Question

Which amino acid does the DNA sequence ATG code for?

Short answer

The DNA sequence ATG codes for methionine.

Step-by-step solution

Understand DNA to RNA Transcription

DNA sequences are transcribed into mRNA sequences. When transcribing, each DNA base is replaced by its complementary RNA base; adenine (A) becomes uracil (U), thymine (T) becomes adenine (A), cytosine (C) becomes guanine (G), and guanine (G) becomes cytosine (C). The DNA sequence ATG will therefore be transcribed to the RNA sequence UAC.

Translate mRNA to Amino Acid

mRNA sequences are translated into amino acids using the genetic code, where each three-base codon corresponds to a specific amino acid. The mRNA codon UAC translates to the amino acid methionine.

Key concepts

DNA Transcription

DNA transcription is a critical process in the flow of genetic information in cells. It is the first step in converting DNA instructions into usable molecules. During transcription, a specific segment of DNA is copied into messenger RNA (mRNA). This is done by the enzyme RNA polymerase, which binds to the DNA and unwinds the double helix. It reads the DNA template strand and synthesizes a single strand of mRNA. The key here is the complementary base pairing rule:

• Adenine (A) in DNA pairs with Uracil (U) in RNA.
• Thymine (T) in DNA pairs with Adenine (A) in RNA.
• Cytosine (C) pairs with Guanine (G), and vice versa.

For example, the DNA sequence ATG would be transcribed into the mRNA sequence UAC. It is important to note that transcription occurs in the nucleus of the cell. Once completed, the mRNA strand is modified and transported out of the nucleus to act as a template for protein synthesis.

mRNA Translation

Following transcription, the mRNA must be translated to synthesize proteins, a process that occurs in the ribosomes located in the cytoplasm. Translation converts the nucleotide sequence of mRNA into a specific sequence of amino acids, forming a polypeptide chain. This process is made possible by the genetic code, which is nearly universal in all organisms.
During translation, ribosomes read the mRNA sequence three nucleotides at a time, which are called codons. Each codon specifies a certain amino acid. For instance, the mRNA codon UAC codes for the amino acid methionine. This process involves two key players:

• Transfer RNA (tRNA), which brings the correct amino acid to the ribosome based on the codon it reads.
• Ribosomal RNA (rRNA), which helps the ribosome bind to mRNA and assemble the amino acids in the correct order.

Through a series of elongation steps, the ribosome connects these amino acids with peptide bonds, growing the polypeptide chain until a stop codon is reached.

Amino Acid Codon

The connection between a sequence of three mRNA bases, known as a codon, and a specific amino acid is the essence of the genetic code. Each codon corresponds to one of the 20 amino acids used in the synthesis of proteins. Because there are 64 possible codon combinations but only 20 amino acids, the genetic code is considered redundant or degenerate. This means that most amino acids are specified by more than one codon.

• For example, the codons AUG, which is derived from the DNA sequence ATG, all code for methionine.
• Some codons have special functions, such as start codons, which signal the start of translation, and stop codons, which signal its end.

The genetic code is crucial for the accurate translation of genetic information from mRNA to amino acids, ensuring that proteins are built correctly based on the DNA blueprint. Understanding this process is fundamental to fields such as genetics, biotechnology, and molecular biology.