From Genes to Proteins

DNA Structure & DNA Replication

This study guide covers the fundamental aspects of DNA structure and the process of DNA replication, essential topics in General Biology. Understanding these concepts is crucial for grasping how genetic information is stored, maintained, and transmitted in living organisms.

The Structure of DNA

Nucleotides: The Monomeric Units of DNA

DNA is a polymer composed of repeating units called nucleotides. Each nucleotide consists of three components:

• Deoxyribose sugar (a five-carbon pentose)

• Phosphate group

• Nitrogenous base (Thymine (T), Adenine (A), Cytosine (C), Guanine (G))

The nucleotides are linked together by phosphodiester bonds between the sugar of one nucleotide and the phosphate of the next, forming the sugar-phosphate backbone of DNA.

Example: The structure of a DNA nucleotide can be represented as:

• 5' end: Phosphate group

• Central: Deoxyribose sugar

• Attached: Nitrogenous base

Nucleotide Bases: Pyrimidines and Purines

The nitrogenous bases in DNA are classified into two groups:

• Pyrimidines: Single-ring structures (Cytosine (C), Thymine (T), and Uracil (U) in RNA)

• Purines: Double-ring structures (Adenine (A), Guanine (G))

Example: Adenine and Guanine are purines, while Cytosine and Thymine are pyrimidines.

Chargaff’s Rules About DNA

Base Composition and Ratios

Erwin Chargaff discovered that the proportion of bases in DNA varies between species but follows specific rules:

• The amount of Adenine (A) is equal to Thymine (T)

• The amount of Guanine (G) is equal to Cytosine (C)

This is summarized as:

• Ratio of A:T = 1

• Ratio of G:C = 1

These ratios are consistent across different organisms, as shown in the table below.

DNA: A Double Helix

Discovery and Structure

DNA is structured as a double helix, with two strands running in opposite directions (antiparallel). The backbone is formed by alternating sugar and phosphate groups, while the bases pair in the center:

• Adenine (A) pairs with Thymine (T) via two hydrogen bonds

• Guanine (G) pairs with Cytosine (C) via three hydrogen bonds

The double helix is stabilized by hydrogen bonding and hydrophobic interactions between bases.

Example: The width of the DNA double helix is approximately 2 nanometers (nm).

Rosalind Franklin and X-ray Diffraction

Rosalind Franklin was instrumental in elucidating the structure of DNA through her work with X-ray diffraction. Her images provided critical evidence for the helical nature of DNA.

• Franklin's X-ray diffraction images showed a regular, repeating pattern consistent with a helical structure.

• Her data was interpreted by James Watson and Francis Crick to build the double helix model.

Example: X-ray diffraction involves directing X-rays at a crystallized molecule and analyzing the pattern of diffracted rays to infer molecular structure.

Summary Table: DNA Base Pairing

Key Terms and Concepts

• Nucleotide: The basic building block of DNA and RNA.

• Pyrimidine: Single-ring nitrogenous base (C, T, U).

• Purine: Double-ring nitrogenous base (A, G).

• Double Helix: The spiral structure formed by two complementary strands of DNA.

• Chargaff’s Rules: The principle that A=T and G=C in DNA.

• X-ray Diffraction: Technique used to determine the structure of molecules.

Equations and Formulas

• Base pairing ratios:

• Width of DNA double helix:

Additional info:

• Uracil (U) is found in RNA, replacing Thymine (T).

• DNA replication and further details (e.g., enzymes, mechanisms) are covered in subsequent sections.