DNA Replication Overview 9.1-9.2

Key Enzymes in DNA Replication

• Topoisomerase: Uncoils the DNA to relieve the tension ahead of the replication fork.
• Primase: Synthesizes short RNA primers needed to initiate DNA synthesis.
• DNA Polymerase: Attaches complementary nucleotides in a 5' to 3' direction to form new DNA strands.
• DNA Ligase: Connects Okazaki fragments on the lagging strand, sealing gaps.

Importance of DNA

• DNA is universal in all living cells and is composed of nucleotides.
• Contains instructions for protein synthesis and carries genetic information inherited from parents.

Functions of DNA

• Acts as a storage medium for genetic information and as a blueprint for protein production.
• Undergoes replication during cell division to ensure continuity of genetic information.
• Genes encode for proteins through two main processes: transcription (DNA → RNA) and translation (RNA → protein).

Structure of DNA

• DNA has a double helix structure with two strands running antiparallel; one end is 5' (phosphate) and the other is 3' (hydroxyl).
• Base pairing occurs between adenine-thymine (A-T) and guanine-cytosine (G-C).

Sugar-Phosphate Backbone

• The DNA backbone consists of alternating sugar and phosphate groups with distinct ends: 5' (phosphate) and 3' (hydroxyl).
• New nucleotides are added exclusively to the 3' end during synthesis, maintaining the 5' to 3' growth direction.

DNA Replication Process

• Involves breaking hydrogen bonds between DNA strands, where each parent strand serves as a template for the formation of new strands.
• Results in a semi-conservative outcome where each new DNA molecule is comprised of one old and one new strand.

Semiconservative Replication

• Requires various enzymes, notably helicases for unzipping DNA strands and creating replication forks.
• Leading strand replication is continuous, while the lagging strand forms in short segments known as Okazaki fragments.

The Polymerase Chain Reaction (PCR)

• A laboratory technique for amplifying specific DNA fragments, utilizing cycles of denaturation and extension akin to natural DNA replication.

Gel Electrophoresis

• A method for analyzing DNA fragments by applying an electrical current through a gel, allowing separation based on size and charge. DNA moves toward positive electrodes due to its negative charge.

DNA Sequencing

• A procedure for determining the precise sequence of nucleotides in a DNA segment. Various sequencing technologies utilize fluorescent labeling for detection.

Errors in DNA Replication

• Mistakes can lead to mutations, potentially encoding defective proteins. The polymerase error rate is approximately 1 per 100,000 nucleotides.
• In humans, with roughly 6 billion base pairs, about 120,000 mistakes may occur during each replication process.

Factors Causing DNA Replication Errors

• Replication Stress: Includes unusual DNA structures and repetitive sequences that interfere with normal replication processes.

Causes of DNA Damage

• UV Radiation: Induces thymine dimers (T-T bonds).
• Chemical Exposure: Hydrocarbons from sources like coal, gasoline, burned meats, and certain drugs can cause DNA adducts.

DNA Repair Mechanisms

• Cells employ various repair strategies which generally include recognition of the error, removal, and replacement of defective sequences.
• DNA polymerase can also correct mistakes by reading the correct template strand.

Consequences of Unrepaired DNA Errors

• Uncorrected mutations can disrupt genetic instructions, potentially leading to diseases such as cancer.