The two replication forks meet at this site, thus, halting the replication process.
In eukaryotes, the linear DNA molecules have several termination sites along the chromosome, corresponding to each origin of replication.
Figure 1. Replication of circular prokaryotic chromosome. Replication of a circular chromosome is schematically shown. Replication is initiated by unwinding of a specific site of the chromosome, the origin of replication (oriC), followed by the synthesis of two leading strands and lagging strands in opposite directions. Elongation proceeds bidirectionally, with the same rate of synthesis at the two replication forks. Elongation terminates at a specific site, the terminus of replication.
RNA polymerase searches DNA for initial site. It unwinds a short stretch double helical DNA to produce a single stranded DNA template from which it takes instructions. Also selects the correct ribonucleotide and catalyzes the formation of phosphodiester bond and detects the termination signals which specify where a transcript ends. RNA polymerase interacts with activated and repressor proteins that modulate the rate of transcription initiation over a wide dynamic range. In prokaryotes, only one type of RNA polymerase is present. It transcribes mRNA, tRNA and rRNA. Eukaryotes possess three RNA polymerases: RNA polymerase I, II and III.
Different replicons of prokaryotes and eukaryotes utilize distinct mechanisms which vary in complexity, depending on the complexity of the organisms. A common feature of replication initiation control in E. coli genomes and plasmids is the presence of repeats of A^T rich sequences which facilitate unwinding of DNA and one or multiple repeats of a "dnaA box" to which the initiator DnaA protein in E. coli or its functional homolog (called Rep in other cases) binds to allow helical unwinding and primer synthesis. The level of DnaA protein regulates the initiation frequency and, in turn, is controlled at the level of transcription of the dnaA gene. Thus, there are complex negative autofeedback loops to control dnaA gene expression. DnaA regulates its own gene, and its steady-state level in the cell is determined by the cellular growth state. The frequency of replicon firing is dependent on the growth rate of the bacteria. As mentioned before, rapidly growing cells can have multiple copies of the genome, while cells with a very low growth rate have only one copy. Furthermore, as expected in cells with multiple genome copies, the genes near the origin will have a higher average copy number than the genes located near the terminus of replication and, therefore, will be more transcriptionally active.
coli's rRNA genes are also repeated, but only about 3-10 times.) This repetition is presumably due to the fact that the cell must be able to make a lot of this transcript (rRNA) in order to make ribosomes.
The initiation of translation of most eukaryotic mRNAsinvolves recognition of the cap followed by either the first downstream AUG orby a 5' proximal AUG with a consensus sequence surrounding it (like thebacterial or the viral sequence).
This ensures that errors only occur about once per 100 000 amino acids.
The large subunit places pearls on the string
The role of the large subunit in the ribosome is primarily to synthesize new protein.
At the other end, there is the specific amino acid which matches the codon.
Thus emerged an image of the most fundamental process of life: the manner in which information flows from DNA to RNA and become enzymes and other proteins.
One profound implication of the specialized telomere structure and its synthesis is that in the absence of telom-erase, the repeat length of telomeres could not be maintained. Telomerase is active in neonatal cells and also in some immortal tumor cells, but is barely detectable in diploid, terminally differentiated mammalian cells. Most such diploid cells can multiply in vitro in specialized culture medium, but have a limited life span. Loss of replica-tive capacity is associated with shortening of telomere repeat lengths. Furthermore, ectopic and stable expression of telomerase in human diploid cells by introduction of its gene confer an indefinite reproductive life on such cells. It is generally believed that cells will senesce if the telomere length is reduced below a critical level after repeated replication of the genome.
The error rate in replication of mammalian genome is about 10-6 to 10-7 per incorporated deoxynucleotide. The catalytic units of the replication machinery, namely, DNA polymerases, have a significantly higher error rate of the order of 10-4 to 10-5 per deoxynucleotide. In fact, some DNA polymerases, notably the reverse transcrip-tases of retroviruses, including HIV, the etiologic agent for AIDS, are highly error prone and incorporate a wrong nucleotide for every 102-103 nucleotides. These mistakes result in a high frequency of mutation in the viral protein, which helps the virus escape from immunosurveil-lance. The overall fidelity of DNA replication is significantly enhanced by several additional means. The editing or proof-reading function of the replication machinery is a 3′ ^ 5′ exonuclease (which is either an intrinsic activity of the core DNA polymerase or is present in another subunit protein of the replication complex) which tests for base pair mismatch during DNA replication and removes the misincorporated base. Such an editing function is also present during RNA synthesis. In addition, after replication is completed, the nascent duplex is scanned for the presence of mispaired bases. Once such mispairs are marked by mismatch recognition proteins, a complex mismatch repair process is initiated, which causes removal of a stretch of the newly synthesized strand spanning the mismatch, followed by resynthesis of the segment, as described later.
The maintenance of genomic integrity in the form of the organism-specific nucleotide sequence of the genome is essential for preservation of the species during propagation. This requires an extremely high fidelity of DNA replication. Errors in RNA synthesis may be tolerated at a significantly higher level because RNAs have a limited half-life, even in nondividing cells, and are redundant. In contrast, any error in DNA sequence is perpetuated in the future, as there is only one or two copies of the genome per cell under most circumstances. Obviously, all organisms have a finite rate of mutation, which may be necessary for evolution. Genetic errors are one likely cause of such mutations. Inactivation of a vital protein function by mutation of its coding sequence will cause cell death. However, mutations that affect nonessential functions could be tolerated. Some of these mutations can still lead to change in the phenotype, which in extreme cases can cause pathological effects. In other cases, these may be responsible for susceptibility to diseases. In many cases, however, such mutations appear to be innocuous and are defined as an allelic polymorphism. The mammalian genome appears to have polymorphism in one out of several hundred base pairs. Such mutations obviously arose during the evolution and subsequent species propagation.
Unlike in bacteria and plasmids, DNA replication in eu-karyotic cells is extremely precise, and replication initiation occurs only once in each cell cycle to ensure genomic stability. "Licensing" is the process of making the chro-matin competent for DNA replication in which a collection of proteins called origin recognition complex (ORC) bind to the ori sequences. This binding is necessary for other proteins required for the onset of the S phase to bind to DNA. ORC is present throughout the cell cycle. However, other proteins required for replication initiation and chain elongation are loaded in a stepwise fashion. The onset of the S phase may be controlled by a minichromo-some maintenance (MCM) complex of proteins which licenses DNA for replication, presumably by making it accessible to the DNA synthesis machinery. Several protein factors are involved in the loading process, which is regulated both positively and negatively. The level of regulator proteins, such as geminin, which blocks licensing, is also regulated by some cell cycle-dependent feedback mechanisms.