Match Each Filament or Constriction to Its Job
Two cell-biology ideas recur: the three filament systems of the cytoskeleton, and the constrictions that help identify chromosomes at metaphase.
Living-system questions are solved by matching a structure or organism to its function, process and stated condition.
Cytoskeletal filaments
Actin microfilaments and microtubules made of α and β tubulin are the main systems that give the cell shape and drive motility (with motor proteins): muscle contraction, cell crawling, and the movement of chromosomes, cilia and flagella. Intermediate filaments such as desmin and keratin give mechanical strength, and lamins form the nuclear lamina that supports the nuclear envelope; they are not the main motility proteins.
Primary and secondary constrictions
The primary constriction is the centromere. A secondary constriction has a constant position on a chromosome, so it identifies that chromosome, and it is the nucleolar organiser region. Chromosomes with a secondary constriction cut off a small knob called a satellite are SAT chromosomes. In humans these are the acrocentric chromosomes 13, 14, 15, 21 and 22, with the satellites on their short arms.
OlympIQ exam lens
- ✓Structure and motility: actin microfilaments and tubulin microtubules
- ✓Intermediate filaments (desmin, keratin) give strength; lamins support the nuclear envelope
- ✓Secondary constriction = nucleolar organiser; satellites occur on SAT chromosomes
- ✓Human SAT chromosomes: 13, 14, 15, 21 and 22 (acrocentric)
Do not count every filament protein as a motility protein. Intermediate filaments and lamins mainly provide strength and nuclear support; actin and tubulin are the two systems behind movement.
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