Okazaki fragments are short segments of DNA on the lagging (3'-5') strand in DNA replication, which are processed by the replication machinery to produce a continuous strand of DNA, and hence a complete daughter DNA helix.
DNA polymerases display an intrinsic 5'-3' polarity, so only one continuous new strand can be synthesized at the 3' end of the leading strand, which reads 5'-3'. Because the lagging strand is the antiparallel complement of the leading strand, its replication utilizes a series of Okazaki fragments.
1. Initiation of lagging-strand replication takes place at the primase-created replication fork of an RNA primer (short complementary RNA strand). Primase is a dnaG-coded RNA polymerase that binds to the DNA helicase, forming a primosome complex. Such priming of initiation is necessary because no known DNA polymerases are able to initiate replication.
2. Extension of Okazaki fragments is performed by DNA polymerase III.
3. RNA primers are removed by nucleolytic enzymes such as ribonuclease H (RNAse H), flap endonucleases (FENs)and Dna2 helicase/nucleases.
4. Okazaki fragments are linked by DNA ligase I, which forms phosphodiester bonds to generated a continuous DNA chain that is complementary to the leading strand.
Showing posts with label Okazaki fragments. Show all posts
Showing posts with label Okazaki fragments. Show all posts
12/10/2007
12/07/2007
replication
Replication results in the copy of the DNA double helix, and, like transcription, proceeds 3' to 5' on the template strand and 5' to 3' on the replica strand. Replication is "semi-conservative" in that each new DNA double helix contains one of the original "parental" strands along with a newly synthesized strand.
Other possibilities for replication would have been
1. Fully conservative replication, with an original parental strand plus a complete new double stranded copy, or
2. Fully nonconservative replication ("dispersive") with disassembly, copying of fragments, and reassembly into two new randomly mixed strands each containing sections of the original and copied segments.)
Helicases are a critical part of the DNA replication process because they unwind double-stranded DNA to create single strands suitable for copying by the replication machinery. This and other helicase activity in the cell depends on the ability of the helicase's protein “engine” to crawl along the DNA strand. This locomotion is powered by ATP, the cell's ubiquitous energy source. Helicase ProteinA helicase protein moves rapidly on a highly flexible single-stranded DNA track. Repetitive movement on the DNA may keep it clear of potentially toxic proteins. Watch Animation 8KB Flash Animation(requires Flash Player)
Rediscovering Biology - Genomics Animations and Images - Genetics of Development - Animations and Images - Biology of Sex & Gender - Animations and Images - Genetically modified organisms - Animations and Images - Biodiversity - Animations and Images :
Other possibilities for replication would have been
1. Fully conservative replication, with an original parental strand plus a complete new double stranded copy, or
2. Fully nonconservative replication ("dispersive") with disassembly, copying of fragments, and reassembly into two new randomly mixed strands each containing sections of the original and copied segments.)
Helicases are a critical part of the DNA replication process because they unwind double-stranded DNA to create single strands suitable for copying by the replication machinery. This and other helicase activity in the cell depends on the ability of the helicase's protein “engine” to crawl along the DNA strand. This locomotion is powered by ATP, the cell's ubiquitous energy source. Helicase ProteinA helicase protein moves rapidly on a highly flexible single-stranded DNA track. Repetitive movement on the DNA may keep it clear of potentially toxic proteins. Watch Animation 8KB Flash Animation(requires Flash Player)
Rediscovering Biology - Genomics Animations and Images - Genetics of Development - Animations and Images - Biology of Sex & Gender - Animations and Images - Genetically modified organisms - Animations and Images - Biodiversity - Animations and Images :