Quick look: Found only in animal cells and some lower plants, a centriole is composed of short lengths of microtubules lying parallel to one another and arranged around a central cavity to form a cylinder.
In animal cells centrioles are located in, and form part of, the centrosome where they are paired structures lying at right angles to one another. In this context they are possibly involved in spindle assembly during mitosis. The centrosome is positioned in the cytoplasm outside the nucleus but often near to it.
A single centriole is also to be found at the basal end of cilia and flagella. In this context it is called a ‘basal body’ and is connected with the growth and operation of the microtubules in a cilium or flagellum.
To view an image of centrioles in sperm of Drosophila, interpreted using CIMR GridPoint technology, CLICK HERE
Centrioles present something of an enigma
Centrioles are present in (1) animal cells and (2) the basal region of cilia and flagella in animals and lower plants (e.g. chlamydomonas). In cilia and flagella centrioles are called ‘basal bodies’ but the two can be considered inter-convertible.
Centrioles are absent from the cells of higher plants.
When animal cells undergo mitosis they are considered by some to benefit from the presence of centrioles which appear to control spindle fibre formation and which later has an effect on chromosome separation. Research however has shown that mitosis can take place in animal cells after centrioles have been destroyed. Sometimes this seems to be at the expense of abnormalities in spindle development and subsequent problems with chromosome separation. Recent research also suggests that embryos of Drosophila arrest very early if centriole replication cannot take place.
In higher plants mitosis takes place perfectly satisfactorily with microtubules forming spindle fibres but without the help of centrioles. The function of centrioles therefore remains something of a mystery.
A centriole is composed of short lengths of microtubules arranged in the form of an open-ended cylinder about 500nm long and 200nm in diameter. The microtubules forming the wall of the cylinder are grouped into nine sets of bundles of three microtubules each.
In cilia and flagella where centrioles are at the base of the structure, and are called basal bodies, the wall and cavity architecture is slightly different. In addition to cylinder walls composed of nine sets of bundles of three microtubules, there are walls of nine sets of two bundles. In both types there is a central matrix from which spokes radiate as in a cart wheel.
In animal cells centrioles usually reside in pairs with the cylindrical centrioles at right angles to each other.
Centrioles organise a ‘cloud’ of protein material around themselves; this is the pericentriolar material (PCM). Together the two constitute the all important centrosome.
Centrioles function as a pair in most cells in animals but as a single centriole or basal body in cilia and flagella.
Centrioles in pairs
Cells entering mitosis have a centrosome containing two pairs of centrioles and associated pericentriolar material (PCM). During prophase the centrosome divides into two parts and a centriole pair migrates to each end or pole on the outside of the nuclear membrane or envelope. At this point microtubules are produced at the outer edge of the pericentriolar material and grow out in a radial form. The centriole pair and PCM is called an aster. Microtubules from the aster at one pole grow towards the aster at the opposite pole. These microtubules are called spindle fibres. Some of these will become attached by centromeres to chromosomes lined up on the ‘equator’ of the dividing cell. Others, though not attached to chromatids/chromosomes by centromeres, will assist in pushing apart the two parts of the dividing cell.
A single centriole or basal body.
At the base of each cilium or flagellum there is a single centriole. This structure and associated pericentriolar material, construct microtubules in a linear direction. These microtubules form most of the inside of cilia and flagella and are largely responsible, using protein motors, for the mechanical aspects of their movement. The centriole at the base of each one also appears to exert some degree of direction and control over the movement of the cilia and flagella.
In cells where centrioles are present as a pair, replication takes place during the whole of the cell cycle. In phase G1 the two centriole cylinders move very slightly apart from one another. During S phase new cylinders of microtubules form near, and at right angles to, the two ‘mother’ cylinders. The two pairs of centrioles keep very close to one another until the prophase stage of mitosis. At this point they separate with both pairs of centrioles moving over the outer surface of the nuclear envelope to opposite ends or ‘poles’ of the cell, to form the astral poles of the dividing cell.
- Centrioles occur as paired cylindrical organelles together with pericentriolar material (PCM) in the centrosome of an animal cell.
- Centrioles are found as single structures in cilia and flagella in animal cells and some lower plant cells.
- Centrioles are constructed of microtubules.
- In animal cells centrioles organise the pericentriolar material to produce microtubules including mitotic spindle fibres.
- Centrioles present something of an enigma; they appear to have an effect on the outcome of mitosis in animal cells. When present there is a satisfactory division but without them mitosis still takes place but sometimes with an unsatisfactory outcome. Centrioles are absent from the cells of higher plants but normal mitosis takes place and with satisfactory results.
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