Chapter 10: The Structure and Function of DNA

Chapter 10: The Structure and Function of DNA

Introduction: Universal Genetic Code

All life on Earth shares a universal genetic code, meaning the DNA of one organism can be used to genetically modify another, such as bacteria. This universality underpins genetic engineering and biotechnology.

• Genetic Code: The set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins by living cells.

• Application: Genes from one species can be expressed in another, enabling advances in medicine, agriculture, and research.

Biology and Society: The Global Threat of Zika Virus

The Zika virus outbreak in Brazil (2015) highlighted the importance of molecular biology in understanding and combating viral diseases.

• Symptoms: Neurological problems, slow growth, feeding difficulties, and joint/muscle issues in affected babies.

• Transmission: By a specific mosquito species; not dangerous to most healthy adults.

• Structure: Like all viruses, Zika consists of nucleic acid (RNA or DNA) within a protein coat.

• Combatting Viruses: Requires molecular-level understanding; no vaccine or cure for Zika as of 2016.

DNA: Structure and Replication

DNA was identified as a cellular chemical in the late 1800s, but its role as hereditary material was confirmed in the 1950s. Its unique three-dimensional structure enables genetic information storage, copying, and inheritance.

• Key Properties: Storage, replication, and transmission of genetic information.

DNA and RNA Structure

DNA and RNA are nucleic acids composed of long chains of nucleotides. These nucleotides form polymers called polynucleotides.

• Nucleotide: Monomer unit consisting of a nitrogenous base, a sugar, and a phosphate group.

• Polynucleotide: Long chain of nucleotides; sequence can vary.

• Sugar-Phosphate Backbone: Covalent bonds between sugar and phosphate groups create a repeating backbone.

Nucleotide Components

• 1. Nitrogenous base

• 2. Sugar (deoxyribose in DNA, ribose in RNA)

• 3. Phosphate group

Deoxyribose is the sugar in DNA, missing one oxygen atom compared to ribose in RNA. DNA stands for deoxyribonucleic acid, referencing its location in eukaryotic nuclei.

Chemical Structure of DNA Polynucleotide

• Single-ring bases: Thymine (T), Cytosine (C)

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

• RNA: Uses uracil (U) instead of thymine and contains ribose sugar.

Table: DNA vs. RNA Components

Component	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Structure	Double helix	Single strand

Watson and Crick’s Discovery of the Double Helix

James Watson and Francis Crick, aided by Rosalind Franklin’s X-ray images, determined that DNA is a double helix—two polynucleotide strands twisted into a spiral.

• Double Helix: Uniform diameter, sugar-phosphate backbones on the outside, base pairs as rungs.

• Base Pairing: A pairs with T, G pairs with C via hydrogen bonds.

• Structure-Function Relationship: The arrangement of DNA’s parts enables its role in heredity and replication.

DNA Replication

DNA replication is essential for cell division, ensuring each new cell receives a complete set of genetic information.

• Template Mechanism: Each strand serves as a template for a new complementary strand.

• Base-Pairing Rules: $A{ pairs with }T$, $G ext{ pairs with } C$

• DNA Polymerases: Enzymes that synthesize new DNA strands and repair damaged DNA.

• Origins of Replication: Specific sites where replication begins, forming replication bubbles.

Diagram: DNA Replication

• Parental DNA strands separate, each serving as a template for daughter strands.

• Replication proceeds in both directions from origins, forming bubbles.

From DNA to RNA to Protein

DNA contains the genetic instructions (genotype) that determine an organism’s physical traits (phenotype) through protein synthesis.

• Genotype: Sequence of nucleotide bases in DNA.

• Phenotype: Physical traits resulting from protein actions.

How an Organism’s Genotype Determines Its Phenotype

Genes specify proteins via two main processes:

• Transcription: DNA information is transferred to RNA.

• Translation: RNA information is used to build polypeptides (proteins).

The Flow of Genetic Information in a Eukaryotic Cell

• Transcription occurs in the nucleus, producing RNA.

• Translation occurs in the cytoplasm, producing proteins.

From Nucleotides to Amino Acids: An Overview

The genetic code is written as a linear sequence of nucleotide bases. Genes are transcribed into RNA, which is then translated into proteins.

• Codons: Three-base words in DNA and RNA that specify amino acids.

• Triplet Code: Each codon corresponds to one amino acid.

Table: Codon Translation

DNA Codon	RNA Codon	Amino Acid
ATG	AUG	Methionine (Start)
TAA	UAA	Stop
GGC	GGC	Glycine

The Genetic Code

The genetic code is nearly universal, shared by all organisms. Of 64 codons, 61 code for amino acids and 3 are stop codons.

• Universality: Allows genetic engineering across species.

• Applications: Production of proteins in bacteria, gene therapy, and biotechnology.

Example: Coding for a Polypeptide

• To code for a polypeptide of 100 amino acids, 300 nucleotides are required (since each amino acid is specified by a triplet codon).

• Equation: $100{ amino acids}\cdot3{ nucleotides/amino acid}=300{ nucleotides}$

Table: Dictionary of the Genetic Code

RNA Codon	Amino Acid
AUG	Methionine (Start)
UUU	Phenylalanine
UAA	Stop
GCU	Alanine

Summary

• DNA’s structure enables its function in heredity and protein synthesis.

• The genetic code is universal, allowing for genetic engineering.

• Understanding DNA, RNA, and protein synthesis is essential for modern biology and medicine.