Chromosomes eukaryotic

Adolph KW. Chromosomes Eukaryotic, Prokaryotic and Viral, CRC Press, Boca Raton, FL, 1991. 

FIGURE 24-5 Eukaryotic chromosomes, (a) A pair of linked and condensed sister chromatids from a human chromosome. Eukaryotic chromosomes are in this state after replication and at metaphase during mitosis, (b) A complete set of chromosomes from a leukocyte from one of the authors. There are 46 chromosomes in every normal human somatic cell. 

Yeasts Two or more chromosomes (eukaryote), single-celled near spherical, 5-12 p-m diameter Usually asexual by budding, under some conditions sexual (via spore cells) Air, water, and soil (beer, wine, spirits, bread)... 

C. W. Schmid, N. Deka, and G. Matera Repetitive human DNA The shape of things to come. In Chromosomes Eukaryotic, Prokaryotic, and Viral. Volume 1. K. W. Adolph, ed., Boca Raton, FL CRC Press (1990). 

Organisms (excluding viruses) can be broadly separated into prokaryotes and eukaryotes on the basis of cellular structure. Prokaryotic cells contain no nuclear membranes and their DNA is not arranged in chromosomes. Eukaryotic cells always contain a nuclear membrane and their DNA is usually in chromosomal form. There are other differences between the two cell types but these are the most useful general distinguishing features. The prokaryotes comprise all bacteria (including cyanobacteria) and are therefore unicellular (also termed the Monera). All other organisms are eukaryotes, either unicellular (the Protista) or multicellular. 

DNA material is found in chromosomes. Eukaryotic (higher level cells) chromosomes are each made of a single strand of DNA and several different kinds of proteins (Hale et al., 1995). These chromosomes exist in pairs (said to be diploid), except in some lower forms of algae or fungi that have single sets of chromosomes only (said to be haploid). Mammalian germ cells are also haploid. Some... 

Eig. 5. Restriction map of the yeast artificial chromosome (YAC) vector used for cloning very large fragments of eukaryotic DNA. Terms defined in text... 

The native form of chromatin in cells assumes a higher order stmcture called the 30-nm filament, which adopts a solenoidal stmcture where the 10-nm filament is arranged in a left-handed cod (Fig. 5). The negative supercoiling of the DNA is manifested by writhing the hehcal axis around the nucleosomes. Chromatin stmcture is an example of toroidal winding whereas eukaryotic chromosomes are linear, the chromatin stmctures, attached to a nuclear matrix, define separate closed-circular topological domains. 

Eukaryotic ceils possess a discrete, membrane-bounded nucleus, the repository of the cell s genetic material, which is distributed among a few or many chromosomes. During ceil division, equivalent copies of this genetic material must be passed to both daughter ceils through duplication and orderly partitioning of the chromosomes by the process known as mitosis. Like prokaryotic... 

The DNA in a eukaryotic cell nucleus during the interphase between cell divisions exists as a nucleoprotein complex called chromatin. The proteins of chromatin fall into two classes histones and nonhistone chromosomal proteins. 

The eukaryotic somatic cell cycle is defined by a sequential order of tasks a dividing cell has to complete it must replicate its DNA, segregate its chromosomes, grow, and divide. The cell cycle can be divided into four discrete phases. DNA replication is restricted to S phase (DNA synthesis phase), which is preceded by a gap phase called G1 and followed by a gap phase called G2. During mitosis (M phase) the sister chromatids are segregated into two new daughter nuclei and mitosis is completed by the division of the cytoplasm termed cytokinesis (Fig. 1). 

Bacterial as well as eukaryotic chromosomes contain too much DNA to fit easily into a cell. Therefore, the DNA must be condensed (compacted) to fit into the cell or nucleus. This is accomplished by supercoiling the DNA into a highly condensed form. When relaxed circular DNA is twisted in the direction that the helix turns, the DNA becomes positively supercoiled, if it is twisted in the opposite direction, it is called negatively supercoiled. Bacterial DNA is normally found in a negatively supercoiled state. Supercoiling reactions are catalyzed by topoisomerases. 

Kornberg RD, Lorch Y (1999). Twenty-five years of the nucleosome, fundamental particle of the eukaryote chromosome. Cell 98 285-294. 

Tubulins arose very early during the course of evolution of unicellular eukaryotes and provide the machinery for the equipartitioning of chromosomes in mitosis, cell locomotion, and the maintenance of cell shape. The primordial genes that coded for tubulins likely were few in number. As metazoan evolution progressed, natural selection processes conserved multiple and mutant tubulin genes in response to the requirements for differentiated cell types (Sullivan, 1988). 

In terms of evolutionary biology, the complex mitotic process of higher animals and plants has evolved through a progression of steps from simple prokaryotic fission sequences. In prokaryotic cells, the two copies of replicated chromosomes become attached to specialized regions of the cell membrane and are separated by the slow intrusion of the membrane between them. In many primitive eukaryotes, the nuclear membrane participates in a similar process and remains intact the spindle microtubules are extranuclear but may indent the nuclear membrane to form parallel channels. In yeasts and diatoms, the nuclear membrane also remains intact, an intranuclear polar spindle forms and attaches at each pole to the nuclear envelope, and a single kinetochore microtubule moves each chromosome to a pole. In the cells of higher animals and plants, the mitotic spindle starts to form outside of the nucleus, the nuclear envelope breaks down, and the spindle microtubules are captured by chromosomes (Kubai, 1975 Heath, 1980 Alberts et al., 1989). 

CHROMATIN IS THE CHROMOSOMAL MATERIAL EXTRACTED FROM NUCLEI OF CELLS OF EUKARYOTIC ORGANISMS... 

From individual to individual within a single species, the pattern of staining (banding) of the entire chromosome complement is highly reproducible nonetheless, it differs significantly from other species, even those closely related. Thus, the packaging of the nucleoproteins in chromosomes of higher eukaryotes must in some way be dependent upon species-specific characteristics of the DNA molecules. 

In diploid eukaryotic organisms such as humans, after cells progress through the S phase they contain a tetraploid content of DNA. This is in the form of sister chromatids of chromosome pairs. Each of these sister... 

A third class of sequence elements can either increase or decrease the rate of transcription initiation of eukaryotic genes. These elements are called either enhancers or repressors (or silencers), depending on which effect they have. They have been found in a variety of locations both upstream and downstream of the transcription start site and even within the transcribed portions of some genes. In contrast to proximal and upstream promoter elements, enhancers and silencers can exert their effects when located hundreds or even thousands of bases away from transcription units located on the same chromosome. Surprisingly, enhancers and silencers can function in an orientation-independent fashion. Literally hundreds of these elements have been described. In some cases, the sequence requirements for binding are rigidly constrained in others, considerable sequence variation is... 

Like some eukaryotic viruses (eg, herpes simplex, HIV), some bacterial viruses can either reside in a dormant state within the host chromosomes or can rephcate... 

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