The Nucleus and Nucleolus

The nucleus

• The nucleus was the first organelle to be discovered .The nucleus also known as the control center is a membrane enclosed organelle found in eukaryotic cells.

• It contains most of the cell’s genetic material.

• The main structures making up the nucleus is the nuclear envelope a double membrane that encloses the entire organelle.

The Nucleolus

• The nucleolus is a non-membrane bound structure composed of proteins and nucleic acids found within the nucleus.

• The ribosomal RNA is transcribed in the nucleolus.

• There are three main parts that are recognized: the fibrillar centers, the dense fibrillar component, and the granular components.

The cell nucleus

• The function of the nucleus is to maintain the integrity of the genes and to control the activities of the cell by regulating gene expression.

• The nucleus is the control center of the cell.

• The cell nucleus has the directions for the proteins.

The nucleolus

• The nucleolus is the main part of the cell that produces the ribosomes that make the proteins.

The cell nucleus

The cell nucleus is surrounded by the nuclear envelope (also called nuclear

membrane, nucleolemma, or karyotheca).

The material within the nuclear envelope is the nucleoplasm or karyoplasm.

The components of the karyoplasm:

• DNA and its associated proteins, the chromatin

• skeletal elements, the nucleo- or karyoskeleton

• the fluid material surrounding the karyoskeleton and the chromatin, the nucleo-

or karyosol

Chromatid: one DNA chain and its associated proteins

Chromosome: densely packed ´transport´ form of chromatin with characteristic

shape during cell division. In functional sense, the term is also used for the

intermitotic phase.

The nuclear envelope: a double lipid bilayer that

encloses the genetic material in eukaryotic cells

The outer membrane is

continuous with the

endoplasmic reticulum

Nuclear lamina (the major component of nucleoskeleton) is

attached to the inside surface of the inner nuclear membrane

Electron microscopic image of the nuclear

lamina: a 2D mesh

The structure of the nucleoplasma

HC - Heterochromatin (electron dense granules): densely packed DNA without transcription

It has 2 forms, the constitutive HC, which is never transcribed, and is usually attached to

the nuclear envelope via the nuclear lamina. In contrast, facultative heterochromatin is

densely packed only in a particular cell type but could also be euchromatin.

EC - Euchromatin: less coiled DNA, the functional part of chromatin with intense mRNA synthesis

NU - Nucleolus: contains genes for ribosomal RNA

A

A

Nucleus

Euchromatine

Heterochromatine

Euchromatine

Heterochromatine

Nucleolus

The compact structure of chromosomes during mitosis

The 23 chromosome pairs of the human genom

Karyotype: chromosomes articially lined

up in order (e.g. for cytogenetic analysis) FISH: fluorescent in situ hybridization

EM01. Ultrastructure of the Cell nuclear morphologies, heterochromatin, euchromatin, and nucleolus

© 2002 Oxford University Press

nucleus: heterochromatin, euchromatin, and nucleolus

Euchromatin is a lightly packed form of chromatin (DNA, RNA and protein)

that is rich in gene concentration, and is often (but not always)is under active transcription,

Heterochromatin is a tightly packed form of DNA there is not RNA synthesis there

© 2002 Oxford University Press

Nuclear membrane

- is a double lipid bilayer that encloses the genetic material in eukaryotic cells.

- The nuclear membrane also serves as the physical barrier, separating the contents

of the nucleus (DNA in particular) from the cytoplasm.

- Many nuclear pores are inserted in the nuclear membrane, which facilitate and regulate

the exchange of materials (proteins such as transcription factors, and RNA) between the

nucleus and the cytoplasm.

Nuclear pores in the nuclear envelope shown by cryofracture EM