Central Dogma and DNA Replication

Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as rRNA genes or tRNA genes, the product is a functional RNA. The process of gene expression is used by all known life - eukaryotes (including multicellular organisms), prokaryotes (bacteria and archaea) and viruses - to generate the macromolecular machinery for life.

When genes are expressed, the genetic information (base sequence) on DNA is first transcribed (copied) to a molecule of messenger RNA (mRNA). The mRNA molecules then leave the cell nucleus and enter the cytoplasm, where they participate in protein synthesis by specifying the particular amino acids that make up individual proteins.

The central dogma is the conceptual framework for understanding everything else that has anything to do with gene expression, DNA, RNA, proteins, or how these all relate to each other. In other words it refers to the flow of information from the genetic store (DNA) to phenotypic expressions.

DNA was always believed to be the sole medium of storage according to the Watson and Crick’s central dogma scheme until 1971, when it was discovered that some viruses can synthesize DNA from RNA. These viruses make proteins in the same way that higher organisms do. During infection, the RNA code is first transcribed using the enzyme reverse transcriptase in a process called reverse transcription to DNA, then mRNA and finally proteins. Such viruses are referred to as retroviruses.

DNA REPLICATION

Prior to cell division, the DNA material in the original cell must be replicated so that after cell division, each new cell contains the full amount of DNA material.

Replication can be broadly defined as genome duplication, an essential process for the propagation of cellular genome and those of ‘Molecular parasites’ – Viruses, plasmids and transposable elements.

culled from nature.com

DNA replication requires the concerted actions of a number of proteins which are clustered to together in a cell. DNA replication begins with the unwinding of the DNA double strand at the origin of replication to form a replication bubble by the enzyme helicase and stabilization by single strand binding proteins (SSB). Primase an RNA polymerase synthesizes short RNA strands that signal the start of DNA replication. DNA polymerase binds to the replication fork and strings new nucleotides to the growing DNA strand a clamp protein helps to hold DNA polymerase in place during this process. At the end RNAase H removes the short RNA strands used to begin the DNA strand. DNA ligase links the short discontinuously synthesized DNA strands. When helicase separates both strands, each strand then serves as a template for the synthesis of new DNA strands. DNA polymerase reads the strands in the 3'-5' direction as such new DNA strands are synthesized in the 5’-3’ direction by DNA polymerase. DNA polymerase does not start synthesis from the scratch it can only extend an existing strand hence primase is the enzyme which synthesizes short complementary RNA primers. Both DNA strands have an anti-parallel configuration. Two DNA polymerases are required for the synthesis of DNA from both strands of the parent DNA. Each of the two new DNA strands are synthesized in the 5’-3’ direction but one strand the leading strand is synthesized continuously while the other is synthesized discontinuously (lagging strand) with several short fragments of DNA called Okazaki fragments interrupted by RNA primers. Before DNA replication is complete, the interrupting RNA primers are removed which is done by RNAse H which recognizes RNA-DNA hybrids and digests the RNA primers, and the created gaps are filled by DNA polymerase in the 5’-3’ direction finally the nicks are then finally joined by DNA ligase which requires in an ATP or NAD dependent reaction. DNA polymerase is so accurate that it makes only about one error in every 107 nucleotide pairs it copies. 

This error rate is much lower than can be accounted for simply by the accuracy of complementary base pairing. Although A-T and C-G are by far the most stable base pairs, other, less stable base pairs—for example, G-T and C-A—can also be formed. Such incorrect base pairs are formed much less frequently than correct ones, but if they occur often enough that they would kill the cell through an accumulation of mistakes in the DNA if they were allowed to remain. This catastrophe is avoided because DNA polymerase can correct its mistakes. As well as catalyzing the polymerization reaction, DNA polymerase has an error-correcting activity called proofreading.

suggested further reading

http://en.wikipedia.org/wiki/DNA_replication

http://www.uq.edu.au/vdu/DNAReplication.htm