1. Online Counseling Resource YCMOU ELearning Drive… School of Architecture, Science and Technology Yashwantrao Chavan Maharashtra Open University, Nashik – 422222, India

2. OC-SBT044-U01-01 Introduction Programmes and Courses  SEP – SBT044 – CP01  SEP – SBI044 – CP01  SEP – SGS044 – CP01

3. School of Science and Technology, Online Counseling Resource… © 2007, YCMOU. All Rights Reserved.3 Credits  Academic Inputs by Mrs. Rasika Bhore  M.sc (Microbiology) rasika.bhore@gmail.com

4. School of Science and Technology, Online Counseling Resource… © 2007, YCMOU. All Rights Reserved.4 How to Use This Resource  Counselor at each study center should use this presentation to deliver lecture of 40-60 minutes during Face-To-Face counseling.  Discussion about students difficulties or tutorial with assignments should follow the lecture for about 40-60 minutes.  Handouts (with 6 slides on each A4 size page) of this presentation should be provided to each student.  Each student should discuss on the discussion forum all the terms which could not be understood. This will improve his writing skills and enhance knowledge level about topics, which shall be immensely useful for end exam.  Appear several times, for all the Self-Tests, available for this course.  Student can use handouts for last minutes preparation just before end exam.

5. School of Science and Technology, Online Counseling Resource… © 2007, YCMOU. All Rights Reserved.5 Learning Objectives  After studying this module, you should be able to: Describe the set of fundamental rules that DNA replication follows.

6. School of Science and Technology, Online Counseling Resource… Introduction  DNA replication is the process by which genetic information is transmitted from one generation of cells to the next.  Replication process involves copying of double stranded DNA to form two identical copies of double stranded DNA.  DNA replication is semi conservative, semi discontinuous & bidirectional. © 2007, YCMOU. All Rights Reserved.6

7. School of Science and Technology, Online Counseling Resource… Replication Exactly Is………….  Each organism create its exact copy by the process of replication.  The DNA of organism unwinds & separates the two strands which serves as template for the production of new complementary strands.  Thus the two new strands are formed where Adenine pairs with Thymine & Cytosine pairs with Guanine.  As each strand of DNA holds the same genetic information it transfers to the next generation. © 2007, YCMOU. All Rights Reserved.7

8. School of Science and Technology, Online Counseling Resource… Replication Is Semiconservative  Each DNA strand serves as a template for the synthesis of a new strand, producing two new DNA molecules.  These DNA molecules have one new strand & one old or parental strand, which is called as Semiconservative replication.  This idea was proposed by Watson & Crick which later proved by Messelson & Stahl experiment. © 2007, YCMOU. All Rights Reserved.8

9. School of Science and Technology, Online Counseling Resource… Replication Is Bidirectional  DNA strands have a directionality, and the different ends of a single strand are called the "3' end" and the "5' end“.  Thus DNA strands are complementary & antiparallel means they are orientated in opposite directions.  This bidirectionality of DNA strands have consequences in DNA synthesis as DNA synthesis always proceeds from 5’-> 3’ direction. © 2007, YCMOU. All Rights Reserved.9

10. School of Science and Technology, Online Counseling Resource… Replication Begins At ‘Origin’  Before the cell begins to divide, DNA replication must takes place.  This process starts from particular point on DNA called as ‘Origin’.  DNA sequences at origin are recognized by replication initiator proteins.  Then next group of proteins starts separating the DNA strands at origin, forming a bubble like structure.  Origin usually is a A=T rich region so that it is easily separated.  As strands are separated RNA primers are created on template strands & DNA synthesis continues. © 2007, YCMOU. All Rights Reserved.10

11. School of Science and Technology, Online Counseling Resource… Replication Fork  As DNA synthesis continues the strands unwinds continuously on each side of bubble forming a ‘replication fork’. © 2007, YCMOU. All Rights Reserved.11

