbook. You will also
find other resources, such as downloadable MP3 tutorials for each chapter, a glossary,
and an electronic copy of your text—you can catch up on your reading anywhere!
KEY TERMS
5' cap
amino acid
anticodon
base-pair substitution
chaperonin
codon
endomembrane system
exon
frameshift mutation
gene expression
insertion
intron
messenger RNA (mRNA)
missense mutation
mutagen
mutation
nonsense mutation
P site
point mutation
poly (A) tail
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polyribosome (polysome)
primary transcript
promoter
reading frame
ribosomal RNA (rRNA)
ribosome
ribozyme
RNA polymerase
RNA splicing
spliceosome
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BIOLOGY SEMESTER ONE
UNIT 17
template strand
terminator
transcription
transcription factor
transcription initiation
complex
transcription unit
transfer RNA (tRNA)
translation
triplet code
ROOT WORDS TO KNOW 1
anti- = opposite (anticodon: a specialized base triplet on one end of a tRNA molecule that
recognizes a particular complementary codon on an mRNA molecule)
exo- = out, outside, without (exon: a coding region of a eukaryotic gene that is expressed)
intro- = within (intron: a noncoding, intervening sequence within a eukaryotic gene)
muta- = change; -gen = producing (mutagen: a physical or chemical agent that causes
mutations)
poly- = many (poly-A tail: the modified end of the 3[H11032] end of an mRNA molecule
consisting of the addition of some 50 to 250 adenine nucleotides)
trans- = across; -script = write (transcription: the synthesis of RNA on a DNA template)
SOURCES
Campbell, N. A. (2008). Biology, Eighth Edition. San Francisco: Pearson, Benjamin Cummings.
Pearson Education. (2010). Retrieved 2010, from Mastering Biology : http://session.masteringbiology.com
University of Leicester. (2009, Feb 11). Gene Regulation and Expression. Retrieved May 2010, from Virtual Genetics
Education Centre: http://www.le.ac.uk/genetics/genie/vgec/he/expression.html
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(Pearson Education, 2010)
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BIOLOGY SEMESTER ONE
UNIT 17
NANSLO Biology Core Units and Laboratory Experiments
by the North American Network of Science Labs Online,
a collaboration between WICHE, CCCS, and BCcampus
is licensed under a Creative Commons Attribution 3.0 Unported License;
based on a work at rwsl.nic.bc.ca.
Funded by a grant from EDUCAUSE through the Next Generation Learning Challenges.
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