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Questions and Answers
Why should each CODON in the genetic code consist of 3 bases?
Why should each CODON in the genetic code consist of 3 bases?
What are polypeptides?
What are polypeptides?
Chains of amino acids joined by covalent peptide bonds
Ribosomes are the sites of translation.
Ribosomes are the sites of translation.
True
Transfer RNAs (tRNAs) serve as adaptors, binding to a specific ______________ on the mRNA template.
Transfer RNAs (tRNAs) serve as adaptors, binding to a specific ______________ on the mRNA template.
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Match the components of translation with their functions:
Match the components of translation with their functions:
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How many nucleotides make a codon?
How many nucleotides make a codon?
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The anticodon must match the _______________ codon.
The anticodon must match the _______________ codon.
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The genetic code is commaless, meaning it uses internal punctuation marks.
The genetic code is commaless, meaning it uses internal punctuation marks.
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Describe the structure and function of a tRNA.
Describe the structure and function of a tRNA.
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What happens at each of the three active sites of the ribosome during translation?
What happens at each of the three active sites of the ribosome during translation?
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Of the 64 codons, 61 specify one of the 20 amino acids. The other 3 codons are chain-terminating codons, also known as ________ codons.
Of the 64 codons, 61 specify one of the 20 amino acids. The other 3 codons are chain-terminating codons, also known as ________ codons.
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What are the 8 characteristics of the genetic code?
What are the 8 characteristics of the genetic code?
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How does a gene encode protein?
How does a gene encode protein?
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What are the differences in gene expression between prokaryotes and eukaryotes?
What are the differences in gene expression between prokaryotes and eukaryotes?
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Study Notes
Translation in Prokaryotes and Eukaryotes
The Genetic Code
- The genetic code is a set of instructions for transferring genetic data from DNA/RNA to proteins.
- It consists of 64 triplets of nucleotides, known as codons.
- Each codon specifies one of the 20 amino acids used in protein synthesis.
Why Triplet Codons?
- There are 20 different amino acids used in protein synthesis.
- To code for each amino acid, there must be at least 20 different codons.
- With 4 bases (A, U, G, and C), a minimum of 3 bases per codon is necessary to have at least 20 different codes (4 x 4 x 4 = 64).
General Structure of an Amino Acid
- There are 20 amino acids used in biological proteins.
- They are divided into subgroups based on the properties of their R groups (acidic, basic, neutral, and polar).
Polypeptides
- Polypeptides are chains of amino acids joined by covalent peptide bonds.
- A peptide bond forms between the carboxyl group of one amino acid and the amino group of another.
- Polypeptides are unbranched, with a free amino group at one end (the N terminus) and a carboxyl group at the other (the C terminus).
Process of Gene Expression
- RNA is the intermediate between genes and the proteins they code for.
- Transcription is the synthesis of RNA under the direction of DNA.
- Transcription produces messenger RNA (mRNA).
- Translation is the synthesis of a polypeptide, which occurs under the direction of mRNA.
- Ribosomes are the sites of translation.
Translation in Prokaryotes and Eukaryotes
- The major events of translation are similar in both prokaryotes and eukaryotes.
- The main differences are in the initiation process, with related effects on mRNA structure.
- Prokaryotes have polycistronic mRNA, with internal initiation by ribosomes.
- Eukaryotes have monocistronic mRNA, with initiation only at the 5’ end cap structures.
- Prokaryotes couple transcription and translation in the same cellular compartment.
- Eukaryotes separate transcription (nuclear) from translation (cytoplasmic).
Components of Translation
- mRNA: carries genetic information from DNA to the ribosome.
- tRNA: adaptor molecules that mediate the transfer of information from nucleic acids to protein.
- Ribosomes: manufacturing units of the cell, located in the cytoplasm.
- Enzymes: required for the attachment of amino acids to the correct tRNA molecule and for peptide bond formation.
- Proteins: soluble factors necessary for proper initiation, elongation, and termination of translation.
Ribosome Structure
- Prokaryotic (70S) and eukaryotic (80S) ribosomes are composed of a large subunit and a small subunit of differing sizes.
