how is translation similar to DNA replication and transcription
- Translation also involves large macromolecular machines
- accessory factors are also required for initiation, elongation and termination of the process
- translation also must be executed with accuracy
- the act of polymerisation is also endergonic (driven by ;high-energy’ phosphoanhydride bond cleavage
Why does the genetic code use triplets (3 nucleotides) to name amino acids instead of just 1 or 2 nucleotides?
It’s a matter of having enough unique “names” for all 20 amino acids:
- The 1-Letter Code (4^1 = 4): If each nucleotide stood for one amino acid, we could only name 4 amino acids. (Not enough!)
- The 2-Letter Code (4^2 = 16): If we used pairs (like AU, CG, etc.), we could name 16 amino acids. (Still not enough!)
- The 3-Letter Code (4^3 = 64): By using triplets, we get 64 possible combinations. This is finally enough to cover all 20 amino acids.
Key term to remember… ‘Degenerate’: Because we have 64 “names” (codons) but only 20 amino acids, some amino acids have more than one name. It’s like a person having three different nicknames that all refer to the same individual.
Does the genetic code overlap when being read, and what are the three definitive characteristics of how codons are organized?
- Non-overlapping:
- The code is not read like ‘ABC’, ‘BCD’ (overlapping).
- Instead, it is read in distinct, successive blocks: ‘ABC’ = 1st amino acid, ‘DEF’ = 2nd amino acid. - The 3 Key Rules of the Genetic Code:
- Non-overlapping: Each- nucleotide is part of only one codon.
- Degenerate: Multiple different codons can specify the same amino acid.
- Triplet: It is always read in groups of three bases.
How did scientists Francis Crick and Sydney Brenner use bacteriophage T4 to prove that the genetic code is read sequentially in triplets?
- Discovery: Deleting a single nucleotide (a mutation) abolished gene function.
- Restoration: A second mutation (an insertion) could restore function by acting as a “suppressor” of the first.
- Mechanism: These insertions/deletions are known as frameshift mutations because they shift the entire “reading frame” of the sequence from a fixed point.
The Proof for Triplets:
- Two closely spaced deletions/insertions could not restore gene function.
- However, three closely spaced deletions/insertions could restore function.
Conclusion: This confirmed the code is read in groups of three (a triplet code).
What is a frameshift mutation, and how did scientists use them to prove the genetic code is a triplet?
Definition: A mutation caused by the insertion or deletion of a number of nucleotides that is not divisible by three.
This shifts the “reading frame,” changing every codon (and thus every amino acid) from the point of mutation onward.
Experimental Proof (Crick & Brenner):
- 1 or 2 Mutations: If you insert or delete 1 or 2 bases, the entire downstream protein sequence is ruined (non-functional).
- 3 Mutations: If you insert or delete 3 bases near each other, the original reading frame is restored after the third mutation site.
Conclusion: Because function is restored only in multiples of three, the code must be read in blocks of three nucleotides.
How does mRNA recognize amino acids, and how did scientists synthesize RNA sequences in the 1960s before modern sequencing was available?
Recognition Mechanism:
- tRNA Intermediary: mRNA does not recognize amino acids directly.
- Anticodons: mRNA binds to tRNA molecules that carry specific amino acids.
- Complementarity: Each tRNA has an anticodon that is complementary to a specific mRNA codon.
- Binding Requirement: Amino acids must be joined to tRNAs so that the tRNA can bind to the mRNA template.
Historical Sequencing (1960s):
- Enzyme used: Scientists utilized polynucleotide phosphorylase from Azotobacter vinelandii.
- Template-independent: This enzyme allowed them to link nucleotides together without needing a DNA template to create synthetic RNA for study.
What is a cell-free system in the context of protein synthesis, and how did Nirenberg and Matthaei use it to identify the first codon?
The System Setup:
- Preparation: E. coli cells are broken open and centrifuged to remove cell walls.
- Components: The remaining mixture contains DNA, mRNA, ribosomes, and enzymes needed for protein synthesis.
- Method: Adding DNase removes the original DNA (halting natural synthesis), allowing scientists to add synthetic mRNA and recover the resulting polypeptide.
The 1961 Discovery (Nirenberg & Matthaei):
- They added poly(U) (synthetic RNA made only of Uracil) to the system.
- They recovered a polypeptide made entirely of the amino acid Phenylalanine (Phe).
- Conclusion: This proved that the codon UUU = Phe.
