Chromosome Structure and Karyotype

Each chromosome packs a single enormous DNA molecule into a compact, organized structure, and arranging an individual's full chromosome set into a karyotype reveals their number, size, and banding at a glance.

Definition

Chromosome structure is the organization of a chromosome's DNA and proteins into a packaged, functional unit, and a karyotype is the systematic arrangement of an individual's complete set of chromosomes by size and banding pattern.

Scope

This topic covers the hierarchical packaging of DNA around histones into nucleosomes and higher-order chromatin, the functional landmarks of a chromosome including the centromere and telomeres, the distinction between euchromatin and heterochromatin, the preparation and reading of a karyotype, chromosome banding and nomenclature, and molecular cytogenetic methods such as fluorescence in situ hybridization. It treats chromosome architecture and identification; chromosome movement during division is covered in the adjacent topic.

Core questions

• How is a meter-long DNA molecule packaged into a microscopic chromosome?
• What are the roles of the centromere and telomeres in a chromosome?
• How is a karyotype prepared and read to identify each chromosome?
• How do banding and molecular cytogenetic methods reveal chromosomal detail?

Key concepts

• Nucleosomes and chromatin packaging
• Centromeres and telomeres
• Euchromatin versus heterochromatin
• Karyotype preparation and chromosome banding
• Fluorescence in situ hybridization

Mechanisms

DNA wraps roughly twice around a histone octamer to form a nucleosome, nucleosomes coil into thicker fibers, and during division these condense further into the visible chromosome; the centromere anchors the spindle attachment and telomeres cap and protect the chromosome ends, while banding stains and labeled probes make individual regions distinguishable.

Clinical relevance

Reading chromosome structure underlies clinical karyotyping and FISH testing used to detect aneuploidies, deletions, and rearrangements in prenatal diagnosis and cancer, and telomere biology connects to cellular aging and certain inherited disorders.

History

Flemming described chromatin and mitosis in the 1880s, Tjio and Levan established the human chromosome number as forty-six in 1956, banding techniques in the early 1970s allowed each chromosome to be uniquely identified, and the nucleosome model of chromatin packaging was established later that decade.

Key figures

• Walther Flemming
• Joe Hin Tjio
• Roger Kornberg

Seminal works

• klug2019

Frequently asked questions

What is the function of a telomere?

Telomeres are repetitive caps at the ends of chromosomes that protect them from degradation and from fusing with other chromosomes, and they buffer the gradual loss of end sequence that occurs with each round of replication.

Why is DNA packaged with histone proteins?

The DNA in a single human cell is about two meters long and must fit inside a tiny nucleus; wrapping it around histones into nucleosomes and further coiling compacts it enormously while keeping it organized and accessible for use.