Premed · Premed · Genetics
Lecture 5: Chromosome Structure and Karyotyping
Genetics
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the hierarchical levels of chromatin packaging from DNA double helix to metaphase chromosome
- Identify the structural components of a chromosome (centromere, telomere, origin of replication)
- Distinguish between euchromatin and heterochromatin and their functional significance
- Explain the process of karyotyping and interpret a human karyogram
- Describe chromosome banding techniques and their utility
- Explain the role of telomeres and telomerase in chromosome stability and aging
Lecture Content
I. Chromatin Organization and Packaging
The human genome contains approximately 6.4 billion base pairs of DNA, which if stretched end to end would measure about 2 meters, yet it must fit within a nucleus roughly 6 micrometers in diameter. This feat is accomplished through a hierarchy of packaging levels.
The fundamental unit of chromatin is the nucleosome, which consists of a core of 8 histone proteins (an octamer comprising 2 each of H2A, H2B, H3, and H4) around which approximately 147 base pairs of DNA are wrapped about 1.7 times. Adjacent nucleosomes are connected by linker DNA of approximately 20-60 base pairs, and histone H1 binds this linker DNA to stabilize higher-order structure. Under the electron microscope, this arrangement has a "beads on a string" appearance and forms the 10 nm fiber. The next level of organization is the 30 nm fiber, in which nucleosomes coil into a solenoid or zigzag structure requiring histone H1, with roughly 6 nucleosomes per turn in the solenoid model. The 30 nm fiber then forms chromatin loops of approximately 300 nm, with each loop spanning 50-200 kb of DNA anchored to a non-histone protein scaffold. Further coiling produces the 700 nm chromonema fiber, and at maximum compaction during mitosis, the metaphase chromosome reaches approximately 1,400 nm in width, consisting of two sister chromatids joined at the centromere. The overall compaction ratio from DNA double helix to metaphase chromosome is approximately 10,000:1.
<image>Panel A: Step-by-step diagram showing the levels of chromatin packaging from double-stranded DNA (2 nm) to nucleosome (10 nm "beads on a string") to 30 nm fiber to looped domains (300 nm) to chromonema fiber (700 nm) to metaphase chromosome (1400 nm), with scale bars and fold-compaction at each level. Panel B: Detailed view of a single nucleosome showing the histone octamer (H2A, H2B, H3, H4 labeled), DNA wrapping, linker DNA, and histone H1 binding. Panel C: Electron micrograph-style illustration showing the "beads on a string" appearance of chromatin after histone H1 removal.</image>
II. Histones and Histone Modifications
Histones are small, highly basic proteins rich in the amino acids lysine and arginine. Their positive charge enables electrostatic interaction with the negatively charged phosphate backbone of DNA. The core histones H2A, H2B, H3, and H4 are highly conserved across eukaryotes, while the linker histone H1 is more variable.
Histones undergo a variety of post-translational modifications that regulate chromatin structure and gene expression. Acetylation of lysine residues is generally associated with gene activation because it loosens chromatin. This modification is added by histone acetyltransferases (HATs) and removed by histone deacetylases (HDACs). Methylation of lysine or arginine residues can either activate or repress genes depending on the specific residue modified: H3K4me3 marks active promoters, H3K27me3 marks Polycomb-repressed genes, and H3K9me3 marks heterochromatin. Phosphorylation of serine and threonine residues is involved in chromosome condensation and DNA repair, while ubiquitination, particularly monoubiquitination of H2B, is associated with transcription. The histone code hypothesis proposes that specific combinations of histone modifications dictate chromatin state and gene expression patterns.
III. Euchromatin and Heterochromatin
Euchromatin is loosely packed and transcriptionally active chromatin that replicates early in S phase, resides in gene-rich regions, stains lightly in chromosome banding, and is dynamic in its condensation state depending on cell type and conditions. Heterochromatin, by contrast, is tightly packed and generally transcriptionally silent chromatin that replicates late in S phase. It comes in two varieties: constitutive heterochromatin is permanently condensed in all cells, found at centromeres and telomeres, and is rich in repetitive DNA sequences such as satellite DNA (examples include pericentromeric regions and the long arm of the Y chromosome); facultative heterochromatin is conditionally condensed, representing genes that are silenced in specific cell types or developmental stages, with the inactive X chromosome (Barr body) in female mammals being the most prominent example.
IV. Chromosome Anatomy
The centromere is the primary constriction of the chromosome and serves as the site of kinetochore assembly, which is essential for chromosome segregation during mitosis and meiosis. In humans, centromeres contain alpha-satellite DNA consisting of repetitive 171 base pair units. The kinetochore is a protein complex that mediates spindle fiber attachment. Centromere position defines chromosome morphology: metacentric chromosomes have the centromere near the middle with roughly equal arms, submetacentric chromosomes have the centromere off-center with a shorter p arm, acrocentric chromosomes (chromosomes 13, 14, 15, 21, and 22 in humans) have the centromere near one end with a very short p arm, and telocentric chromosomes have the centromere at the very end (not found in humans).
