Problem 2 - Campbell Biology

Question

Which of the following is true of a codon? (A) It never codes for the same amino acid as another codon. (B) It can code for more than one amino acid. (C) It can be either in DNA or in RNA (D) It is the basic unit of protein structure.

Step-by-Step Solution

Answer

Option C is correct. Codons can be in either DNA or RNA.

Step 1: Understand Codons

A codon is a sequence of three nucleotides that correspond to a specific amino acid or a start/stop signal during protein synthesis.

Step 2: Analyze Option A

Option A states that a codon never codes for the same amino acid as another codon. This is incorrect because multiple codons can code for the same amino acid due to the redundancy of the genetic code.

Step 3: Analyze Option B

Option B states that a codon can code for more than one amino acid. This is incorrect because each codon specifically codes for only one amino acid or a start/stop signal.

Step 4: Analyze Option C

Option C states that a codon can be either in DNA or RNA. This is correct because codons are found in both mRNA during transcription and in DNA codons before transcription.

Step 5: Analyze Option D

Option D states that a codon is the basic unit of protein structure. This is incorrect because the basic unit of protein structure is the amino acid, not the codon.

Step 6: Conclusion

After evaluating all the options, the correct statement about codons is found in Option C.

Key Concepts

protein synthesis

Protein synthesis is a fundamental biological process during which cells build proteins. This process has two main stages: transcription and translation. During transcription, an mRNA copy is made from a DNA template, which then travels to the ribosome where translation occurs.
In translation, the ribosome reads the mRNA sequence and translates it into an amino acid sequence, creating a protein. This sequence is formed by reading groups of three nucleotides known as codons. Each codon specifies a particular amino acid or a start/stop signal. For example, the codon AUG is both the start signal and codes for the amino acid methionine. Codons are universal in nearly all organisms, which means the same codon codes for the same amino acid across different species.

redundancy of genetic code

The redundancy of the genetic code means that multiple codons can code for the same amino acid. This is also known as the degeneracy of the genetic code.
For instance, the amino acid leucine is encoded by six different codons: UUA, UUG, CUU, CUC, CUA, and CUG. This redundancy is important for minimizing the effects of mutations. If a mutation occurs and changes one nucleotide in the codon, it might still code for the same amino acid, thereby preventing any change in the protein’s structure. This feature of the genetic code helps in maintaining protein function and stability within biological systems.

transcription

Transcription is the first step in protein synthesis where an mRNA copy is made from a DNA template. This process occurs in the cell nucleus. The enzyme RNA polymerase binds to a specific region of the DNA called the promoter and unwinds the DNA strands.
RNA polymerase then uses one of the strands as a template to synthesize a complementary strand of RNA. This newly formed RNA strand is called messenger RNA (mRNA) and carries the genetic instructions from the DNA in the nucleus to the ribosome in the cytoplasm.
The mRNA sequence is a series of codons that will be read during translation to build a corresponding sequence of amino acids, forming a protein. Thus, transcription is a critical step that determines how the genetic code stored in DNA is expressed as proteins, which are the functional molecules in cells.