Robert Brown described the nucleus in 1831. It has a double nuclear envelope with pores, a nucleoplasm, nucleoli that make ribosomal RNA, and chromatin of DNA and histones. In a dividing cell the chromatin forms chromosomes, named by where the centromere lies. Microbodies are small vesicles of enzymes.
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Finding the nucleus
Robert Brown first described the nucleus as a cell organelle in 1831. Later, the material of the nucleus that takes up basic dyes was named chromatin by Flemming.
A nucleus that is not dividing is the interphase nucleus. It holds three things:
chromatin, long, thin threads of DNA and protein;
the nuclear matrix, also called the nucleoplasm;
one or more round bodies, the nucleoli (singular, nucleolus).
The nuclear envelope
The electron microscope shows that the nucleus is wrapped in a nuclear envelope of two parallel membranes.
The space between them is the perinuclear space, 10 to 50 nm wide.
The envelope is a barrier between what is inside the nucleus and the cytoplasm.
The outer membrane is usually continuous with the endoplasmic reticulum, and it bears ribosomes.
In places the two membranes fuse, leaving tiny nuclear pores. Through them, RNA and protein molecules move both ways, between the nucleus and the cytoplasm.
How many nuclei?
A cell normally has one nucleus, but the number can vary. Some mature cells have no nucleus at all.
The red blood cells (erythrocytes) of many mammals lose their nucleus as they mature.
The sieve tube cells of vascular plants have none either.
Would you call such a cell living? It still carries out the work of life for a while, though it cannot divide.
The nucleolus
The nucleoplasm holds the nucleolus and the chromatin.
Nucleoli are spherical.
They are not bound by a membrane, so their content is continuous with the nucleoplasm.
A nucleolus is the site of active ribosomal RNA (rRNA) synthesis.
Cells that make a lot of protein have larger and more numerous nucleoli.
Chromatin and chromosomes
In the interphase nucleus, the chromatin is a loose, tangled network of fibres. During cell division, it coils up into compact chromosomes.
Chromatin contains DNA, some basic proteins called histones, some non-histone proteins and some RNA. A single human cell has about two metres of DNA. It is shared among 46 chromosomes, which make 23 pairs.
The parts of a chromosome
Every chromosome has a primary constriction, the centromere. On its sides lie disc-shaped structures, the kinetochores. The centromere holds the two chromatids of the chromosome together.
By where the centromere lies, there are four kinds of chromosome.
Metacentric: the centromere is in the middle, giving two equal arms.
Sub-metacentric: it is slightly away from the middle, giving one shorter arm and one longer arm.
Acrocentric: it is close to one end, giving one extremely short arm and one very long arm.
Telocentric: it is at the end, so there is only one arm.
Four chromosomes drawn side by side, each as two blue chromatids with a red dot, the centromere, where they join. In the metacentric one the red dot is in the middle, so the two arms are equal. In the sub-metacentric one it is a little above the middle, giving a short arm and a long arm. In the acrocentric one it is close to the top, so the short arm is tiny. In the telocentric one it is at the very top, so there is no short arm.
Some chromosomes have a secondary constriction at a fixed place, which does not stain. It sets off a small piece called the satellite.
One more: microbodies
Many tiny vesicles, each bound by a membrane and full of enzymes, are found in both plant and animal cells. They are called microbodies.
Where is the centromere?
Slide the centromere from the middle to the end. Watch the arms and the name.
Try it in Python
The program names three chromosomes from the lengths of their arms. Its cut-offs are rough ones, chosen only for this program.
Practise with new numbers
Common mistakes
"The nucleolus has its own membrane." It has none. Its content merges with the nucleoplasm.
"Nuclear pores let things go only out of the nucleus." RNA and proteins pass both ways.
"Chromatin and chromosomes are different substances." They are the same material: loose in interphase, coiled up in a dividing cell.
"An acrocentric chromosome has no short arm." It has a very short one. A telocentric chromosome has none.
"Every cell has exactly one nucleus." Mature mammalian red blood cells and sieve tube cells have none.
Remember these
Robert Brown described the nucleus in 1831. Flemming named chromatin.
Nuclear envelope: two membranes, a perinuclear space, the outer one joined to the ER, and pores for two-way traffic.
Nucleoli: no membrane, the site of rRNA synthesis, larger in cells that make much protein.
Chromatin: DNA, histones, non-histone proteins and RNA. A human cell has about two metres of DNA in 46 chromosomes.
The centromere is the primary constriction, with kinetochores on its sides.
Metacentric (middle), sub-metacentric (a little off), acrocentric (near one end), telocentric (at the end).
A satellite lies beyond a non-staining secondary constriction.
Microbodies: small membrane-bound vesicles of enzymes, in plant and animal cells.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
Name the three things found in an interphase nucleus.
What passes through the nuclear pores, and in which direction?
Why does a cell that makes much protein have large nucleoli?
What is the difference between chromatin and a chromosome?
Where is the centromere in an acrocentric chromosome, and what arms does it give?
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