Question

Where is the start codon located? a. at the very start ( \(5^{\prime}\) end) of the mRNA b. in the DNA just upstream of where transcription starts c. at the downstream end of the \(5^{\prime}\) untranslated region (UTR) d. at the upstream end of the \(3^{\prime}\) untranslated region (UTR)

Short answer

c. at the downstream end of the \(5^{\prime}\) untranslated region (UTR).

Step-by-step solution

Understanding the Gene Structure

A gene can be roughly divided into three main regions: the \(5^{\prime}\) untranslated region (UTR), the coding region, and the \(3^{\prime}\) untranslated region (UTR). Transcription starts at the beginning of the \(5^{\prime}\) UTR and continues through the coding region until it reaches the end of the \(3^{\prime}\) UTR. The start codon, which signifies the beginning of the translation process, is located within the coding region, right after the \(5^{\prime}\) UTR. Now let's examine each option to identify the correct one:

Option a

Option a states that the start codon is at the very start (\(5^{\prime}\) end) of the mRNA. This option is incorrect because the start codon is actually found within the coding region, right after the \(5^{\prime}\) UTR.

Option b

Option b says the start codon is in the DNA just upstream of where transcription starts. This is not correct, as the start codon is found within the coding region of the mRNA, not in the DNA directly.

Option c

Option c states that the start codon is at the downstream end of the \(5^{\prime}\) untranslated region (UTR). This option is correct because the start codon is indeed found within the coding region, right after the \(5^{\prime}\) UTR.

Option d

According to option d, the start codon is at the upstream end of the \(3^{\prime}\) untranslated region (UTR). This option is incorrect because the start codon is located within the coding region, not in the \(3^{\prime}\) UTR. Based on the analysis above, the correct answer is: c. at the downstream end of the \(5^{\prime}\) untranslated region (UTR).

Key concepts

Start Codon

The start codon is a crucial element in the genetic coding process. It's the signal that kicks off the translation of mRNA into a protein. The most common start codon is the sequence AUG. This specific sequence is essential as it codes for methionine, an amino acid that is often the first building block in the synthesis of a new protein.

In simpler terms, think of the start codon as the "go" signal for building proteins. Without it, the ribosome would not know where to begin translating the mRNA sequence into a chain of amino acids.
Its position is typically right after the 5' untranslated region (UTR) of the mRNA transcript, marking the start of the coding region.

For students, remembering where the start codon is located—at the downstream end of the 5' UTR—can help in understanding how gene translation starts.

5' Untranslated Region (UTR)

The 5' untranslated region (UTR) is a section of the mRNA that lies upstream of the coding sequence. It is a non-coding region, meaning it does not encode for protein but plays vital roles in the regulation of gene expression.

Some of the essential functions of the 5' UTR include:

• Stabilizing the mRNA molecule, making it less susceptible to degradation.
• Helping the ribosome attach to the mRNA for translation to occur.
• Regulating the frequency and efficiency of translation initiation.

The 5' UTR varies considerably in length between different mRNAs and can contain regulatory elements that influence how quickly or slowly the coding region will be translated. Understanding the role of the 5' UTR is vital for comprehending how genetic information is regulated and not all parts of mRNA are involved in direct protein synthesis.

Translation Process

The translation process is the method by which mRNA is decoded to produce a specific protein. It occurs in the ribosome, the cell's protein factory, in a sequence of steps that ensure the correct amino acids are linked in the proper order.

Here's a simplified overview of the translation process:

• Initiation: The ribosome binds to the mRNA at the start codon (AUG), setting the stage for translation.
• Elongation: tRNA molecules bring amino acids to the ribosome, matching them to the codons on the mRNA. These amino acids are then joined together to form a growing protein chain.
• Termination: When the ribosome encounters a stop codon, the translation ends, and the newly formed protein is released for folding and modification.

The translation process is essential for life, as proteins perform numerous critical functions in a cell including structural roles, enzymatic activities, and cell signaling. Understanding translation helps clarify how genes dictate cellular behavior through creating functional proteins.