Telomeres are protective caps at chromosome ends consisting of the repeated sequence TTAGGG, present in thousands of copies. The 3' G-rich strand extends as a single-stranded overhang that folds back and invades the double-stranded region, forming a T-loop structure. Telomeres prevent chromosome end fusion, protect against degradation, and help solve the end-replication problem. The shelterin complex, a six-protein assembly, protects telomeres from being recognized as damaged DNA.
Every chromosome also contains multiple origins of replication, which are sites where DNA replication initiates and are essential for complete chromosome duplication. The chromosome is divided by the centromere into a short arm (p) and a long arm (q).
<image>Panel A: Labeled diagram of a metaphase chromosome showing sister chromatids, centromere, kinetochore, p arm, q arm, telomeres, and satellite (for acrocentric chromosomes). Panel B: Four chromosome types based on centromere position — metacentric, submetacentric, acrocentric, and telocentric — with proportional arm lengths drawn. Panel C: Detailed diagram of telomere structure showing the TTAGGG repeat sequence, the 3' overhang, T-loop formation, D-loop, and the shelterin complex proteins (TRF1, TRF2, POT1, TIN2, TPP1, RAP1).</image>
V. Karyotyping: Techniques and Applications
A karyotype is the complete set of chromosomes in an organism, arranged by size and morphology. The karyotyping procedure begins with culturing cells, usually peripheral blood lymphocytes stimulated with phytohemagglutinin. Colchicine or colcemid is then added to arrest cells in metaphase by disrupting spindle formation. The cells are treated with a hypotonic solution to swell and spread the chromosomes, then fixed, dropped onto slides, and stained. Finally, the chromosomes are photographed and arranged into a karyogram. The normal human karyotype consists of 46 chromosomes: 22 pairs of autosomes plus 1 pair of sex chromosomes (46,XX for females and 46,XY for males).
VI. Chromosome Banding Techniques
G-banding (Giemsa banding) is the most commonly used banding technique. Chromosomes are treated with trypsin and then stained with Giemsa dye. AT-rich regions stain dark (G-bands) while GC-rich regions stain light (R-bands), producing a unique banding pattern for each chromosome. The resolution is approximately 5-10 Mb, allowing detection of deletions or duplications of about 5 Mb or larger. Q-banding (Quinacrine banding) uses fluorescent staining and produces a pattern similar to G-banding. R-banding (Reverse banding) reverses the G-banding pattern so that GC-rich regions stain dark. C-banding specifically stains constitutive heterochromatin in centromeric regions.
FISH (Fluorescence In Situ Hybridization) uses fluorescently labeled DNA probes that hybridize to specific chromosome regions, enabling detection of specific sequences, translocations, and microdeletions at higher resolution (approximately 100 kb to 1 Mb) than traditional banding. Spectral karyotyping (SKY) and M-FISH use combinatorial labeling with multiple fluorophores to paint each chromosome pair a different color, which is particularly useful for detecting complex rearrangements.
Chromosome nomenclature uses p for the short arm (from French "petite") and q for the long arm, with bands numbered outward from the centromere. For example, 17p13.1 indicates chromosome 17, short arm, region 1, band 3, sub-band 1.
VII. Telomeres and Telomerase
The end-replication problem arises because conventional DNA polymerase cannot fully replicate the 3' end of a linear chromosome. Each round of replication results in slight shortening of the lagging strand, and without a mechanism to counteract this, chromosomes would progressively shorten with every cell division.
Telomerase is a ribonucleoprotein enzyme that solves this problem by extending telomeric DNA. It consists of an RNA component (TERC) that serves as a template for telomere synthesis and a protein component (TERT) that possesses reverse transcriptase activity. Telomerase adds TTAGGG repeats to the 3' overhang, and after extension, primase and DNA polymerase synthesize the complementary strand to complete the process.
Telomerase activity varies dramatically among cell types. Germ cells and stem cells maintain high telomerase activity to preserve telomere length. Most somatic cells have low or absent telomerase activity, meaning their telomeres shorten with age. Approximately 85-90% of cancer cells reactivate telomerase to enable unlimited proliferation, while the remaining 10-15% use an alternative mechanism called ALT (Alternative Lengthening of Telomeres). The progressive shortening of telomeres in somatic cells underlies the Hayflick limit, the observation that somatic cells have a finite number of divisions (approximately 50-70). When telomeres become critically short, the p53 pathway triggers senescence or apoptosis. Telomere length has therefore garnered interest as a potential biomarker of cellular aging.
<image>Panel A: A standard human male karyogram (46,XY) with all 22 autosome pairs and the sex chromosomes arranged by group (A through G), showing G-banding patterns. Panel B: Diagram of the end-replication problem showing progressive telomere shortening over successive rounds of DNA replication on the lagging strand. Panel C: Mechanism of telomerase action: step-by-step illustration showing the TERC RNA template aligning with the 3' overhang, TERT extending the telomere by reverse transcription, translocation, and subsequent fill-in synthesis by DNA polymerase.</image>
