Biology Today — May 2017

9 Triplets

Centriole

Centrosphere

Fig.: Centrosome with pair of centrioles (Diplosome)

Centriole

Massule or

pericentriolar

satellite

C-A connective (proteinaceous linker)

Central rod (HUB)

Cart-wheel structure

9 Spokes

Triplet fibril

Fig.: Ultrastructure of centriole

BC

A

X

Y

• Centrioles replicate with the help of massules in S or G 2 phase to coordinate in animal cell division. Before nuclear division,
the two centrosomes separate and move to opposite ends at the place of spindle pole formation.

Functions

• Centrioles help in cell division by forming microtubule-organising centres (MTOCs).

• Out of the two centrioles in a spermatozoan, the distal one forms axial filament or tail.

• Centrioles can be transformed into basal bodies to give rise to cilia and flagella.

• Centrioles are capable of forming new centrioles with the help of massules which function as nucleating centres.

NUCLEUS

• Nucleus was first observed by Leeuwenhoek in RBCs and was first studied by Robert Brown in orchid root cells.

• It is the largest cell organelle having double membrane and carry all the genetic information.

• Commonly cells are uninucleate, Paramecium caudatum is binucleate having (macronucleus for controlling metabolic activities
and micronucleus that contains hereditary information). Cells of bone narrow, striated muscles, several fungi and algae are
multinucleate.

• A typical interphase nucleus is 5-25 μm in diameter and differentiated into nuclear envelope, nucleoplasm, nuclear matrix,
chromatin and nucleolus.

Chromatin

Nucleolus

Nuclear envelope

Ribosomes

Fibrous lamina

Heterochromatin

Nucleoplasm

Fig.: Ultrastructure of interphase nucleus

Nuclear pore

Euchromatin

Endoplasmic

reticulum

Outer nuclear membrane

Inner nuclear membrane

Perinuclear space

Karyosome

(False nucleolus)

Chromocentre