DNA Structure and Replication

DNA Structure

• DNA is a polymer of nucleotides
• Each nucleotide comprises a nitrogenous base, a pentose sugar (deoxyribose), and a phosphate group
• Purines: adenine and guanine
• Pyrimidines: thymine and cytosine
• The backbone is formed by alternating sugar and phosphate groups linked by phosphodiester linkages
• The 5' carbon of one deoxyribose is attached to the phosphate group, and the 3' carbon of the next deoxyribose is attached to the phosphate group
• DNA strands run antiparallel
• 5' end to 3' end
• 3' end to 5' end
• The strands are held together by hydrogen bonds between bases (A with T, and G with C)

DNA Replication

• Replication is semi-conservative: each new DNA molecule consists of one original strand and one newly synthesized strand
• DNA replication begins at specific sites (origins of replication) on the DNA molecule
• The two strands of the double helix unwind, each strand serves as a template for the complementary strand
• DNA polymerase synthesizes the new strands
• DNA polymerase adds nucleotides to the 3' end of the existing growing strand.
• Leading strand is synthesized continuously toward the replication fork while lagging strand synthesis is discontinuous in small Okazaki fragments, synthesized away from the fork
• RNA primers are required where new strands are synthesized
• DNA ligase joins the Okazaki fragments
• DNA replication occurs in both directions
• Eukaryotic chromosomes have multiple origins of replication
• Prokaryotic chromosomes are circular and have a single origin of replication

Models of DNA Replication

• Semiconservative: One original strand and one new complementary strand
• Conservative: Both parent strands remain together and two new strands form a separate helix
• Dispersive: Parental and newly synthesized strands get randomly mixed

Meselson-Stahl Experiment

• Used isotopes of nitrogen (15N and 14N) to track DNA replication
• Demonstrated that DNA replication is semi-conservative
• DNA molecules in the first generation of replication were hybrid, containing both heavy and light nitrogen

Proteins in DNA Replication

• Helicase: opens the double helix at replication forks
• Single-strand binding proteins (SSB): prevent the separated strands from re-joining
• Topoisomerases: relieve tension caused by unwinding
• DNA polymerase: synthesizes new DNA strands
• DNA primase: synthesizes RNA primers
• DNA ligase: joins Okazaki fragments

DNA Replication Steps

• Unwinding of the double helix
• Stabilizing single-stranded DNA
• Preventing supercoiling
• Synthesizing RNA primers
• Elongating new DNA strands
• Removing and replacing RNA primers with DNA
• Joining Okazaki fragments

DNA replication direction and characteristics

• DNA synthesis always proceeds in the 5' to 3' direction.
• The replication proceeds in both directions in a circular chromosome.
• Eukaryotic chromosomes contain multiple origins of replication and replication proceeds in bubbles.

This quiz covers the essential concepts of DNA structure, including the composition of nucleotides, base pairing, and the antiparallel nature of DNA strands. It also delves into the process of DNA replication, emphasizing its semi-conservative nature and the role of DNA polymerase. Test your understanding of these fundamental genetic concepts.