DNA Structure and Replication

Nucleotide Connection in Nucleic Acids

  • Nucleotides must be connected on the 3' side.

  • Nucleic acids are synthesized by adding nucleotides at the 3' end.

  • Polynucleotide Definition: A strand of nucleotides connected together.

Formation of Bonds

  • The bond that forms between nucleotides is called a phosphodiester bond.

  • Every new nucleotide must be added to the 3' end of the existing nucleotide within a polynucleotide strand.

  • Connection of nucleotides occurs by formation of phosphodiester bonds.

Structure of DNA

  • DNA consists of two polynucleotide strands.

  • Phosphates and sugars form the backbone (represented as blue ribbons).

  • Nitrogenous bases reside in the interior.

Types of Bonds Between DNA Strands

  • The strands of DNA are held together by hydrogen bonds between complementary bases.

  • Base Pairing Rules:

    • Guanine (G) pairs with Cytosine (C).

    • Adenine (A) pairs with Thymine (T).

  • Importance of base pairing rules:

    1. Distance Maintenance: Essential for correct connection.

    2. Hydrogen Bond Variation:

    • G and C form 3 hydrogen bonds.

    • A and T form 2 hydrogen bonds.

  • Cannot pair A with G or C with T due to hydrogen bond differences and distance issues.

Percent Composition of DNA Bases

  • If 20% of the bases are guanine (G), then cytosine (C) must also be 20% due to pairing rules.

  • Therefore:

    • %A + %T + %C + %G = 100%

    • A and T together must then account for 60%, so both A and T are 30% each.

Introduction to DNA Replication

  • DNA replication involves copying the DNA molecule.

  • Essential for cell division to ensure that new cells receive copies of DNA.

  • Known method of DNA replication is the semi-conservative hypothesis:

    • Each new DNA molecule contains one original strand and one newly synthesized strand.

  • Base pairing is vital as original strands serve as templates for new strands.

Prokaryotic vs. Eukaryotic DNA Replication

  • Prokaryotes have a circular chromosome, whereas eukaryotes have linear chromosomes.

  • Most replication details are similar between prokaryotes and eukaryotes.

Steps of DNA Replication

  1. Separation of Strands:

    • DNA helix is unwound and separated at the replication fork.

    • Parent strands serve as templates.

  2. DNA Polymerase:

    • Can only add nucleotides to the 3' end of the growing strand.

    • Builds new strands antiparallel to the template strands.

Leading and Lagging Strands

  • Leading Strand:

    • Built continuously towards the replication fork.

  • Lagging Strand:

    • Built in fragments (Okazaki fragments) because it is built away from the replication fork, requiring multiple RNA primers.

Enzymes in DNA Replication

  • Helicase:

    • Unwinds and separates DNA strands by breaking hydrogen bonds.

  • Single-Strand Binding Protein:

    • Stabilizes separated strands to prevent re-association.

  • Topoisomerase:

    • Relieves the tension of coiling ahead of the replication fork.

  • Primase:

    • Synthesizes short RNA primers to initiate DNA synthesis.

  • DNA Polymerase III:

    • Main enzyme that synthesizes new DNA strands.

  • DNA Ligase:

    • Joins Okazaki fragments on the lagging strand by forming phosphodiester bonds.

Final Steps and Completion of DNA Replication

  • RNA primers must be replaced with DNA nucleotides, carried out by DNA Polymerase I.

  • After replacing RNA primers, ligase connects any remaining fragments, completing DNA replication.

  • Result is two identical DNA molecules, each containing one original and one new strand.