DNA replication

• DNA replication


  Production of exact copies of DNA with identical base sequences
• DNA replication


  • Required for reproduction and for growth and tissue replacement in multicellular organisms
• DNA replication


  1. Making exact copies of DNA molecules with the same base sequences

  2. Allows life to continue through reproduction and cell division

  3. Essential for both sexual and asexual reproduction, as well as for growth and tissue repair in multicellular organisms

  4. Happens in the S phase of the cell cycle and not right before or during mitosis
• Semi-conservative nature of DNA replication

  Each new double helix contains one original and one newly synthesized strand
• Complementary base pairing


  Adenine pairs with Thymine, Cytosine with Guanine
• Complementary base pairing ensures new strands are identical copies of the original DNA molecule
• High accuracy is achieved with only 1 error in 10 billion bases, allowing for genetic continuity across generations
• Mispairing errors can be detected and corrected, further ensuring replication fidelity
• Helicase


  • Acts like a zipper opener, unwinding the DNA double helix and separating the strands for copying
• DNA polymerase


  • Responsible for building new DNA strands, using the separated strands as templates
  • Ensures only the correct nucleotides (A-T or C-G) are added, guaranteeing accurate copying
  • Links the phosphate group of a new nucleotide to the 3' end of the growing strand
• Polymerase Chain Reaction (PCR)

  1. An automated technique that rapidly copies specific DNA sequences, starting with tiny samples

  2. Thermal cycling: Repeated temperature changes within a PCR machine drive the copying process, doubling DNA with each cycle

  3. Primers: Short DNA strands guide the copying process, ensuring only desired sequences are amplified
• Gel electrophoresis


  1. Separates DNA molecules by size using an electric field and a gel matrix

  2. Smaller DNA molecules move faster, allowing researchers to identify the length and quantity of DNA fragments in a sample
• Applications of PCR and gel electrophoresis

  • Coronavirus testing
  • Paternity testing & forensics investigations
• Short tandem repeats (STRs) are regions of non-coding DNA that contain repeats of the same nucleotide sequence, such as GATA
• STRs are useful in genetic profiling because they are highly variable among individuals and can be used as genetic markers to identify or compare DNA samples
• Directionality of DNA polymerases

  Always building 5' to 3'
• DNA strands


  • Have a direction: 5' end with a free phosphate and 3' end with a free hydroxyl group
• DNA polymerase


  • Links the phosphate group of a new nucleotide to the 3' end of the growing strand
• DNA replication on the leading strand and the lagging strand

  1. Leading strand: Polymerase moves with the fork, continuously adding nucleotides towards it (faster)

  2. Lagging strand: Polymerase moves away from the fork, building short fragments (Okazaki fragments) in stages (slower)

  3. Restarting synthesis makes the lagging strand slower overall
• DNA primase


  • Makes short RNA primers (start sites) for DNA polymerase on both strands
• DNA polymerase III


  • The main builder, adding nucleotides to the 3' end of growing DNA strands, proofreading for errors along the way
• DNA polymerase I


  • Cleans up on the lagging strand, removing RNA primers and replacing them with DNA
• DNA ligase


  • Seals the gaps left by removed RNA primers, creating a continuous sugar-phosphate backbone
• DNA proofreading


  DNA polymerase III identifies mismatched base pairs in the newly added nucleotide, removes the mistake, realigns with the template, and inserts the correct base
• Okazaki fragments are joined together by ligases to form one continuous strand.
• The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously as short fragments called Okazaki fragments.
• Lagging strand grows discontinuously due to its movement toward the origin of replication.
• RNA primers are used during DNA replication to initiate new segments of DNA.
• Replication fork is where two new strands grow from opposite directions towards each other.
• DNA polymerase adds complementary bases to the primer sequence.
• Leading strand moves away from the replication fork at a constant rate, while the lagging strand moves towards it.
• DNA polymerase III is the primary enzyme responsible for synthesizing new strands during DNA replication.
• Leading strand grows continuously as it moves away from the origin of replication.
• DNA polymerase I replaces the RNA primers with DNA sequences.
• Once the RNA primer has been laid down, DNA polymerase can add complementary bases to the growing chain.
• Okazaki fragments are small pieces of newly synthesized DNA that form on the lagging strand.
• DNA ligase joins adjacent Okazaki fragments together into one continuous strand.
• The lagging strand is discontinuous and grows in short fragments called Okazaki fragments.
• The leading strand has no gaps or discontinuities, while the lagging strand does.
• Primase adds primers to the 3' end of the template DNA strand