Question

The base sequences in mRNA that code for certain amino acids are Glu: \(\quad\) GAA, GAG Val: GUU, GUC, GUA, GUG Met: \(\quad\) AUG Trp: \(\quad\) UGG Phe: UUU, UUC Asp: \(\quad\) GAU, GAC These sequences are complementary to the sequences in DNA. a. Give the corresponding sequences in DNA for the amino acids listed above. b. Give a DNA sequence that would code for the peptide trp-glu-phe-met. c. How many different DNA sequences can code for the tetrapeptide in part \(\mathrm{b}\) ? d. What is the peptide that is produced from the DNA sequence \(\mathrm{T}-\mathrm{A}-\mathrm{C}-\mathrm{C}-\mathrm{T}-\mathrm{G}-\mathrm{A}-\mathrm{A}-\mathrm{G} ?\) e. What other DNA sequences would yield the same tripeptide as in part \(\mathrm{d}\) ?

Short answer

a. Glu: CTT, CTC; Val: CAA, CAG, CAT, CAC; Met: TAC; Trp: ACC; Phe: AAA, AAG; Asp: CTA, CTG b. ACCTTTAAATAC c. 4 sequences d. Met-Asp-Phe e. TACCTAAAAA, TACCTAAAG, TACCTGAAA, TACCTGAAG

Step-by-step solution

a. Corresponding DNA sequences for given amino acids

To find the corresponding DNA sequences for the listed amino acids, we need to remember that RNA bases A, C, G, and U are complementary to the DNA bases T, A, C, and G, respectively. Using these complementary bases, we can find the corresponding DNA sequences: - Glu: CTT, CTC - Val: CAA, CAG, CAT, CAC - Met: TAC - Trp: ACC - Phe: AAA, AAG - Asp: CTA, CTG

b. DNA sequence for the peptide trp-glu-phe-met

To find the DNA sequence that codes for the peptide trp-glu-phe-met, we need to find the DNA sequences for each of those amino acids and combine them in the given order: - Trp: ACC - Glu: CTT or CTC - Phe: AAA or AAG - Met: TAC So, one of the possible DNA sequences for the peptide trp-glu-phe-met would be: ACCTTTAAATAC

c. Number of different DNA sequences for the tetrapeptide

To find out how many different DNA sequences can code for the tetrapeptide, we need to count the number of possibilities for each amino acid and multiply them together: - Trp: 1 possibility (ACC) - Glu: 2 possibilities (CTT, CTC) - Phe: 2 possibilities (AAA, AAG) - Met: 1 possibility (TAC) So, the number of different DNA sequences for the tetrapeptide = 1 * 2 * 2 * 1 = 4 sequences.

d. Peptide from the given DNA sequence

Given DNA sequence: T-A-C-C-T-G-A-A-G To find the peptide produced from this DNA sequence, we need to divide the sequence into groups of three to represent the different amino acids: TAC - CTG - AAG Now, we need to find the corresponding amino acids for these DNA sequences: - TAC: Met - CTG: Asp - AAG: Phe So, the peptide produced from the given DNA sequence is: Met-Asp-Phe

e. Other DNA sequences yielding the same tripeptide

To find other DNA sequences that yield the same tripeptide as in part d (Met-Asp-Phe), we need to consider all possible DNA sequences for each amino acid in the peptide and combine them: - Met: TAC (1 possibility) - Asp: CTA or CTG (2 possibilities) - Phe: AAA or AAG (2 possibilities) Consider all these possibilities: 1. TACCTAAAAA 2. TACCTAAAG 3. TACCTGAAA 4. TACCTGAAG So, there are 4 different DNA sequences that yield the same tripeptide as the one given in part d.

Key concepts

Complementary Base Pairing

Understanding the concept of complementary base pairing is crucial when deciphering the connection between DNA sequences and the proteins they code for. In the double helical structure of DNA, bases pair up in a specific way: adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). However, in the world of RNA, uracil (U) takes the place of thymine and pairs with adenine.

When instructions from DNA are transcribed to messenger RNA (mRNA), complementary base pairing guides the process. For example, if a DNA sequence had the base adenine (A), during transcription, mRNA would form with a corresponding uracil (U). This pairing is fundamental to the transfer of genetic information, ensuring that the appropriate amino acids are assembled to form proteins.

Considering the exercise, we see that to determine the DNA sequences corresponding to mRNA codons, one must apply this base pairing rule. For instance, the mRNA codon GAA translates to a DNA sequence CTT — the 'G' in mRNA pairs with 'C' in DNA, and 'A' pairs with 'T'. This principle forms the backbone of genetic translation and is an essential topic for students to master.

mRNA Codons

The genetic information for the synthesis of proteins is encoded in mRNA in the form of codons. These codons are sequences of three nucleotides, and each codon corresponds to a specific amino acid or a stop signal during protein synthesis. The mRNA strand is read in the 5' to 3' direction, and the codons direct the ribosome's assembly of amino acids in the correct order to form a functional protein.

Reading Frames and Codons

Each grouping of three nucleotides in mRNA comprises a codon. Since the reading of codons is sequential, there is no overlap. For example, if an mRNA strand begins with the sequence AUGUGG, it would be read as two separate codons: AUG (methionine) and UGG (tryptophan), in that order.

The exercise provided illustrates this concept by listing specific amino acids, such as valine (Val) and their corresponding mRNA codons (GUU, GUC, GUA, GUG). Students should be able to recognize that each codon is unique to an amino acid and that understanding mRNA codons is integral to interpreting genetic information.

Genetic Code

The genetic code is the set of rules by which information encoded within genetic material is translated into proteins by living cells. This code defines how sequences of three nucleotides, called codons, correspond to specific amino acids or to signals for the start of the start or stop of protein synthesis. Remarkably, the genetic code is nearly universal, with almost all organisms using the same code.

There are 64 codons in the genetic code, with 61 coding for the 20 standard amino acids and 3 serving as stop signals. One of the most notable aspects of the genetic code is its redundancy; most amino acids are encoded by more than one codon. This feature, also known as degeneracy, offers a certain level of protection against mutations.

In reference to the problem in the exercise, understanding the genetic code is key to determining the peptide sequence produced from a given DNA sequence. Students must learn to transcribe the DNA sequence to its complementary mRNA, translate that to the appropriate amino acids using the genetic code, and finally, deduce which peptides or proteins are being coded for.