Additional Findings:
- Poly(A) = poly(Lysine)
- Poly(C) = poly(Proline)
How were trinucleotides used to identify the amino acids specified by different codons, and how many codons were deciphered using this method?
The Experimental Method:
- Setup: Ribosomes were bound to a nitrocellulose filter.
- Process: Specific trinucleotides (3-base sequences) were tested for their ability to promote the binding of tRNA to these ribosomes.
- Identification: While free tRNAs passed through the filter, the specific bound tRNA was identified by its attached labeled amino acid.
Key Results:
- UUU stimulates only Phe (Phenylalanine) tRNA binding.
- UUG = Leu (Leucine) tRNA binding.
- UGU = Cys (Cysteine) tRNA binding.
- GUU = Val (Valine) tRNA binding.
Conclusion:
- This method was successfully used to determine the amino acids specified by 50 codons.
- For the remaining codons, there was either no binding or the results were ambiguous.
How did H. Gobind Khorana use synthetic, repeating RNA sequences to identify codons, and what did his results with Poly(UAC) reveal?
Experimental Approach:
- Synthesis: Khorana chemically synthesized polynucleotides with specific repeating sequences.
- Translation: These were added to a cell-free translation system to see which polypeptides they produced.
Key Examples:
- Repeating Dinucleotide (UCU CUC…): Stimulated the production of a repeating polypeptide: Ser-Leu-Ser-Leu….
- Repeating Trinucleotide Poly(UAC): Produced three different homopolypeptides (Poly-Tyr, Poly-Thr, and Poly-Leu).
Major Conclusion:
The Poly(UAC) result proved that ribosomes can initiate translation in any of the three possible reading frames.
What does it mean for the genetic code to be degenerate, and what are the specific roles of start and stop codons?
Features of the genetic code:
- Degenerate: Multiple codons can specify the same amino acid.
- Three amino acids (Arg, Leu, Ser) are specified by 6 different codons.
- Met & Trp are the only ones represented by a single codon.
- Synonymous Codons: Codons coding for the same amino acid, usually differing only in their third nucleotide.
Patterns in Coding: (Changes in the 1st position specify similar amino acids)
- 2nd position pyrimidines encode mostly hydrophobic amino acids.
- 2nd position purines encode mostly polar amino acids.
Start and Stop Signals:
- Start Codons (AUG & GUG): Specify the starting point for synthesis and can also specify Met & Val at internal positions.
- Stop Codons (UAG, UAA, & UGA): Also called ‘nonsense’ codons; they do not specify amino acids but signal the end of translation.
Is the genetic code the exact same for every living thing on Earth, and what are some notable exceptions
The General Rule:
For many years, the code was considered universal (standard for all life) because E. coli can accurately translate genes from other organisms.
The Exceptions:
- Mitochondria: DNA studies in 1981 showed that certain mitochondria use variants of the “standard” code.
- Mammalian Mitochondria Examples:
> AUA: Acts as a Start (Met) codon instead of coding for Ile.
> UGA: Codes for Trp (an amino acid) instead of being a “Stop” signal.
> AGA & AGG: Act as Stop signals instead of coding for Arg.
- Ciliated Protozoa: These also use an alternate genetic code because they branched off early in eukaryotic evolution.
What are the four main structural features of a tRNA molecule, and how is it shaped?
General Structure:
- Size: 54 to 100 nucleotides long.
- Shape: Arranged in a cloverleaf structure.
The 4 Key Parts:
- 5’-terminal phosphate group: The start of the RNA chain.
- Acceptor Stem: A 7-bp stem containing the 3’-terminal nucleotide where the amino acid attaches.
- D-arm: A loop containing the unusual base dihydrouridine.
- Anticodon Arm: A 5-bp stem ending in a loop that contains the anticodon (which matches the mRNA).
What is the actual 3D shape of a tRNA molecule, and what is the role of its modified bases?
Complex 3D Structure:
- L-shape: Although the 2D view looks like a cloverleaf, the actual 3D molecule is folded into an “L-shape”.
- Structure Formation: The Acceptor and T stems fold to form one leg of the “L,” while the D and Anticodon stems form the other leg.
- Variant Arm: A site of variability that can range from 3 to 21 nucleotides in length.
Modified Bases:
- Over 25% of the bases in tRNA are modified after transcription.
Key Roles:
- Strengthen the interaction between the codon (on mRNA) and the anticodon (on tRNA).
- Help the correct amino acid attach to the acceptor stem.
Note: These modifications are helpful but not essential for the tRNA to stay physically intact.
