DNA and Chromosome Structure: Organization, Packaging, and Genome Complexity

DNA Structure and Forms

Essential Characteristics of Hereditary Material

Hereditary material must fulfill several criteria to serve as the basis for genetic inheritance and evolution:

• Localization: Found in the nucleus and as a component of chromosomes.

• Stability: Maintains a stable form within cells.

• Complexity: Contains sufficient information for structure, function, development, and reproduction.

• Replication: Able to accurately replicate so daughter cells inherit identical information.

• Mutability: Undergoes low rates of mutation, providing genetic variation for evolution.

Historical Discovery of DNA

DNA was first isolated in the 1860s by Friedrich Miescher from nuclei of white blood cells and salmon sperm. Early 20th-century experiments localized genes to chromosomes, and subsequent work identified DNA (not RNA) as the hereditary material.

• Key contributors: Sutton, Boveri, Fleming, Avery, MacCleod, McCarty, Watson, Crick, Franklin, Wilkins.

Nucleotides: The Building Blocks of DNA

Nucleotide Structure

Nucleotides are the monomers of nucleic acids, forming the primary structure of DNA and RNA. Each nucleotide consists of three major parts:

• Phosphate group

• Pentose sugar: D-deoxyribose (DNA) or D-ribose (RNA)

• Nitrogenous base: Purines (adenine, guanine) or Pyrimidines (cytosine, thymine, uracil)

Key differences: DNA contains deoxyribose and thymine; RNA contains ribose and uracil.

The sugar carbons are numbered (1' to 5'), with the 1' carbon attached to the base, 2' distinguishing DNA from RNA, and 3' being the site for new nucleotide attachment.

Complementary Base Pairing

Base pairing is fundamental to the double helix structure:

• Adenine (A) pairs with Thymine (T) in DNA (2 hydrogen bonds).

• Guanine (G) pairs with Cytosine (C) (3 hydrogen bonds).

• In RNA, Uracil (U) replaces thymine.

Single Strand of DNA

DNA strands have directionality based on the sugar's carbon atoms:

• 5' end: Terminates with a phosphate group.

• 3' end: Terminates with a hydroxyl (-OH) group.

• New nucleotides are always added to the 3' end during synthesis.

• Strands are connected by phosphodiester linkages.

DNA Double Helix and Higher-Order Structure

Features of the DNA Double Helix

The most common form of DNA is a right-handed double helix:

• Each complete turn contains 10.5 base pairs (bp), spanning 3.4 nm.

• Diameter is consistently 2 nm.

• Strands are antiparallel (5' to 3' and 3' to 5').

• Major and minor grooves are exposed to water and serve as protein interaction sites.

Forces stabilizing DNA: Hydrogen bonds, base-stacking/pi-stacking, hydrophobic and van der Waals interactions.

Alternative Forms of DNA

DNA can adopt several helical forms:

• B-DNA: Most common, right-handed, 10.4 bp per turn, 2 nm diameter.

• A-DNA: Right-handed, 11 bp per turn, wider diameter, occasional in cells.

• Z-DNA: Left-handed, zig-zag appearance, found near transcription start sites.

• H-DNA: Triple-stranded DNA, occurs in short segments, may regulate gene expression.

Genomic Complexity and Packaging

Genome Definition and Processes

A genome is the complete set of genetic material in an organism. Four key processes are associated with genomes:

• Synthesis: RNA and protein production, regulated by proteins.

• Replication: DNA duplication, stabilized by proteins.

• Segregation: Chromosome separation during cell division, aided by condensation.

• Compaction: DNA packaging to fit within cells or nuclei.

Comparison of Genome Complexity and Packaging

Genome size and packaging vary across biological domains:

• Viruses: 5–200 Kbp, DNA or RNA, single/double-stranded, linear/circular.

• Bacteria: 1–10 Mbp, circular chromosome, looped and supercoiled.

• Archaea: 500 Kbp–6 Mbp, circular chromosome, coiled with histones.

• Eukaryotes: 5 Mbp–>100 Gbp, linear chromosomes, coiled with histones, supercoiled.

Virus Structure and Genomes

Virus Structure

Viruses range from 20 to 300 nm in diameter and consist of:

• Internal nucleic acid core (DNA or RNA)

• Protein coat (capsid)

• Some have a lipid envelope derived from host cells

• Surface spikes for receptor binding

Organization of Viral Genomes

Viral genomes are highly diverse:

• Double or single-stranded RNA or DNA

• Circular or linear, single or multiple molecules

• Viruses are obligate intracellular parasites, not cellular organisms

Virus	Nucleic Acid	Genome Size	Number of Genes	Chromosome	Form Host
Parvovirus	ssDNA	5176 bases	5	Linear	Animals
Herpes simplex virus	dsDNA	152,000 bp	162	Linear	Animals
Influenza virus	ssRNA	13,500 bases	11	Linear	Animals
HIV	ssRNA	9700 bases	9	Linear	Animals
Bacteriophage lambda	dsDNA	48,502 bp	37	Linear	Bacteria
Cauliflower mosaic virus	dsDNA	8245 bp	7	Circular	Plants
Poliovirus	ssRNA	7,500 bases	11	Linear	Animals
Additional info: Table includes selected viral genomes, showing diversity in nucleic acid type, size, and host.

