Eukaryotic Chromosome Abnormalities and Molecular Organization: Study Notes

Chapter 10: Eukaryotic Chromosome Abnormalities and Molecular Organization

Chromosome Number in Selected Animal Species

Chromosome number varies widely among animal species, reflecting evolutionary diversity and genetic complexity. The diploid chromosome number (2n) is characteristic for each species.

Species	Diploid Chromosome Number (2n)
Carp (Cyprinus carpio)	104
Cat (Felis catus)	38
Chicken (Gallus gallus domesticus)	78
Chimpanzee (Pan troglodytes)	48
Cow (Bos taurus)	60
Dog (Canis familiaris)	78
Frog (Rana pipiens)	26
Fruit fly (Drosophila melanogaster)	8
Horse (Equus caballus)	64
Human (Homo sapiens)	46
Mouse (Mus musculus)	40
Rat (Rattus norvegicus)	42
Rhesus monkey (Macaca mulatta)	42

Interphase Chromosome Territories

During interphase, chromosomes occupy distinct regions called chromosome territories within the nucleus. These territories are visualized using techniques such as fluorescent in situ hybridization.

• Chromosomes do not occupy the same territory in each nucleus, but once confined, they remain until M phase.

• Chromosomes are active within their territories, moving and twisting during transcription and DNA replication.

• Centromeres anchor chromosomes in their territories.

• Interchromosomal domains are regions between territories, serving as channels for movement of proteins, enzymes, and RNA molecules.

• Larger, gene-rich chromosomes are generally near the center of the nucleus; smaller, gene-poor chromosomes are near the periphery.

Karyotypes

A karyotype is an organized visual display of chromosomes, used to identify abnormalities in chromosome number or structure.

• Autosomal homologs are numbered 1 through 22, in descending order of size.

• Sex chromosomes (X and Y) are identified separately.

• Chromosomes may be stained with different compounds to distinguish them.

Chromosome Shape

Chromosomes are classified by centromere position:

• Metacentric: centromere near the middle; arms are equal length.

• Submetacentric: centromere slightly off-center; arms are unequal.

• Acrocentric: centromere near one end; one very short arm (p), one long arm (q), often with a satellite.

• Telocentric: centromere at the end; only one arm visible.

Standardized Human Chromosome Banding Patterns

Chromosome banding patterns, produced by staining, allow identification of individual chromosomes and detection of structural abnormalities. Each band may contain multiple genes.

Heterochromatin and Euchromatin

Chromosome condensation varies along the chromosome, affecting gene expression.

• Euchromatin: less condensed, contains actively expressed genes.

• Heterochromatin: tightly condensed, contains fewer expressed genes.

Chromosome Nondisjunction

Nondisjunction is a major source of chromosome number abnormalities.

• The euploid number is the normal number of complete chromosome sets (e.g., n, 2n, 3n).

• Aneuploidy refers to cells with chromosome numbers that are not euploid.

• Nondisjunction is the failure of chromosomes or sister chromatids to separate properly during cell division, leading to aneuploidy.

Nondisjunction in Germ-Line Cells

Nondisjunction during meiosis produces aneuploid gametes, which can result in aneuploid zygotes.

• Meiosis I nondisjunction: failure of homologs to separate; gametes are n + 1 or n - 1.

• Fusion with normal gametes produces trisomic (2n + 1) or monosomic (2n - 1) offspring.

Phenotypic Effects of Aneuploidy: Jimson Weed Example

Trisomic lines of Datura stramonium (Jimson weed) show distinct seed head morphologies compared to wild-type diploid plants.

Type	Seed Head Morphology
Wild type (2n)	Normal
Trisomic (2n + A)	Rolled
Trisomic (2n + B)	Glossy
Trisomic (2n + C)	Buckling
Trisomic (2n + D)	Elongate

Aneuploidy in Humans

Humans are highly sensitive to gene dosage changes; most aneuploidies are lethal.

• Only autosomal trisomies of chromosomes 13, 18, and 21 are seen in newborns; no autosomal monosomies are observed.

• Multiple forms of sex-chromosome trisomies and one type of sex-chromosome monosomy (Turner syndrome) occur.