- Each subunit is composed of rRNA and protein.
- Ribosomes are responsible for aligning tRNA anticodons with mRNA codons.
tRNA Structure
- tRNA molecules are structural RNA molecules that serve as adaptors.
- Each tRNA type binds to a specific codon on the mRNA template and adds the corresponding amino acid to the polypeptide chain.
- The tRNA molecule interacts with three factors: aminoacyl-tRNA synthetases, ribosomes, and mRNA.
Overview of Translation
- Three stages of translation: initiation, elongation, and termination.
- Initiation: the ribosome binds to the mRNA and the initiator tRNA.
- Elongation: the amino acid chain is extended.
- Termination: the ribosome encounters a stop codon, and the polypeptide is released.
Initiation of Translation
- In prokaryotes, the small ribosomal subunit (30S) binds to the mRNA near the AUG codon.
- The initiating tRNA (tRNAfMet) binds to the AUG codon.
- The large subunit of the ribosome (50S) attaches to the small subunit, completing the initiation process.
Initiation of Translation in Eukaryotes
- In eukaryotes, the small ribosomal subunit binds to the methylated cap (7-methyl guanosine) at the 5’ end of the mRNA.
- The small subunit migrates to the initiation site, usually the first AUG it encounters.
- The methionine need not be modified.
Elongation of Translation
- The ribosome reads the mRNA in a 5’ to 3’ direction.
- The ribosome has three tRNA binding sites: the A site, P site, and E site.
- The peptidyl transferase enzyme catalyzes the formation of a peptide bond between the amino acids.
Termination of Translation
- When the ribosome encounters a stop codon, there is no tRNA available to bind to the A site.
- A release factor binds to the A site, and the ribosome unit falls apart, releasing the polypeptide product.
Comparison of Translation in Bacteria versus Eukaryotes
- Ribosomes: 70S in prokaryotes, 80S in eukaryotes.
- Amino acid carried by initiator tRNA: fMet in prokaryotes, Met in eukaryotes.
- Shine-Dalgarno sequence: present in prokaryotes, absent in eukaryotes.
- Simultaneous transcription and translation: yes in prokaryotes, no in eukaryotes.### Characteristics of the Genetic Code
- The genetic code is a triplet code, where each three-nucleotide codon in the mRNA specifies one amino acid in the polypeptide.
- The code is written in linear form, derived from the complementary nucleotide bases in DNA.
- Each "word" within the mRNA consists of 3 ribonucleotide letters, with only three exceptions where each codon specifies one amino acid.
- The code is non-overlapping, where each nucleotide is part of only one codon and is read only once during translation.
- The code is commaless, with no internal punctuation, and codons are read one after the other with no breaks between them until a STOP signal is reached.
- The code is unambiguous, where each triplet specifies only a single amino acid, except for AUG, UGA, and UAG.
- The code is degenerate, where most amino acids are specified by more than one codon, except for Met and Trp.
- The code contains one "START" signal (AUG) and three "STOP" signals (UAA, UAG, and UGA), which initiate and terminate translation, respectively.
- The code is universal, with the same codons assigned to the same amino acids and START and STOP signals in the vast majority of genes in animals, plants, and microorganisms, with some exceptions.
RNA Codon
- Of the 64 codons, 61 specify one of the 20 amino acids, while the other 3 codons are chain-terminating codons (STOP CODONS).
- AUG, one of the 61 codons that specify an amino acid, is used in the initiation of protein synthesis.
Degeneracy
- Degeneracy is found only in the third nucleotide of the codon.
- The number of codons that specify each amino acid varies, with some amino acids having 1, 2, 3, 4, or 6 codons.
Universality
- The genetic code is remarkably the same in all organisms, with some exceptions involving assigning one or two of the three STOP codons to an amino acid instead.
- The most common exception is the use of UGA as a codon for Tryptophan in mitochondria.
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Description
Understand the genetic code, its composition, and the role of triplet codons in protein synthesis. Learn how the 64 codons specify 20 amino acids.
