Question

A gene encodes a protein with 150 amino acids. There is one intron of 1000 base pairs (bp), a 5?-untranslated region of 100 bp, and a 3?- untranslated region of 200 bp. In the final mRNA, about how many bases lie between the start AUG codon and final termination codon? (A) 150 (B) 450 (C) 650 (D) 1750

Short answer

B) 450

Step-by-step solution

Understand the Coding Sequence

Each amino acid is encoded by a codon, which consists of 3 bases. Therefore, a protein with 150 amino acids will be encoded by a coding sequence of \[150 \times 3 = 450 \text{ bases}.\]

Identify Relevant Sequences in mRNA

In the mature mRNA, the intron (which is 1000 bases) and the untranslated regions are not part of the sequence between the start AUG codon and the final termination codon. Thus, the untranslated regions (100 bp at 5'-end and 200 bp at 3'-end) do not count towards the sequence length between the start and stop codons.

Calculate the Final Length

Since the intron and untranslated regions are excluded from the coding sequence, only the 450 bases of the coding sequence remain between the start AUG codon and the final termination codon.

Key concepts

Protein Synthesis

Protein synthesis is the process by which cells build proteins. This process involves two main stages: transcription and translation. During transcription, a segment of DNA is transcribed into mRNA (messenger RNA). Then, the mRNA is translated into a protein in the ribosome.

A crucial part of this process is the recognition and use of codons. Codons are sequences of three nucleotides in the mRNA that correspond to specific amino acids or a stop signal during protein synthesis. Each of the 20 amino acids is encoded by one or more of these codons.

To illustrate, in the given exercise, the gene encodes a protein with 150 amino acids. Each amino acid corresponds to a codon, where each codon is made up of three bases. Hence, the coding sequence for 150 amino acids would be 150 codons, equaling 450 bases in total.

Understanding how codons work and the role of mRNA in carrying this information from DNA to the ribosome is crucial in grasping the essence of protein synthesis.

Introns and Exons

In eukaryotic organisms, genes are made up of segments called introns and exons. Exons are the coding sequences that are expressed, whereas introns are non-coding sequences that need to be removed during RNA processing.

During the transcription of DNA to mRNA, both introns and exons are initially copied. But before the mRNA can be translated into a protein, the introns are spliced out in a process called RNA splicing, leaving only the exons in the mature mRNA. This mature mRNA is then used as a template for protein synthesis.

In the exercise given, the gene contains one intron of 1000 base pairs. However, during the production of the mature mRNA, this intron is removed. Therefore, the sequence containing the intron does not count towards the final number of bases between the start AUG codon and the termination codon.

• Introns: Non-coding sequences, spliced out.
• Exons: Coding sequences, remain in mRNA.

Untranslated Regions (UTRs)

Untranslated regions (UTRs) are sequences found on the mRNA that are not translated into protein. There are two main types of UTRs: the 5'-UTR and the 3'-UTR.

The 5'-UTR is located upstream of the start codon (AUG) and plays a role in the regulation of translation. The 3'-UTR is found downstream of the stop codon and is important for stability and localization of the mRNA. These regions can have various regulatory elements that influence the efficiency of translation and mRNA stability.

According to the exercise, the mRNA has a 5'-UTR of 100 base pairs and a 3'-UTR of 200 base pairs. These UTRs are not part of the coding sequence contributing to the final protein. Thus, they are not included when counting the bases between the start codon and the termination codon.

• 5'-UTR: Located before the start codon.
• 3'-UTR: Located after the stop codon.
• Regulate translation and mRNA stability.