Question 1

What are the four types of nitrogen-containing bases found in DNA?

Accepted Answer

DNA contains four nitrogen-containing bases: adenine (A), cytosine (C), guanine (G), and thymine (T). Each base attaches to a deoxyribose sugar as part of a nucleotide subunit. These bases pair specifically with each other—C pairs with G and A pairs with T—through different numbers of hydrogen bonds, which is fundamental to DNA's double helix structure.

Question 2

How are nucleotides linked together in a DNA strand?

Accepted Answer

Nucleotides are linked by phosphodiester bonds that covalently connect the third carbon atom of one sugar to the fifth carbon atom of the next sugar molecule. This bonding pattern creates the sugar-phosphate backbone of each DNA strand. The backbone provides structural stability while bases extend inward to form base pairs between the two antiparallel strands.

Question 3

Why do adenine and thymine pair together in DNA?

Accepted Answer

Adenine and thymine pair together because they form a specific number of hydrogen bonds between them. Similarly, cytosine and guanine form a different number of hydrogen bonds. These chemical differences in hydrogen bonding ensure that base pairing is specific and consistent, which is critical for accurate DNA replication and maintaining the integrity of genetic information across generations.

Question 4

What is the significance of the major and minor grooves in DNA?

Accepted Answer

The major groove occurs where the sugar-phosphate backbones are relatively far apart, providing accessible space for DNA-binding proteins such as transcription factors to interact with the DNA. The minor groove, where backbones are close together, accommodates relatively few proteins. These grooves are essential for regulating gene expression and other cellular processes that depend on protein-DNA interactions.

Question 5

How many base pairs are present in each complete turn of the DNA double helix?

Accepted Answer

Each complete turn of the DNA double helix contains ten base pairs. This regular spacing results from the antiparallel arrangement of the two polynucleotide chains winding around each other. The consistent geometry of the double helix, with its predictable base pair spacing, allows the structure to efficiently pack genetic information while remaining accessible to cellular machinery.

Question 6

What role did Chargaff's Rule play in determining DNA structure?

Accepted Answer

Chargaff's Rule, discovered in the late 1940s, established that the amount of adenine in DNA equals thymine, and guanine equals cytosine. This pattern provided crucial evidence for understanding base pairing and was instrumental in enabling Watson and Crick to construct accurate physical models of DNA's three-dimensional structure in the early 1950s.

Question 7

How does DNA function as genetic material across different organisms?

Accepted Answer

DNA is the genetic material responsible for passing traits from generation to generation in all organisms and most viruses. Its double helix structure, with specific base pairing and antiparallel strands, ensures stable storage and accurate replication of genetic information. This universal role makes DNA fundamental to understanding life across the tree of life bacteria archaea and eukaryotes.

DNA ヘリックス（二重らせん）

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