Cell Nucleus Structure & Function

Cell Nucleus Structure and Function

The nucleus is the cell’s genetic control center. Its two main structural components—the nuclear envelope and the nuclear contents—contribute to gene expression, DNA organization, and ribosome production.

Cell Types and the Nucleus

• Eukaryotic cells have a nucleus and other membrane-enclosed organelles.
• Prokaryotic cells lack a nucleus and membrane-enclosed organelles.
• In eukaryotic cells, DNA is contained within the nucleus. It provides instructions that guide cellular activity and protein production.

Structure of the Nucleus

Nuclear Envelope

The nuclear envelope surrounds the contents of the nucleus. It consists of:

• Outer membrane: A phospholipid bilayer that is continuous with the rough endoplasmic reticulum (rough ER).
• Inner membrane: A phospholipid bilayer associated with the nuclear lamina, a supportive layer made of lamin proteins.
• Nuclear pores: Protein complexes that span the envelope and regulate movement between the nucleus and cytoplasm.

Nuclear Contents

The main nuclear contents discussed are:

• Chromatin: DNA associated with proteins, particularly histones.
• Nucleolus: A region involved in producing ribosomal components.

Functions of the Nuclear Envelope

• Outer membrane and rough ER: Their continuity links nuclear genetic instructions to protein production. DNA is transcribed into RNA, and messenger RNA (mRNA) can be translated by ribosomes, including those on the rough ER. Proteins entering the rough ER may be modified and sent onward, such as to the Golgi apparatus.
• Inner membrane and lamins: Lamins help with DNA replication, transcription, and the organization and compaction of DNA.
• Nuclear pores: These control import into and export from the nucleus. Smaller molecules can move passively, while larger cargo, such as RNA or proteins, requires energy-dependent transport.

Chromatin and DNA

Chromatin is made primarily of DNA and histone proteins. DNA is negatively charged, while histones are positively charged. DNA wraps around groups of histone proteins to form nucleosomes. This packaging compacts DNA so it can fit in the nucleus and be organized for processes such as cell division.

When a cell divides, DNA becomes more tightly condensed into chromosomes. DNA has two major roles:

• Replication: Copying DNA so it can be passed on when cells divide.
• Transcription: Using DNA instructions to produce RNA, including mRNA, transfer RNA (tRNA), and ribosomal RNA (rRNA).

Histones also help regulate gene expression by influencing how accessible DNA is:

• Heterochromatin is tightly packed and generally less available for transcription. Histone methylation is associated with this condensed state and reduced transcription.
• Euchromatin is more open and generally more available for transcription. Histone acetylation is associated with this state and increased transcription.

Nucleolus and Ribosome Production

The nucleolus is the site of rRNA production and ribosomal assembly. The process proceeds as follows:

1. DNA is used to produce rRNA in the nucleus.
2. Ribosomal proteins are produced through protein synthesis outside the nucleus.
3. These proteins are imported into the nucleus and combine with rRNA in the nucleolus to form ribosomal subunits.
4. The subunits are exported through nuclear pores and form functional ribosomes.
5. Ribosomes—free in the cytoplasm or bound to the rough ER—use mRNA and tRNA to synthesize proteins.

Speakers and Sources Featured

• Professor Zach Murphy, presenting for Ninja Nerd.
• Nucleus Medical Media, whose animated segment provides an overview of the nucleus.