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Chapter 9: Problem 14

Describe the various characteristics of the Watson-Crick double helix model for DNA.

Short Answer

Expert verified
Answer: The key characteristics of the Watson-Crick double helix model for DNA include its nucleotide components (adenine, thymine, guanine, and cytosine), complementary base pairing (A-T and G-C), hydrogen bonds holding the base pairs together, the antiparallel orientation of the DNA strands (5' to 3' and 3' to 5'), the right-handed twist of the helix (10 base pairs per turn), and the major and minor grooves that allow proteins to access and interact with the DNA molecule.

Step by step solution

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1. Components of DNA

DNA, or deoxyribonucleic acid, is a molecule that holds the genetic information for every living organism. The Watson-Crick double helix model of DNA consists of two long, linear strands made of smaller units called nucleotides. Each nucleotide contains a sugar molecule called deoxyribose, a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), or cytosine (C).
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2. Base pairing

In the Watson-Crick model, the nitrogenous bases of the nucleotides form pairs between the two DNA strands, creating a ladder-like structure. The bases pair up according to a specific pattern: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). This is called complementary base pairing and is due to hydrogen bonding.
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3. Hydrogen bonds

The complementary base pairs are held together by hydrogen bonds. Two hydrogen bonds form between adenine and thymine, while three hydrogen bonds form between guanine and cytosine. These hydrogen bonds provide stability to the DNA molecule and ensure that the two strands stay together.
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4. Antiparallel orientation of the DNA strands

The two DNA strands are antiparallel, which means they run in opposite directions. One strand is oriented in the 5' to 3' direction, while the other strand is oriented in the 3' to 5' direction. This orientation is important for the replication and transcription processes of DNA.
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5. Double helix structure

The two DNA strands wind around each other in a twisted manner, forming a double helix structure. This twisting is due to the sugar-phosphate backbone of DNA, which creates the structural framework for the molecule. The helix has a right-handed twist, with 10 base pairs per turn.
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6. Major and minor grooves

The double helix structure of DNA also contains major and minor grooves, which are the spaces between the two strands. The grooves are important for the binding of proteins involved in various cellular processes, such as DNA replication, transcription, and repair, as they provide access points for these proteins to interact with the DNA molecule. In summary, the characteristics of the Watson-Crick double helix model for DNA include its nucleotide components, complementary base pairing, hydrogen bonds, antiparallel orientation, twist of the helix, and major and minor grooves. These features help maintain the stability, functionality, and accessibility of DNA for various cellular processes.

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