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Ch. 10 - DNA Structure and Analysis
Chapter 10, Problem 30

Consider the drawing of a dinucleotide below. Is the arrow closest to the 5' or the 3' end?

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Identify the structure of a nucleotide, which consists of a phosphate group, a sugar (deoxyribose in DNA), and a nitrogenous base.
Understand that in a dinucleotide, two nucleotides are connected by a phosphodiester bond between the 3' carbon of one sugar and the 5' carbon of the next sugar.
Recognize that the 5' end of a nucleotide has a free phosphate group attached to the 5' carbon of the sugar.
Recognize that the 3' end of a nucleotide has a free hydroxyl group (-OH) attached to the 3' carbon of the sugar.
Examine the direction of the arrow in the drawing: if it points towards the free phosphate group, it is closest to the 5' end; if it points towards the free hydroxyl group, it is closest to the 3' end.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

DNA Structure

DNA is composed of nucleotides, which include a phosphate group, a sugar (deoxyribose), and a nitrogenous base. The structure of DNA is directional, with one end designated as the 5' end, where a phosphate group is attached to the fifth carbon of the sugar, and the other end as the 3' end, where a hydroxyl group is attached to the third carbon. Understanding this orientation is crucial for interpreting molecular diagrams and the directionality of DNA strands.
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Nucleotide Orientation

In a dinucleotide, two nucleotides are linked by a phosphodiester bond, which connects the 5' phosphate group of one nucleotide to the 3' hydroxyl group of another. This linkage creates a continuous sugar-phosphate backbone with distinct 5' and 3' ends. Recognizing the orientation of these ends is essential for understanding how nucleotides are arranged and how they interact during processes like replication and transcription.
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Directionality

Molecular Diagrams

Molecular diagrams, such as those depicting dinucleotides, often use arrows to indicate directionality or specific features. In the context of DNA, an arrow may point towards the 5' or 3' end, which helps in identifying the orientation of the molecule. Accurately interpreting these diagrams is vital for grasping the functional aspects of DNA, including how enzymes interact with the molecule during various biological processes.
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Branch Diagram