What is the role of aminoacyl-tRNA synthetase, and what are the two steps of aminoacylation?
Role of the Enzyme:
- Each enzyme is specific to one amino acid.
- It selects the correct amino acid and attaches it covalently to the correct tRNA at its 3’-terminal ribose residue.
- The resulting molecule is called aa-tRNA.
The Two-Step Process:
1. Activation: The amino acid reacts with ATP to form an aminoacyl-adenylate (aminoacyl-AMP), releasing pyrophosphate (PPi)
2. Formation of aa-tRNA: The aminoacyl-AMP then reacts with the tRNA to form aminoacyl-tRNA and releases AMP.
How does the aminoacyl-tRNA synthetase (aaRS) enzyme recognize and bind to the correct tRNA molecules?
Recognition Points:
- Due to the degeneracy of the genetic code, more than one tRNA can carry a specific amino acid.
- These are called ‘isoaccepting tRNAs’, and they must all be recognized by the same specific aaRS enzyme.
- To ensure accuracy, the synthetase makes physical contact with the tRNA at two main points: the acceptor stem and the anticodon loop.
Binding Mechanics:
- Enzymes that interact with both the anticodon and the acceptor stem must be able to bind to both legs of the L-shaped tRNA molecule.
- Example (Yeast AspRS): This is a homodimer that symmetrically binds two tRNA molecules.
- Induced Fit: The final shape (conformation) of a tRNA when bound is dictated by its protein interactions (induced fit) rather than just its sequence.
How does the cell ensure that the correct amino acid is attached to its matching tRNA, and what are the two active sites of the IleRS enzyme?
Accuracy of tRNA Charging:
- Charging is highly accurate; for example, IleRS transfers roughly 40,000 isoleucines for every one incorrect valine.
- This is impressive because valine is chemically very similar to isoleucine, differing only by a single methyl group.
The Two-Site Proofreading Mechanism: tRNA synthetase enzymes, like IleRS, have two distinct active sites to filter out mistakes…
1. Synthesis Site: This site activates both isoleucine and valine because they both have high affinity for it.
2. Editing Site: This site acts as a filter; it hydrolyses (breaks down) aminoacylated tRNA if the amino acid is incorrect. It only admits aminoacyl groups that are smaller than isoleucine (like valine) to be removed.
What is “wobble” hypothesis in protein synthesis, and why is it necessary?
The Concept:
- Constraint: Protein synthesis requires selecting the correct tRNA for each mRNA codon.
- The Problem: There are 61 different codons that code for amino acids, but cells do not have 61 different tRNAs.
- The Solution: Many tRNAs can bind to 2 or 3 different codons.
How it Works:
- Flexible Pairing: “Non-Watson-Crick” base pairing can occur at the 3rd position of the codon-anticodon interaction.
- Special Bases: This flexible position often contains modified bases like Inosine (I) or Gm (G with a methyl group), which allow for more versatile binding.
What are the specific pairing rules for the “wobble” position, and how many tRNAs are needed to translate the genetic code?
Pairing Rules:
- Standard Pairing: The first two positions of the codon-anticodon interaction follow strict Watson-Crick rules (A-U, G-C).
- Wobble Pairing: The 3rd position allows for “play” or “wobble”…
> U can pair with A or G.
>G can pair with C or U.
> I (Inosine) is the most flexible and can pair with U, C, or A.
Key Requirements:
- Minimum tRNAs: At least 31 tRNAs are required to translate all 61 triplets (not including initiation), though most cells have more than 32.
- Efficiency: The most frequently used codons are usually complementary to the most abundant tRNA species in the cell.
SUMMARY OF THE GENETIC CODE
- the ribosome is the site of protein synthesis; mRNA is translated at this site, with the help of an adaptor molecule (tRNA)
- mRNA is deciphered in codons of 3 nucleotides
- these codons are sequential and non-overlapping
- the genetic code is degenerate as a several codon may code for the same amino acid
- many tRNAs bind to more than 1 codon thank to the ‘wobble’
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Cell Theory & Microscopy35
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cell types & specialisation19
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Boundaries & Membranes11
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Organelles13
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Mendelian Genetics 122
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DNA & RNA28
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Genetic information and transfer20
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Mendelian Genetics 220
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Advanced Mendelian Genetics 112
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Advanced Mendelian Genetics 219
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DNA Replication 121
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DNA replication 225
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Transcription in Prokaryotes37
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Transcription in Eukaryotes48
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The Genetic Code19
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Translation23
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Amino Acids 185
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Amino Acids 255
