Nucleus

Nucleus

"Nucleus acts as the cell's information center"

                The largest and most easily seen organelle within a eukaryotic 

cell is the nucleus, first described by the botanist Robert Brown 

in 1831. Nuclei are roughly spherical in shape, and in animal 

cells they are typically located in the central region of the cell 

(figure below). In some cells, a network of fine cytoplasmic filaments 

seems to cradle the nucleus in this position.

Most mature plant cells contain large central  vacuoles that occupy a major portion of the 

internal volume of the cell, as well as organelles called chloroplasts, within which photosynthesis takes place. The cells of plants, fungi, 

and some protists have cell walls. Flagella occur in sperm of a few plant species but are otherwise absent in plant and fungal cells. 

Centrioles are also absent in plant and fungal cells.See figure

                    The nucleus is the repository of the genetic information that 

enables the synthesis of nearly all proteins of a living eukaryotic 

cell. Most eukaryotic cells possess a single  nucleus, although the 

cells of fungi and some other groups may have several to many 

nuclei. Mammalian erythrocytes (red blood cells) lose their nuclei 

when they mature. Many nuclei exhibit a dark-staining zone called 

a  nucleolus, which is a region where intensive synthesis of 

ribosomal RNA (rRNA)is taking place.

The Nuclear Envelope

            The surface of the nucleus is bounded by two phospholipid bilayer membranes , which together make up the nuclear envelope

(figure below). The outer membrane of the nuclear envelope is 

continuous with the cytoplasm’s interior membrane system, 

called the  endoplasmic reticulum(described in an earlier post).

Scattered over the surface of the nuclear envelope are what 

appear as shallow depressions in the electron micrograph but are, 

in fact, structures called nuclear pores (see figures)

These pores form 50 to 80 nm apart at locations where the two membrane 

layers of the nuclear envelope pinch together. The structure consists of a central framework with eightfold symmetry that is 

embedded in the nuclear envelope. This is bounded by a 

cytoplasmic face with eight fibers and a nuclear face with a complex ring that forms a basket beneath the central ring. The pore 

allows ions and small molecules to diffuse freely between 

nucleoplasm and cytoplasm while controlling the passage of proteins  and RNA-protein complexes. Transport across the pore is 

controlled and consists mainly of the import of proteins that 

function in the nucleus and the export to the cytoplasm of RNA 

and RNA–protein complexes formed in the nucleus.

                 The inner surface of the nuclear envelope is covered with a 

network of fibers that make up the nuclear lamina  (figure below). 

This is composed of intermediate filament fibers called nucleat lamins . This structure gives the nucleus its shape and is involved 

in the deconstruction and reconstruction of the nuclear envelope 

that accompanies cell division.

DNA Packaging

   

                                 In both prokaryotes and eukaryotes, DNA is the 

molecule that stores genetic information. In 

eukaryotes, the DNA is divided into multiple linear chromosomrs , which are organized with 

proteins into a complex structure called 

chromatin. It is becoming clear that the very structure of chromatin affects the function of DNA. 

Changes in gene expression that do not involve changes 

in DNA sequence, so-called epigenetic changes, involve 

alterations in chromatin structure. Although 

still not fully understood, this offers an  exciting new view of 

many old ideas.

                             Chromatin is usually in a more extended form that is 

organized in the nucleus, although we still do not completely 

understand this organization. When cells divide, the chromatin 

must be further compacted into a more highly condensed state 

that forms the X-shaped chromosomes visible in the light 

microscope.

The Nucleolus

                             Before cells can synthesize proteins in large quantity, they must 

first construct a large number of ribosomes  to carry out this synthesis. Hundreds of copies of the genes encoding the ribosomal 

RNAs are clustered together on the chromosome, facilitating ribosome construction. By transcribing RNA molecules from this 

cluster, the cell rapidly generates large numbers of the molecules 

needed to assemble ribosomes.

                                       The clusters of ribosomal RNA genes, the RNAs they produce, and the ribosomal proteins all come together within the 

nucleus during ribosome production. These ribosomal assembly 

areas are easily visible within the nucleus as one or more darkstaining regions called nucleoli (singular, nucleolus). Nucleoli can 

be seen under the light microscope even when the chromosomes 

are uncoiled.
Press the Bell-icon and Subscribe