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Definition of Nuclear Membrane
All eukaryotic cells (those found in animals, plants, protists, and fungi) have a control center called the nucleus. This is where genetic material, or DNA, is stored. Surrounding every nucleus is a double-layered membrane called the nuclear membrane or nuclear envelope. This membrane separates the nucleoplasm, or fluid inside the nucleus, from the cytoplasm, or the fluid outside the nucleus.
Appearance of Nuclear Membrane
A nuclear envelope is made up of two membranes: an outer membrane and an inner membrane. Each membrane is composed of phospholipids arranged in a bilayer. This means that the entire nuclear membrane has four rows of phospholipids. The inner and outer membranes are separated by the perinuclear space.
The outer nuclear membrane is continuous with the rough endoplasmic reticulum, an organelle important in making and transporting proteins. Both the rough endoplasmic reticulum and the outer nuclear membrane are covered in ribosomes, which are the actual sites of protein synthesis.
Attached to the inner nuclear membrane on the nucleoplasm side is the nuclear lamina. This is a sheet of proteins that provides support for and strengthens the nuclear envelope. The nuclear lamina also attaches to and anchors chromatin, loosely arranged DNA and protein.
Function of Nuclear Membrane
The nuclear membrane keeps your DNA inside the nucleus to protect it from surrounding substances in the cytoplasm. Additionally, the nuclear envelope can regulate what materials enter or exit the nucleus. Anything that needs to pass between the nucleoplasm and the cytoplasm can only do so through holes in the membrane called nuclear pores.
Nuclear pores only allow very small molecules, ions, or proteins to freely move into or out of the nucleus. Any large molecules that need to cross the nuclear membrane must have the appropriate labels. Proteins lining the nuclear pores will recognize these labels or tags and let the molecules cross. There are two kinds of tags: the nuclear localization signal and the nuclear export signal.
A nuclear localization signal (NLS) is a short amino acid sequence that tags a molecule for entry into the nucleus. Large molecules that are needed for DNA replication or transcription in the nucleus must have the NLS. This will allow them to be recognized and escorted into the nucleoplasm by nuclear pore proteins.
In contrast, large molecules made in the nucleus, like the different types of RNA, usually need to leave the nucleoplasm. Nuclear pore proteins will only let these molecules cross the membrane and enter the cytoplasm if they have a nuclear export signal attached to them. The nuclear export signal (NES) is a short amino acid sequence, similar to the NLS, but it has the opposite effect. The NES only allows molecules to exit the nucleus, while the NLS only allows molecules to enter the nucleus.
A nuclear membrane or envelope surrounds every nucleus. It's composed of an inner membrane and an outer membrane separated by the perinuclear space. The nuclear membrane keeps DNA inside the nucleus and protects it from materials in the cytoplasm. Nuclear pores determine which substances can enter or leave the nucleus by recognizing only those molecules with the appropriate tags, including nuclear localization signal (NLS) or nuclear export signal (NES).
Terminology to Remember
|Nuclear Membrane||double layer that surrounds every nucleus|
|Nucleus||central control center of all eukaryotic cells|
|Nucleoplasm||membrane separates the fluid inside the nucleus|
|Cytoplasm||the fluid outside the nucleus|
|Rough Endoplasmic Reticulum||an organelle that's important in making and transporting proteins|
|Ribosomes||sites of protein synthesis|
|Nuclear Lamina||a sheet of proteins that provides support for and strengthens the nuclear envelope|
|Chromatin||loosely arranged DNA and protein|
|Nuclear Pores||only allow very small molecules, ions, or proteins to move into or out of the nucleus|
|Nuclear Localization Signal (NLS)||a short amino acid sequence that tags a molecule for entry into the nucleus|
|Nuclear Export Signal (NES)||a short amino acid sequence similar to the NLS; has the opposite effect|
Watch the lesson's video until you can:
- Define nuclear membrane
- Illustrate the appearance of the nuclear membrane
- Understand its function
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