Explain what a reading frame is and what effects it has when the reading frame is changed
A reading frame is the way in which the nucleotide sequence of DNA or RNA is read during transcription and translation. The genetic code consists of triplet codons, and each codon codes for a specific amino acid. The nucleotide sequence is read in groups of three (codons), and the reading frame determines the order in which codons are read.
When the reading frame is changed, the order in which codons are read is altered, which can have significant effects on protein synthesis. A frameshift mutation occurs when nucleotides are inserted or deleted, resulting in a shift in the reading frame. This can cause codons to be read incorrectly, leading to changes in the amino acid sequence of the protein.
Frameshift mutations can also affect the regulation of protein expression. For example, in bacteria, the Shine-Dalgarno sequence is a ribosome-binding site that is located upstream of the start codon. If a frameshift mutation occurs in this region, the ribosome may not be able to bind properly, leading to reduced protein expression. In eukaryotes, changes in the reading frame can result in premature termination codons, which can lead to the degradation of mRNA molecules and reduced protein expression.
In summary, the reading frame is essential for accurate protein synthesis, and any changes to the reading frame can have significant effects on protein expression and function.
A reading frame refers to the sequential grouping of nucleotides, usually in sets of three called codons, during protein synthesis. The reading frame determines how the mRNA (messenger RNA) is "read" and translated into amino acids to form a protein. Any change in the reading frame can have significant effects on protein synthesis.
In normal circumstances, the reading frame begins at the start codon (AUG) and continues until a stop codon is encountered, usually one of the three codons: UAA, UAG, or UGA. This process is known as open reading frame (ORF). The ribosomes read the mRNA in this frame, ensuring that the correct amino acids are added to the growing polypeptide chain.
When the reading frame is changed, usually due to a mutation or an error during transcription or translation, the ribosomes start reading the mRNA in a different set of three nucleotides. This alteration leads to three potential effects:
1. Frameshift mutation: Insertion or deletion of one or two nucleotides in the DNA sequence causes a shift in the reading frame, disrupting the codon alignment. As a result, most of the codons downstream from the mutation are changed, leading to an incorrect protein translation. Frameshift mutations often result in non-functional proteins.
2. Missense mutation: A single nucleotide substitution occurs, leading to the replacement of one amino acid with another. This substitution can alter the protein's structure and function, and its impact depends on the specific amino acid change and its location within the protein.
3. Nonsense mutation: A premature stop codon (UAA, UAG, or UGA) is introduced into the mRNA due to a mutation. This results in truncation of the protein, as the ribosomes terminate translation prematurely. The resulting protein is often non-functional or significantly altered in its structure.
Overall, any change in the reading frame, whether through frameshift, missense, or nonsense mutation, can have profound effects on protein synthesis, potentially leading to abnormal or non-functional proteins. The cellular processes relying on these proteins may be disrupted, affecting normal biological functions.