Bacterial Genomes and Chromosome Packaging

Bacterial Genome Organization

Bacteria typically have a haploid genome with a single circular chromosome of double-stranded DNA. They may also possess plasmids—autonomously replicating DNA molecules.

• Genomes carry thousands of protein-coding genes with short intergenic regions.

• Repetitive sequences may be interspersed.

• Nucleoid: Dense, non-membrane region where the chromosome is supercoiled.

Bacterial Chromosome Condensation

Chromosome packaging involves:

• Loops anchored by proteins (HU, H-NS, SMC proteins)

• Supercoiling: DNA can be negatively or positively supercoiled, with negative supercoiling being most common.

• Supercoiling is regulated by DNA gyrase/topoisomerase II (introduces supercoils) and topoisomerase I (relaxes supercoils).

Eukaryotic Chromosome Organization and Packaging

Chromatin Structure

Eukaryotic chromosomes are organized into chromatin—a complex of DNA and proteins (primarily histones). Chromatin condensation varies:

• Euchromatin: Less condensed, actively expressed genes.

• Heterochromatin: More condensed, fewer expressed genes.

• Facultative heterochromatin: Variable condensation, related to gene transcription.

• Constitutive heterochromatin: Permanently condensed, found in centromeres and telomeres.

Levels of Chromatin Packaging

• Nucleosome (10 nm fiber): dsDNA wrapped around histone octamer (H2A, H2B, H3, H4).

• 30 nm fiber: Coiling of nucleosomes into solenoid structure, stabilized by H1 histone.

• 300 nm fiber (radial loops): Loops of 30 nm fibers attached to chromosome scaffold.

• 700 nm chromatid: Further compaction mediated by condensin proteins.

Chromatin structure changes through the cell cycle, becoming maximally condensed during metaphase.

Eukaryotic Genome Complexity

Genome Size and Gene Number

Eukaryotic genome size varies greatly and does not always correlate with organismal complexity (C-value paradox). Gene number is more closely associated with complexity, but exceptions exist (e.g., humans vs. plants).

Human Genome Composition

• Approximately 3.2 billion base pairs

• ~20,000 to 25,000 protein-coding genes

• ~40% unique sequence DNA, ~58% repetitive sequence DNA

Repetitive Sequence DNA

Repetitive DNA is hierarchically classified:

• Transposons: DNA segments that move within the genome.

• Retrotransposons: Use RNA intermediates to copy themselves.

• LTR retrotransposons: Endogenous retroviruses (ERVs), flanked by long terminal repeats.

• Non-LTR retrotransposons: LINEs (autonomous, code for reverse transcriptase), SINEs (non-autonomous, rely on LINEs).

• Alu SINEs: Unique to primates, 1 million copies in humans (~10% of genome).

Satellite DNA: Tandem Repeats

• Microsatellites: 1–6 bp, repeated many times.

• Minisatellites: 10–100 bp, repeated 5–50 times.

• Macrosatellites: Several thousand bp, much larger repeats.

• VNTRs: Variable number tandem repeats, used in DNA fingerprinting.

• Telomeres: Repetitive sequence (TTAGGG in vertebrates).

• Centromeres: Imperfect satellite repeats, most common is alpha satellite DNA (171 bp).

Archaeal Chromosomes and Histones

Archaeal Genome Organization

Archaea have a single, usually circular chromosome, often with plasmids. Genome size ranges from 500 Kbp to 6 Mbp, with ~87% protein-coding sequences. Some repetitive and intergenic regions are present.

Archaeal Histones and Nucleosomes

Archaeal histone proteins show homology to eukaryotic histones and form nucleosome-like complexes. Their functional role is not fully understood but likely involves regulation of gene expression.

Summary Table: Genome Complexity Across Domains

Domain	Genome Size	Chromosome Structure	Packaging
Virus	5–200 Kbp	Linear/circular, DNA/RNA	Capsid, envelope
Bacteria	1–10 Mbp	Circular, dsDNA	Supercoiling, nucleoid
Archaea	500 Kbp–6 Mbp	Circular, dsDNA	Histones, supercoiling
Eukaryotes	5 Mbp–>100 Gbp	Linear, multiple chromosomes	Histones, chromatin, supercoiling