Building Block of DNA: The basic unit of DNA is a nucleotide.
Molecular Structure:
Composed of three parts:
Sugar:
Contains five carbon atoms.
Can be ribose or deoxyribose (lacks a hydroxyl group on 2' carbon).
Carbons are numbered 1' to 5'.
Phosphate Group:
Attached to the 5' carbon of the sugar.
Nitrogen-Containing Base:
Five different bases exist, but only four are used in DNA.
Types of Bases:
Purines: Adenine (A), Guanine (G)
Pyrimidines: Thymine (T), Cytosine (C), Uracil (U) (Note: Uracil is found in RNA, not DNA).
Functionality: Nucleotides also serve as energy carriers (e.g., ATP).
Nucleic Acids
Formation: Nucleic acids are polymers formed by nucleotides through condensation reactions (dehydration synthesis).
Bonding:
Phosphate of one nucleotide bonds to the -OH of another's 3' carbon, creating a phosphodiester bond.
Directionality:
Nucleic acid chains are asymmetrical with distinct ends:
5' end (free phosphate)
3' end (free hydroxyl group)
DNA Structure
Polymer Structure:
DNA is a polymer made exclusively of deoxyribose nucleotides.
Strands: A single chain of nucleotides.
Backbone consists of sugars and phosphates held together by covalent bonds.
Double-Stranded: Chromosomes consist of two strands of DNA that are complementary and anti-parallel.
Base Pairing:
A pairs with T (2 Hydrogen bonds)
G pairs with C (3 Hydrogen bonds)
This complementarity allows one strand's information to dictate the other strand.
Twisted Structure: DNA forms a double helix structure with alternating major and minor grooves.
Major groove: larger gap.
Minor groove: smaller gap.
Functions of DNA
Genetic Information Storage:
DNA contains the information necessary to synthesize RNA and proteins, organized into sequences known as genes.
Genes are segments of DNA that code for specific proteins.
Gene Expression: The process by which information from genes is used to synthesize RNA and proteins.
Chromosomal Organization:
Human cells contain 23 pairs of chromosomes, totaling 46 per cell, comprising about 3.2 billion base pairs.
Approximately 20% of this DNA is coding DNA (genes), while 80% is non-coding DNA with various functions.
DNA Replication
Semi-Conservative Process:
Replication of DNA results in two double-stranded molecules, each containing one original and one new strand.
Replication Basics:
Occurs during the S phase of interphase before cell division.
Involves unwinding the DNA strands and synthesizing new complementary strands using original strands as templates.
Initiation of Replication:
Begins at replication origins where replication bubbles form.
Bidirectional replication occurs through forks originating from the bubbles until the entire chromosome is copied.
DNA Polymerase Functionality
Synthesis:
DNA polymerase synthesizes DNA by catalyzing nucleotide incorporation onto a growing strand.
It also proofreads newly synthesized DNA to correct mismatched bases, ensuring fidelity in DNA replication.
Proofreading Mechanism: If an incorrect base is added, polymerase identifies the error, halts synthesis, and uses its exonuclease activity to remove the wrong base before continuing synthesis.