A cell has no bones, yet it holds its shape, moves, and pulls its chromosomes to opposite ends when it divides. Behind all of that sit one small tube and three kinds of thread.
1. The centriole and the centrosome
Near the nucleus of an animal cell, and in some plant cells too, there is a short tube. It can duplicate itself. It is made of nine groups of peripheral microtubules around a cavity, and it is called the centriole.
Von Benden discovered the centriole in 1887, and Theodor Boveri named it in 1888. It is present in algae, fungi, mosses (bryophytes), ferns (pteridophytes), gymnosperms and most animal cells. It is absent from prokaryotic cells, diatoms, yeast and angiosperms.
A pair of centrioles together is a diplosome. One centriole is tubular, about 0.15-0.25 μm across, like a barrel open at both ends. It has no envelope and no DNA or RNA. It is made of protein, lipid and ATP.
A centriole has three parts: the cylinder wall, the triplets and the linkers. The wall is built of nine triplets. Each triplet has three subtubules, which Threadgold in 1968 labelled A, B and C from inside to outside.
The dense fluid around the centriole is the centrosphere. The centrosphere together with the centriole is the centrosome, a name Boveri gave in 1888. Do not mix them up: the centriole is only the tube, the centrosome is the tube plus the fluid around it. Tap the four points on Figure 1.14.
Figure 1.14Tap a point
Parts of the centrosome
The centriole. A tube of nine triplets.
Subtubules (A, B, C). Three in each triplet, named from inside to outside: A, then B, then C.
The centrosphere. The dense fluid around the centriole.
The centrosome. The centrosphere and the centriole together.
All four seen. The centriole is just the tube; the tube and the fluid around it make the centrosome.
Drawn by hand after the book's figure, not to scale.
The centriole does four jobs: it forms the spindle fibres in cell division, helps the division, forms cilia and flagella, and forms the tail of a sperm.
2. The cytoskeleton and microtubules
Among the organelles of a eukaryotic cell, protein filaments join up into a net-like framework. It is the cytoskeleton, a term Koltzoff used in 1928.
The framework is built from three kinds of filament: microtubules, microfilaments and intermediate filaments.
Robert and Franchi found microtubules in animal nerve cells in 1953. Ledbetter and Porter first saw them in a plant cell in 1963.
A microtubule is an unbranched, long, hollow tube that runs all through the cytoplasm, 18-20 millimicron across. One end is marked “+” and the other “−”. Each has 13 protofilaments arranged in a spiral, and each protofilament is made of α-β tubulin dimer protein.
Microtubules are sub-components of flagella and cilia. They lie in spindle fibres, centrioles and basal bodies, and attach to the centromere.
Their jobs: moving cilia and flagella, forming the mitotic apparatus and separating chromosomes, directing the arrangement of microfibrils, helping to build the cell wall, and giving the cell rigidity.
Watch the tube being built, then see what it does when the cell divides.
microtubulerun it
From dimer to spindle
3. Microfilaments and intermediate filaments
The very fine, contractile protein threads that take part in cell movement are microfilaments, also called actin filaments. Paleviz first saw them in 1974.
Microfilaments are thin, contractile, twisted double strands, 30-60 Å across. They are made of the proteins actin and myosin and lie like bands under the cell membrane.
They give the cell shape and strength, and control cytoplasmic movement, phagocytosis, pinocytosis and cytokinesis.
Intermediate filaments lie between microtubules and microfilaments in size, about 10 nm across. Four kinds occur: keratin, lamin, neurofilament and vimentin.
Put the three diameters side by side. The thinnest is the microfilament, 30-60 Å. In the middle is the intermediate filament, 10 nm. The thickest is the microtubule, 18-20 millimicron.
sort themTap a chip, then pick a bin
Which filament?
Pick up a chip, then drop it in a bin. Ask: which protein, what diameter, what job.
Right. Next one.
Not yet. Recall which filament is a hollow tube, which is contractile, and which is the middle size.
All in place. Tubulin, the hollow tube and the spindle fibre are microtubules. Actin and myosin, and phagocytosis, belong to microfilaments. Keratin, vimentin and 10 nm belong to intermediate filaments.
4. Who and when
Pairs of years and names come straight into exam questions. Tap the six years and read them in order.
yearsRead them in order
Who and when?
1887. Von Benden discovers the centriole.
1888. Boveri names the centriole and the centrosome.
1928. Koltzoff uses the word “cytoskeleton”.
1953. Robert and Franchi find microtubules in animal nerve cells.
1963. Ledbetter and Porter first see microtubules in a plant cell.
1974. Paleviz first sees microfilaments.
5. Check yourself
Three questions in the exam's style. Start with the first.
check yourselfpick one answer
Question 1
The functions of the centriole are:
i. Forming spindle fibres during cell division ii. Helping the polar movement of chromosomes iii. Synthesising enzymes needed for the Calvin cycle
Right. i and ii are right, but iii is claimed too: it makes Calvin cycle enzymes.
The centriole forms spindle fibres and aids cell division; it does not make Calvin cycle enzymes.
ii and iii: iii is wrong.
Not all three, because iii is wrong.
In the second, tell the filaments apart.
check yourselfpick one answer
Question 2
In which does a linker exist?
A plastid has no linkers.
The lysosome has no linkers.
The vacuole has no linkers.
Right. A centriole has three parts: the wall, the triplets and the linkers.
Read the last one with care.
check yourselfpick one answer
Question 3
Who discovered the centriole?
Strasburger worked on cell division and plant cytology but did not discover the centriole.
Right. Van Beneden discovered the centriole in 1887.
Boveri gave the centriole its name; he did not discover it.
De Duve discovered the lysosome and the peroxisome.
The whole note on seven cards.
cut from the notetap to turn over
Seven cards
QuestionWhat is a centriole?
AnswerA short self-duplicating tube near the nucleus of animal (and some plant) cells, made of nine groups of microtubules around a cavity.
QuestionWhat are the centrosphere and the centrosome?
AnswerCentrosphere: the dense fluid around the centriole. Centrosome: centrosphere and centriole together.
QuestionName the three filaments of the cytoskeleton.
AnswerMicrotubules, microfilaments and intermediate filaments.
QuestionWhat is the shape and diameter of a microtubule?