Eukaryotic Chromosome Abnormalities and Molecular Organization: Study Notes

Chapter 10: Eukaryotic Chromosome Abnormalities and Molecular Organization

10.1 Chromosome Number and Shape Vary Among Organisms

Chromosome number, size, and shape are species-specific and play a fundamental role in heredity and genetic variation. Chromosomes are distributed non-uniformly within the nucleus, and their organization is crucial for gene expression and cellular function.

• Genome Content: The total chromosome number in a nucleus varies widely among species, but closely related species tend to have similar numbers.

• Chromosome Territories: Chromosomes occupy distinct regions within the nucleus during interphase, known as chromosome territories.

• Chromosome Pairing: Each chromosome pair in a diploid genome has unique size, shape, and genetic content.

Karyotypes

A karyotype is an organized visual display of chromosomes, used to identify abnormalities in chromosome number or structure. Autosomal homologs are numbered 1–22, and sex chromosomes are identified separately. Chromosomes can be stained to distinguish them.

Chromosome Shapes

Chromosomes are classified based on the position of the centromere:

• Metacentric: Centromere near the middle.

• Submetacentric: Centromere between center and tip.

• Acrocentric: Centromere close to one end.

• Telocentric: Centromere at the tip, no p arm.

Fluorescent in Situ Hybridization (FISH)

FISH uses fluorescently labeled molecular probes to detect specific DNA sequences on chromosomes. Multiple probes emitting different wavelengths can be used simultaneously to identify various sequences.

Heterochromatin and Euchromatin

Chromosome condensation varies along its length, affecting gene expression:

• Euchromatin: Less condensed, actively expressed genes.

• Heterochromatin: Highly condensed, fewer expressed genes.

• Each G-band contains 1–10 million base pairs and multiple genes.

10.2 Nondisjunction Leads to Changes in Chromosome Number

Chromosome Nondisjunction

Nondisjunction is the failure of chromosomes or chromatids to separate properly during cell division, resulting in abnormal chromosome numbers (aneuploidy).

• Euploid: Complete sets of chromosomes.

• Aneuploid: Chromosome number not a complete set.

Nondisjunction in Germ-Line Cells

Nondisjunction during meiosis produces aneuploid gametes, which can lead to aneuploid zygotes. Fusion with normal gametes results in trisomic or monosomic offspring.

Aneuploidy in Humans

Humans are sensitive to gene dosage changes; most aneuploids do not survive. Only trisomies of chromosomes 13, 18, and 21 are seen in newborns, and no autosomal monosomies are observed. Sex chromosome aneuploidies are more common.

Trisomy 21 (Down Syndrome)

Trisomy 21 is linked to maternal age. Prolonged meiotic arrest in oocytes may degrade cohesin proteins, increasing nondisjunction risk. A few genes on chromosome 21 cause the main symptoms.

Turner Syndrome

Turner syndrome (45, XO) results from monosomy of the X chromosome. Haploinsufficiency of the SHOX gene, which escapes X-inactivation, is a primary cause of short stature and developmental abnormalities.

10.3 Changes in Euploid Content Lead to Polyploidy

Polyploidy

Polyploidy is the presence of three or more sets of chromosomes. It can arise from duplication within a species (autopolyploidy) or from combining sets of different species (allopolyploidy).

• Polyploidy is common in plants and can lead to new species.

• Polyploid organisms often have increased size and vigor.

10.4 Chromosome Breakage Causes Mutation by Loss, Gain, and Rearrangement of Chromosomes

Partial Chromosome Deletion

Chromosome breakage at a break point can lead to terminal deletions. Acentric fragments (lacking centromeres) are often lost during cell division.

Unequal Crossover

Unequal crossover between homologs results in partial duplication on one homolog and partial deletion on the other. This can cause syndromes such as Williams-Beuren syndrome.

10.5 Chromosome Breakage Leads to Inversion and Translocation of Chromosomes

Chromosome Inversion

Chromosome breakage and reattachment in the wrong orientation leads to inversion. Two types:

• Paracentric inversion: Centromere outside inverted region.

• Pericentric inversion: Centromere within inverted region.

Chromosome Pairing and Recombination

Alignment of normal and inverted homologs forms an inversion loop. Crossing over outside the inverted region is normal, but within the region causes duplications and deletions.

Crossing Over Within a Paracentric Inversion

Results in a dicentric chromosome (pulled to both poles, breaks randomly) and an acentric fragment (lost).

Crossing Over Within a Pericentric Inversion

Results in duplicated and deleted regions in recombinant chromosomes. Two normal and two abnormal gametes are produced.

Three Types of Translocations

• Nonreciprocal translocation: Piece of one chromosome moves to a nonhomolog.

• Reciprocal balanced translocation: Pieces of two nonhomologs switch places.

• Robertsonian translocation: Fusion of two nonhomologous chromosomes.

10.6 Eukaryotic Chromosomes Are Organized into Chromatin

Chromatin Organization

Chromatin structure is essential for chromosome function, gene regulation, and compaction within the nucleus.

• Chromatin is composed of DNA and proteins, mainly histones.

• Histone proteins are highly conserved and critical for nucleosome formation.

Histone Proteins and Nucleosomes

There are five types of histone proteins: H1, H2A, H2B, H3, and H4. Nucleosome core particles consist of an octamer of histones (two each of H2A, H2B, H3, and H4), with DNA wrapped around them.

Nucleosome Assembly

• Two H3 and two H4 form a tetramer, which binds DNA.

• Two H2A-H2B dimers associate to complete the octamer.

Chromatin Structure

Least condensed chromatin appears as a 10-nm fiber (“beads-on-a-string”), with nucleosomes as beads and linker DNA as string.

Higher-Order Chromatin Organization and Chromosome Structure

Chromatin forms 30-nm fibers during interphase and becomes maximally condensed in metaphase. Loops of 30-nm fibers attach to a chromosome scaffold, forming 300-nm fibers and giving chromosomes their shape.

Nucleosome Disassembly, Synthesis, and Reassembly During Replication

During DNA replication, old histones are retained as single molecules, dimers, or tetramers. New nucleosomes are assembled from both old and new histones.

Nucleosome Dynamics During Transcription

Transcription requires chromatin remodeling to convert chromatin from a closed (condensed) to an open (accessible) state. Remodeling complexes and transcription factors slide or eject nucleosomes, allowing RNA polymerase access. After transcription, nucleosomes are restored, silencing the gene.

Key Equations and Concepts

• Chromosome Number: $2n$ for diploid organisms, where $n$ is the number of unique chromosomes.

• Aneuploidy: Trisomy ($2n+1$), Monosomy ($2n-1$).

• Polyploidy: Triploid ($3n$), Tetraploid ($4n$), etc.

Example Table: Types of Chromosome Abnormalities

Type	Description	Example
Aneuploidy	Abnormal number of chromosomes	Trisomy 21 (Down syndrome)
Polyploidy	Extra sets of chromosomes	Triploid plants
Deletion	Loss of chromosome segment	Cri du chat syndrome
Duplication	Extra copy of chromosome segment	Charcot-Marie-Tooth disease
Inversion	Reversed segment within chromosome	Paracentric inversion
Translocation	Segment moved to another chromosome	Robertsonian translocation

Additional info: Table entries and some examples are inferred for completeness and academic context.