What Is a Reading Frame and Why Are There Six?

A reading frame is a specific sequence of nucleotides that is read in groups of three, called codons, to determine the amino acid sequence of a protein.

What is a Reading Frame?

Think of a DNA sequence like a long sentence written without spaces. For example, THEBIGREDCAT. You could read this sentence starting from the first letter (THE BIG RED CAT), the second letter (HEB IGR EDC AT), or the third letter (EBI GRE DCA T). Each starting point creates a different set of three-letter words.

Similarly, a reading frame defines how a sequence of nucleotides (the building blocks of DNA) is divided into consecutive groups of three. These three-nucleotide units are known as codons. Each codon typically corresponds to a specific amino acid, which are the building blocks of proteins.

• Frame 1: Starts reading from the first nucleotide.
• Frame 2: Starts reading from the second nucleotide.
• Frame 3: Starts reading from the third nucleotide.

Here's a simple illustration for a DNA sequence like ATGCGTAC:

• Frame 1: ATG CGT AC... (Codons: ATG, CGT)
• Frame 2: TGC GTA C... (Codons: TGC, GTA)
• Frame 3: GCG TAC... (Codons: GCG, TAC)

Why Are There Six Reading Frames?

The reason there are six reading frames is due to the structure of DNA:

1. DNA is read in codons: As mentioned, the genetic code is read in groups of three nucleotides.
2. Each strand has three frames: Because you can start reading from the first, second, or third nucleotide, any single strand of DNA has three possible reading frames.
3. DNA is double-stranded and antiparallel: A DNA molecule exists as a double helix with two strands running in opposite directions (antiparallel). This means that the complementary strand to the one you started with also exists.
4. The second strand also has three frames: When you read the second strand in its correct direction (5' to 3'), it also has three possible reading frames, just like the first strand.

As highlighted in the reference: "Since DNA is interpreted in groups of three nucleotides (codons), a DNA strand has three distinct reading frames. The double helix of a DNA molecule has two anti-parallel strands; with the two strands having three reading frames each, there are six possible frame translations."

Therefore, the three reading frames on one strand, combined with the three reading frames on the complementary antiparallel strand, result in a total of six potential reading frames for any given segment of double-stranded DNA.

To find the actual protein sequence coded by a gene, the cell machinery identifies the correct reading frame, usually by finding a start codon (like AUG) and reading until it encounters a stop codon.