Mechanism of Translation
Supplemental Instruction
Iowa State University / Leader: / Lilli Howard
Course: / BIOL/GEN 313
Instructor: / Dr. Rodermel/Dr. Tuggle
Date: / 10/21/14
  1. Translation is the process of synthesizing polypeptidesfrom an mRNA template.
  1. In which direction do ribosomes move?5’  3’
  1. Where are ribosomes located?Cytoplasm, RER
  1. 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

  1. In eukaryotes, ribosomes carry out protein synthesis in the cytoplasm, but their assembly occurs in a part of the nucleus called the nucleolus.
  1. Translation takes place in four stages:
  2. tRNA charging
  3. initiation
  4. elongation
  5. termination
  1. 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

  1. 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.

  1. Required components for prokaryotic translation initiation:
  2. mRNA
  3. 30S (small) and 50S (large) ribosomal subunits
  4. Initiation Factors (IFs): IF1, IF2, IF3
  5. fMet-tRNAfMet(initiator tRNA)
  6. GTP (for energy)
  1. 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

  1. Prokaryotic translation initiation happens in three steps.
  2. mRNA binds to 30S subunit via Shine Dalgarno sequence
  3. fMet-tRNAfMetbinds to initiation codon of mRNA
  4. 50S joins complex
  1. Summarize each of the above three steps.
  1. What are the major differences between eukaryotic and prokaryotic translation initiation?
  2. In eukaryotic mRNAs, there are NO Shine Dalgarno sequences to position the mRNA on the ribosome
  3. Instead, the 5’-CAP of eukaryotic mRNAs binds proteins (CAP-binding proteins) that attract the 40S subunit of the ribosome to the mRNA
  4. The poly(A) tail also plays a role in initiation
  5. 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
  6. There are seven IF (versus 3 in prokaryotes) to coordinate the initiation process (you do not need to know their names)
  7. 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
  1. Describe the Kozak Sequence.

Kozak consensus sequence facilitates identification of the start codon by the ribosome

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