DNA and Chromatin Organization – Key Vocabulary

Vocabulary flashcards covering DNA structure, chromatin organization, histone and DNA modifications, repeat sequences, and clinical correlations.

Deoxyribonucleic Acid (DNA)

The hereditary molecule composed of a double‐stranded helix carrying genetic information in all living cells.

Double Helix

Watson-Crick model of two antiparallel DNA strands wound around a common axis.

Antiparallel Strands

Orientation of DNA strands running in opposite 5'→3' directions.

Sugar-Phosphate Backbone

Repeating deoxyribose sugars and phosphate groups forming DNA’s structural framework.

Nitrogenous Bases

Adenine, Thymine, Guanine, and Cytosine—the information-containing components of DNA.

Base Pairing

Hydrogen bonding of A with T (2 bonds) and G with C (3 bonds).

Purines

Two-ring bases adenine and guanine.

Pyrimidines

Single-ring bases thymine and cytosine.

Nucleoside

Nitrogenous base linked to a pentose sugar without phosphate.

Nucleotide

Nitrogenous base + deoxyribose + phosphate; the monomer of DNA.

Phosphodiester Bond

Covalent linkage between 3'-OH of one nucleotide and 5'-phosphate of the next.

5′ End

Terminus of a nucleic acid strand bearing a free phosphate group.

3′ End

Terminus of a nucleic acid strand bearing a free hydroxyl group.

B-DNA

Right-handed DNA helix most common in vivo; ~10 bp per turn.

A-DNA

Right-handed, shorter, and more compact DNA form observed under dehydrating conditions.

Z-DNA

Left-handed DNA helix associated with active transcription and gene regulation.

Major Groove

Wider groove of DNA where most sequence-specific proteins bind.

Minor Groove

Narrower groove running along the DNA helix.

High GC Content

DNA region rich in G and C; confers higher melting temperature.

DNA Packaging

Multilevel condensation allowing ~2 m of DNA to fit in the nucleus.

Nucleosome

Fundamental chromatin unit: ~147 bp DNA wrapped around a histone octamer.

Histone Octamer

Protein core of 2×H2A, 2×H2B, 2×H3, and 2×H4 around which DNA winds.

Linker DNA

Stretch of DNA connecting adjacent nucleosomes; bound by H1 histone.

H1 Histone

Linker histone that stabilizes higher-order chromatin folding.

30-nm Fiber (Solenoid)

Second-level chromatin coil of ~6 nucleosomes per turn.

Chromatin Loop

300-nm loop of 30-nm fiber anchored to a protein scaffold.

Scaffold Attachment Region (SAR)

DNA sequence where chromatin loops bind non-histone scaffold proteins.

Cohesin

Protein complex that holds sister chromatids and organizes chromatin loops.

Chromosome

Highest-order DNA-protein structure visible during mitosis/meiosis.

Euchromatin

Light-staining, loosely packed, transcriptionally active chromatin.

Heterochromatin

Dark-staining, densely packed, transcriptionally silent chromatin.

Histone Acetylation

Addition of acetyl groups that neutralize lysine charge, open chromatin, and activate transcription.

Histone Methylation

Addition of methyl groups often leading to transcriptional repression.

Histone Phosphorylation

Addition of phosphate groups involved in DNA repair and chromatin remodeling.

DNA Methylation

Covalent addition of methyl groups to cytosine, generally silencing gene expression.

Satellite DNA

Highly repetitive tandem sequences located at centromeres and telomeres.

Minisatellite

Tandem repeats 20–70 bp long, totaling a few kilobases.

Microsatellite (STR)

Short tandem repeat of 2–6 bp used in forensic CODIS profiling.

Trinucleotide Repeat Expansion

Pathogenic lengthening of a 3-bp microsatellite within a gene.

LINE (Long Interspersed Element)

~7 kb transcribed repeat present 20–50 000 times in the genome.

SINE (Short Interspersed Element)

90–500 bp transcribed repeat present ~100 000 times (e.g., Alu).

Alpha Satellite

171-bp repeat forming large arrays at human centromeres.

Telomere

Repetitive DNA at chromosome ends protecting against degradation.

Centromere

Chromosome region where kinetochores form and sister chromatids attach.

Hutchinson-Gilford Progeria Syndrome

Premature aging disorder linked to defective nuclear lamina and chromatin packaging.

Tumor Suppressor Gene Hypermethylation

Excessive promoter methylation leading to cancer-related gene silencing.

DNA Melting Temperature (Tm)

Temperature at which half of DNA duplexes dissociate; increases with GC content.

Protein Scaffold

Non-histone framework organizing chromatin loops inside chromosomes.

Diploid

Cell containing two complete sets of chromosomes (46 in humans).

Haploid

Cell containing one set of chromosomes (23 in human gametes).