University:
Kentucky State University
Course:
CHE 315 | Biochemistry
Academic year:

2019
Views:

404

Pages:

12
Author:

Lamghari6kx

DNA REPLICATION 27 Semiconservative model • The process of replication follows the semiconservative model. • It means that the new DNA molecule is made of an old strand and a new strand each time a cell replicates its DNA. 28 DNA template • The process of DNA templating involves the recognition of each nucleotide in the DNA template strand by a free complementary nucleotide; that is A for T, C for G, T for A, and G for C 29 Enzyme and substrate • The nucleotide polymerizing enzyme is called DNA polymerase. • The free nucleotides that serve as substrates for this enzyme are deoxyribonucleoside triphosphates. 30 Origin of replication • DNA replication begins in the nucleus with partial unwinding of the double helix. • This process involves enzymes known as helicases that use unwind DNA. • The unwinding occurs in many specific locations known as origins of replication. “Bubbles” are formed and each bubble is composed of replication forks. 31 The replication fork and types of new strands • Within a bubble, replication moves progressively along the parental DNA double helix bidirectionally. • Because of its Y-shaped structure, this active region is called a replication fork. 32 Direction of replication • DNA polymerase catalyzes the reaction between the phosphate on an incoming nucleotide and the free 3’-OH on the growing polynucleotide. • Therefore, the template strand can only be read in the 3’ –to-5’ direction, and the new DNA strand can grow only in the 5’ to 3’ direction. 33 New DNA (long vs short) • During replication, there is a long strand and shorter pieces of DNA at the growing replication fork – The small DNA fragments are known as Okazaki fragments • The Okazaki fragments polymerize in the 5-to-3 direction and join together after their synthesis to create long DNA chains 34 Leading vs. lagging strand • The DNA daughter strand that is synthesized continuously (the long strand) is known as the leading strand • On the other hand, the daughter strand synthesized discontinuously is known as the lagging strand – its synthesis is delayed waiting for the leading strand to expose the template strand. 35 A primer is needed • A primer is a short nucleic acid sequence that provides a starting point for DNA synthesis. • Primers are short strands of RNA. A primer must be synthesized by an enzyme called primase, which is a type of RNA polymerase, before DNA replication can occur. 36 The whole process 37 DNA ligase • To form the lagging strand from the Okazaki fragments, these short fragments are joined by the action of an enzyme known as DNA ligase. 38

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