DNA Structure and Replication

Overview of DNA Structure

DNA (Deoxyribonucleic Acid) is the hereditary material in almost all living organisms. Its structure is a double helix composed of two strands running in opposite directions, held together by complementary base pairing.

• Sugar-phosphate backbone: The sides of the DNA ladder are formed by alternating deoxyribose sugars and phosphate groups.

• Nitrogenous bases: The rungs of the ladder are pairs of nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).

• Base pairing: A pairs with T, and G pairs with C via hydrogen bonds.

• Antiparallel orientation: The two DNA strands run in opposite directions (5' to 3' and 3' to 5').

Example: The image provided shows the double helix structure, with labeled bases and backbone.

Antiparallel Nature of DNA

DNA is described as antiparallel because the two strands run in opposite directions. One strand runs from the 5' to 3' end, while the other runs from 3' to 5'. This orientation is crucial for replication and enzyme function.

• 5' and 3' ends: Refer to the carbon positions in the deoxyribose sugar.

• Importance: DNA polymerases can only add nucleotides to the 3' end, influencing replication direction.

Proteins Involved in DNA Replication

Several enzymes and proteins coordinate the process of DNA replication, each with a specific role:

Replication Fork and Leading/Lagging Strands

During DNA replication, the double helix is unwound to form a replication fork. DNA synthesis occurs differently on the two strands:

• Leading strand: Synthesized continuously in the 5' to 3' direction.

• Lagging strand: Synthesized discontinuously as short fragments (Okazaki fragments) in the 5' to 3' direction, which are later joined by DNA ligase.

• Reason for discontinuity: DNA polymerase can only add nucleotides to the 3' end, so the lagging strand must be synthesized in segments as the fork opens.

Telomeres and Cellular Aging

Telomeres are repetitive nucleotide sequences at the ends of chromosomes that protect genetic data during replication. Their shortening is associated with cellular aging and disease.

• Shortening effects: Can lead to cell aging, increased risk of cancer, and decreased likelihood of apoptosis.

• Telomerase: An enzyme that can extend telomeres in certain cell types.

DNA vs. RNA Reactivity

RNA (Ribonucleic Acid) is generally more reactive than DNA due to the presence of a hydroxyl group (-OH) on the 2' carbon of ribose, making it more susceptible to hydrolysis.

• DNA: Contains deoxyribose, which lacks the 2' hydroxyl group, making it more stable.

• RNA: Contains ribose, which has the 2' hydroxyl group, increasing reactivity.

Central Dogma: Replication, Transcription, and Translation

Genetic information flows from DNA to RNA to protein through three main processes:

• Replication: Copying DNA to produce identical DNA molecules.

• Transcription: Synthesizing RNA from a DNA template.

• Translation: Synthesizing proteins from an mRNA template.

In eukaryotic cells, these processes occur in specific locations:

• Replication: Nucleus

• Transcription: Nucleus

• Translation: Cytoplasm (at ribosomes)

mRNA Complementary Sequence

To determine the complementary mRNA sequence for a DNA strand, replace each base with its RNA complement:

• A (Adenine) → U (Uracil)

• T (Thymine) → A (Adenine)

• G (Guanine) → C (Cytosine)

• C (Cytosine) → G (Guanine)

Example: For DNA strand 5' TGCTAC 3', the mRNA sequence is 3' ACGAUG 5'.

Hairpin Structure of RNA

RNA molecules can fold into secondary structures such as hairpins, where complementary sequences within the same strand base pair to form a stem-loop.

• Function: Hairpins are important in regulation and stability of RNA.

• Structure: A stem of paired bases and a loop of unpaired bases.

Additional info: Hairpin structures are common in tRNA and regulatory RNAs.

Definition: Replisome

The replisome is a complex of proteins that carries out DNA replication at the replication fork.

• Components: Includes DNA polymerases, helicase, primase, ligase, and other accessory proteins.

• Function: Coordinates the synthesis of both leading and lagging strands.

Summary Table: DNA vs. RNA

Key Equations

• Base pairing:

• Direction of synthesis: