Question

Sometimes knowing the DNA sequence of a gene that codes for a protein does not tell you the amino acid sequence. Suggest several reasons why this is so.

Short answer

Degeneracy of the genetic code, RNA splicing, alternative splicing, and post-translational modifications can alter the amino acid sequence derived from the DNA sequence.

Step-by-step solution

Understanding the Genetic Code

The genetic code is redundant, meaning that multiple codons can code for the same amino acid. This redundancy is called codon degeneracy. For example, the amino acid leucine is coded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG).

Post-Transcriptional Modifications

After the DNA is transcribed into mRNA, the mRNA undergoes modifications. One important modification is RNA splicing, where introns (non-coding regions) are removed and exons (coding regions) are joined together. The sequence of the amino acids in the final protein depends on which exons are included or excluded, which can vary.

Alternative Splicing

Alternative splicing allows a single gene to produce multiple mRNA variants, leading to different protein products from the same gene. This means the DNA sequence alone does not provide complete information about the amino acid sequence of the resulting proteins.

Post-Translational Modifications

Even after translation into an amino acid sequence, the final protein may undergo several post-translational modifications such as phosphorylation, glycosylation, or cleavage. These modifications affect the final structure and function of the protein and are not encoded directly by the DNA sequence.

Key concepts

Codon Degeneracy

The genetic code is made up of sequences of three nucleotides called codons. Each codon corresponds to a specific amino acid or a stop signal during translation. However, the code is redundant, which means multiple codons can code for the same amino acid. This redundancy is called codon degeneracy. For instance, the amino acid leucine can be coded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG).
Codon degeneracy provides flexibility and reduces the impact of mutations. For example, a change in the third nucleotide of a codon may not alter the amino acid it codes for, making the genetic code more robust against mutations.

RNA Splicing

After DNA is transcribed into mRNA, the mRNA undergoes several modifications before it can be translated into a protein. One critical modification is RNA splicing. Genes are often split into coding regions called exons and non-coding regions called introns. During RNA splicing, introns are removed from the pre-mRNA, and exons are joined together to form the mature mRNA.
The arrangement of exons can influence which segments are included in the mRNA, affecting the final protein product. Splicing ensures that only the necessary coding information is expressed in the form of protein.

Alternative Splicing

Alternative splicing is a mechanism allowing a single gene to code for multiple proteins. This process involves the removal and rejoining of exons in different combinations to produce various mRNA transcripts from the same gene.
Alternative splicing can produce different protein isoforms with distinct functional properties. For example, the inclusion or exclusion of certain exons can change the binding sites or functional domains of a protein. This diversity in protein products plays a crucial role in cellular complexity and adaptability.

Post-Translational Modifications

Even after a protein has been synthesized through translation, its final structure and function are not determined solely by the amino acid sequence. Post-translational modifications (PTMs) are critical processes that modify proteins after translation.
Common types of PTMs include:

• Phosphorylation: The addition of phosphate groups
• Glycosylation: The attachment of sugar molecules
• Cleavage: Cutting the protein at specific sites

These modifications can impact the protein’s activity, stability, interaction with other molecules, and cellular localization. PTMs are vital for functional protein maturation and regulation.