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Transcription and Translation
Supplemental Instruction
Iowa State University
Leader:
Course:
Instructor:
Date:
Brandon
Biol 212
Dr. Coffman/Manz
3/9/2017
1. How does transcription differ in eukaryotes in comparison to bacteria?
- Eukaryotes have three polymerases – RNA polymerase I, RNA polymerase II, RNA
polymerase III
o Each polymerase transcribes only certain types of RNA in eukaryotes.
o RNA polymerase II is the only polymerase that transcribes protein-coding
genes.
- Promoters in eukaryotic DNA are more diverse than bacterial promoters.
o Eukaryotic promoters include the TATA box and other important diverse
sequences.
- Eukaryotic RNA polymerases recognize promoters using basal transcription factors
instead of a sigma protein.
- Termination of eukaryotic protein-coding genes involve a poly-A signal.
2. Describe the process of RNA splicing.
- SnRNPs (small nuclear ribonucleoproteins) bind to the start of the intron marked by
bases G and U, and bind towards the end of the intron marked by base A.
- Other snRNPs assemble with the initial SnRNPs bound to bases G and U, and bound
to base A. This assembly forms a spliceosome.
- The intron is cut between the G base and the U base, forming a loop and a single
stranded stem of the intron called a lariat.
- The lariat is released, the exons that were located on each side of the intron join
together via a phosphodiester linkage.
o This produces the continuous coding sequence known as mRNA.
3. What is the function of the 5’ cap and the poly(A) tail in mRNA?
- Protect mRNAs from degradation by ribonucleases and enhance the efficiency of
translation.
o Ribonucleases = enzymes that degrade RNA.
4. Compare and contrast where and when the process of translation occurs in bacteria and
eukaryotes.
- In bacteria, translation occurs at the same time that transcription occurs
o This happens because there is no nuclear envelope to separate the processes.
- In eukaryotes, translation and transcription occur at separate times and in separate
locations.
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o Transcription is conducted in the nucleus to produce a mature mRNA; the
mRNA is then exported to the cytoplasm of the cell where translation occurs.
5. What is the function of tRNAs?
- Transfer amino acids from the RNA to a growing polypeptide.
- tRNAs act as an interpreter during translation
o This allows amino acids to interact with an mRNA template.
6. The overall structure of tRNA is divided into three substructures.
a. Describe the primary structure of tRNA.
o tRNA sequences are relatively short, ranging from 75 to 85 nucleotides in
length.
b. Describe the secondary structure of tRNA.
o tRNA has stem and loop structures via the formation of hydrogen bonds
between complementary bases.
 The stems are short stretches of double-stranded RNA.
 The loops are single stranded.
o *A CCA sequence at the 3’ end of each tRNA offers a site for amino acid
attachment, while a triplet on the loop at the other end of the structure could
serve as an anticodon.
 *Anticodon = set of three ribonucleotides that forms base pairs with
the mRNA codon.
c. Describe the tertiary structure of tRNA.
o tRNAs fold into an upside-down L shape.
7. What allows the correct amino acid for a particular tRNA to be attached?
- Aminoacyl-tRNA synthetases
o Catalyze the addition of amino acids to tRNAs.
- ATP
o Required to attach an amino acid to a tRNA.
- *Each aminoacyl-tRNA synthetase has a binding site for a particular amino acid and
a particular tRNA; so small differences in the shape and the base sequence of tRNAs
allow aminoacyl-tRNA synthetases (enzymes) to recognize the correct tRNA for the
correct amino acid.
8. Fill in the blanks to make the following paragraph correct.
Ribosomes can be separated into ___two___ major substructures called the ___large___
subunit and the ___small____ subunit.
The ___small____ subunit holds ___mRNA___ in place during ___translation___. The
___large_____ subunit is where ____peptide-bond formation_____ takes place.
9. Describe the three step protein synthesis sequence by ribosomes?
1. An aminoacyl tRNA diffuses into the A site; if its anticodon matches a codon in
mRNA, it stays in the ribosome.
2. A peptide bond forms between the amino acid held by the aminoacyl tRNA in the A
site and the growing polypeptide, which was held by a tRNA in the P site.
3. The ribosome moves down the mRNA by one codon, and all three tRNAs move one
position within the ribosome. The tRNA in the E site exits; the tRNA in the P site
moves to the E site; and the tRNA in the A site switches to the P site.