Chromosome Structure and DNA Sequence Organization

Chromosome Structure and Organization

Overview

This section explores the structural organization of chromosomes in viruses, bacteria, and eukaryotes, as well as the unique features of organellar DNA and the various types of DNA sequence organization found in eukaryotic genomes.

Chromosomes in Viruses and Bacteria

General Features

• Bacterial and viral chromosomes are usually composed of a single nucleic acid molecule.

• They are largely devoid of associated proteins, unlike eukaryotic chromosomes.

• These chromosomes are much smaller than those found in eukaryotes.

• Viral chromosomes can be made of single- or double-stranded DNA or RNA, and may be linear or circular.

Examples: Bacteriophage Chromosomes

• Phage λ (lambda):

  • Double-stranded DNA.

  • DNA is linear before infection, circularizes after entering the host.

  • DNA molecule length: 17 μm; phage head diameter: <0.1 μm.

  • DNA is inert (inactive) when packaged in the phage head.

• Phage T2:

  • Double-stranded DNA.

  • Linear DNA molecule: 52 μm; phage head: 0.1 μm.

Bacterial Chromosomes

• Escherichia coli (E. coli):

  • Chromosome is a circular, double-stranded DNA molecule (~2000 μm in length).

  • Compacted into a region called the nucleoid.

  • DNA binding proteins (e.g., HU and H1, which are positively charged) help compact the DNA.

  • Bacterial cell size: ~2 μm.

  • Unlike viruses, bacterial DNA is active even when highly compacted.

Organelle DNA: Mitochondria and Chloroplasts

Mitochondrial DNA (mtDNA)

• Double-stranded and lacks chromosomal proteins (such as histones).

• Introns are mostly absent from mitochondrial genes.

• Gene repetition is rare.

• Replication of mtDNA depends on enzymes encoded by nuclear DNA.

Chloroplast DNA (cpDNA)

• Shares many similarities with prokaryotic DNA.

• Circular, double-stranded, and free of the associated proteins found in eukaryotic nuclear DNA.

• cpDNA is larger than mtDNA and contains more genes.

• cpDNA contains both introns and gene duplications.

Eukaryotic Chromosomes

General Features

• Larger and more complex than prokaryotic chromosomes.

• Composed of chromatin: DNA complexed with proteins (mainly histones).

• Highly compacted to fit within the nucleus (nucleus diameter: 5–10 μm; total DNA length: ~2 meters in humans).

Specialized Chromosomes

• Polytene Chromosomes:

  • Found in salivary glands and guts of some flies, protozoans, and plants (somatic cells).

  • Very large, visible by light microscopy.

  • Consist of many DNA strands; paired homologs.

  • Show distinctive banding patterns.

  • Undergo multiple rounds of replication without cell division (endomitosis).

  • Contain puff regions where DNA is uncoiled, indicating high gene activity.

• Lampbrush Chromosomes:

  • Found in oocytes of vertebrates during prophase I of meiosis.

  • Large chromosomes with extensive DNA looping.

  • Transcriptionally active, extended, and uncoiled versions of meiotic chromosomes.

Chromatin Structure

• DNA is packaged into a nucleoprotein complex called chromatin.

• Chromatin is organized into nucleosomes, which consist of DNA wrapped around histone proteins (H2A, H2B, H3, H4).

• Nucleosomes are further condensed to form chromatids.

Histone Proteins

Histone Type	Lysine-Arginine Content	Molecular Weight (Da)
H1	Lysine-rich	23,000
H2A	Highly lysine-rich	14,000
H2B	Highly lysine-rich	13,800
H3	Arginine-rich	15,300
H4	Arginine-rich	11,300

Chromatin Remodeling

• Chromatin must be remodeled to allow access to DNA by binding proteins (e.g., during transcription).

• Histone tails are subject to modifications such as acetylation, methylation, and phosphorylation.

• These modifications play a key role in gene regulation.

Euchromatin vs. Heterochromatin

• Euchromatin: Less condensed, transcriptionally active.

• Heterochromatin: Highly condensed, transcriptionally inactive.

• Examples of heterochromatic regions:

  • Centromeres

  • Telomeres

  • Mammalian Y chromosome

  • Inactive X chromosome (Barr body)

Repetitive DNA and Satellite DNA

General Features

• Repetitive DNA sequences are repeated many times within eukaryotic chromosomes.

• Categories include multi-copy genes, satellite DNA, minisatellites, microsatellites, SINEs, and LINEs.

Types of Repetitive DNA

• Multi-copy genes: Some coding genes, such as those for ribosomal RNA, occur in multiple copies.

• Satellite DNA: Highly repetitive, often found at centromeres and telomeres.

• Minisatellites (VNTRs): 15–100 bp sequences, repeated hundreds of times.

• Microsatellites: 2–4 bp sequences, repeated 5–100 times.

• VNTRs and microsatellites are used in genetic identity and kinship analysis due to high variability among individuals.

• SINEs (Short Interspersed Elements): ~500 bp, up to 500,000 copies in the human genome.

• LINEs (Long Interspersed Elements): ~6,000 bp, up to 850,000 copies in the human genome; act as retrotransposons.

Functions of Repetitive DNA

• Centromeres: Primary constriction points on chromosomes, composed of satellite DNA.

• Telomeres: Consist of short tandem repeats (e.g., 5'-TTAGGG-3' in vertebrates), up to 1,000 repeats, providing chromosome stability.

Organization of Eukaryotic Genomes

• Only 2–10% of the eukaryotic genome encodes proteins.

• Large portions are single-copy noncoding regions, including pseudogenes (nonfunctional remnants of once-functional genes).

• Pseudogenes are evolutionary vestiges and do not produce functional products.

Summary Table: Types of DNA Sequences in Eukaryotic Genomes

Type	Features	Example/Function
Protein-coding genes	2–10% of genome	Structural and enzymatic proteins
Multi-copy genes	Repeated coding sequences	rRNA genes
Satellite DNA	Highly repetitive, tandem repeats	Centromeres, telomeres
Minisatellites (VNTRs)	Moderately repetitive, 15–100 bp	DNA fingerprinting
Microsatellites	Short repeats, 2–4 bp	Genetic mapping
SINEs	Short, interspersed, noncoding	Alu elements
LINEs	Long, interspersed, retrotransposons	L1 elements
Pseudogenes	Nonfunctional, single-copy	Evolutionary remnants

Additional info: Chromatin remodeling and histone modifications are active areas of research, with implications for epigenetic regulation and disease.