DNA Replication Flashcards
DNA Replication
D1.1 Standard Level
Continuity and Change
Living things maintain equilibrium and undergo transformation. Environmental change drives evolution through natural selection.
Guiding Questions
How has knowledge of DNA replication enabled applications in biotechnology?
How is new DNA produced?
DNA Replication
DNA replication is the production of exact copies of DNA with identical base sequences. Each new molecule consists of one new strand and one old strand conserved from the parent DNA molecule. The result is two DNA molecules, each with an original and a newly synthesized strand.
Importance of DNA Replication
DNA replication is required for organism growth, reproduction, and repair:
Growth: Multicellular organisms increase in size by increasing their number of cells. Individual cell size cannot grow due to surface area to volume ratio.
Reproduction: Offspring need copies of the base sequences of their parents, so parents must replicate their DNA during sexual or asexual reproduction.
Tissue replacement and repair: Multicellular organisms require cell division to replace lost or damaged tissues (e.g., skin cells, wounds). New cells need a full set of an organism’s base sequences, necessitating DNA replication.
Semi-Conservative Nature of DNA Replication
The two strands of DNA separate, and each acts as a template for replication. New strands are formed by adding nucleotides one by one and linking them together. When replication is complete, there are two DNA molecules, each composed of an original strand and a newly synthesized strand. This is why DNA replication is referred to as semi-conservative.
Meselson-Stahl Experiment
Experiments (like the Meselson-Stahl experiment) have proven that DNA replication is semi-conservative. The experiment involved:
Growing bacteria in ^{15}N (heavy isotope of nitrogen).
Transferring cells to ^{14}N (light isotope of nitrogen) and growing for one generation.
Growing for a second generation in ^{14}N. Centrifugation in CsCl showed different DNA densities, supporting the semi-conservative model.
Role of Complementary Base Pairing
The DNA base sequence of the parent strands acts as a template for the new strands. Complementary bases form hydrogen bonds, stabilizing the structure. Adenine pairs only with Thymine and Cytosine pairs only with Guanine. Complementary base pairing ensures a high degree of accuracy during strand assembly.
Role of Helicase and DNA Polymerase
Helicase: Unwinds the DNA double helix and separates the two strands by breaking hydrogen bonds. This process requires energy (ATP).
DNA Polymerase: Adds free nucleotides one by one in positions on the template strand where hydrogen bonds between complementary bases can form. DNA polymerase forms a covalent bond between the sugar of the previous nucleotide and the phosphate group of the new nucleotide.
PCR (Polymerase Chain Reaction)
PCR is an automated procedure used to copy specific sections of DNA molecules in a cell sample (e.g., blood, tissue cells, semen). PCR machines use repeated cycles of heating and cooling to replicate a small quantity of DNA using a special type of polymerase (Taq).
Thermus aquaticus (Taq): An extremophile bacteria living in harsh conditions such as hot springs has a heat-resistant DNA polymerase. This Thermus aquaticus polymerase (