Structure of Nucleus

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Chapter: Anatomy and Physiology for Health Professionals: Levels of Organization : Cells

1. How does the nucleus control the activities of a cell? 2. What is chromatin? 3. What is the nuclear envelope?


Nucleus

The nucleus is the largest, most visible organelle inside a cell. The nucleus serves as the control center for cellular operations. It contains DNA or genetic material that controls cell activities. A single nucleus stores all required information that directs the synthe-sis of the 100,000 proteins in the human body. A cell without a nucleus cannot repair itself, disintegrating within 3 to 4 months. The nucleus contains the genetic instructions needed to synthesize the proteins that determine cell structures and functions. These instruc-tions are stored in the chromosomes­. These structures consist of DNA and various proteins­ that control and access genetic information. Most cells contain a sin-gle nucleus, with the exception of skeletal muscle cells (with numerous nuclei) and mature red blood cells (with no nuclei). Along DNA strands, information is stored in a sequence of nitrogenous bases, which include adenine (A), thymine (T), ­cytosine (C), and guanine (G). Genes are the ­functional units of DNA.

The nucleus of a cell is usually round and is enclosed in a double nuclear envelope, with inner and outer lipid membranes. This envelope also has a protein lining, allowing certain molecules to exit the nucleus. Communication between the nucleus and cytosol occurs through nuclear pore complexes, which are large protein complexes that have a central channel. Ions and small molecules move freely through the cen-tral channel while it is open. However, large molecules (proteins, RNA, etc.) require energy for their transport process. Inside the nucleus is a fluid called nucleoplasm that suspends the following structures:

Nucleolus: A transient “mini nucleus” made upmostly of RNA, enzymes, and protein molecules are called histones; nucleoli have no surrounding membranes. Ribosomal RNA forms in the nucleo-lus, and ribosomes and their subunits migrate out to the cell’s cytoplasm via carrier-mediated trans-port at the nuclear pores. Nucleoli form around parts of DNA containing instructions for produc-ing ribosomal proteins and RNA as these processes are occurring. The nucleoli are prominent in cells making large amounts of proteins, which include those of the liver, muscles, and nerves, since these cells require large amounts of ribosomes. When stained, nucleoli appear dark in color.

Nucleosome: Nuclear DNA stores instructions for protein synthesis. Each molecule of DNAs thousands of genes and holds informationrequired to synthesize thousands of proteins.The DNA interacts with histones to help determine information that can be supplied to the cellwhen needed. The chemical “language” used bycells is called the genetic code. The genetic codeis a triplet code because of its sequence of threenitrogenousbases that specifies a single aminoacid. At certain ­intervals, strands of DNA wind around the histones to form a complex called cleosome. This winding allows more DNA tofit in smaller spaces. Chains of nucleosomes maycoil around various proteins. The amount of coiling is based on whether cell division is occurring. Chromatin:Loosely coiled DNA andproteinfibers that condenseforming chromosomes(­FIGURE 3-12). The DNA controls proteinsynthesis, and when the cell starts to divide, the chromatin fibers coil tightly to form the chro-mosomes. Chromatin causes a cell nucleus to have a grainy, clumped appearance.



Gene Activation in Protein Synthesis

Genes are kept inactive by bound histones, and in order for a gene to affect a cell, the histones must be tempo-rarily removed. The part of the DNA molecule contain-ing that gene can then uncoil. This partially understood process is called gene activation. Every gene has seg-ments that regulate its own activity. These segments are nitrogenous-based triplets. Signals from the triplets that control activities form regions known as controlsegments or promoters at the start of each gene. Everygene ends with a “stop” signal. Temporary disruption of weak hydrogen bonds between nitrogenous bases of two DNA strands, followed by the removal of the his-tone guarding the promoter, results in gene activation.

Then, the enzyme called RNA polymerase binds to the promoter. This is the first step intranscription, in which all three types of RNA are synthesized from a DNA template. Transcription is the copying or rewrit-ing of information. The most important type of RNA to understand is called messenger RNA or mRNA. It carries the information required to synthesize ­proteins. Its synthesis is essential since DNA cannot leave the cell nucleus. Information is copied to mRNA, which leaves the nucleus, carrying information to the cytoplasm, where protein synthesis occurs.


Translation

When protein synthesis occurs, functional poly-peptides are assembled in the cytoplasm. It occurs through translation, which is the forming of a linear amino acid chain. It uses information from an mRNA strand. Messages from nucleic acids are translated by the ribosomes so that they can be utilized by proteins. Every mRNA codon designates a certain amino acid to be put into the polypeptide chain. The amino acids are provided via transfer RNA or tRNA, which is comparatively small and mobile. The molecules of tRNA bind and deliver a specific amino acid. There are more than 20 types of tRNA—at least one for each amino acid used in protein synthesis.

Molecules of tRNA have tails that bind amino acids. At approximately the middle section, a nucle-otide chain of tRNA forms a tight loop. This loop can interact with a strand of mRNA. In the loop are three nitrogenous bases, which form an anticodon. During translation, there is complementary bonding of the anticodon with an appropriate mRNA codon. The anticodon’s base sequence indicates which type of amino acid is carried by the tRNA. The moleculesof tRNA provide a physical link between codons and amino acids. During translation, every codon in the mRNA strand binds a complementary anticodon on a molecule of tRNA. The sequence of amino acids in the polypeptide chain is based on the arrangement of codons on the mRNA strand.

The process of translation can produce a typi-cal protein in approximately 20 seconds. The mRNA strand stays intact and is able to interact with other ribosomes. It can also create extra copies of the same polypeptide chain. After a few minutes or hours, the mRNA strands are broken down and there is recycling of the nucleotides. Large numbers of protein chains can be produced simultaneously. Though the entire strand can have thousands of codons, only two mRNA codons are examined by a ribosome at one time. Therefore, many ribosomes can bind to just one mRNA strand.

At a single moment, each ribosome examines a different part of the same overall message. Each of them will construct a copy of the same protein as the other ribosomes. A polyribosome, also called a poly-some, is a series of ribosomes that are attached to thesame mRNA strand.


Nuclear Control of Cell Structure and Function

In a cell’s nucleus, the DNA directs specific protein synthesis and is able to control almost all variable of cell structure and function. The nucleus has two levels of control:

Direct control: over structural protein synthesis,including cytoskeletal components, receptors, and other membrane proteins, and secretory products. Via instructions in mRNA strands, the nucleus can change the internal structure of the cell, its sensitivity to environmental substances, and its secretory functions.

Indirect control: over all other factors of cellularmetabolism. This occurs by regulation of enzyme synthesis. All metabolic activities and cell func-tions can be regulated when the nucleus orders or stops production of certain enzymes. One exam-ple is when the nucleus speeds up glycolysis by increasing certain enzymes in the cytoplasm.

Often, gene activation or deactivation is trig-gered by cytoplasmic changes. The nucleoplasm may be sufficiently affected to turn certain genes on or off. Oppositely, hormones or messengers can enter the nucleus through nuclear pores, binding to ­specific promoters or receptors on the DNA strands. Therefore, continued but relatively selective chemi-cal communication occurs between the nucleus and cytoplasm. This selectivity is due to the restrictiveactions of the nuclear pores and the barrier of the nuclear envelope. Communication also continues between the cytoplasm and extracellular fluid, across the plasma membrane. Certain communications may later alter gene activity.


1. How does the nucleus control the activities of a cell?

2. What is chromatin?

3. What is the nuclear envelope?

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