Nucleus animal cell

The nucleus, nucleolus, and nuclear envelope of plant cells are like those of animal cells. 

Because the carotenoids favour hydrophobic domains they are generally localised in the membranes and lipoproteins of animal cells. In this location they can influence the oxidation of membrane lipids and prevent the passage of free radicals from one cellular compartment to another. Thus, DNA in the nucleus is protected from intracellularly generated ROS by (at least) the nuclear membrane and from extracellular ROS by a number of membranes. Should ROS reach the nucleus, base oxidation can occur. The base most susceptible to oxidation is guanine, although all other bases can also be affected. The cell has the ability to detect damaged bases, excise them. 

Bacteria, being procaryotic, do not show compartmentation of the biosynthetic processes. The genome of a bacterium relates directly to the cytoplasm of the cell. Transcription into mRNA can lead directly to translation, and the processes of transcription and translation are not carried out in separate organelles. Animal cells, being eucaryotic, show compartmentation of the transcription and translation processes. Transcription of the genome into mRNA occurs in the nucleus, whereas translation occurs in the cytoplasm. The messenger RNA in the eucaryote is usually modified by adding to it... 

Most of the DNA of animal cells is found in the nucleus, where DNA is the major constituent of the chromosomes. On the other hand, most of the RNA is located in the cytoplasm. Nuclear DNA exists as a thin, double helix only 2 nm wide. The double helix is folded and complexed with protein to form chromosomal strands approxim-ately 100 to 200 nm in diameter. Each chromosome contains a single DNA duplex. The human chromosomes vary in size the smallest contains approximately 4.6 X 10 base pairs of DNA, and the largest 2.4 X 10 base pairs. In contrast, the Escherichia coli chromosome has 4.5 x 106 base pairs. The DNA of die chromosomes is tightly packed and associated with both histone and nonhistone proteins. 

Not all the cellular DNA is in the nucleus some is found in the mitochondria. In addition, mitochondria contain RNA as well as several enzymes used for protein synthesis. Interestingly, mitochond-rial RNA and DNA bear a closer resemblance to the nucleic acid of bacterial cells than they do to animal cells. For example, the rather small DNA molecule of the mitochondrion is circular and does not form nucleosomes. Its information is contained in approximately 16,500 nucleotides that func-tion in the synthesis of two ribosomal and 22 transfer RNAs (tRNAs). In addition, mitochondrial DNA codes for the synthesis of 13 proteins, all components of the respiratory chain and the oxidative phosphorylation system. Still, mitochondrial DNA does not contain sufficient information for the synthesis of all mitochondrial proteins most are coded by nuclear genes. Most mitochondrial proteins are synthesized in the cytosol from nuclear-derived messenger RNAs (mRNAs) and then transported into the mito-chondria, where they contribute to both the structural and the functional elements of this organelle. Because mitochondria are inherited cytoplasmically, an individual does not necessarily receive mitochondrial nucleic acid equally from each parent. In fact, mito-chondria are inherited maternally. 

In spite of the variety of appearances of eukaryotic cells, their intracellular structures are essentially the same. Because of their extensive internal membrane structure, however, the problem of precise protein sorting for eukaryotic cells becomes much more difficult than that for bacteria. Figure 4 schematically illustrates this situation. There are various membrane-bound compartments within the cell. Such compartments are called organelles. Besides the plasma membrane, a typical animal cell has the nucleus, the mitochondrion (which has two membranes see Fig. 6), the peroxisome, the ER, the Golgi apparatus, the lysosome, and the endosome, among others. As for the Golgi apparatus, there are more precise distinctions between the cis, medial, and trans cisternae, and the TGN trans Golgi network) (see Fig. 8). In typical plant cells, the chloroplast (which has three membranes see Fig. 7) and the cell wall are added, and the lysosome is replaced with the vacuole. 

Available methods for carrying DNA into an animal cell vary in efficiency and convenience. Some success has been achieved with spontaneous uptake of DNA or electroporation, techniques roughly comparable to the common methods used to transform bacteria. They are inefficient in animal cells, however, transforming only 1 in 100 to 10,000 cells. Microiqjection—the injection of DNA directly into a nucleus, using a very fine needle—has a high success rate for skilled practitioners, but the total number of cells that can be treated is small, because each must be injected individually. 

FIGURE 21-37 Ring closure converts linear squalene to the condensed steroid nucleus. The first step in this sequence is catalyzed by a mixed-function oxidase (a monooxygenase), for which the cosubstrate is NADPH. The product is an epoxide, which in the next step is cyclized to the steroid nucleus. The final product of these reactions in animal cells is cholesterol in other organisms, slightly different sterols are produced, as shown. 

Many cells contain centrioles,53 little cylinders about 0.15 pm in diameter and 0.5 pm long, which are not enclosed by membranes. Each centriole contains a series of fine microtubules of 25 nm diameter. A pair of centrioles are present near the nucleus in most animal cells and play an important role in cell division. Together with surrounding materials they form the centrosome. However, centrioles have never been observed in plant cells. 

A eukaryotic cell is surrounded by a plasma membrane, has a membrane-bound nucleus and contains a number of other distinct subcellular organelles (Fig. 1). These organelles are membrane-bounded structures, each having a unique role and each containing a specific complement of proteins and other molecules. Animal and plant cells have the same basic structure, although some organelles and structures are found in one and not the other (e.g. chloroplasts, vacuoles and cell wall in plant cells, lysosomes in animal cells). 

Acentric fragments often are not incorporated into daughter nuclei at anaphase, but rather appear as micronuclei in the cytoplasm separate from the cell nucleus after cell division. The appearance of such micronuclei is a convenient manifestation of earlier chromosomal breakage, with loss of the distal fragment. Micronuclei can thus be used as an index of chromosomal breakage.392 Schmid394 has developed the bone marrow micronucleus test for use in experimental animals. 

Animal cells have two very distinct compartments, the cytoplasm and the nucleus, with a constant flow of distinct chemical compounds between them. While the cytoplasm is enclosed in a plasma membrane, the nucleus is enclosed by a nuclear envelope. The different organelles of the cell are immersed in the cytoplasm. Each of these cellular components has specific characteristics and functions that will be discussed below. 

Main intracellular compartments of an animal cell. Cytosol, endoplasmic reticulum, Golgi complex, nucleus, mitochondria, endosome, lysosome, and peroxisome are distinct compartments that are isolated from the rest of the cell by at least one selective membrane. 

Animal cells are bounded by a cell membrane or PM and within the interior cytosol are various membrane-bound organelles, namely the nucleus (containing the genome and surrounded by a double membrane having elaborate pore structures) the ER network the cis- and trans-Go g network of membranes (involved in processing, folding and glycosylation of... 

The nucleus is a large membrane-bound cell organelle which houses the chromosomes and which occupies roughly 10% of the volume of all eukaryotic cells. The nucleus is separated from the rest of the cell and the cytoplasm by a double membraue known as the nuclear envelope. The outer layer of the nuclear membrane is studded with small openings called nuclear pores, which allow for the controlled movement of selected molecules in and out of the nucleus. Most of a eukaryotic cell s DNA is found in the chromosomes of the nucleus, while a very small amount of DNA is present in the mitochondria. All plant and animal cells with a nucleus and known as eukaryotic cells, (meaning true nucleus) while bacterial cells which lack a nucleus are known as prokaryotic cells. 

Fig. 1.2 Typical Animal Cell - Eukaryotes The role of the nucleus is three-fold ...

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