3.1 DNA structure and replication

DNA structure and replication form the foundation of genetics. The double helix, made of nucleotides, carries our genetic code. Base pairing rules ensure accurate information storage, while phosphodiester bonds provide stability.

DNA replication is a complex process involving various enzymes. DNA polymerase builds new strands, while helicase, primase, and ligase play crucial supporting roles. The semiconservative nature of replication ensures accurate genetic transmission to daughter cells.

DNA Structure

Nucleotide Composition and Structure

Top images from around the web for Nucleotide Composition and Structure

• 

  The Nucleus and DNA Replication | Anatomy and Physiology I View original

  Is this image relevant?

• 

  Francis Crick - wikidoc View original

  Is this image relevant?

• 

  Structure of DNA | Biology for Majors I View original

  Is this image relevant?

• 

  The Nucleus and DNA Replication | Anatomy and Physiology I View original

  Is this image relevant?

• 

  Francis Crick - wikidoc View original

  Is this image relevant?

1 of 3

Top images from around the web for Nucleotide Composition and Structure

• 

  The Nucleus and DNA Replication | Anatomy and Physiology I View original

  Is this image relevant?

• 

  Francis Crick - wikidoc View original

  Is this image relevant?

• 

  Structure of DNA | Biology for Majors I View original

  Is this image relevant?

• 

  The Nucleus and DNA Replication | Anatomy and Physiology I View original

  Is this image relevant?

• 

  Francis Crick - wikidoc View original

  Is this image relevant?

1 of 3

• Nucleotides building blocks of DNA consist of a phosphate group, a sugar (deoxyribose), and one of four nitrogenous bases (adenine, thymine, guanine, or cytosine)
• Phosphate group and deoxyribose sugar form the backbone of the DNA strand
• Nitrogenous bases attached to the sugar molecule project inward from the backbone
• Adenine (A) and guanine (G) purines have a double-ring structure
• Thymine (T) and cytosine (C) pyrimidines have a single-ring structure

Double Helix Configuration and Base Pairing

• DNA exists as a double helix structure with two complementary strands running antiparallel to each other
• Strands held together by hydrogen bonds between complementary base pairs (A with T and G with C)
• Base pairing rules dictate that adenine always pairs with thymine and guanine always pairs with cytosine
• Double helix structure allows for efficient packaging of genetic material and protection from damage
• Complementary base pairing enables accurate replication and transmission of genetic information

Phosphodiester Bonds and Structural Stability

• Phosphodiester bonds covalent bonds that link the phosphate group of one nucleotide to the sugar molecule of the adjacent nucleotide
• Phosphodiester bonds form the backbone of the DNA strand and provide structural stability
• Bonds are formed through a condensation reaction, releasing a water molecule
• Phosphodiester bonds are strong and resistant to cleavage, ensuring the integrity of the DNA molecule
• Bonds can be broken by specific enzymes (nucleases) during processes like replication and repair

DNA Replication Enzymes

DNA Polymerase and Its Functions

• DNA polymerase enzyme responsible for catalyzing the synthesis of new DNA strands during replication
• Catalyzes the formation of phosphodiester bonds between adjacent nucleotides in the 5' to 3' direction
• Requires a primer (short RNA or DNA sequence) to initiate replication
• Ensures accuracy by proofreading and correcting errors during synthesis
• Different types of DNA polymerases (I, II, III) play specific roles in replication, repair, and recombination

Additional Enzymes Involved in DNA Replication

• Helicase unwinds the double helix by breaking hydrogen bonds between base pairs, creating single-stranded templates for replication
• Primase synthesizes short RNA primers that provide a starting point for DNA polymerase
• Ligase seals nicks between Okazaki fragments on the lagging strand, creating a continuous strand of DNA
• Topoisomerases relieve tension and supercoiling caused by the unwinding of the double helix during replication

DNA Replication Process

Leading and Lagging Strand Synthesis

• DNA replication occurs simultaneously on both strands, but in opposite directions due to the antiparallel nature of the double helix
• Leading strand synthesized continuously in the 5' to 3' direction, following the movement of the replication fork
• Lagging strand synthesized discontinuously in short fragments (Okazaki fragments) in the 5' to 3' direction, opposite to the movement of the replication fork
• Okazaki fragments later joined together by DNA ligase to form a continuous strand

Okazaki Fragments and Discontinuous Replication

• Okazaki fragments short segments of DNA (100-200 nucleotides) synthesized on the lagging strand during replication
• Synthesis of Okazaki fragments initiated by RNA primers, which are later removed and replaced with DNA
• Fragments are synthesized in the 5' to 3' direction, but the overall direction of lagging strand synthesis is 3' to 5'
• Okazaki fragments are named after their discoverer, Reiji Okazaki, who first observed discontinuous replication in the 1960s

Semiconservative Nature of DNA Replication

• Semiconservative replication model proposed by Watson and Crick, based on the complementary nature of the double helix
• During replication, the two strands of the parent DNA molecule separate, and each strand serves as a template for the synthesis of a new complementary strand
• Resulting daughter DNA molecules consist of one original (conserved) strand and one newly synthesized strand
• Semiconservative replication ensures that genetic information is accurately passed on to daughter cells
• Experimentally proven by Meselson and Stahl using density gradient centrifugation and isotope labeling of DNA in E. coli