Question

If a DNA strand is TAC AAT AAA CGT GTC ATT, what are the codons of \(\mathrm{mRNA}\), the anticodons of \(\mathrm{tRNA}\), and the amino acid sequence? (pages \(495-97\) )

Short answer

The mRNA sequence is AUG UUA UUU GCA CAG UAA, with codons AUG - UUA - UUU - GCA - CAG - UAA. The corresponding tRNA anticodons are UAC - AAU - AAA - CGU - GUC - AUU. The amino acid sequence is Methionine - Leucine - Phenylalanine - Alanine - Glutamine, with a STOP codon at the end.

Step-by-step solution

Find the mRNA sequence

To find the mRNA sequence, write out the complementary base sequence, replacing Thymine (T) with Uracil (U). Remember to use the following base pairs A-U and C-G. DNA: TAC AAT AAA CGT GTC ATT mRNA: AUG UUA UUU GCA CAG UAA

Group mRNA sequence into codons

Group the mRNA sequence into units of three bases, called codons: mRNA Codons: AUG - UUA - UUU - GCA - CAG - UAA

Find the anticodons of tRNA

To find the anticodons of tRNA, write out the complementary base sequence of each mRNA codon: mRNA Codons: AUG - UUA - UUU - GCA - CAG - UAA tRNA Anticodons: UAC - AAU - AAA - CGU - GUC - AUU

Find the corresponding amino acid sequence

Translate each mRNA codon into an amino acid using the genetic code table. The amino acids for the given sequence are: mRNA Codons: AUG - UUA - UUU - GCA - CAG - UAA Amino Acids: Methionine - Leucine - Phenylalanine - Alanine - Glutamine - STOP Note that UAA is a stop codon, which indicates the end of the polypeptide chain. So the amino acid sequence is: Methionine - Leucine - Phenylalanine - Alanine - Glutamine

Key concepts

mRNA sequence

The mRNA sequence is a direct translation from a DNA template. DNA uses the bases Adenine (A), Cytosine (C), Guanine (G), and Thymine (T). However, mRNA replaces Thymine with Uracil (U). This is essential to remember as it plays a critical role in the base-pairing process. To transcribe the mRNA from DNA, match each DNA base with its RNA complement:

• A pairs with U
• T pairs with A
• C pairs with G
• G pairs with C

For example, if the original DNA sequence is TAC AAT AAA CGT GTC ATT, the mRNA sequence becomes AUG UUA UUU GCA CAG UAA. This sequence of nucleotides is then split into codons, each consisting of three nucleotides, e.g., AUG, UUA, etc.

tRNA anticodons

tRNA anticodons serve an essential function in translation. Each tRNA molecule has a tri-nucleotide sequence complementary to the mRNA codon. This complementarity ensures that the correct amino acids are added to the growing polypeptide chain. For every mRNA codon, the tRNA anticodon binds to it:

• mRNA AUG binds to tRNA UAC
• mRNA UUA binds to tRNA AAU
• mRNA UUU binds to tRNA AAA
• mRNA GCA binds to tRNA CGU
• mRNA CAG binds to tRNA GUC
• mRNA UAA binds to tRNA AUU

This precise base pairing is vital for translation accuracy, ensuring that the mRNA's instructions are followed precisely.

amino acid sequence

The amino acid sequence is determined during the translation of mRNA into proteins. As ribosomes move along the mRNA sequence, each codon is read and tRNA delivers the corresponding amino acid. This happens sequentially, adding to a growing polypeptide chain. For our mRNA series AUG - UUA - UUU - GCA - CAG - UAA:

• AUG translates to Methionine
• UUA translates to Leucine
• UUU translates to Phenylalanine
• GCA translates to Alanine
• CAG translates to Glutamine
• UAA is a stop codon, signalling the end of translation

Thus, the resulting amino acid chain is Methionine - Leucine - Phenylalanine - Alanine - Glutamine.

stop codon

Stop codons are crucial in the genetic translation process as they signal termination of protein synthesis. There are three stop codons: UAA, UAG, and UGA. In our sequence, UAA acts as the stop codon. When a ribosome encounters a stop codon:

• The translation process halts
• The completed polypeptide chain is released
• Protein synthesis machinery is disassembled

This termination is an essential step, ensuring that proteins are synthesized to the appropriate length and functionality, preventing errors that could occur from an overly extended protein chain.

genetic code table

The genetic code table is a reference for translating mRNA sequences into amino acids. It lists the correspondence between mRNA codons and their resultant amino acids. This code is universal among nearly all organisms, reflecting the evolutionary conservation of genetic translation processes. Using the genetic code table:

• Identify the mRNA codon (a series of three nucleotides)
• Match it to the corresponding amino acid as specified in the table
• Recognize special cases, like stop codons, which do not code for an amino acid but signal termination

By consulting the genetic code table, the conversion from mRNA to a coherent and functional sequence of amino acids becomes systematic and accurate.