Why do nucleus have pores

Without this detailed information access is denied. Why is a nuclear pore complex needed? In the item about the nuclear envelope we wrote of the need to keep the chromosomes and chemical reactions taking place in the nucleus separate from reactions taking place in the cytoplasm. This is true, but for the cell to function properly certain chemicals need to flow between organelles. This is called nucleocytoplasmic transport. The nucleus requires nucleotide triphosphates, proteins and various water soluble ions to make different types of RNA.

These are also needed to make ribosome sub-units for export to the cytoplasm and to enable the cell to divide when needed. Nuclear pore complexes control the flow into and out of the nucleus and check the credentials of all large molecules attempting to pass through. Imports to the nucleus The pore complex is constructed from more than one hundred different proteins and is a watery channel that can be as small as 9 nm in diameter.

Nuclear pores are fully permeable to small molecules up to the size of the smallest proteins, but form a barrier keeping most large molecules out of the nucleus. Yet, some larger proteins, such as histones, are granted admission into the nucleus despite the fact that that the pores should be too small to let them through. It is generally thought that the elaborate protein structure called the nuclear pore complex see Figure 1 that surrounds each pore plays a key role in allowing the active transport of a select set of large molecules into and out of the nucleus.

The nuclear pore complex is comprised of several subunits. Surrounding the inside of the pore is a nonmembranous material organized into an annulus that extends spoke-like structures toward the center of the small channel. The actual pore wall is predominantly comprised of columnar subunits, and lumenal subunits, with the help of transmembrane proteins, anchor the entire pore complex into the nuclear envelope. Also, tiny fibrils usually extend form both sides of the complex and congregate into basket-like configurations on the nuclear side of the complex.

The proteinaceous composition of these fibrils is different on opposing sides of the structure. In addition to their role in nuclear transport, nuclear pores are important as sites where the outer membrane and inner membrane of the nuclear envelope are fused together.

Due to this fusion, the membranes can be considered continuous with one another although they have different biochemical characteristics and can function in distinctive ways. Since the outer nuclear membrane is also continuous with the membrane of the endoplasmic reticulum ER , both it and the inner nuclear membrane can exchange membranous materials with the ER. Each nuclear pore is a large complex of proteins that allows small molecules and ions to freely pass, or diffuse, into or out of the nucleus.

Nuclear pores also allow necessary proteins to enter the nucleus from the cytoplasm if the proteins have special sequences that indicate they belong in the nucleus. These sequence tags are known as nuclear localization signals. Similarly, RNA transcribed in the nucleus and proteins that are destined to enter the cytoplasm have nuclear export sequences that tag them for release through the nuclear pores.

A nuclear membrane is a double membrane that encloses the cell nucleus. It serves to separate the chromosomes from the rest of the cell. The nuclear membrane includes an array of small holes or pores that permit the passage of certain materials, such as nucleic acids and proteins, between the nucleus and cytoplasm.