12. School of Science and Technology, Online Counseling Resource… DNA Synthesis Is Semi-discontinuous (?)  Experiments have proven that DNA synthesis proceeds only in 5' -> 3' direction.  This means the template is read from 3’ end toward its 5’ end.  Incoming nucleotide added at 3’OH end.  The problem arose was if the both DNA strands synthesized simultaneously & continuously while the replication fork moved, one strand would have to undergo 3’->5’ synthesis.  The solution to this problem was given by Reiji Okazaki in 1960s, from which the idea of semi- discontinuous replication has established. © 2007, YCMOU. All Rights Reserved.12

13. School of Science and Technology, Online Counseling Resource… Okazaki Fragments  Reiji Okazaki proposed and demonstrated that synthesis of one strand (the leading strand) is continuous, synthesis of the other strand (the lagging strand) is discontinuous.  In fact, he showed that lagging strand replication proceeded through the synthesis of relatively short polynucleotide segments that were then ligated together to form a continuous strand.  These short segments are called Okazaki fragments.  In bacteria they are approximately 1000 nt in length; in eukaryotes they are approximately 200 nt in length. © 2007, YCMOU. All Rights Reserved.13

14. School of Science and Technology, Online Counseling Resource… Leading Strand Synthesis  In DNA replication, the leading strand is defined as the new DNA strand at the replication fork that is synthesized in the 5' → 3' direction in a continuous manner.  On the leading strand DNA polymerase III is able to synthesize DNA using the free 3' OH group donated by a single RNA primer and continuous synthesis occurs in the direction in which the replication fork is moving. © 2007, YCMOU. All Rights Reserved.14

15. School of Science and Technology, Online Counseling Resource… Lagging Strand Synthesis  The lagging strand is the DNA strand at the opposite side of the replication fork from the leading strand, running in the 3' to 5' direction.  Because DNA polymerase cannot synthesize in the 3' → 5' direction, the lagging strand is synthesized in short segments known as Okazaki fragments.  Short RNA primers provides the free 3' OH groups used by DNA polymerase III to synthesize DNA in the 5' → 3' direction.  The RNA fragments are then removed and new deoxyribonucleotides are added to fill the gaps where the RNA was present.  DNA ligase then joins the deoxyribonucleotides together, completing the synthesis of the lagging strand. © 2007, YCMOU. All Rights Reserved.15

16. School of Science and Technology, Online Counseling Resource… Overview Of DNA Synthesis © 2007, YCMOU. All Rights Reserved.16

17. School of Science and Technology, Online Counseling Resource… What We Learn………….  Replication is the process of copying DNA after which cell division occurs.  It is semiconservative as the newly formed DNA contain one old & one new strand.  Replication initiates from origin & form a replication fork.  Always takes place from 5’->3’ direction by DNA polymerase.  It is bidirectional, a leading strand synthesizes continuously while lagging strand synthesizes discontinuously by okazaki fragments.

18. School of Science and Technology, Online Counseling Resource… Critical Thinking Questions 1.Why the origin of replication is usually A=T rich region instead of G ≡ C? 2.Why the lagging strand synthesis is discontinuous? © 2007, YCMOU. All Rights Reserved.18

19. School of Science and Technology, Online Counseling Resource… © 2007, YCMOU. All Rights Reserved. Tips For Critical Thinking Questions 1.There are only 2 double bonds present between A & T while 3 bonds lies between C& G. 2.Replication takes place only from 5’->3’ direction.

20. School of Science and Technology, Online Counseling Resource… © 2007, YCMOU. All Rights Reserved.20 Study Tips  Book Title: Principles Of Biochemistry Author: David Nelson & Michael Cox  Book Title: Genetics Author: Russel

21. School of Science and Technology, Online Counseling Resource… © 2007, YCMOU. All Rights Reserved.21 Acknowledgement www.en.wikipedia.org www.biology.arizona.edu www.library.thinkquest.org

22. End of the Presentation Thank You !