DNA Structure and Function: Study Notes for General Chemistry

DNA Structure and Function

Overview

This section introduces the chemical and structural basis of DNA, the molecule responsible for genetic information in living organisms. Understanding DNA is fundamental to both biology and chemistry, as it involves molecular structure, bonding, and the chemical processes of replication.

Genes are Made of DNA

Historical Context and Experimental Evidence

• Genetic Material Debate: In the early 20th century, scientists debated whether DNA or protein was the genetic material.

• Proteins are composed of 20 different amino acids, while nucleic acids (DNA and RNA) are made from only 4 types of nucleotides.

• Despite the apparent simplicity of DNA, a series of experiments in the 1940s (notably by Avery, MacLeod, and McCarty) demonstrated that DNA, not protein, carries genetic information.

• Transformation Experiments: Bacterial transformation showed that DNA from a pathogenic strain could transfer disease-causing ability to a non-pathogenic strain, proving DNA's role as the genetic material.

Example: The classic experiment with Streptococcus pneumoniae showed that heat-killed pathogenic bacteria could "transform" harmless bacteria into disease-causing forms when DNA was transferred.

DNA Structure

Nucleic Acids and Nucleotides

• Nucleic acids are polymers called polynucleotides, made from repeating units called nucleotides.

• Each nucleotide consists of three components:

  • A phosphate group

  • A five-carbon sugar (deoxyribose in DNA)

  • A nitrogenous base (adenine, thymine, cytosine, or guanine)

• Nucleotides are linked by phosphodiester bonds between the phosphate of one nucleotide and the sugar of the next, forming a sugar-phosphate backbone.

Example: The four nitrogenous bases in DNA are adenine (A), thymine (T), cytosine (C), and guanine (G).

Double Helix Structure

• DNA consists of two polynucleotide strands that run in opposite directions (antiparallel), forming a double helix.

• The strands are held together by hydrogen bonds between complementary bases:

  • A pairs with T (2 hydrogen bonds)

  • G pairs with C (3 hydrogen bonds)

• The double helix is right-handed and twists around an imaginary axis.

• The 5' and 3' ends refer to the carbon positions in the sugar molecule, indicating the directionality of each strand.

Example: If one strand has the sequence 5'-ATCG-3', the complementary strand will be 3'-TAGC-5'.

Chargaff's Rules and Experimental Data

• Chargaff's Rules: Analytical chemistry showed that in DNA, the amount of adenine (A) equals thymine (T), and the amount of guanine (G) equals cytosine (C):

  • $[A] = [T]$

  • $[G] = [C]$

• X-ray diffraction experiments by Rosalind Franklin revealed the helical structure and the arrangement of bases and sugar-phosphate backbone.

Example: If a DNA sample contains 22% guanine, it must also contain 22% cytosine, and the remaining 56% is split equally between adenine and thymine (28% each).

DNA Measurements and Nomenclature

• DNA length is measured in base pairs (bp), where one base pair consists of two complementary nucleotides.

• Larger units include:

  • Kilobase (kb): 1,000 base pairs

  • Megabase (Mb): 1,000,000 base pairs

• The human genome contains approximately 3 billion base pairs.

DNA Synthesis

Overview of DNA Replication

• DNA replication is the process by which a cell copies its DNA before cell division.

• The two strands of the double helix separate, and each serves as a template for the synthesis of a new complementary strand.

• The result is two identical double-stranded DNA molecules, each with one old (parental) and one new (daughter) strand. This is called semiconservative replication.

Example: During cell division, each daughter cell receives an exact copy of the DNA.

DNA and Chromosomes

Organization of DNA in Cells

• In eukaryotic cells, DNA is packaged into structures called chromosomes.

• Each chromosome contains a single, long DNA molecule associated with proteins (histones) that help compact and organize the DNA.

• Genes are specific sequences of DNA that code for proteins or functional RNA molecules.

Example: Humans have 23 pairs of chromosomes, each containing thousands of genes.

Summary Table: Key Features of DNA

Feature	Description
Monomer	Nucleotide (phosphate, deoxyribose sugar, nitrogenous base)
Strand Directionality	5' to 3' and 3' to 5' (antiparallel)
Base Pairing	A-T (2 H-bonds), G-C (3 H-bonds)
Structure	Double helix
Replication	Semiconservative (each new DNA has one old and one new strand)
Measurement Units	Base pair (bp), kilobase (kb), megabase (Mb)

Additional info: These notes expand on the original slides and handwritten notes by providing definitions, examples, and context for key concepts in DNA chemistry, suitable for General Chemistry students.