Aneuploidy	Syndrome	Frequency at Birth	Syndrome Characteristics
Trisomy 13	Patau syndrome	1 in 15,000	Mental retardation, developmental delay, organ abnormalities, early death
Trisomy 18	Edwards syndrome	1 in 8,000	Mental retardation, developmental delay, skeletal and facial abnormalities, early death
Trisomy 21	Down syndrome	1 in 1,500	Mental retardation, developmental delay, characteristic facial features, short stature, variable life span
47, XXY	Klinefelter syndrome (males)	1 in 1,000	Variable secondary sexual characteristics, infertility, reduced testis size, no impact on mental capacity
47, XYY	Jacobs syndrome (males)	1 in 1,000	Tall stature, sometimes reduced fertility, no impact on mental capacity
47, XXX	Triple X syndrome (females)	1 in 1,000	Tall stature, sometimes reduced fertility, menstrual irregularity, no impact on mental capacity
45, XO	Turner syndrome (females)	1 in 5,000	Short stature, infertility, webbed neck, no impact on mental capacity

Mosaicism

Mosaicism arises from mitotic nondisjunction early in embryogenesis, resulting in individuals with two or more genetically distinct cell lines.

• Example: 25–30% of Turner syndrome cases are mosaics, with some 45, XO cells and some 46, XX cells.

• Some Turner syndrome individuals may also carry 47, XXX cells.

Uniparental Disomy

Uniparental disomy is a rare abnormality where both copies of a homologous chromosome pair are inherited from the same parent.

• First identified in Angelman syndrome and Prader-Willi syndrome, usually caused by deletion of a region on chromosome 15.

Polyploidy

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

• Meiotic nondisjunction can produce diploid gametes, leading to triploid (3n) or tetraploid (4n) offspring.

• Mitotic nondisjunction can double chromosome number in somatic cells.

• Polyploidy increases fruit and flower size, but often decreases fertility in odd-numbered polyploids (e.g., 3n, 5n).

• Hybrid vigor: polyploids may show increased growth, yield, and disease resistance.

• Polyploidy can lead to rapid speciation.

Example: Production of new allopolyploid species in wheat involves chromosome doubling and hybridization between species.

Chromosome Breakage: Mutation by Loss, Gain, and Rearrangement

Chromosome breakage can result in loss, gain, or rearrangement of chromosome segments, leading to gene dosage imbalances and severe abnormalities.

• Terminal deletions: loss of chromosome ends.

• Interstitial deletions: loss of internal segments due to two breaks.

• Unequal crossover can create syndromes such as Williams-Beuren syndrome.

Chromosome Terminal and Interstitial Deletions

• Terminal deletion: loss of a chromosome end, as seen in cri-du-chat syndrome (chromosome 5).

• Interstitial deletion: loss of an internal segment, as in WAGR syndrome (chromosome 11).

Deletion Mapping

Pseudodominance occurs when a recessive allele is expressed due to deletion of the dominant allele. Deletion mapping is used to locate genes by observing phenotypes in individuals with specific deletions.

Example: Mapping the Drosophila Notch (n) gene using partial deletion mutants.

Chromosome Inversion and Translocation

Chromosome breakage can lead to reattachment of segments in incorrect orientations or to nonhomologous chromosomes.

• Inversion: segment reattached in reverse orientation; can be paracentric (does not include centromere) or pericentric (includes centromere).

• Translocation: segment attached to a nonhomologous chromosome; can be nonreciprocal, reciprocal balanced, or Robertsonian (chromosome fusion).

• Inversions suppress recombination within the inverted region, potentially affecting fertility.

• Translocation heterozygotes may be phenotypically normal but can experience semisterility due to segregation abnormalities.

Chromatin Organization and Chromosome Structure

Eukaryotic chromosomes are organized into chromatin, which is essential for chromosome function, gene regulation, and compaction within the nucleus.

• Five types of histone proteins: H1, H2A, H2B, H3, H4.

• Nucleosome: fundamental unit, consisting of an octamer of histones (2 each of H2A, H2B, H3, H4) wrapped by ~146 bp of DNA.

• Histone H1 stabilizes higher-order structures.

• Chromatin exists as a 10-nm "beads-on-a-string" fiber and condenses into a 30-nm solenoid structure.

• Further condensation forms 300-nm loops attached to a protein scaffold, giving chromosomes their shape.

• Chromosome compaction is crucial for efficient segregation during cell division and regulation of gene expression.

Nucleosome Disassembly and Reassembly During Replication

During DNA replication, nucleosomes are partially disassembled and reassembled, with old histones retained and new histones incorporated.

• Most nucleosomes after replication are assembled from both old and new histone components.

Additional info: These notes expand on the original slides by providing definitions, examples, and context for key genetic concepts, including mechanisms and consequences of chromosome abnormalities and molecular organization in eukaryotes.