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1.6 Nucleic Acids

Keywords

English Term 中文翻译 Definition & Explanation
Nucleic Acid 核酸 A polymer (DNA or RNA) consisting of many nucleotide monomers; serves as a blueprint for proteins and cellular activities.
Nucleotide 核苷酸 The building block (monomer) of a nucleic acid, consisting of a five-carbon sugar, a nitrogenous base, and a phosphate group.
Nitrogenous Base 含氮碱基 An organic molecule with a nitrogen atom that has the chemical properties of a base; forms the "rungs" of the DNA ladder (A, T, C, G, U).
Antiparallel 反向平行 The opposite arrangement of the sugar-phosphate backbones in a DNA double helix (one strand runs 5' to 3', the other 3' to 5').
Double Helix 双螺旋 The form of native DNA, referring to its two adjacent antiparallel polynucleotide strands wound around an imaginary axis.

1. The Structure of a Nucleotide

In living systems, biological information is encoded in sequences of monomers called nucleotides. When connected, these monomers form the polymers known as nucleic acids (DNA and RNA).

Every single nucleotide, regardless of whether it is in DNA or RNA, is composed of three fundamental structural components: 1. A Five-Carbon Sugar: Either deoxyribose (in DNA) or ribose (in RNA). 2. A Phosphate Group: Provides a negative charge to the molecule. 3. A Nitrogenous Base: The "letters" of the genetic code. There are five main bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C), and Uracil (U).

AP Exam Exclusion Statement: Nucleotide Structures

Memorization of the specific molecular structures of individual nucleotides (e.g., drawing the exact atomic ring structure of Guanine vs. Adenine) is beyond the scope of the AP Exam. Focus on the three main components and how they connect!

2. Directionality and Nucleic Acid Synthesis

Nucleic acids are not just random clumps of nucleotides; they have a highly specific linear sequence and a clear directionality.

  • The 5' and 3' Ends: A polynucleotide strand has two distinct ends, defined by the carbon atoms of the 5-carbon sugar.
    • The 5' (five prime) end is the side where the phosphate group is attached.
    • The 3' (three prime) end is the side where a free hydroxyl (\(-\ce{OH}\)) group is located on the sugar.
  • Synthesis: During nucleic acid synthesis (such as DNA replication or RNA transcription), biological enzymes can only build in one direction. Nucleotides are ALWAYS added to the 3' end of the growing strand. This results in the formation of strong covalent bonds between the phosphate of the new nucleotide and the 3' sugar of the existing chain.
(Placeholder: Diagram highlighting the 5' carbon attached to the phosphate and the 3' carbon attached to the hydroxyl group, with an arrow showing a new nucleotide being added to the 3' end.)

3. DNA Structure and Base Pairing

DNA is structured as an antiparallel double helix.

  • Antiparallel: The two strands of nucleotides run side-by-side but in opposite directions (one strand runs 5' to 3', while the complementary strand runs 3' to 5').
  • Base Pairing: The two strands are held together in the middle by relatively weak hydrogen bonds between specific nitrogenous bases. This is governed by strict pairing rules:
    • Adenine (A) pairs with Thymine (T).
    • Cytosine (C) pairs with Guanine (G).
    • (Note: In RNA, Thymine is replaced by Uracil, so Adenine pairs with Uracil (U)).
(Placeholder: A 2D 'ladder' view of DNA showing the 5' and 3' ends flipped on opposite sides, with dotted lines representing the hydrogen bonds between A-T and C-G.)

4. Comparing DNA and RNA

While DNA and RNA are both nucleic acids involved in genetic information, they have three primary structural differences that dictate their different functions:

Feature DNA (Deoxyribonucleic Acid) RNA (Ribonucleic Acid)
Sugar Component Contains deoxyribose (lacks one oxygen atom). Contains ribose.
Nitrogenous Bases Contains Adenine, Guanine, Cytosine, and Thymine (T). Contains Adenine, Guanine, Cytosine, and Uracil (U).
Overall Structure Typically double-stranded (forms a double helix). Typically single-stranded.

Quiz

Campbell Biology Chapter 5 Practice Test: Large Biological Molecules

Click the link above to practice related multiple-choice questions (opens in a new tab).