The Nucleus

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The two-dimensional image depicts the nucleus of a cell as a circular object with two membranes; several gaps appear in the circle, representing nuclear pores. Surrounding the nucleus are membranous sacks representing the endoplasmic reticulum. Inside the nucleus is another circle, approximately ten percent of the total size of the nucleus, representing the nucleolus.
The nucleus stores chromatin (DNA plus proteins) in a gel-like substance called the nucleoplasm. The nucleolus is a condensed chromatin region where ribosome synthesis occurs. We call the nucleus’ boundary the nuclear envelope. It consists of two phospholipid bilayers: an outer and an inner membrane. The nuclear membrane is continuous with the endoplasmic reticulum. Nuclear pores allow substances to enter and exit the nucleus.

Source: OpenStax Biology 2e

OpenStax Biology 2e

Typically, the nucleus is the most prominent organelle in a cell. The nucleus (plural = nuclei) houses the cell’s DNA and directs the synthesis of ribosomes and proteins.

The Nuclear Envelope

The nuclear envelope is a double-membrane structure that constitutes the nucleus’ outermost portion. Both the nuclear envelope’s inner and outer membranes are phospholipid bilayers.

The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and cytoplasm. The nucleoplasm is the semi-solid fluid inside the nucleus, where we find the chromatin and the nucleolus.

– What is a dense fibrillar network inside the nucleus of most cells and is composed of intermediate filaments and membrane associated proteins?

Chromatin and Chromosomes

To understand chromatin, it is helpful to first explore chromosomes, structures within the nucleus that are made up of DNA, the hereditary material. You may remember that in prokaryotes, DNA is organized into a single circular chromosome. In eukaryotes, chromosomes are linear structures. Every eukaryotic species has a specific number of chromosomes in the nucleus of each cell. For example, in humans, the chromosome number is 46, while in fruit flies, it is eight. Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, proteins attach to chromosomes, and they resemble an unwound, jumbled bunch of threads. We call these unwound protein-chromosome complexes chromatin. Chromatin describes the material that makes up the chromosomes both when condensed and decondensed.

– What are highly basic proteins found in eukaryotic cell nuclei that pack and order the DNA into structural units called nucleosomes?

Part a: In this illustration, DNA tightly coiled into two thick cylinders is shown in the upper right. A close-up shows how the DNA is coiled around proteins called histones. Part b: This image shows paired chromosomes.  The chromosomes are shown as a collection of slender tubes.
(a) This image shows various levels of chromatin’s organization (DNA and protein). (b) This image shows paired chromosomes. (credit b: modification of work by NIH; scale-bar data from Matt Russell)

The Nucleolus

We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus called the nucleolus (plural = nucleoli) aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported out through the pores in the nuclear envelope to the cytoplasm.

– What is the process of making ribosomes which occurs in nucleolus?

Viral eukaryogenesis is the hypothesis that the cell nucleus of eukaryotic life forms evolved from a large DNA virus in a form of endosymbiosis within a methanogenic archaeon. The virus later evolved into the eukaryotic nucleus by acquiring genes from the host genome and eventually usurping its role.

Source:

Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e

https://en.wikipedia.org/wiki/Nuclear_lamina

https://en.wikipedia.org/wiki/Histone

https://en.wikipedia.org/wiki/Nucleolus

https://en.wikipedia.org/wiki/Viral_eukaryogenesis


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