Problem 6 - AP Biology Premium Prep 2021

Question

Which of the following represents the maximum number of amino acids that could be incorporated into a polypeptide encoded by 21 nucleotides of messenger RNA? (A) 3 (B) 7 (C) 21 (D) 42

Step-by-Step Solution

Answer

The maximum number of amino acids that could be incorporated into a polypeptide encoded by 21 nucleotides of messenger RNA is 7. This is because each group of three nucleotides, called a codon, codes for one amino acid, and \( \frac{21 \text{ nucleotides}}{3 \text{ nucleotides/codon}} \) = 7 codons.

Step 1: Determine the number of codons in 21 nucleotides

Divide the total number of nucleotides (21) by the number of nucleotides in a codon (3) to find the number of codons present in 21 nucleotides. Number of codons = \( \frac{21 \text{ nucleotides}}{3 \text{ nucleotides/codon}} \)

Step 2: Calculate the number of amino acids encoded by the given nucleotides

Since each codon codes for a single amino acid, the number of codons can represent the maximum number of amino acids that could be incorporated into a polypeptide. Number of amino acids = Number of codons

Step 3: Find the maximum number of amino acids

Now, simply divide the given 21 nucleotides by 3 nucleotides/codon to find the maximum number of amino acids. Number of amino acids = \( \frac{21 \text{ nucleotides}}{3 \text{ nucleotides/codon}} \) = 7 amino acids Based on this calculation, the correct answer is: (B) 7

Key Concepts

Amino Acids

Amino acids are the building blocks of proteins, much like bricks are to a building. Each amino acid has a decisive role in shaping the protein's structure and determining its function. There are 20 different amino acids that combine in various sequences to form an incredibly wide variety of proteins. Each amino acid is distinguished by its unique side chain, known as an R group, which can affect the amino acid's properties such as polarity, charge, and hydrophobicity.

When discussing polypeptide synthesis, it's essential to consider that a string of amino acids forms a polypeptide chain through peptide bonds. This process occurs in cells through a series of complex biological mechanisms, translating genetic information into functional proteins. Understanding the nature of amino acids is pivotal as they not only contribute to the structure but also play a significant part in the biochemical functions within living organisms.

mRNA Codons

Messenger RNA (mRNA) is a type of RNA that conveys genetic information from DNA to the ribosome, where proteins are synthesized. This information is translated into a sequence of amino acids via codons. Each mRNA codon is comprised of three nucleotides, and each codon specifies a particular amino acid.

For example, the codon AUG codes for the amino acid methionine and also serves as the start signal for protein synthesis. The genetic code, which maps mRNA codons to amino acids, is nearly universal, exhibiting only slight variations across different organisms. The codons act as a triplet code, a three-nucleotide sequence, and there are 64 possible codons encoding for the 20 amino acids, with some redundancy (certain amino acids can be encoded by more than one codon) and three stop codons signaling the end of translation.

Protein Encoding

Protein encoding is a fundamental concept that explains how genetic information is used to create proteins, the workhorses of the cell. It starts with transcription, where a segment of DNA is copied into mRNA. The mRNA then undergoes translation, a process through which ribosomes read the sequence of mRNA codons and assemble amino acids into a polypeptide chain that will fold into a functional protein.

The genetic code on the mRNA is like the blueprint for constructing the protein with a specific sequence of amino acids. Protein encoding is highly efficient; for instance, in the given exercise, 21 nucleotides of mRNA, through the genetic code, have the potential to be translated into a polypeptide of 7 amino acids. Practically, protein synthesis is a highly regulated process and involves complex interaction among various cellular components, ensuring that proteins are synthesized accurately according to the genetic instructions.