The cell. 4. Nucleus.
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Chromatin is composed of DNA and associated proteins, mainly histones.
Morphologically, there are two types of chromatin:
Euchromatin shows a clear loose aspect, is less condensed, and there is a higher gene expression.
Heterochromatin is dark and compact, is more condensed, and it is supposed that gene expression is lower.
Chromatin is located in the nucleoplasm, surrounded by the nuclear envelope. Chromatin is DNA plus associated molecules, mainly histones, involved in the DNA organization. DNA is composed of 4 deoxyribonucleotides (abbreviated as nucleotides). Every nucleotide contains a nitrogenous base, a pentose and a phosphate group. Nitrogenous base is either a purine base: adenine (A) and guanine (G), or a pyrimidine base: thymine (T) and cytosine (C). Pentose is a deoxyribose. Each nitrogenous base is bonded to a pentose resulting in a deoxyribonucleoside. Each deoxyribonucleoside is bonded to a phosphate group through the pentose molecule, together they form a deoxyribonucleotide. In this way, DNA is composed of a chain of nucleotides linked by phosphate groups. This is a single chain, but DNA is made of two strands, which are paired by the complementarity of the nucleotide sequence. A-T and G-C, each nucleotide of each par in one chain, establish hydrogen bridges that keep the two DNA chains tightly joined. It is said that both chains join in an antiparallel maner because one of their ends is 3' in one chain and 5' in the other, whereas the other end is 5' in one chain and 3' in the other chain. Both chains forms a double helix of about 2.5 nm in width.
Nucleotides are not exclusively located in the DNA. They can be found in other molecules with different functions. For example, ATP (adenosine triphosphate) is the principal molecule for energy transfer, and cAMP (cyclic adenosine monophosphate) is the second messenger of many molecular processes in the cell.
Cells of the intestine of a mammal at light microscopy. They are stained with haematoxylin. Nuclei are rounded and show intense and clear stained areas. Nuclear dense areas are heterochromatin, where more dye is present, whereas the more faint areas are euchromatin, where less dye is observed.
DNA is not naked in the nucleoplasm, but associated to proteins, most of them being histones. Other proteins which are part of chromatin are those involved in the DNA processing. All of them together are known as chromatin. Histones are DNA-associated proteins mainly involved in chromatin organization. There are two types of histones: nucleosome histones (H2A, H2B, H3 y H4) and H1 histone. The four nucleosome histones, along with DNA, form the nucleosome, which is the structural unit of chromatin organization. Other proteins are transiently associated to DNA, such as those involved in the DNA transcription (gene expression), DNA synthesis (replication), or chromatin compaction into heterochromatin and chromosomes.
When a nucleus is observed at interphase (the non mitotic part of the cell cycle) at transmission electron microscopy, clear and dark areas are observed. Dark areas mostly correspond to compact chromatin, referred as heterochromatin. Heterochromatin is usually located close to the nuclear envelope. Clear areas contain more loose chromatin, known as euchromatin. Sometimes, a rounded very dense area is observed in the nucleoplasm. It is the nucleolus. Nucleolus is composed of chromatin involved in the transcription of ribosomal RNA and ribosomal subunits assembling, as we will see in the next page.
Image at transmission electron microscope. Nuclei (N) of ependymal cells of a fish spinal cord. Black asterisks indicate euchromatin, loose and more clear. White asterisks indicate heterochromatin, more dense and compact.
Euchromatin usually corresponds to DNA regions being transcripted, whereas heterochromatin to DNA regions not being transcripted or of low transcription. Heterochromatin is classified into facultative heterochromatin, it can change between the euchromatin and heterochromatin forms, and constitutive heterochromatine, which is always condensed and it is up to 10-20 % of the total heterochromatin. Although it have been found some genes to be expressed at heterochromatin, most of genes at heterochromatin are not expressed. There is even a higher level of chromatin compactation when chromatin is going to form chromosomes, either in mitosis or meiosis. During this process, both euchromatin and heterochromatin are packed into chromosomes. It is noteworthy that when the unpacking of chromosomes occurs, each chromosome occupies one particular region of the nucleoplasm. In other words, chromatins from different chromosomes are not freely overlapped during interphase.
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Updated: 2018-01-28. 15:16
Atlas of Plant and Animal Histology
Dep. of Functional Biology and Health Sciences.
Faculty of Biology.
University of Vigo