DNA Complementary Base Pairing and Replication Notes

Complementary base pairing rules
- DNA: Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C).
- RNA (transcription, using DNA as template): Adenine (A) pairs with Uracil (U); Thymine (T) in DNA pairs with Adenine (A) in RNA; Guanine (G) pairs with Cytosine (C); Cytosine (C) pairs with Guanine (G).
Transcript-specific mapping described in the lecture
- RNA complementary base pairing mentioned: A → U, T → A, C → G, G → C.
- Statement summarized: when making RNA complementary to a DNA template, adenine in DNA pairs with uracil in RNA, and thymine in DNA pairs with adenine in RNA.
Significance
- Ensures accurate transcription of genetic information from DNA to RNA, which is then translated into proteins.

Starting point: Original double-stranded DNA (double helix).
Unwinding enzyme: DNA helicase
- Function: Opens the double helix to form the replication fork, creating two single-stranded DNA templates.
Role of DNA polymerase
- Function: Extracts nucleotides from the nucleoplasm and attaches them to the growing new strands, following complementary base-pairing rules.
- Directionality: Works in one direction on each strand; two polymerases operate in opposite directions to synthesize the two new strands.
Role of DNA ligase
- Function: Seals nicks and joins the newly formed DNA fragments by forming covalent phosphodiester bonds, ensuring hydrogen bonds form between bases and the backbone is continuous.
Outcome: Semiconservative replication
- Each new DNA molecule consists of one original (parent) strand and one newly synthesized (daughter) strand.
Visual description from the lecture
- Original parent strand is split; red new strand is built off the parent; replication proceeds until the entire molecule is copied.

Semiconservative model
- Each daughter DNA molecule contains one old strand and one new strand.
Chromosome assembly context
- Newly replicated DNA associates with histones to form chromatin (nucleosomes) and circulates (in the nucleoplasm) until condensation into chromosomes.
Chromosome count in humans (as stated in the lecture)
- Total chromosomes: $46$ .
- Parental contribution: $23$ from mom + $23$ from dad.
Replication timing note (per lecturer)
- It is described as a large, simultaneous process occurring in many nuclei at once; the statement cites roughly $68$ hours to replicate chromosomes across many cells, highlighting the scale of the process.

DNA helicase
- Unwinds the parental DNA strands to create replication forks.
DNA polymerase (the heavy lifter)
- Synthesizes new DNA by adding nucleotides to the growing strand.
- Proofreading function: checks base-pairing fidelity as the strand is synthesized.
- Reported accuracy in the lecture: about one error per $10^9$ bases copied (approximately $1/10^9$ ).
DNA ligase
- Seals the backbone by joining the sugar-phosphate strands and stabilizes newly formed segments by ensuring proper hydrogen bonding between bases.
Histones and chromatin context
- Newly formed DNA interacts with histones to form chromatin; later condenses into chromosomes for cell division.

Fidelity
- Polymerase proofreading reduces errors during replication.
Error rate stated in the lecture
- Approximately one error per $10^9$ bases copied, reflecting high fidelity.
Post-replication repair mechanisms
- The cell has mechanisms to fix replication errors; however, some mutations escape repair.

Causes of mutations
- Environmental factors: radiation, chemical exposure, viruses.
- Random replication errors that escape proofreading.
Consequences of mutations
- Some mutations have no effect; others can terminate the cell via lysosomal pathways if the cell is not functioning properly.
- Mutations can contribute to cancer if cells begin to proliferate abnormally.
- Some genetic defects can affect future generations (inherited mutations).

Sickle cell anemia as a mutation example
- Described as a mutation that conferred a selective advantage in malaria-endemic environments.
Effect on red blood cells (RBCs)
- Hemoglobin mutation causes RBCs to become misshapen (sickle-shaped), which can reduce parasite adherence but also causes blood flow issues.
Clinical implications
- Increased risk of blood clots and other complications due to misshapen cells.
Ethical and human context noted
- Personal reflection mentioned about family members; emphasizes the real-world impact of genetic mutations.

Fidelity of DNA replication is crucial for genetic stability and organismal health.
Mutations drive evolution but can cause disease (e.g., cancer, inherited disorders).
Environmental exposures can influence mutation rates and outcomes.
Understanding base pairing and replication mechanics underpins genetic engineering, medicine, and biotechnology.

Complementary base pairing: A-T, G-C in DNA; A-U, G-C in RNA (with T in DNA pairing to A in RNA).
Replication fork: Point at which the two DNA strands separate for copying.
Helicase: Enzyme that unwinds DNA.
DNA polymerase: Enzyme that adds nucleotides and proofreads.
DNA ligase: Enzyme that seals gaps in the sugar-phosphate backbone.
Semiconservative replication: Each new DNA molecule contains one old and one new strand.
Chromatin and histones: Packaging of DNA into nucleosomes and chromosomes.
Chromosome count: $46$ total, with $23$ from each parent.
Replication fidelity: Approximately $1/10^9$ errors per base copied.
Mutations: Can arise from environmental factors or replication errors; may be harmless, harmful, or inherited.
Sickle cell anemia: Mutation that provided malaria resistance in some populations but causes health issues.

Error rate in DNA replication: $ext{error rate} \approx \frac{1}{10^9}$ errors per base copied.
Human chromosome count: $46$ chromosomes total ( $23$ from each parent).
Time reference (as stated in the lecture): $68\text{ hours}$ to replicate chromosomes across nuclei.