The nucleus is a membrane bound organelle found in the majority of eukaryotic cells. It is the largest organelle of the eukaryotic cell, accounting for around 10% of its volume. It houses the genome, and through gene expression, it co-ordinates the activities of the cell.
In this article, we will consider the structure and function of the nucleus.
Structure
The nucleus is a relatively large and spherical membrane-bound organelle. The nucleus itself is comprised of distinct components, and understanding their structure allows a deeper understanding of their function.
Fig 1 – The nucleus
Nuclear envelope
The nucleus is completely surrounded by the nuclear envelope. This consists of both an inner and outer membrane which run parallel to each other. The envelope is perforated by small gaps known as the nuclear pores. These pores are around 100nm wide in true diameter, however due to the presence of central regulatory proteins the true size of the gap is around 9nm.
This small size controls the passage of molecules into and out of the nucleus. Larger molecules such as larger proteins and nucleic acid are unable to pass through these pores, and so the function of the nuclear envelope is to selectively separate the contents of the nucleus from that of the cytoplasm.
Nuclear lamina
Mechanical support for the nucleus is provided by the nuclear lamina. This is a protein mesh, which is more organised on the internal surface on the nucleus than on the cytoplasmic surface.
Chromatin
Chromatin describes DNA that is complexed with proteins. The primary protein components of chromatin are histones, which are highly basic proteins that associate readily with DNA. Histones combined with DNA form nucleosomes, which are the subunit of chromatin. Specifically, a nucleosome describes a segment of DNA associated with 8 histone proteins. By associating with histones, DNA is more compact and able to fit into the nucleus.
Chromatin can exist as either euchromatin or heterochromatin. Euchromatin is the form of chromatin present during gene expression, and has a characteristic ‘beads on a string’ appearance. It is activated by acetylation. In contrast, heterochromatin is the ‘inactive’ form, and is densely packed. On electron microscopy, euchromatin stains lighter than heterochromatin which reflects their relative densities.
Fig 2 – Schematic diagram of euchromatin and heterochromatin
Nucleolus
The nucleolus is the site of ribosome and ribosomal RNA production. On microscopy, it appears as a large dense spot within the nucleus. After a cell divides, a nucleolus is formed when chromosomes are brought together into nucleolar organising regions. During cell division, the nucleolus disappears.
Function
The information above can be simplified into three key functions:
- Cell compartmentalisation: The presence of a selectively permeable nuclear envelope separates the contents of the nucleus from that of the cytoplasm.
- Gene expression: Gene expression first requires transcription, which is the process by which DNA is transcribed into mRNA. As the nucleus is the site of transcription, proteins within the nucleus play a key role in regulating the process.
- Processing of pre-mRNA: Newly synthesised mRNA molecules are known as pre-mRNA. Before they exit the nucleus, they undergo a process known as post-transcriptional modification where molecules are added or removed from the structure.
Fig 3 – Nucleus with the cisternae of a continuous endoplasmic reticulum highlighting its main features.
