DNA Structure and Replication 

DNA structure and replication
Ibrar Ahmed, PhD
CEO, Alpha Genomics Private Limited
Islamabad
www.alphagenomics.co
Conceptual recap of the relevant topics:
DNA and RNA as hereditary materials
Structure of DNA,
Organization of genome in cells
Basic building blocks,
Types of bonds which keep these building blocks together
Denaturation and renaturation of DNA with changes in temperature
DNA replication and repair
Mutations
Hereditary material
The hereditary material contains the information needed for the continuity of life
Central Dogma of Molecular Biology
Cellular organization of the genome
Chromosomes = DNA plus proteins
Each chromosome contains one continuous piece of DNA (double stranded)
Sister chromatids contain two copies of the same DNA molecule
Chromosomes visible at cell division stage
During interphase, Chromosomes are arranged in the form of euchromatin and heterochromatin
Euchromatin is the part of genome which is transcriptionally active
Heterochromatin in the par of genome which is transcriptionally inactive
Coiling and supercoiling of chromatin to fit in the 3D nuclear space
Polarity of a DNA strand
Building blocks of DNA
Building blocks of DNA
Types of bonds present in DNA
Covalent bonds and hydrogen bonds
Covalent bonds are strong and can be made or broken enzymatically
Phosphodiester bonds hold the backbone
N-glucosidic bonds join ribose sugar with the nitrogenous base
Phosphodiester bonds can potentially join unlimited number of nucleotides in linear arrangement
Weak hydrogen bonds to hold two strands together
Two H bonds between A and T; three hydrogen bonds between G and C
Hydrogen bonds can be affected by temperature, in addition to enzymes
DNA denaturation and renaturation
DNA Replication
Watson-Crick model of DNA replication:
Hydrogen bonds between DNA bases break to allow strand separation
Each DNA strand is a template for the synthesis of a new strand
Template (parental) strand determines the sequence of bases in the new strand (daughter)= complementary base pairing rules
DNA Synthesis
One strand of the newly made DNA is synthesized continuously = leading strand
The other, lagging strand is made in small precursor fragments = Okazaki fragments
The size of Okazaki fragments is 1000-2000 base pairs in prokaryotic cells and 100-200 base pairs in eukaryotic cells.
Comprehensive DNA repair system to eliminate / minimize errors in DNA replication