Supplemental Instruction
Iowa State University / Leader: / Lilli Howard
Course: / BIOL/GEN 313
Instructor: / Dr. Rodermel/Dr. Tuggle
Date: / 10/21/14
- Translation is the process of synthesizing polypeptidesfrom an mRNA template.
- In which direction do ribosomes move?5’ 3’
- Where are ribosomes located?Cytoplasm, RER
- Describe the structure of the chromosome and how it is different in prokaryotes and eukaryotes.
Ribosomes have 2 subunits, the large ribosomal subunit and the small ribosomal subunit
Ribosome composition differs in prokaryotes versus eukaryotes:
Prokaryotes: 30S (small) + 50S (large)= 70S
Eukaryotes: 40S (small) + 60S (large) = 80S
- In eukaryotes, ribosomes carry out protein synthesis in the cytoplasm, but their assembly occurs in a part of the nucleus called the nucleolus.
- Translation takes place in four stages:
- tRNA charging
- initiation
- elongation
- termination
- Translation is an energy intensive process. Where does the cell get it’s energy?
GTP or ATP hydrolysis provides ~ 5-7 kcal/mol of energy in the cell, just enough energy to drive many biological reactions; any left-over energy is released as heat
- Describe the process of charging tRNAs. What is the product of this process?
Aminoacyl-tRNA synthetases : enzymes that charge tRNAs with amino acids. This reaction requires energy to proceed (provided by ATP hydrolysis). The below occurs in the active site of the aminoacyl-tRNA synthetase. There are 20 different aminoacyl-tRNA synthetases, one for each amino acid. The active site recognizes a particular amino acid + all isoacceptingtRNAs for that amino acid.
- Required components for prokaryotic translation initiation:
- mRNA
- 30S (small) and 50S (large) ribosomal subunits
- Initiation Factors (IFs): IF1, IF2, IF3
- fMet-tRNAfMet(initiator tRNA)
- GTP (for energy)
- What is the Shine-Dalgarno sequence? What is its consensus sequence? Where is it located?
The Shine–Dalgarno sequence is in the 5’-UTR of prokaryote mRNAs, ~8 bp upstream from START codon; consensus is AGGAGG ; it is the sequence to which the ribosome binds to begin translation
- Prokaryotic translation initiation happens in three steps.
- mRNA binds to 30S subunit via Shine Dalgarno sequence
- fMet-tRNAfMetbinds to initiation codon of mRNA
- 50S joins complex
- Summarize each of the above three steps.
- What are the major differences between eukaryotic and prokaryotic translation initiation?
- In eukaryotic mRNAs, there are NO Shine Dalgarno sequences to position the mRNA on the ribosome
- Instead, the 5’-CAP of eukaryotic mRNAs binds proteins (CAP-binding proteins) that attract the 40S subunit of the ribosome to the mRNA
- The poly(A) tail also plays a role in initiation
- The tRNA that recognizes the first AUG codon during initiation is called the initiator tRNA (tRNAi): it is charged with a normal Met, hence termed Met-tRNAiMet
- There are seven IF (versus 3 in prokaryotes) to coordinate the initiation process (you do not need to know their names)
- Proteins that bind the poly-A tail interact with CAP-binding proteins to enhance interactions between the 40S & 60S subunits and the 5’- end of the mRNA
- Describe the Kozak Sequence.
Kozak consensus sequence facilitates identification of the start codon by the ribosome
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