Introduction
Karyotype
- Definition: The number and visual appearance of chromosomes in an organism's cell nuclei.
- Sets:
- Haploid: One copy of each chromosome.
- Diploid: Two copies of each chromosome (one paternal, one maternal).
- Origins: Maternal chromosomes from oocytes, paternal from spermatozoa.
- Types: Gonosomes (sex chromosomes), Autosomes (non-sex chromosomes).
- Karyogram: Chromosomes arranged in homologous pairs, numbered by length and centromere position.
- Idiogram: Diagrammatic representation of a karyotype.
Classification of chromosomes based on centromere position
- telocentric chromosomes are not normal - they can be result of chromosomal aberration
Metaphase Chromosomal set
Idiogram
Karyogram
Chromosome Classification & Evolution
- Centromere Position: Telocentric chromosomes, often abnormal.
- Evolution: Due to chromosomal aberrations like deletions, duplications, inversions, affecting fertility and potentially leading to new species.
- Crossing over can occur; however, the majority of gametes will possess extra deletions and duplications, subsequently diminishing fertility.
Chromosomal Aberrations
- Inversion: Causes loops in meiotic synapsis, affecting gamete integrity and fertility, leading to reproductive isolation in homozygotes.
- Translocation: Creates crosses in meiotic synapsis, causing segregation issues, with homozygotes having normal fertility but being reproductively isolated.
- Robertsonian Translocation: Fusion of two acrocentric chromosomes alters number and causes reproductive issues, linked to Down's syndrome.
Chromosomal Comparisons and Polyploidy
- Human vs. Great Ape Chromosomes: Similarities with differences mainly in inversions.
- Polyploidy:
- Autopolyploidy: More than two genome copies from one species.
- Allopolyploidy: Genomes from two or more different species, causing spindle blocking and conjugation issues.
Polytene Chromosomes
- Location: Common in dipteran larvae's secretory cells.
- Characteristics:
- Large, containing thousands of DNA strands.
- Facilitate high gene expression levels.
- Result from multiple DNA synthesis cycles without cell division, keeping daughter chromatids together.
- Formation: Occurs during endomitosis.
- Structure:
- Length: 0.5 mm.
- Diameter: 20µm.
- Contains two types of bands:
- Dark Bands: Higher DNA content, lower RNA.
- Interbands: Lower DNA content, higher RNA.
- Bands and Gene Expression:
- Puffs indicate active gene sites, where mRNA synthesis occurs, appearing and disappearing during larval development.
Lampbrush Chromosomes
- Occurrence: In meiotically paired chromosomes of growing oocytes in many animals (excluding mammals).
- Structure: Loops of decondensed chromatin correspond to active DNA sequences, while most DNA remains condensed.
Barr Body in Oral Mucosa Cells
- Also Known As: Sex chromatin.
- Function: Inactivates one X chromosome in female somatic cells to balance gene expression.
- Mechanism:
- Females have two X chromosomes; one is inactivated to regulate the amount of expressed genetic information.
- Packaged into heterochromatin, making it less accessible for transcription.