DNA REPLICATION (STAGES & ENZYMES)

=DNA Replication= [image:http://i.imgur.com/TNXsxrk.png?2] '''Step One''' [image:http://i.imgur.com/JcZWwwM.png] An enzyme called'''Helicase''' breaks the hydrogen bonds between the bases of the two antiparallel strands. The strands are initially split apart in areas that are rich in A-T base paris (there are only two bonds between Adenine and Thymine) forming a '''replication fork'''. '''DNA Gyrase (also called Topoisomerase)''' relieves tension that builds up as a result of unwinding. Single strand binding proteins (SSBs) help to stabilise the single stranded DNA. '''Step Two''' [image:http://i.imgur.com/Gkewnis.png] RNA polymerase (also known as RNA Primase) synthesizes short RNA nucleotides sequences that act as primers (starters). These essentially provide a starting point for DNA replication. '''Step Three''' [image:http://i.imgur.com/pqSbjHD.png] DNA Polymerase III can now start synthesising the new DNA strand using free DNA nucleotides. However, DNA polymerase can only read the original template (parent strand) in the 3’ → 5’ direction (making DNA 5’ → 3’). This is not a problem on the leading strand, because the DNA polymerase can simply continue to read along as the two parent stands continue to unzip. '''Step Four''' [image:http://i.imgur.com/uWdYnYT.png] On the lagging strand DNA polymerase moves away from the replication fork. As the strands continue to unzip more DNA is exposed and new RNA primers must be added. As a result the lagging strand is synthesised in short bursts as DNA polymerase synthesizes DNA in-between each of the RNA primers. [image:http://i.imgur.com/SUPzlar.png]The newly synthesised lagging strand now consists of both RNA and DNA fragments. The DNA fragments are known as Okazaki fragments, after a Japanese scientist who noticed that heating DNA during replication, which separates the strands, gave many small fragments of DNA. From this he concluded that one stand must be synthesized in short bursts of DNA. '''Step Six''' DNA Polymerase I now removes the RNA primers and replaces them with DNA [image:http://i.imgur.com/kXSYjSR.png] '''Step Seven''' DNA Ligase joins the DNA fragments of the lagging strand together to form one continuous length of DNA. [image:http://i.imgur.com/GMvp0XM.png] '''Telomeres''' During DNA replication the enzymes (polymerase) that duplicate the chromosome and its DNA can't continue their duplication all the way to the end of the chromosome. At the very ends of the DNA, are long non-coding region of Repeats known as telomeres. Every time the DNA is replicated the telomeres shorten slightly. It is believed that this may be the genetic basis for the aging process. '''Proof Reading and Correction''' DNA replication occurs at a surprisingly fast rate. Despite this, errors are very rare; occurring at a rate of approx. 1 in every 10,000,000,000 base pairs. This is much lower than the expected value of about 1 in every 100 bp. This is the result of a complex series of enzymes that proof-read the new DNA strands and make corrections where needed.