Nucleus
The nucleus is the controlling and coordinating centre of the cellular factory. This usually occupies about one-tenth (1/10) of the volume of the cell. The nucleus of the cell which is not undergoing cell division is called interphase nucleus. The nucleus is enveloped by the porous nuclear membrane and the fluid substance present in the nucleus is called karyolymph or nucleoplasm. The nucleus encloses a spherical body called nucleolus. Inside the interphase nucleus is a thread-like network called chromatin, which form chromosomes during the cell division. The chromatin mainly consists of DNA (deoxyribonucleic acid), the hereditary material.
Nuclear membrane
The nuclear membrane is a double membrane structure, with pores or openings at intervals. Each membrane is 90 Å thick and the space between the two membranes is called perinuclear space, 100 to 300 Å wide. Each nuclear pore or opening is about 200 to 400 Å in diameter. The nuclear membrane is connected with the membranes of ER at many places, so the space between inner and outer nuclear membranes is directly connected with the lumen of endoplasmic reticulum. The nuclear membrane acts as a barrier that separates the contents of nucleus from the cytoplasm as well as permits the passage of materials between the nucleus and cytoplasm.
Nucleolus
The nucleolus is a spherical body found within the nucleus and it contains proteins and RNA. The nucleolus is constituted of four components: particulate component, fibrillar component, amorphous part, and chromatin part. The particulate component or the granular part is constituted of ribonucleoprotein particles (RNPs) which are 150 to 200 Å in diameter. The fibrillar part is composed of fibrils, which are 50 to 150 Å in diameter. The particulate and fibrillar components together constitute the nucleolonema. The amorphous part is rich in proteins and may be composed of closely packed fibrils and granules. The chromatin part is composed of the network of chromatin, which contains abundant DNA. The nucleolus plays a vital role in protein synthesis by producing RNA. This nucleolar RNA is a precursor for the formation of ribosomal RNA. The nucleolus disappears at the end of prophase and reappears at the end of telophase of cell division.
Chromatin
Chromatin appears as a network of thread-like material in the interphase nucleus, which condenses into chromosomes during cell division. Chromatin exhibiting differential thickening (or heteropycnosis) during certain stages of cell cycle is called heterochromatin, while the remaining which does not show heteropycnosis is called euchromatin. The heterochromatin is generally associated with kinetochore (or centromere), the nucleolar organizer, and telomeres or chromosomal ends. The euchromatin accounts for the bulk of the chromosome, which shows normal chromosomal coiling (isopycnosis).
Chromosomes
Chromosomes are nuclear components, which appear as rods with two longitudinal functional subunits termed chromatids. Chromosomes in condensed metaphase and anaphase stages appear as being composed of chromatids (chromonemata), kinetochore, constrictions, and satellite bodies. Chromosomes of somatic cells occur in pairs called homologous chromosomes, which exhibit characteristic behavior during meiosis. A pair of chromosomes, associated with the determination of sex is called sex-chromosomes or allosomes, and the rest of the chromosomes are known as autosomes. For example, in humans 2n = 46 = 44 autosomes + 2 allosomes (XY in male and XX in female). The major chemical components of the metaphasic chromosomes are DNA, RNA, and proteins. Chromosomes play an important role in variation, heredity, mutation and evolution. They are hence considered as the principal vehicles of hereditary transmission.
DNA
DNA is the primary hereditary material found in almost all living systems. It mainly occurs in chromosomes, but traces of DNA are also found in mitochondria and centrioles. DNA is a macromolecule made up of a chain of nucleotides. A nucleotide is a fundamental unit of DNA and is composed of a nitrogenous base, the pentose sugar, and the phosphate. The #nitrogenous bases of DNA are Adenine (A), #Guanine (G) (Purine bases), and Thymine (T) and Cytosine (C) (the Pyrimidine bases). The pentose sugar in DNA is the #deoxyribose type. The double helical model of DNA was proposed by Watson and Crick, and is built like a spiral #staircase with the #phosphate and sugar constituting the twisted frame of the staircase, and the #nitrogenous bases forming the steps. The base pairs of the double helixes are joined by weak hydrogen bonds. #Adenine pairs with thymine by two hydrogen bonds, while guanine pairs with #cytosine by three hydrogen bonds.
RNA is another genetic material found in all living systems, its main function being directing protein synthesis. RNA is also the primary #hereditary material in some viruses. RNA is a #macromolecule made up of a chain of #nucleotides
Anatomy of a nucleus biology regents curve | |
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