THE NUCLEUS (pl. nuclei)

The Nucleus

The nucleus is an essential part of a eukaryotic cell. It acts as the "brain" of the cell, serving two main functions: storing the cell's hereditary information (its DNA) and coordinating the cell's activities.

  • A double-layer membrane (Nuclear Envelope)
  • The nucleoplasm
  • Chromatin
  • Nucleolus
  • Nuclear pores

  • A basis for future development
  • Coordinates the cell's activities: growth, metabolism, protein synthesis, and cell division (reproduction)
  • Mitosis: Mitosis Animation

Biologists separate cells into categories: prokaryotes and eukaryotes. Prokaryotes are cells without a nucleus. Their genetic material (DNA) is stored in a region known as the nucleoid. This region, however, has no membrane that separates itself from the rest of the cell. Eukaryotes, on the other hand, literally means "true" and kernel" or nucleus. This means that a eukaryotic cell has a distinguished region where the nucleus is found that is bound by a double membrane.

Prokaryotic Cell

Eurkaryotic Cell

Only the cells of advanced organisms, the eukaryotes, have nuclei. Normally, there is only one nucleus per cell, but some organisms are an exception (such as some slime molds).

The nuclues takes a spherical shape, and it occupies about 10% of the volume of the cell (aka a big amount!). It is generally found close to the center of the cell, but not always! It would not, however, be found near the edge of the cell because it could possibly be damaged by an external item. Through the microscope, the nucleus looks like a dark spot within the ctyoplasm. the cytoplasm is the fluid that fills the cell.

  • The Nuclear Envelope: This is a double membrane that surrounds the nucleus. There is a layer between the layers of the membrane called the pernicular space, and this space attaches to the Endoplasmic Reticulum on the outside of the nucleus. The nuclear envelope is dotted with pores (see below). During cell division (mitosis), the nuclear envelope distintegrates but reforms when the two cells complete their formation.Nuclear Envelope
  • The Nucleoplasm: The nucleoplasm is a highly viscous fluid inside the nucleus that suspends and protects the nucleolus. Nucleoplasm Summary
  • Chromatin: Located inside the nucleus, chromatin is made of DNA and proteins. They make the cell what it is; they carry all the information that a cell needs to help it grow, thrive, and reproduce. Chromatin is the dense, fiber-like structure of DNA and proteins. Chromatin and DNA

DNA in Chromatin

  • Nucleolus: For information about the nucleolus, please see the section below.
  • Nuclear Pores: The nuclear envelope is perforated with nuclear pores. These pores regulates the entry and exit of certain macromolecules and particles. Nuclear Pore

Nuclear Envelope(ne) & Nuclear Pores (nep)

  • Endoplasmic Reticulum: The endoplasmic reticulum is continuous with the nuclear envelope. In this oragnelle secretory proteins are built and new proteins and phospholipids are added to the cell.


Nucleus location in a Plant Cell

Nucleus Location in Plant Cell

To quiz yourself on the nucleus, go to: Nucleus Structure Quiz
For further explaination and imagery:
Go to following site and click "next page" in green on the bottom of the page up to 3 times to see all Nucleus pages
Nucleus Structure and Function

General Information
Nuclear Envelope and Pore
Interactive Nucleus Animation
Nucleus Animation Video

THE NUCLEOLUS (pl. Nucleoli)

  • Lacks a membranenucleolus_image.jpg
  • Consists of granular and fibrillar components and roughly spherical
  • Size depends on protein requirements of the cell
  • Structure

  • To manufacture the subunits that combine to form ribosomes
    • ribosomes produce specific proteins to the cell
Ribosome producing specific proteins

The nucleolus is where ribosomes are synthesized and assembled. Once completely built they then pass through the nuclear pores to the cytoplasm. The ribosomes are the sites where the cell builds proteins. So essentially the nucleus controls the synthesis of proteins by sending genetic messages from DNA through mRNA into the cytoplasm through the nuclear pores.


The dark purple cirle in the right center is the Nucleolus

For more Information about the nucleolus, go to:
Mocecular Expression: Nucleolus
Nucleolus Structure and Function
Nucleolus Structure


Assembly & Reproduction of Peroxisomes

Microbodies are a a diverse group of organelles that are found in the cytoplasm of the cell. There are multiple types of microbodies, but they most common type are peroxisomes. All eukaryotic cells contain at least one peroxisome. Peroxisomes are similar to lysosomes (another type of microbody). Peroxisomes are assembled from proteins and consist of enyzme with specific functions, usually involved with metabolism. Originally discovered and defined as organelles, they complete oxidation of hydrogen peroxide, which is harmful to the human body.
Peroxisomes are bound a single membrane, not the more common double membrane.
Brown Trout Peroxisome
Marine Snail Peroxisome

  • Absorb nutrients that the cell has acquired (very well known for digesting fatty acids)
  • Their enzymes attack complex molecules and break them down into smaller molecules
  • Involved in lipid biosynthesis
  • In plants, peroxisomes in seeds are responsible for converting fatty acids to carbohydrates, and peroxisomes in leaves are involved in photorespiration
  • Peroxisome Function

Peroxisomes contain at least 50 different enzyme reactions that occur entirely or partially inside them. They are similar to lysosomes, but they hold on to enzymes that require oxygen instead of having enzymes that work in oxygen-poor areas (lysosomes).

A peroxisome is round or oval in shape, and a cell will contain several hundred of them. Their life span is not very long, lasting only a day or two. With this being said, a cell must constantly reproduce them. They were first discovered by Belgian scientist Christian de Duve (Christian René, burgrave de Duve).

The reproduction of peroxisomes is known as peroxisomal assembly, and it occurs as follows:
1) The proteins which will make up the peroxisome's membrane and matrix are synthesized by free ribosomes. The ribosome is the site at which messenger RNA, bringing genetic information from the DNA in the cell nucleus, is translated into the variety of proteins which make up the cell and its organelles.
2) The completed proteins enter the cytosol, which is that portion of the cell's interior that isn't either the nucleus or an organelle.
3) From the cytosol, the peroxisomal membrane and matrix proteins are imported into pre-existing peroxisomes, which exist either singly or in a network called a peroxisomal reticulum. These expand with the upload of the new material and at a certain point new peroxisomes are formed either by division or budding from the reticulum.

(Image in Video) African Mongoose Peroxisome

For more peroxisome information and projected images of peroxisomes visit:
Function and Assembly
Structure and Functions
Peroxisome Database

A real life look at when too many alcohol toxins accumulate and peroxisomes can't do their job in the liver!

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