Question

A part of an mRNA molecule with the sequence \(5^{\prime}\) -UGC GCA-3 is being translated by a ribosome. The following activated tRNA molecules are available. tRNA Anticodon \(3^{\prime}-\mathrm{GGC}-5^{\prime}\) \(3^{\prime}-\mathrm{CGU}-5^{\prime}\) \(3^{\prime}-\mathrm{UGC}-5^{\prime}\) \(3^{\prime}-\mathrm{CCG}-5^{\prime}\) \(3^{\prime}-\mathrm{ACG}-5^{\prime}\) \(3^{\prime}-\mathrm{CGG}-5^{\prime}\) Amino Acid Proline Alanine 'Treonine Glycine Cysteine Alanine Which two of them can bind correctly to the mRNA so that a dipeptide can form? a. cysteine-alanine b. proline-cysteine c. glycine-cysteine d. alanine-alanine e. threonine-glycine

Short answer

The two tRNAs that can bind correctly to the mRNA to form a dipeptide are those with anticodons for Cysteine and Alanine, leading to option (a) cysteine-alanine.

Step-by-step solution

Identify Corresponding Anticodons

To find which tRNA molecules can bind to the mRNA sequence, match the codons on the mRNA to the anticodons on the tRNA. Remember, RNA pairs are: Adenine (A) with Uracil (U), and Guanine (G) with Cytosine (C).

Match the First Codon of mRNA with tRNA

The first codon on the mRNA is 'UGC'. Look for the tRNA with the complementary anticodon 'ACG'. We find that 'ACG' corresponds to the amino acid Cysteine.

Match the Second Codon of mRNA with tRNA

The second codon on the mRNA is 'GCA'. Look for the tRNA with the complementary anticodon 'CGU'. We find that 'CGU' corresponds to the amino acid Alanine.

Determine the Dipeptide

Since the tRNA that matches the first codon 'UGC' carries Cysteine and the tRNA that matches the second codon 'GCA' carries Alanine, the dipeptide formed is Cysteine-Alanine.

Key concepts

Anticodon-Codon Pairing

The genetic code is a set of rules by which information encoded in genetic material (DNA or mRNA sequences) is translated into proteins by living cells. Anticodon-codon pairing is a crucial aspect of this translation process. It involves the specific matching of nucleotide triplets, known as codons on mRNA, with complimentary triplets, called anticodons on tRNA molecules.

The pairing follows the rules of complementary base-pairing: adenine (A) pairs with uracil (U) in RNA, and cytosine (C) pairs with guanine (G). For example, if the codon on the mRNA is UGC, the anticodon must be ACG to correctly pair with it. This precise matching is critical because it ensures that the correct amino acids are added to the growing polypeptide chain, thus forming the protein indicated by the gene.

The anticodons on tRNA are not randomly assigned but are specifically designed to ensure accurate protein synthesis. In the exercise, finding the correct anticodon that corresponds to each codon on the mRNA was the first step, allowing us to predict which tRNA molecule would bind and bring the correct amino acid for the growing chain.

Translation Process

The translation process is the synthesis of proteins in the cytoplasm of a cell. This complex process involves various molecular components, including mRNA, ribosomes, tRNA, and numerous enzymes. During translation, an mRNA molecule is 'read' by a ribosome in the 5' to 3' direction, and the information it contains is used to synthesize a polypeptide chain with a specific sequence of amino acids.

To elaborate, the ribosome facilitates the binding of complementary tRNA molecules which carry specific amino acids to the mRNA codons. A peptide bond is formed between the amino acids as they are aligned by the tRNA, creating a chain that forms the primary structure of a protein. The ribosome moves along the mRNA, catalyzing the addition of amino acids provided by tRNA molecules until it reaches a stop codon, signaling the end of protein synthesis. The resulting polypeptide chain then undergoes folding and possible post-translational modifications to become a functioning protein.

In the given exercise, the translation process begins with identifying the correct tRNA molecules that can bind to the mRNA strand, based on anticodon-codon pairing.

Dipeptide Formation

Dipeptides are the simplest peptides, formed when two amino acids are linked together by a single peptide bond. This bond is a covalent chemical bond that forms when the amine group of one amino acid reacts with the carboxyl group of another. During translation, dipeptide formation is a part of the wider process of protein synthesis, where amino acids are continuously added to a growing polypeptide chain.

Once the correct tRNA's pair with their corresponding mRNA codons at the ribosome, their carried amino acids are positioned close together, and the ribosome catalyzes the formation of a peptide bond between them. This reaction not only forms a dipeptide but also moves the nascent polypeptide chain from the tRNA at the P site of the ribosome to the tRNA at the A site, allowing the next tRNA-amino acid complex to bind to the next codon of the mRNA.

In the context of the exercise, by identifying the tRNA molecules with anticodons complementary to two consecutive mRNA codons and understanding which amino acids they would add to the chain, it was determined that a dipeptide consisting of cysteine followed by alanine would be formed.