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Chapter-40 Deoxyribo Nucleic Acid (DNA): Structure and Replication

BOOK TITLE: Textbook of Biochemistry for Medical Students

Author
1. Vasudevan DM
2. S Sreekumari
3. Vaidyanathan Kannan
ISBN
9789350250167
DOI
10.5005/jp/books/11359_40
Edition
6/e
Publishing Year
2011
Pages
12
Author Affiliations
1. Faculty of Medicine, Amrita Vishwa Vidyapeetham, (Amrita University), Kochi, Kerala, Formerly Principal, College of Medicine, Amrita, Kerala; Formerly, Dean, Sikkim Manipal Institute of Medical Sciences, Gangtok, Sikkim, Amrita Vishwa Vidyapeetham (Deemed University), Cochin, Kerala, E-mail: dmvasudevan@aims.amrita.edu, PG Programs and Research College of Medicine, Amrita Institute of Medical Sciences, Kochi, Kerala, India, College of Medicine, Amrita Institute of Medical Sciences, Kochi, Kerala, India; Sikkim Manipal Institute of Medical Sciences, Gangtok, Sikkim, India
2. Jubilee Mission Medical College, Trissur, Kerala, Sree Gokulam Medical College and Research Foundation, Thiruvananthapuram, Kerala, India, Jubilee Mission Medical College and Research Institute, Thrissur, Kerala, India, Government Medical College, Thrissur and Thiruvananthapuram, Kerala, India; Sree Gokulam Medical College and Research Foundation, Thiruvananthapuram, Kerala, India; Jubilee Mission Medical College and Research Institute, Thrissur, Kerala, India
3. Amrita Institute of Medical Sciences, Kochi, Kerala, India, Pushpagiri Institute of Medical Sciences and Research Centre, Thiruvalla, Kerala, India, Believers Church Medical College and Hospital, Thiruvalla, Kerala, India
Chapter keywords

Abstract

DNA sequence is always written from the 5’ end to 3’ end. This is called polarity of the DNA chain. Chargaff’s rule states that the number of purines is equal to the number of pyrimidines. The two strands run antiparallel to each other. Supercoiling of DNA is mediated by the enzymes topoisomerases and gyrase. DNA is assembled into nucleosomes. Histones are unusually rich in basic amino acids. Transcriptionally active chromatin is called ‘euchromatin’ and stains less densely as compared to ‘heterochromatin’ which is the inactive region of the chromatin. DNA replication in vivo is semi-conservative. In mammals, the DNA polymerase is called alpha polymerase. There are five DNA polymerases (alpha, beta, gamma delta and epsilon). The DNA polymerase requires a RNA primer synthesized by RNA polymerase for initiation of its activity. This forms the first step in DNA synthesis. DNA synthesis is continuous on the leading strand and discontinuous on the lagging strand. This results in the formation of the Replication fork. The small DNA molecules attached to their own RNA primers are called Okazaki fragments. Xeroderma pigmentosum is an autosomal recessive condition caused due to a defective nucleotide excision repair mechanism. In every replication the 3’ end of the parent strands cannot be replicated. This end piece of the chromosome is called telomere. This shortening is prevented by telomeres. Telomeres have been implicated in ageing process and cancers.

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