Cell Nucleus: The Center of Information

Advertisements
Advertisements

Related Posts:


Cell Nucleus: The Center of Information (Campbell Biology)

The nucleus contains most of the genes in the eukaryotic cell. (Some genes are located in mitochondria and chloroplasts.) It is usually the most conspicuous organelle, averaging about 5 µm in diameter. The nuclear envelope encloses the nucleus, separating its contents from the cytoplasm.

The nuclear envelope is a double membrane. The two membranes, each a lipid bilayer with associated proteins, are separated by a space of 20–40 nm. The envelope is perforated by pore structures that are about 100 nm in diameter. At the lip of each pore, the inner and outer membranes of the nuclear envelope are continuous. An intricate protein structure called a pore complex lines each pore and plays an important role in the cell by regulating the entry and exit of proteins and RNAs, as well as large complexes of macromolecules. Except at the pores, the nuclear side of the envelope is lined by the nuclear lamina, a netlike array of protein filaments (in animal cells, called intermediate filaments) that maintains the shape of the nucleus by mechanically supporting the nuclear envelope. There is also much evidence for a nuclear matrix, a framework of protein fibers extending throughout the nuclear interior. The nuclear lamina and matrix may help organize the genetic material so it functions efficiently.

Within the nucleus, the DNA is organized into discrete units called chromosomes, structures that carry the genetic information. Each chromosome contains one long DNA molecule associated with many proteins. Some of the proteins help coil the DNA molecule of each chromosome, reducing its length and allowing it to fit into the nucleus. The complex of DNA and proteins making up chromosomes is called chromatin. When a cell is not dividing, stained chromatin appears as a diffuse mass in micrographs, and the chromosomes cannot be distinguished from one another, even though discrete chromosomes are present. As a cell prepares to divide, however, the chromosomes coil (condense) further, becoming thick enough to be distinguished under a microscope as separate structures. Each eukaryotic species has a characteristic number of chromosomes. For example, a typical human cell has 46 chromosomes in its nucleus; the exceptions are the sex cells (eggs and sperm), which have only 23 chromosomes in humans. A fruit fly cell has 8 chromosomes in most cells and 4 in the sex cells.

A prominent structure within the non-dividing nucleus is the nucleolus (plural, nucleoli), which appears through the electron microscope as a mass of densely stained granules and fibers adjoining part of the chromatin. Here a type of RNA called ribosomal RNA (rRNA) is synthesized from instructions in the DNA. Also in the nucleolus, proteins imported from the cytoplasm are assembled with rRNA into large and small subunits of ribosomes. These subunits then exit the nucleus through the nuclear pores to the cytoplasm, where a large and a small subunit can assemble into a ribosome. Sometimes there are two or more nucleoli; the number depends on the species and the stage in the cell’s reproductive cycle.

The nucleus directs protein synthesis by synthesizing messenger RNA (mRNA) according to instructions provided by the DNA. The mRNA is then transported to the cytoplasm via the nuclear pores. Once an mRNA molecule reaches the cytoplasm, ribosomes translate the mRNA’s genetic message into the primary structure of a specific polypeptide.

Source:

Urry, Lisa A.. Campbell Biology. Pearson Education. Kindle Edition. https://www.pearson.com/us/higher-education/series/Campbell-Biology-Series/2244849.html

Advertisements
Advertisements

Related Research

Research Article: Quantitative Analysis of Cell Nucleus Organisation

Date Published: July 27, 2007 Publisher: Public Library of Science Author(s): Carol Shiels, Niall M Adams, Suhail A Islam, David A Stephens, Paul S Freemont, Johanna McEntyre Abstract: There are almost 1,300 entries for higher eukaryotes in the Nuclear Protein Database. The proteins’ subcellular distribution patterns within interphase nuclei can be complex, ranging from diffuse … Continue reading

Research Article: An automated quantitative analysis of cell, nucleus and focal adhesion morphology

Date Published: March 30, 2018 Publisher: Public Library of Science Author(s): Antonetta B. C. Buskermolen, Nicholas A. Kurniawan, Carlijn V. C. Bouten, Thomas Abraham. http://doi.org/10.1371/journal.pone.0195201 Abstract: Adherent cells sense the physical properties of their environment via focal adhesions. Improved understanding of how cells sense and response to their physical surroundings is aided by quantitative evaluation … Continue reading

Research Article: The Cell Nucleus and Aging: Tantalizing Clues and Hopeful Promises

Date Published: November 15, 2005 Publisher: Public Library of Science Author(s): Paola Scaffidi, Leslie Gordon, Tom Misteli Abstract: Recent evidence links structural proteins in the cell nucleus with aging. Partial Text: There are a handful of biological questions that affect all of us directly in everyday life. How are emotions formed, what is the basis … Continue reading

Research Article: Virion Assembly Factories in the Nucleus of Polyomavirus-Infected Cells

Date Published: April 5, 2012 Publisher: Public Library of Science Author(s): Kimberly D. Erickson, Cedric Bouchet-Marquis, Katie Heiser, Eva Szomolanyi-Tsuda, Rabinarayan Mishra, Benjamin Lamothe, Andreas Hoenger, Robert L. Garcea, Walter J. Atwood. http://doi.org/10.1371/journal.ppat.1002630 Abstract: Most DNA viruses replicate in the cell nucleus, although the specific sites of virion assembly are as yet poorly defined. Electron … Continue reading

Research Article: RAG2 localization and dynamics in the pre-B cell nucleus

Date Published: May 10, 2019 Publisher: Public Library of Science Author(s): William Rodgers, Jennifer N. Byrum, Destiny A. Simpson, Walker Hoolehan, Karla K. Rodgers, Albert Jeltsch. http://doi.org/10.1371/journal.pone.0216137 Abstract: RAG2 of the V(D)J recombinase is essential for lymphocyte development. Within the RAG2 noncore region is a plant homeodomain (PHD) that interacts with the modified histone H3K4me3, … Continue reading