Question

One of the possibilities that researchers interested in the genetic code considered was that the code was overlapping, meaning that a single base could be part of up to three codons. How many amino acids would be encoded in the sequence 5'-AUGUUACGGAAU-3' by a non-overlapping and maximally overlapping triplet code? a. 4 (non-overlapping) and 16 (overlapping) b. 4 and 12 c. 4 and 10 d. 12 and 4

Short answer

c. 4 and 10

Step-by-step solution

Understanding the Sequence

The initial sequence given is 5'-AUGUUACGGAAU-3'. It contains 12 nucleotides. Codons are formed by groups of three nucleotides.

Non-overlapping Sequencing

In a non-overlapping sequence, each codon is independent and does not share nucleotides with another codon. Considering that each codon consists of three nucleotides, the given 12-nucleotide sequence would result in 4 codons (12/3 = 4). Therefore, 4 amino acids would be encoded in the non-overlapping triplet code.

Overlapping Sequencing

In a maximally overlapping sequence, a single base could be part of up to three codons. This means that after the first codon, every subsequent nucleotide begins a new codon. So, after forming the first codon (which requires 3 nucleotides), each additional individual nucleotide adds another codon. As there are 12 nucleotides in our given sequence, the first 3 form the first codon, leaving 9. Hence, these 9 further nucleotides would each form a new codon together with the previous two. As a result, there would be 10 codons in total in the overlapping sequence (1 + 9 = 10). Therefore, 10 amino acids would be encoded in the maximally overlapping triplet code. So the answer is: 4 (non-overlapping) and 10 (overlapping), which corresponds to choice 'c'.

Key concepts

Codons

When we talk about genetic information, codons stand front and center. These are sequences of three nucleotides that come together to form the building blocks of DNA or RNA. Essentially, they are like the words in the language of genes, directing the synthesis of proteins by encoding amino acids.

In the non-overlapping genetic code, each set of three nucleotides, or a triplet, constructs a message for the cell to produce a specific amino acid. Imagine codons as non-overlapping, individual units on a string – each unit is read separately to convey a message. The sequence given in our exercise, 5'-AUGUUACGGAAU-3', when divided into non-overlapping codons, forms four distinct 'words' or codons, each directing the production of an amino acid.

Nucleotides

Nucleotides are the alphabet of the genetic language. They are the molecules that make up the structural units of RNA and DNA. Each nucleotide contains a sugar molecule, a phosphate group, and a nitrogenous base. There are four types of nitrogenous bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). RNA replaces thymine with uracil (U).

The sequence of these bases determines the genetic instructions within a cell. The order of nucleotides in a gene is analogous to the sequence of letters that form sentences; changing the sequence can change the instruction, just like changing letters can alter the meaning of words.

Amino Acid Encoding

The process of amino acid encoding is akin to translating the genetic code into functional proteins. Each codon, a triplet of nucleotides, corresponds to a specific amino acid or a signal to start or stop protein synthesis. This correlation between codons and amino acids is nearly universal across all organisms and is known as the genetic code.

During the process of translation, ribosomes read the messenger RNA (mRNA) codons and use this code to assemble amino acids in the correct sequence to form proteins. In our exercise, the codons within the 12-nucleotide sequence instruct the cell to assemble a chain of amino acids, which are the building blocks of proteins. The length of the resulting protein chain depends on whether the genetic code is read in a non-overlapping or overlapping manner.

Overlapping Sequence

An overlapping genetic code is one where a single nucleotide can be part of multiple codons. This concept is quite intriguing because it implies a more complex decoding process, with a single nucleotide contributing to different amino acids during protein synthesis.

For the maximally overlapping sequence, as mentioned in our exercise, after forming the first codon from the first three nucleotides, each subsequent nucleotide starts a new codon by combining with the previous two. This creates a densely packed set of instructions, much like a puzzle where each piece links to multiple others. However, in nature, the non-overlapping code is predominant, as overlapping codes can limit the evolution of genes due to constraints on possible mutations that won't disrupt the protein coding.