Choose the correct sequence of different regions on a typical eukaryotic DNA segment when we go from the upstream to the downstream of a transcription start point.
A. Initiation site
B. CAAT box
C. Enhancer
D. TATA box Choose the correct answer from the options given below:
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Think of approaching a gene like landing a plane. The \textbf{Enhancer} is the distant airport beacon. The \textbf{CAAT box} is the outer marker. The \textbf{TATA box} is the runway threshold. The \textbf{Initiation site} is the touchdown point. The sequence is always from far to near: Enhancer \(\rightarrow\) Proximal Elements (CAAT) \(\rightarrow\) Core Promoter (TATA) \(\rightarrow\) Start Site.
Step 1: Understanding the Concept:
Eukaryotic genes have complex regulatory regions. The question asks for the typical arrangement of these elements relative to the transcription start point, moving from upstream (further away, in the 5' direction) to downstream (towards and past the start site). Step 2: Detailed Explanation:
Let's locate each element relative to the transcription start point (designated as +1):
C. Enhancer: These are distal control elements. They can be located thousands of base pairs upstream or even downstream of the gene they regulate. They are generally the farthest upstream element.
B. CAAT box: This is a proximal control element, typically located around -80 to -70 base pairs upstream of the start site.
D. TATA box: This is a core promoter element, located very close to the start site, typically at position -35 to -25 upstream.
A. Initiation site (INR): This is the specific point where transcription begins, designated as +1. It is the most downstream of all the listed elements.
Step 3: Final Answer:
Arranging these from most upstream (most negative position or farthest away) to most downstream (+1), the correct sequence is Enhancer \(\rightarrow\) CAAT box \(\rightarrow\) TATA box \(\rightarrow\) Initiation site. This corresponds to the order C, B, D, A.
