DNA Structure and Function: Study Notes for General Biology

DNA Structure and Function

Introduction

Deoxyribonucleic acid (DNA) is the hereditary material in almost all living organisms. Its structure enables it to store genetic information, replicate accurately, and undergo mutation, which are essential for life and evolution.

Functions of DNA

• Information Storage: DNA stores genetic information in the sequence of its nitrogenous bases. This information is used to build and maintain an organism.

• Replication: DNA can make exact copies of itself, ensuring genetic continuity during cell division.

• Mutation: DNA can change its sequence through mutation, providing genetic variation necessary for evolution.

• Example: Changing a single base in a DNA sequence (e.g., 'yam' to 'ham') can alter the genetic message, demonstrating how mutations can affect information.

DNA Structure

The Components of DNA

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

• 5-carbon sugar: Deoxyribose

• Phosphate group: Provides structural support and links nucleotides together.

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

Nitrogenous bases are classified as:

• Purines: Adenine and Guanine

• Pyrimidines: Cytosine and Thymine

The Structure of a Nucleotide

• Each nucleotide contains one deoxyribose sugar, one phosphate group, and one nitrogenous base.

• Nucleotides are joined together by covalent bonds between the sugar of one nucleotide and the phosphate of the next, forming a strong sugar-phosphate backbone.

• Formula: The general structure of a nucleotide can be represented as: $\text{Nucleotide} = \text{Deoxyribose} + \text{Phosphate} + \text{Base}$

The Double Helix

DNA consists of two strands that wind around each other to form a double helix. The strands are held together by hydrogen bonds between complementary nitrogenous bases.

• Base-pairing rules: A pairs with T (2 hydrogen bonds), C pairs with G (3 hydrogen bonds).

• Hydrogen bonds are weak compared to covalent bonds, allowing the strands to separate during replication.

• The two strands run in opposite directions (antiparallel).

Base-Pairing Rules and GC Content

The base-pairing rules ensure that the DNA molecule maintains a uniform width and allows for accurate replication.

• Purine always pairs with pyrimidine: A-T and C-G.

• GC Content: The percentage of guanine and cytosine bases in DNA. In mammals, GC content is typically about 40%.

• Example Calculation: If a mammal's DNA is 20% C, then G is also 20%. The remaining 60% is split equally between A and T (30% each).

• Organisms living in high-temperature environments (e.g., Thermophilus aquaticus) often have higher GC content, which stabilizes the DNA helix.

How Structure Allows Function

The unique structure of DNA enables its three main functions:

• Information Storage: The sequence of bases encodes genetic instructions. Because bases are not involved in linking nucleotides, any sequence is possible, allowing for vast information storage.

• Replication: Complementary base pairing allows each strand to serve as a template for creating a new strand. DNA replication is semi-conservative, meaning each new DNA molecule contains one old and one new strand.

• Mutation: Errors during replication (e.g., incorrect base addition by DNA polymerase) can result in mutations. Most errors are corrected by proofreading, but some persist, providing genetic variation.

Formula for Semi-Conservative Replication: $\text{Parent DNA} \rightarrow \text{Daughter DNA}_1 + \text{Daughter DNA}_2$

Importance of Mutation

• Mutations are rare (about 1 in 1 billion bases), but essential for evolution.

• If DNA polymerase never made mistakes, there would be no genetic variation, no natural selection, and no evolution.

Summary Table: DNA Structure Components

Component	Description	Role in DNA
Deoxyribose Sugar	5-carbon sugar	Forms backbone, links to phosphate and base
Phosphate Group	Phosphoric acid	Links sugars, forms backbone
Nitrogenous Base	A, T, C, G	Stores genetic information, base pairing

Additional info:

• Recitation and project assignments (e.g., disease spread, sickle cell disease) are mentioned but not detailed in the provided content.

• For further study, refer to Campbell Biology, Sections 16.1 and 16.2 (pages 320-328).