A portion of a DNA template strand has the base sequence
5′-...ACGCGATGCGTGATGTATAGAGCT...-3′
Which is the third amino acid added to the polypeptide chain?

Verified step by step guidance

1. Identify the direction of the DNA template strand. The given sequence is 5′ to 3′, but transcription occurs in the 3′ to 5′ direction. Therefore, the complementary mRNA strand will be synthesized in the 5′ to 3′ direction.
2. Write the complementary mRNA sequence by pairing the bases of the DNA template strand. Use the base-pairing rules: Adenine (A) pairs with Uracil (U), Cytosine (C) pairs with Guanine (G), Guanine (G) pairs with Cytosine (C), and Thymine (T) pairs with Adenine (A).
3. Divide the mRNA sequence into codons (groups of three nucleotides) starting from the 5′ end. Each codon corresponds to one amino acid.
4. Use the genetic code table to determine the amino acid for each codon. Locate the third codon in the sequence and identify the corresponding amino acid.
5. Understand that the polypeptide chain is synthesized starting from the N-terminus to the C-terminus. The third amino acid added to the chain corresponds to the amino acid encoded by the third codon in the mRNA sequence.

Key Concepts

DNA Transcription

DNA transcription is the process by which a segment of DNA is copied into RNA by the enzyme RNA polymerase. During transcription, the DNA template strand is read in the 3' to 5' direction, and a complementary RNA strand is synthesized in the 5' to 3' direction. This process is crucial for converting genetic information stored in DNA into messenger RNA (mRNA), which carries the code for protein synthesis.

Codons and Amino Acids

Codons are sequences of three nucleotides in mRNA that correspond to specific amino acids or stop signals during protein synthesis. Each codon is translated into an amino acid by the ribosome, which reads the mRNA sequence. Understanding the genetic code, which maps codons to their respective amino acids, is essential for determining the sequence of a polypeptide chain.

Translation

Translation is the process by which the sequence of codons in mRNA is decoded to synthesize a polypeptide chain. This occurs in the ribosome, where transfer RNA (tRNA) molecules bring the appropriate amino acids to the growing polypeptide based on the codon sequence. The order of amino acids in the polypeptide determines its structure and function, making translation a critical step in gene expression.