“DNA Structure, Function, Replication and Repair” ( Seminar synopsis)

UNIVERSITY OF AGRICULTURAL SCIENCES, BANGALORE

DEPARTMENT OF GENETICS AND PLANT BREEDING

Seminar - GPB 582 (0+1)

“DNA Structure, Function, Replication and Repair”

SYNOPSIS

    The story of DNA dates back to 1665, when Robert Hooke discovered the “cell,” laying the foundation for cell theory by Schleiden and Schwann. In 1866, Gregor Mendel’s pea plant experiments revealed the laws of inheritance, introducing the concept of hereditary “factors” now known as genes. Around the same time, Robert Brown discovered the nucleus, and Walther Flemming observed chromosomes. Later, Sutton and Boveri’s chromosomal theory of inheritance connected Mendel’s laws to chromosomes, leading to the key question: What is the genetic material.

DNA is the predominant genetic material due to unique properties that favor its stability and function over RNA. Watson and Crick’s discovery of its double-helix structure revealed two key features: base sequence complementarity (A–T, G–C) and the double-helical nature of the polymer. Complementarity explained how DNA could replicate faithfully and supported earlier findings that DNA is the “transforming principle.” Chargaff’s rules further confirmed the equivalence of base pairs. The double helix, beyond complementarity, imparts crucial physical and chemical stability, allowing efficient packaging, accessibility, and replication. Thus, DNA’s role as both an information tape and a stable helical polymer underpins its central role in genetics and heredity.¹

DNA replication is a multistep enzymatic process in which helicases unwind the double helix. The leading strand is synthesized continuously by DNA polymerase δ, while the lagging strand is synthesized discontinuously by DNA polymerase α with primase, forming Okazaki fragments. Propagation is ensured by the 3’→5’ exonuclease activity of polymerase δ. RNA primers are removed by RNase H and exonucleases, gaps filled by polymerase, and sealed by DNA ligase. Topoisomerases relieve torsional stress during fork movement, and in eukaryotes replication must also preserve chromatin structure. In E. coli, replication requires about 30 proteins working together. Helicase unwinds the DNA, and single-stranded binding proteins stabilize the unwound regions. DNA polymerase III holoenzyme ensures rapid synthesis at about 1000 nucleotides per second, while primase provides RNA primers and ligase seals the nicks. These processes are tightly coupled to cell cycle control and DNA structural regulation to ensure faithful chromosome replication.² ³

Helicases, functioning in an ATP-dependent manner, are crucial for nucleic acid transactions and are affected during stress, which impairs protein synthesis. Pea DNA helicase 45 (PDH45), a homolog of translation initiation factor eIF4A, has been linked to salinity stress tolerance in tobacco and rice. Using Agrobacterium-mediated transformation, PDH45 was overexpressed in the rice variety IR64, showing stable integration in T1 plants. Transgenics displayed enhanced salt tolerance, better physiological traits, and improved yield compared to wild type and controls. PDH45 likely acts at the translational level, stabilizing protein synthesis under stress.⁴

REFERENCES:

1. TRAVERS, A. AND MUSKHELISHVILI, G., 2015, DNA structure and function. FEBS J., 282(12):2279-2295.

2. THÖMMES, P. AND HÜBSCHER, U., 1990, Eukaryotic DNA replication: enzymes and proteins acting at the fork. Eur. J. Biochem, 194(3):699-712.

3. MARIANS, K.J., 1992, Prokaryotic DNA replication. Annu. Rev. Biochem, 61(1),:673-715.

4. SAHOO, R.K., GILL, S.S. AND TUTEJA, N., 2012, Pea DNA helicase 45 promotes salinity stress tolerance in IR64 rice with improved yield. Plant Signal Behav, 7(8):1042-1046.