Gene Expression: Protein Synthesis and Genetic Mutation
Why make proteins?
- Proteins are made of 20 kinds of amino acids linked together in a specific order. (2 or
- All organisms contain and make proteins. Major structural components of your cells,
and your body as a whole. i.e., keratin- hair, fingernails, hemoglobin- rbc’s, insulin –
hormones, enzymes – control chemical rxns.
- DNA determines how amino acids are strung together and how proteins are made.
- Genes are coded DNA instructions that control the production of proteins.
- Genetic messages can be decoded by copying part of the nucleotide sequence from
DNA into RNA.
- RNA contains coded information for making proteins.
The structure of RNA
- RNA consists of a long chain of nucleotides.
- Each nucleotide is made up of a 5-carbon sugar, a phosphate group, and a nitrogenous
There are three main differences between RNA and DNA:
- The sugar in RNA is ribose instead of deoxyribose.
- RNA is generally single-stranded.
- RNA contains uracil in place of thymine.
Types of RNA
There are three main types of RNA:
Messenger RNA (mRNA)
Messenger RNA (mRNA) carries copies of instructions for assembling amino acids into
Ribosomal RNA (rRNA)
Ribosomes are made up of proteins and ribosomal RNA (rRNA).
Transfer RNA (tRNA)
During protein construction, transfer RNA (tRNA) transfers each amino acid to the
Protein Synthesis: overview
Transcription: the information in DNA is copied onto an RNA molecule (mRNA).
Eukaryotes (nucleus) Prokaryotes (cytoplasm).
Translation: actual synthesis of a polypeptide under the direction of mRNA.
(cytoplasm of the cell).
The Genetic Code
The genetic code is the “language” of mRNA instructions.
The code is written using four “letters” (the bases: A, U, C, and G).
- A codon consists of three consecutive nucleotides on mRNA that specify a particular
- Each codon specifies a particular amino acid that is to be placed on the polypeptide
- Some amino acids can be specified by more than one codon.
(64 possible codons and only 20 amino acids).
- There is one codon AUG that can either specify the amino acid methionine or serve as a
“start” codon for protein synthesis.
- There are three “stop” codons that do not code for any amino acid. These “stop” codons
signify the end of a polypeptide. UAA, UGA, UAG
- RNA polymerase: pries DNA apart and hooks RNA nucleotides together from the DNA
- Promoter region on DNA: where RNA polymerase attaches and where initiation of
- Terminator region: sequence that signals the end of transcription.
- Transcription unit: stretch of DNA transcribed into an RNA molecule.
- Initiation~ transcription factors mediate the binding of RNA polymerase to an initiation
sequence (TATA box)
- Elongation~ RNA polymerase continues unwinding DNA and adding nucleotides to the
- Termination~ RNA polymerase reaches terminator sequence
1) 5’ cap: modified guanine; protection; recognition site for ribosomes
2) 3’ tail: poly(A) tail (adenine); protection; recognition; transport
3) RNA splicing: exons (expressed sequences) kept, introns (intervening sequences)
spliced out; spliceosome
mRNA from nucleus is ‘read’ along its codons by tRNA’s anticodons at the ribosome
tRNA anticodon (nucleotide triplet); amino acid
rRNA- site of mRNA codon & tRNA anticodon coupling
P site- holds the tRNA carrying the growing polypeptide chain
A site- holds the tRNA carrying the next amino acid to be added to the chain
E site- discharged tRNA’s
Initiation~ union of mRNA, tRNA, small ribosomal subunit; followed by large subunit
•peptide bond formation
Termination~ ‘stop’ codon reaches ‘A’ site
Polyribosomes: translation of mRNA by many ribosomes (many copies of a polypeptide
Regulating Gene Expression
- Every living cell has the ability to respond to its env’t. by changing the kinds and
amounts of polypeptides it produces.
- Many factors can affect the rate of transcription and translation in living cells including:
- Changes in temperature or light: animals can change colour or grow thicker fur
coats in the winter.
- The presence or absence of nutrients in the env’t: lactose present triggers E.coli to
produce specific lactose synthesizing enzymes
- The presence of hormones in the body: hormones trigger protein synthesis
mechanisms in other cells.
Mutations: genetic material changes in a cell
A mutation is a permanent change in the genetic material of an organism. All mutations
are heritable and are copied during DNA replication. Not all are passed on to future
generations. (reproductive cells).
Point mutations…. Changes in one or a few base pairs in a single gene (minor effect)
•silent mutations: no effect on protein CCT -> CCC
•missense: a substitution that leads to an altered but functional protein. Can be harmful,
one amino acid change alters protein making up hemoglobin is responsible for sickle cell
•nonsense: erases a start codon or a change to a stop codon results in a nonfunctional
- Base-pair insertions or deletions: the insertion or deletion of one or two nucleotides
within a sequence of codons produces a second type of point mutation known as a
frameshift mutation. These mutations cause the entire reading frame of the gene to be
altered and usually result in a nonsense mutation.
Causes of Mutations
- A substance or event that increases the rate of mutation in an organism is called a
- Physical Mutagens: High energy radiation; X-ray, gamma rays and UV radiation. UV
affects C and T bases and interferes with DNA replication (melanoma).
- Chemical Mutagens: a molecule that enters the cell nucleus and reacts chemically with
DNA causing mutations.
1. Using the genetic code wheel answer the following questions.
a) What amino acids are coded for by each of the following codons?
i) UUC (ii) ACU (iii) GCG (iv) UAA
b) What codons could code for the amino acid serine? aspartate?
c) Write all the possible codon sequences that code for the polypeptide serine-
2. A portion of an mRNA molecule has the sequence CCUAGGCUA. What is the
sequence of the antisense strand of the corresponding DNA molecule?
3. Name the two major steps in protein synthesis. Briefly, what is accomplished by each?
4. Where in the cell does each one occur?
5. Compare and contrast DNA replication and transcription.
6. How much of a molecule of DNA is untwisted during replication? During
7. Name three types of RNA we described and explain the function of each.
8. How many different DNA triplets are possible?
9. The DNA triplet “CGA” is transcribed into which RNA codon?
a) GUT (b) GUC (c) GCU (d) AUG
10. Which enzyme “reads” the mRNA?
11. Explain the role played by each of the following in protein synthesis.
a) template strand of DNA (b) RNA codon (c) enzymes (d) rRNA (e) tRNA
12. During the process of translation what language change occurs? How is it possible to
put together a polypeptide with the correct sequence of amino acids.
13. What is the first codon in the mRNA of any polypeptide?
14. Name the two important regions of a tRNA molecule.
15. For the DNA triplet CGT, write the complementary mRNA codon and the tRNA
16. What amino acid does the triplet GCA represent?
17. Explain the functions of start and stop codons.
18. What would be the effect on translation if the termination codon were changed by
mutation? If the start codon were mutated?
19. Suppose that during protein synthesis, a cell is starved of uracil and another base is
supplied in its place. How will the proteins produced be affected by this substitution?
20. A molecular biologist discovers a drug that blocks the site of attachment of the
ribosome to mRNA. How will the drug affect the functioning of the cell?
21.What kinds of factors act as mutagens? What effects could they have? Where or when
might one be exposed to them?