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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1:47 minutes

Problem 1a

Textbook Question

Why did geneticists believe, even before direct experimental evidence was obtained, that the genetic code would turn out to be composed of triplet sequences and be nonoverlapping? Experimentally, how were these suppositions shown to be correct?

Verified step by step guidance

Understand the concept of the genetic code: The genetic code is the set of rules by which information encoded in DNA or RNA sequences is translated into proteins. Each amino acid is specified by a sequence of nucleotides in the mRNA, known as a codon.

Explain why geneticists hypothesized a triplet code: Geneticists reasoned that a triplet code was necessary because there are 20 amino acids to encode, and a single nucleotide (4 possibilities: A, U, G, C) or a doublet (4^2 = 16 combinations) would not provide enough combinations to specify all amino acids. A triplet code (4^3 = 64 combinations) provides sufficient combinations to encode all amino acids.

Discuss the nonoverlapping nature of the genetic code: Geneticists proposed that the code is nonoverlapping because overlapping sequences would lead to ambiguity in translation. For example, if codons overlapped, a single nucleotide change could affect multiple amino acids, which is not consistent with observed protein synthesis.

Describe experimental evidence for the triplet code: Experiments by Crick and colleagues using frameshift mutations demonstrated that inserting or deleting one or two nucleotides disrupted the reading frame, leading to nonfunctional proteins. However, inserting or deleting three nucleotides restored the reading frame, supporting the triplet nature of the code.

Explain experimental evidence for nonoverlapping codons: Studies of point mutations showed that a single nucleotide change affected only one amino acid in the protein sequence, rather than multiple amino acids. This provided strong evidence that codons are read sequentially and do not overlap.

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Key Concepts

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

Genetic Code

The genetic code is a set of rules that defines how the sequence of nucleotides in DNA is translated into the amino acid sequence of proteins. It is composed of codons, which are triplet sequences of nucleotides. Each codon corresponds to a specific amino acid or a stop signal during protein synthesis, making the code essential for understanding how genetic information is expressed.

Triplet Codons

Triplet codons are sequences of three nucleotides that encode a single amino acid in the process of translation. Geneticists hypothesized that the code was triplet-based because there are 20 standard amino acids, and using combinations of three nucleotides (4^3 = 64 possible combinations) provides sufficient diversity to encode all amino acids and stop signals. This triplet nature was crucial for the nonoverlapping characteristic of the genetic code.

Experimental Validation

Experimental validation of the genetic code's triplet and nonoverlapping nature was achieved through various methods, including the use of synthetic RNA and ribosomes. By introducing specific RNA sequences and observing the resulting protein synthesis, researchers demonstrated that the reading frame of the RNA was maintained without overlap, confirming that each codon was read independently and sequentially, thus supporting the initial hypotheses.