Question

An open reading frame (ORF) is defined as a sequence that has about 100 codons that lack stop codons. Why is finding ORFs helpful in determining the gene number in an unknown sequence?

A. A random sequence is expected have a stop codon every 20 codons.

B. Stop codons are subject to positive selection and are very frequent.

C. Open reading frames are always genes that are expressed.

D. Open reading frames never occur randomly.

Answer 1

Finding ORFs helps identify potential protein-coding genes in DNA or mRNA sequences as ORFs lack stop codons over a significant stretch (~100 codons), indicating a higher likelihood of a gene being present. The genetic code is universal, with AUG marking the start of the reading frame and UAA, UAG, & UGA as stop codons, signaling the end of protein synthesis.

Step-by-step explanation:

Finding open reading frames (ORFs) is helpful in determining the gene number in an unknown sequence because ORFs suggest the presence of potential protein-coding regions within a DNA or mRNA sequence. Since a random sequence is expected to have a stop codon every 20 codons and ORFs have about 100 codons without a stop codon, finding them indicates a higher likelihood of a gene being present. While not all ORFs may be genes, the absence of stop codons in such a large stretch makes it statistically significant and worthy of further investigation.

It's also important to note that the genetic code is universal and redundant, and AUG, the start codon, establishes the reading frame for translation. This means that once the mRNA beings translation with AUG, it continues to read in sets of three nucleotides (codons) until a stop codon is encountered. These terminating sequences, or stop codons, are UAA, UAG & UGA, and they signal the end of the protein synthesis.