Chromosomes circular

Question What is the form taken by a replicating bacterial chromosome (circular DNA molecule) ... 

Circular chromosomes. Circular DNA has no end, therefore has no need for telomeres ... 

Plasmid DNAs. Plasmids are nucleic acid molecules capable of intracellular extrachromosomal repHcation. Usually plasmids are circular DNA species, but linear and RNA plasmids are known. In nature, plasmids can assume a variety of lifestyles. Plasmids can recombine into the host chromosome, be packaged into vims particles, and repHcate at high or low copy number relative to the host chromosome. Additionally, their information can affect the host phenotype. Whereas no single plasmid is usually capable of all these behaviors, the properties of various plasmids have been used to constmct vectors for a variety of purposes. 

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. 

Prokaryotic cells have only a single membrane, the plasma membrane or cell membrane. Because they have no other membranes, prokaryotic cells contain no nucleus or organelles. Nevertheless, they possess a distinct nuclear area where a single circular chromosome is localized, and some have an internal membranous structure called a mesosome that is derived from and continuous with the cell membrane. Reactions of cellular respiration are localized on these membranes. In photosynthetic prokaryotes such as the cyanobacteria,... 

Conjugative transposons are self-transmissible large DNA elements (up to 150kbp) located in the donor chromosome. After excision a circular intermediate is formed that is unable to replicate autonomously. It is nicked at a origin of transfer site and one strand is then transferred to the recipient cell. After generating a double-stranded circle the transposon integrates into the recipients chromosome. 

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. 

The majority of bacteria reproduce-by simple binary fission the circular chromosome divides into two identical circles which segregate at opposite ends ofthe cell. At the same time, the cell wall is laid down in the middle ofthe cell, which finally grows to produce two new cells each with its own wall and nucleus. Each ofthe two new cells will be an exact copy of the original cell from which they arose and no new genetic material is received and none lost. 

The linear RCA method can use both target and signal amplification. A DNA circle (such as a plasmid, circular vims or circular chromosome) is amplified by polymerase extension of a complementary primer. Up to 10 tandemly repeated, concantemerized copies of the DNA circle are generated by each primer, resulting in one single-stranded, concantemerized product. ... 

Eukaryotic chromosomes, unlike their bacterial counterparts, are linear rather than- circular. Since RNA oligonucleotides prime both prokaryotic and eukaryotic DNA synthesis, the 5 termini of the daughter... 

Bacteria normally harbour a single, circular chromosome that tends to be tethered to the bacterial plasma membrane and tends to have few if any closely associated proteins. Many bacteria also contain extra-chromosomal DNA in the form of plasmids, as will be discussed later. Eukaryotes (plants, animals and yeasts) posses multiple linear chromosomes contained within a cell nucleus, and these chromosomes are normally closely associated with proteins termed histones (the pro-tein-DNA complex is termed chromatin). Eukaryotes also invariably possess DNA sequences within mitochondria and in chloroplasts in plants. The (usually circular) DNA molecules are much... 

An essential feature of the cloning vector used is that it must be capable of self-replication in the cell into which it is introduced, which is usually E. coli. Two of the most commonly used types of vector in conjunction with E. coli are plasmids and bacteriophage X. Plasmids are circular extra-chromosomal DNA molecules, generally between 5000 and 350 0000 bp in length, that are found naturally in a wide range of bacteria. They generally house several... 

As well as chromosomal DNA many bacteria contain closed circular double-stranded DNA structures in their cytoplasm called plasmids. These plasmids are able to replicate independently of the chromosome and usually carry one or more genes that are responsible for a useful characteristic displayed by the host bacteria, e.g. antibiotic resistance. The number of plasmid molecules in each bacterium is known as the copy number and varies between different species of bacteria and types of plasmid. 

When genetic information (nucleic acid) is transferred between different cells, species or genera it is often carried by a specialized DNA molecule called a vector. Viruses are natural vectors, as are some kinds of small independently replicating circular extra-chromosomal DNA molecules (plasmids). A few of the basic features of plasmids used in molecular biology are reviewed in Figure 2.6. 

Plasmid A relatively (cf. the chromosome) small (>20 Kb), usually circular, double-stranded DNA molecule found in prokaryotes capable of replicating independently of the chromosome. Plasmids carry genes which are usually not essential for the growth of the organism except under special conditions. Some plasmids carry genes for antibiotic resistance. See also Ti-plasmid. Some plasmids however can be very large, e.g. the plasmids in Rhizobium species. 

---