Nuclear envelopeThe boundary of the nucleus. Two membranes, an outer and an inner, broken by pores.
NucleoplasmThe semi-solid fluid inside the nucleus. The chromatin and the nucleolus sit in it.
NucleolusA darkly staining region where ribosomal RNA is joined with proteins. This builds the parts of ribosomes. The plural is nucleoli.
EuchromatinThe less compact form of chromatin (DNA and its proteins). It holds genes the cell uses often.
Rough endoplasmic reticulumMembrane sacs studded with ribosomes. Its membrane runs on from the outer nuclear membrane.
Outer nuclear membraneThe outer of the envelope's two membranes. It is continuous with the rough ER and carries ribosomes.
Inner nuclear membraneThe inner of the envelope's two membranes. The edge of the nucleus's contents.
Nuclear poreA channel through both membranes, lined by a protein complex. It lets RNA, proteins and ions pass between nucleus and cytoplasm.
The whole nucleus: 6 1
HeterochromatinThe more compact form of chromatin, holding DNA that is rarely read.
The envelope close up: 5 2
Perinuclear spaceThe space between the two nuclear membranes. It is continuous with the inside of the ER.
Nuclear laminaA meshwork of filaments on the inner face of the envelope. It gives the nucleus its shape.
How this list was compiled: Read 2026-09-26. OpenStax Biology 2e, section 4.3 Eukaryotic Cells, https://openstax.org/books/biology-2e/pages/4-3-eukaryotic-cells : chromatin (DNA plus proteins) in a gel-like nucleoplasm, the semi-solid fluid where the chromatin and nucleolus are found; the nucleolus as a darkly staining area that builds ribosome parts; the nuclear envelope of two phospholipid bilayers, an outer and an inner membrane, continuous with the endoplasmic reticulum and punctuated with pores that control the passage of ions, molecules and RNA. Wikipedia, Cell nucleus, https://en.wikipedia.org/wiki/Cell_nucleus : the outer membrane continuous with the endoplasmic reticulum membrane and studded with ribosomes; the perinuclear space between the membranes, continuous with the ER lumen; nuclear pore complexes where the inner and outer membranes fuse; the nuclear lamina as a meshwork on the internal face of the envelope; euchromatin as the less compact form with frequently expressed genes, heterochromatin as the more compact form, infrequently transcribed. Licence: own work
The drawing: Drawn for this site by script, 2026-09-26. Own work, nothing traced. Licence: own work
How it works
A nucleus diagram labeled for biology: the whole nucleus cut open, with the rough ER running out of it, and a close-up of the envelope showing both membranes, the space between and a pore. Start with the 8 main parts.
Read the full guide
The envelope is two membranes
The nuclear envelope is an outer and an inner membrane with the perinuclear space between them. Where they meet they form nuclear pores. The outer membrane runs on into the rough ER, and the lamina lines the inner one.
What is inside
The nucleoplasm holds the chromatin, DNA wound with proteins: loose euchromatin and tightly packed heterochromatin. The nucleolus is where the parts of ribosomes are built.
Nucleus and nucleolus sound alike: the nucleolus is the dark blob inside the nucleus.
Questions and answers
What does the nuclear envelope do?
It separates the DNA from the cytoplasm. Its pores control what passes in and out: RNA leaves for the ribosomes, and proteins and ions come in.
What is the difference between euchromatin and heterochromatin?
Both are chromatin, DNA wound with proteins. Euchromatin is loosely packed and holds genes the cell reads often; heterochromatin is densely packed and rarely read.
Is the nucleus connected to the endoplasmic reticulum?
Yes. The outer nuclear membrane is continuous with the rough ER membrane and carries ribosomes, and the space between the two nuclear membranes opens into the inside of the ER.