Table of contents

1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

11. Translation

The Genetic Code

Genetics

11. Translation

The Genetic Code

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1:53 minutes

Problem 14

Textbook Question

The genetic code contains 61 codons to specify the 20 common amino acids. Many organisms carry fewer than 61 different tRNA genes in their genomes. These genomes take advantage of isoaccepting tRNAs and the rules governing third-base wobble to encode fewer than 61 tRNA genes. Use these rules to calculate the minimal number of tRNA genes required to specify all 20 of the common amino acids.

Verified step by step guidance

Understand the concept of codons and tRNAs: Codons are sequences of three nucleotides that specify amino acids. There are 61 codons for amino acids, but due to the degeneracy of the genetic code, multiple codons can specify the same amino acid. tRNAs are molecules that match codons with their corresponding amino acids during translation.

Learn about isoaccepting tRNAs: Isoaccepting tRNAs are different tRNA molecules that carry the same amino acid but recognize different codons. This reduces the number of distinct tRNA genes required in the genome.

Understand the wobble hypothesis: The wobble hypothesis explains that the third base of a codon can form non-standard base pairs with the first base of the anticodon in tRNA. This allows one tRNA to recognize multiple codons, further reducing the number of tRNA genes needed.

Determine the minimal number of tRNA genes: To calculate the minimal number of tRNA genes, consider the number of codons for each amino acid and how many of these can be recognized by a single tRNA due to wobble pairing. For example, codons ending in U or C can often be recognized by the same tRNA, and codons ending in A or G can be recognized by another tRNA.

Sum the minimal number of tRNAs required for all amino acids: For each of the 20 amino acids, calculate the minimum number of tRNAs needed based on the wobble rules and isoaccepting tRNAs. Add these numbers together to find the total minimal number of tRNA genes required to specify all 20 amino acids.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Codons and Amino Acids

Codons are sequences of three nucleotides in mRNA that correspond to specific amino acids during protein synthesis. There are 64 possible codons (including stop codons), but only 20 common amino acids. This redundancy in the genetic code allows for multiple codons to code for the same amino acid, which is crucial for understanding how fewer tRNA genes can still specify all amino acids.

tRNA and Isoaccepting tRNAs

Transfer RNA (tRNA) molecules are responsible for bringing amino acids to the ribosome during translation. Isoaccepting tRNAs are different tRNA species that can carry the same amino acid but have different anticodons. This allows a single amino acid to be encoded by multiple tRNA genes, reducing the total number of tRNA genes needed in an organism's genome.

Wobble Hypothesis

The wobble hypothesis explains how the third base of a codon can pair loosely with the corresponding base of a tRNA anticodon, allowing for flexibility in base pairing. This means that a single tRNA can recognize multiple codons that differ in their third nucleotide, further decreasing the number of tRNA genes required to translate the genetic code into proteins.

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Related Practice

Textbook Question

Third-base wobble allows some tRNAs to recognize more than one mRNA codon. Based on this chapter's discussion of wobble, what is the minimal number of tRNA molecules necessary to recognize the following amino acids?

Arginine

Leucine

A short RNA molecule was isolated that demonstrated a hyperchromic shift, indicating secondary structure. Its sequence was determined to be

5'-AGGCGCCGACUCUACU-3'

If the molecule were an internal part of a message, what amino acid sequence would result from it following translation?

440

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Textbook Question

A short RNA molecule was isolated that demonstrated a hyperchromic shift, indicating secondary structure. Its sequence was determined to be

5'-AGGCGCCGACUCUACU-3'

If the molecule were a tRNA fragment containing a CGA anticodon, what would the corresponding codon be?

493

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Textbook Question

Shown here is a hypothetical viral mRNA sequence:

5'-AUGCAUACCUAUGAGACCCUUGGA-3'

Assuming that it could arise from overlapping genes, how many different polypeptide sequences can be produced? What are the sequences?

460

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Textbook Question

Most proteins have more leucine than histidine residues, but more histidine than tryptophan residues. Correlate the number of codons for these three amino acids with this information.

516

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Textbook Question

In an experiment to decipher the genetic code, a poly-AC mRNA (ACACACAC...) is synthesized. What pattern of amino acids would appear if this sequence were to be translated by a mechanism that reads the genetic code as

A quadruplet with overlaps?

504

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Textbook Question

A quadruplet without overlaps?

475

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