ADVERTISEMENTS:
The following points highlight the four main replications of DNA. The replications are: 1. Plasmid 2. Mitochondrial DNA 3. Kinetoplast DNA 4. Chloroplast DNA.
Replication # 1. Plasmid:
In addition to the nucleoid DNA, the cytoplasm of bacterial cell contains one or more extra chromosomal mini-circular DNA duplexes which are known as plasmids that can replicate independently of the nucleoid DNA.
Plasmid DNA replicates from an unique origin but in one direction. Bubble structures are formed at the point of origin. At least three proteins bind near the origin and bring about supercoiling of the cyclic molecule, thereby exposing single-stranded origin to which RNA polymerase binds.
An RNA transcript (RNA II) is initiated from the point of origin. As a result a D-loop is formed with DNA/RNA transcript hybrid and single-stranded DNA (Fig. 20.15).
This hybrid is sensitive to RNAase H which fragments the transcript RNA producing several primers. Now DNA polymerase I initiates the synthesis of pieces of DNA with the help of RNA primers. On the lagging side of the fork, there is an n’ binding site (rriA) at which lagging-strand synthesis is initiated by DNA polymerase III and primo-some.
However, lagging-strand synthesis cannot pass origin region if a tail of un-hybridized RNA II remains and so plasmid replication remains unidirectional. Following replication, conversion into a duplex molecule is initiated by the binding of n’ protein to a recognition site (rriB).
When plasmid is not transferred to other cell or when it is not integrated into bacterial chromosome, then plasmid replication takes place within as the cell described above and no nick is produced.
But some bacterial cells contain a type of plasmid, called a sex or F-factor which is transferred to a recipient bacterial cell during conjugation. A bacterium possessing an F factor (F+) is male while a bacterium without F– factor acts as female.
ADVERTISEMENTS:
For conjugal transfer the F-factor replicates partly within the donor cell and partly within recipient cell. In conjugal transfer a nick is produced in one strand, particularly in H-strand [plasmid contains one H-strand (Heavy strand) and one L strand (Light strand) to make a duplex] and the endonuclease remains bound to the 5′-side of the nick.
Rolling circle replication then leads to the synthesis of a new duplex, .cyclic molecule and the parental H-strand can be passed to another bacterium through conjugation tube in the form of single-strand. The complementary strand of the single-stranded plasmid DNA is synthesised in the recipient cell and a circular duplex molecule is formed.
Replication # 2. Mitochondrial DNA:
Eukaryotic cell mitochondria contain a closed circular duplex DNA molecule. This DNA replicates unidirectionally from a unique origin. This origin region also contains the start site for the synthesis of both heavy and light strands of mitochondrial DNA. For this activity, mitochondrial DNA needs a mitochondrial RNA polymerase and a mitochondrial transcription factor.
A RNA transcript is synthesised all the way around the mitochondrial genome but it can be processed by RNAase H-like activity. This enzyme is a ribonucleic protein containing a nuclear-coded RNA and it cleaves transcript RNA into a number of primers for DNA synthesis.
Primers are extended by mitochondrial DNA polymerase y. On the other hand, a small RNA primer is synthesised by mitochondrial DNA primase which leads to initiation of L- strand synthesis. Both strands are synthesised and two daughter molecules separate.
Mitochondrial DNA in Tetrahymena and Paramoecium is not cyclic but is a linear duplex. In Tetrahymena, replication is initiated near the centre of the molecule and eye-shaped replication intermediates are formed. Replication is bidirectional. In Paramoecium, replication of mitochondrial DNA proceeds unidirectional from a cross- linked terminus.
Replication # 3. Kinetoplast DNA:
Kinetoplasts contain a mini-circle DNA which may have more than one origin of replication of heavy strand synthesis. The initiation of lagging strand synthesis appears to take place at multiple random sites.
Replication # 4. Chloroplast DNA:
The chloroplast DNA of higher plants and Chlamydomonas also contains circular DNA. This DNA replicates via D-loop structure.
