DNA and RNA Structure

• DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are vital nucleic acids that store and transmit genetic information
• DNA contains the genetic instructions for all known living organisms and many viruses
• RNA transfers genetic information from DNA to ribosomes for protein synthesis

DNA Structure

• DNA is a double-stranded helix
• It comprises nucleotides with a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T)
• The two strands run antiparallel to each other
• The nitrogenous bases pair via hydrogen bonds: A with T, and C with G (Chargaff's rules)
• The sequence of these bases encodes genetic information

RNA Structure

• RNA is typically single-stranded
• It consists of nucleotides with a ribose sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), guanine (G), cytosine (C), and uracil (U)
• Uracil replaces thymine found in DNA
• RNA folds into complex 3D structures due to intramolecular base pairing

Types of RNA

• Messenger RNA (mRNA): Carries genetic code from DNA to ribosomes
• Transfer RNA (tRNA): Transfers amino acids to the ribosome during protein synthesis
• Ribosomal RNA (rRNA): A component of ribosomes

DNA Replication

• DNA replication is the process by which a DNA molecule is copied to produce two identical DNA molecules
• It is essential for cell division during growth and repair of tissues

Mechanism of DNA Replication

• Initiation: Replication begins at specific sites called origins of replication
• DNA helicase unwinds the double helix
• Single-strand binding proteins stabilize the unwound DNA strands
• Elongation: DNA polymerase adds nucleotides to the 3' end of the new strand, synthesizing a new DNA strand complementary to the template strand
• DNA polymerase requires a primer, which is synthesized by primase (an RNA polymerase)
• Leading strand: Synthesized continuously in the 5' to 3' direction towards the replication fork
• Lagging strand: Synthesized discontinuously in short fragments (Okazaki fragments) in the 5' to 3' direction away from the replication fork
• Okazaki fragments are joined together by DNA ligase
• Termination: Replication continues until the entire DNA molecule is copied
• DNA polymerase proofreads the new strand and corrects errors
• Topoisomerase relieves strain ahead of the replication fork by breaking, swiveling, and rejoining DNA strands

Enzymes Involved in DNA Replication

• DNA Helicase: Unwinds the DNA double helix
• Single-strand Binding Proteins (SSB): Prevent the separated DNA strands from re-annealing
• Primase: Synthesizes RNA primers to initiate DNA synthesis
• DNA Polymerase: Adds nucleotides to the 3' end of the growing DNA strand
• DNA Ligase: Joins Okazaki fragments together on the lagging strand
• Topoisomerase: Relieves strain ahead of the replication fork

RNA Replication

• RNA replication is the process of making copies of RNA molecules
• RNA viruses (e.g., influenza, SARS-CoV-2) use RNA replication to multiply within host cells

Mechanism of RNA Replication

• RNA replication is catalyzed by RNA-dependent RNA polymerase (RNA replicase)
• RNA replicase uses an RNA template to synthesize a complementary RNA strand
• The process involves initiation, elongation, and termination
• Initiation: RNA replicase binds to the RNA template at a specific promoter sequence
• Elongation: RNA replicase adds nucleotides to the 3' end of the new RNA strand
• Termination: Replication continues until a termination signal is reached
• The resulting RNA molecules can be used for various purposes, such as protein synthesis or packaging into new viral particles

Comparison of DNA and RNA Replication

• Template: DNA replication uses a DNA template, while RNA replication uses an RNA template
• Enzyme: DNA replication uses DNA polymerase, while RNA replication uses RNA replicase
• Product: DNA replication produces DNA molecules, while RNA replication produces RNA molecules
• Primer: DNA replication requires a primer to initiate synthesis, while RNA replication may or may not require a primer, depending on the specific enzyme and template
• Error rate: DNA replication has a lower error rate due to proofreading mechanisms, while RNA replication has a higher error rate

DNA and RNA are nucleic acids essential for storing and transmitting genetic information. DNA is a double helix with deoxyribose sugar, while RNA is single-stranded with ribose sugar. Key differences include the bases: DNA uses thymine, whereas RNA uses uracil.