Okazaki fragment
In molecular biology, the Okazaki fragment is the name of one of the short sections of the subsequent strand of DNA that arise during DNA replication . In prokaryotes such a fragment is 1000-2000 nucleotides long in eukaryotes 100 to 200. It is named after the Japanese scientist Tsuneko Okazaki and her husband Reiji Okazaki , who proposed the replication mechanism of the 1968th
In the process of replication, a double-stranded DNA molecule is doubled, so that two identical double-stranded DNA molecules are created. For the beginning of the replication, the existing DNA double helix must first be untied at certain points by a helicase activity and the binding of the hydrogen bonds between the base pairs of the two antiparallel strands must be broken .
At the open position, the origin of replication , or origin , the DNA strands are thus separated from each other than two single sections. These are now used as a template for the construction of a complementary strand determined by base pairing . Each of the two old strands is thus supplemented by a newly formed strand to form a double strand. The two DNA double strands formed in this way semiconservatively have the same base sequence - apart from errors that occur rarely - since the original strands were also complementary to one another. The genetic information is then available in two copies.
Starting from the origin of replication, the division of the double strand into two single-stranded areas proceeds as a Y-shaped fork during replication along the DNA in one direction and, bidirectionally, also in the opposite direction. If you take a closer look at the process of new synthesis of complementary DNA strands taking place in a replication fork , one difference becomes clear: From the origin, the enzyme DNA polymerase can continuously build up the complementary strand on one template strand, but not - here on the other template strand discontinuously one after the other, synthesized complementary DNA strands, in so-called Okazaki fragments, and later connected to one another.
This is because the polymerization catalyzed by DNA polymerases takes place step by step by adding an activated nucleotide ( dNTP ) to the 3 'end of the nucleic acid strand that has already been synthesized, and is therefore only possible in the 5' → 3 'direction. The matrix required for the complementary structure is oriented anti-parallel, i.e. in the direction 3 '→ 5'. Also, since the two separated template strands are oriented antiparallel to each other, a DNA polymerase can only proceed to the template strand in a direction of movement of the traveling replication fork and continuously the so-called leading strand (English leading beach build). On the other hand, on the other strand of the template, a DNA polymerase has to process in the opposite direction and thus against the direction of movement of the replication fork. Therefore be constructed here on the exposed first template strand regularly fragmentary sections and these Okazaki fragments subsequently to the so-called lagging strand (English lagging beach ) linked. In the case of the replication forks that usually move bidirectionally in the opposite direction, the following strand of one adjoins the leading strand of the other replication fork.
In the small ring-shaped genomes of prokaryotes or mitochondria there is usually only one origin of replication, in large eukaryotic genomes, on the other hand, there are often several thousand, in order to increase the speed of replication by means of many simultaneously advancing replication forks. The responsible for the polymerization of the DNA enzyme - in bacteria, the protein complex DNA polymerase III (Pol III), δ in mammalian cells of DNA polymerase and DNA polymerase ε or in mitochondrial DNA polymerase γ - needs to kick-start a short primer called Oligonucleotide. This is provided by a primase as a small RNA molecule bound to the DNA template . The DNA polymerase then adds further nucleotides to it, but now building blocks of DNA (dNTP) - in the fragmentary synthesis of the following strand until it hits the 5 'end of the previously formed short fragment. These short sections of DNA, which are formed repeatedly and discontinuously, are called Okazaki fragments .
Another enzyme with nuclease activity removes the RNA primer, a repair polymerase - the prokaryotic DNA polymerase I (Pol I) or the eukaryotic DNA polymerase α - replaces it with DNA and fills the gap between fragments with complementary deoxynucleotides. A DNA ligase finally links the individual completed fragments through an ester bond between the hydroxyl group of the sugar deoxyribose at the 3 'end of one and the phosphate residue at the 5' end of the next partial strand to form a subsequent strand with a seamless backbone . These procedural steps necessary for the integration of the Okazaki fragments are similar to those of a DNA repair .
literature
- Alberts , Bray, Johnson, Lewis: Textbook of Molecular Cell Biology. 2nd, corrected edition. Wiley-VCH, Weinheim 2001, ISBN 3-527-30493-2 .
- (English edition: B. Alberts, A. Johnson, J. Lewis et al .: Molecular Biology of the Cell. 4th Edition. Garland Science, New York 2002, Chapter: DNA Replication Mechanisms. Online on the NCBI bookshelf )
Individual evidence
- ^ Graw, Jochen .: Genetics . 6th, revised and updated edition. Springer Berlin Heidelberg, Berlin, Heidelberg 2015, ISBN 978-3-662-44816-8 .
- ↑ D. Voet, JG Voet, CW Pratt: Textbook of Biochemistry. 2nd Edition. Wiley-VCH, Weinheim 2010, p. 963.
- ↑ 『岡 崎 フ ラ グ メ ン ト と 私』 岡 崎 恒 子. Retrieved December 21, 2019 (Japanese).