YIJC [H2] CI2.2_DNA Replication (N)

YISHUN INNOVA JUNIOR COLLEGE 9744 H2 BIOLOGY LECTURE NOTES 1 CORE IDEA 2: GENETICS AND INHERITANCE TOPIC 2.2: DNA REPLICATION Learning Outcomes You should be able to: (b) describe the process of DNA replication and how the end replication problem arises Use the knowledge gained in this section in new situations or to solve related problems. References Reece, J B, Urry, L A, Cain, M L, Wasserman, S A, Minorsky, P V and Jackson, R B (201 7) Campbell Biology (11th Edition) (Pearson Higher Education) ISBN 0321739752 I. Introduction The structure of DNA was proposed by Watson and Crick in 195 3. With an understanding of DNA structure, experimental evidence supported the proposal that DNA replicates in a semi-conservative manner. During S phase of interphase, DNA content is doubled to form two sister chromatids joined at the centromere, in preparation of subsequent nuclear division. II. Evidence of Semi-Conservative DNA replication Matthew Meselson and Frank Stahl obtained evidence for the semi-conservative replication of DNA and published their results in 1958. The Meselson and Stahl experiment was as follows: ● Cultures of Escherichia coli (E. coli) bacteria were grown with the presence of ‘heavy’ isotope of nitrogen, 15N, instead of the ordinary ‘light’ isotope of nitrogen, 14N. ● After many generations, the bacteria culture all contained the heavy form, 15N. ● Some bacteria from the culture were treated to release their DNA into a solution. The solution with DNA was then centrifuged at high speed. ● The result was a single band of DNA, a heavy band that contained the 15N. This is known as the parental generation. H2

2 Fig. 2 Parental generation compared to control experiment with both 14N and 15N ● The remaining bacteria were placed in a 14N culture medium and allowed to grow for 20 minutes, which is the generation time for E. coli grown in optimal conditions. ● A sample of bacteria was then taken, processed, and centrifuged to produce the first generation (generation 1). ● This same process was continued so that the second and third generations were obtained, each being processed and centrifuged. Fig. 3 Results of second and third generations The Meselson and Stahl experimental results were as follows: • In the first generation, the DNA band occurred as an intermediate band in the middle of positions of 14N and 15N bands. This indicates that the DNA molecules (double helix) comprises of one 15N strand and one 14N strand. This shows that two 15N parental strands of DNA separated to act as a template for the replication of a new 14N daughter strand. After replication, the parental 15N strand rewinds with a newly synthesised 14N strand to form a new 14N15N daughter DNA molecule.

3 • In the second generation, DNA molecules from the first generation separated and each act as a template for the synthesis of a new 14N daughter strand.