A human red blood cell has no nucleus, yet almost every other true cell carries exactly one, and it is called the brain of the cell. In this note we open that brain up and look at its four parts: what each looks like and what each does.
1. The brain of the cell
The organelle of a eukaryotic cell that is wrapped in a double-layered covering and holds protoplasmic fluid and a net of chromatin is the nucleus. It is called the brain, or life centre, of the cell.
In 1831 Robert Brown found the nucleus in the leaf cells of an orchid and gave it its name. The word Nucleus comes from the Latin Nux/nut.
A cell usually has a single nucleus. Prokaryotic cells have none. Mature sieve cells and human red blood cells also lose theirs once they are fully formed.
A structure with many nuclei is a coenocyte. Examples are the algae Vaucheria, Botrydium and Sphaeroplea, and the fungus Penicillium.
2. Shape, size and work
The nucleus is usually round, though it can be oval, fusiform or branched. It normally sits in the middle of the cell, but a large vacuole pushes it out to the edge.
A round nucleus is usually 5-25 micron across and takes up 10-15% of the cell. In a sperm, about 90% of the cell is nucleus.
The nucleus does three jobs: it controls every biological activity of the cell, it keeps hereditary traits safe through the chromosomes, and it takes part in making RNA and protein.
It is made mainly of nucleic acid and protein. It also holds water, lipid, enzyme, DNA, a little RNA and coenzyme.
3. The four parts
The nucleus has four parts: the nuclear envelope, the nucleoplasm, the nucleolus, and the nuclear reticulum, also called chromatin fibre. Tap the five points on Figure 1.16 to see who sits where. The pore gets its own point because it is a special feature of the envelope.
Figure 1.16Tap a point
Inside the nucleus
Nuclear envelope (double). The covering of the nucleus: two membranes, each a phospholipid bilayer.
Nuclear pore. A transport channel. Other envelopes do not have such pores.
Nucleoplasm. The inner fluid, in which the nucleolus and chromosomes lie.
Nucleolus. It makes RNA. Small, dense and round, with no membrane.
Chromatin fibre. DNA and protein, spread like a net while the cell is not dividing.
All five seen. This is the figure you may be asked to draw and label.
Drawn by hand, not to scale.
4. The envelope and its pores
The nuclear envelope is built of two membranes, and each membrane is a phospholipid bilayer. It carries countless special holes called pores, which no other cell envelope has.
A pore can contract and expand, and a protein network controls it. Eight protein granules keep the pore under control. The protein in the centre is the transporter, and it carries large molecules such as RNA in and out.
In places, the outer membrane of the nuclear envelope joins the endoplasmic reticulum.
Run the traffic yourself. Send an RNA out, and make the pore contract or expand.
pore trafficsend, then drag
Through the pore, out and in
The envelope has three jobs: it keeps the nuclear contents apart from the cytoplasm, it lets the two sides talk and trade through the pores, and by joining the ER it holds the nucleus firmly in place. Now learn the eight parts of the pore in Figure 1.17.
Figure 1.17Tap a point
Parts of the pore
Nuclear interior. The inside.
Fibre cage. Holds the proteins.
Anchor protein. Ties the transporter to the envelope.
Perinuclear space. The space between the two membranes.
Transporter. The central protein; it carries RNA in and out.
Ring subunit. A ring-shaped protein.
Spoke. Joins the proteins together.
Cytosol. The fluid outside the nucleus.
All eight seen. The transporter is in the middle, with rings and spokes beside it and the cage on top.
A hand-drawn side cut, not to scale.
5. The nucleoplasm
The clear, dense, granular fluid inside the envelope is the nucleoplasm, also called karyolymph. The nucleolus and the chromosomes lie in it.
Keep its difference from cytoplasm in mind. Nucleoplasm is amorphous, not colloidal. It is rich in protein and nucleic acid, and it has no pigment and no organelles.
6. The nucleolus
The small, dense, round body inside the nucleus is the nucleolus. Fontana first saw it in 1781, and Bowman gave it its name in 1840. Walk through the three dates in order.
datesTap a year
Who and when?
1781: Fontana sees the nucleolus, even before the nucleus itself is discovered.
1831: Robert Brown discovers the nucleus in orchid leaf cells.
1840: Bowman names the nucleolus.
The nucleolus is attached at the SAT (satellite) region of a particular chromosome. It has no membrane. It has three parts: fibrous, granular and matrix. Genes in the satellite of the SAT chromosome play a direct part in forming the nucleolus; that is its origin.
A nucleus usually has one nucleolus. Where no protein is made there is none, and where a lot of protein is made there may be several.
It is made mainly of protein, RNA and a little DNA. Its jobs are to make various RNAs, to make and store protein, and to act as a store of nucleotides.
7. The chromatin fibre
While a cell is not dividing, the net of fibres spread through the nucleus is the nuclear reticulum, or chromatin fibre. The mixture of DNA and its protein is the chromatin, and it takes Feulgen stain.
When the cell divides, the chromatin fibre coils up and shows as short, thick chromosomes. Each chromosome is made of DNA, RNA, histone and non-histone protein, with a little Ca and Mg.
The nuclear reticulum holds and carries hereditary traits. It also plays the main part in causing mutation and variation.
The features of the four parts are easy to mix up. Put each of the eight features in its right box.
sort themTap a chip, then pick a bin
Which part?
Pick up a chip, then drop it in a box. Ask: is this about the covering, the inner fluid, the small round body, or the fibre?
Right. Next one.
Not yet. Think again about which part this belongs to.
All in place. Pores and the ER link belong to the envelope. Karyolymph is another name for nucleoplasm, and the chromosomes lie in that inner fluid. The SAT region, RNA synthesis and the missing membrane belong to the nucleolus. The fibre of DNA and protein takes Feulgen stain.
8. Check yourself
Three questions in the exam's style. Start with the first.
check yourselfpick one answer
Question 1
The main chemical components of the nucleolus are:
i. Protein ii. RNA iii. Lipid
Right. i and ii are right: the nucleolus is made mainly of protein, RNA and some DNA.
ii and iii: iii is wrong, since lipid is not a main component.
i and iii: iii is wrong.
Not all three, because iii is wrong.
The second makes you match the parts again.
check yourselfpick one answer
Question 2
What is the function of the nuclear membrane?
Right. The nuclear membrane keeps the nuclear contents apart from the cytoplasm and protects them.
Ribosomes are made at the nucleolus.
Storing RNA is not a job of the nuclear membrane.
Forming microtubules is not its job.
Take the last one slowly.
check yourselfpick one answer
Question 3
Which structure is not of the cytoplasm?
The Golgi body is a cytoplasmic organelle.
Ribosomes lie free in the cytoplasm or on the ER.
Right. Chromatin fibres belong to the nuclear reticulum inside the nucleus, not to the cytoplasm.
The ER is the network lying in the cytoplasm.
The whole note on eight cards.
cut from the notetap to turn over
Eight cards
QuestionWhat is the nucleus? What is it called?
AnswerThe double-membrane-bound organelle of a eukaryotic cell holding protoplasmic fluid and chromatin network. It is the brain or life centre of the cell.
QuestionWhich cells lack a nucleus?
AnswerProkaryotic cells; mature sieve cells and human red blood cells.