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DNA Structure, Replication, Mitosis, and Transcription: Study Guide

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DNA Structure

Section 2.11: DNA Structure

Deoxyribonucleic acid (DNA) is the hereditary material in almost all living organisms. Its structure is essential for its function in storing and transmitting genetic information.

  • Double Helix: DNA consists of two antiparallel strands forming a right-handed double helix.

  • Nucleotides: Each strand is composed of nucleotides, which include a phosphate group, a deoxyribose sugar, and a nitrogenous base (adenine, thymine, cytosine, or guanine).

  • Base Pairing: Adenine pairs with thymine (A-T) and cytosine pairs with guanine (C-G) via hydrogen bonds.

  • Complementarity: The sequence of one strand determines the sequence of the other, enabling accurate replication.

Example: If one DNA strand has the sequence 5'-ATCG-3', the complementary strand will be 3'-TAGC-5'.

Chromatin and Nuclear Structure

Nuclear Envelope

The nuclear envelope is a double-membrane structure that surrounds the nucleus, separating the genetic material from the cytoplasm.

  • Function: Protects DNA and regulates transport of molecules in and out of the nucleus via nuclear pores.

Chromatin, Nucleosomes, and Histone Proteins

Chromatin is the complex of DNA and proteins that forms chromosomes within the nucleus.

  • Nucleosome: The basic unit of chromatin, consisting of DNA wrapped around a core of eight histone proteins.

  • Histone Proteins: Positively charged proteins that help package DNA into a compact, organized structure.

  • Function: Chromatin organization regulates gene expression and DNA accessibility.

Additional info: Chromatin can be further classified as euchromatin (loosely packed, transcriptionally active) and heterochromatin (densely packed, transcriptionally inactive).

Cell Cycle: G1, S, G2 Phases and Checkpoints

The cell cycle is the series of events that cells go through as they grow and divide.

  • G1 Phase (Gap 1): Cell grows and carries out normal functions.

  • S Phase (Synthesis): DNA replication occurs.

  • G2 Phase (Gap 2): Cell prepares for mitosis; checks for DNA damage and ensures all DNA is replicated.

  • Checkpoints: G1 and G2 checkpoints ensure the cell is ready to proceed to the next phase, preventing damaged or incomplete DNA from being passed on.

DNA Replication

Leading vs. Lagging Strand

During DNA replication, the two strands are copied differently due to the antiparallel nature of DNA and the directionality of DNA polymerase.

  • Leading Strand: Synthesized continuously in the 5' to 3' direction toward the replication fork.

  • Lagging Strand: Synthesized discontinuously in short fragments (Okazaki fragments) away from the replication fork.

  • Reason: DNA polymerase can only add nucleotides to the 3' end, necessitating different mechanisms for each strand.

Steps of Leading and Lagging Strand Synthesis

  • Leading Strand: Single RNA primer is laid down; DNA polymerase synthesizes continuously.

  • Lagging Strand: Multiple RNA primers are laid down; DNA polymerase synthesizes Okazaki fragments, which are later joined.

Enzymes Involved in DNA Replication

  • Primer: Short RNA sequence synthesized by primase to provide a starting point for DNA polymerase.

  • DNA Polymerase: Enzyme that adds nucleotides to the growing DNA strand.

  • Topoisomerase: Relieves supercoiling ahead of the replication fork.

  • Helicase: Unwinds the DNA double helix.

  • SSBP (Single-Strand Binding Proteins): Stabilize unwound DNA strands.

  • Ligase: Joins Okazaki fragments on the lagging strand.

Additional info: DNA replication is semiconservative, meaning each new DNA molecule consists of one old and one new strand.

Mitosis

Steps of Mitosis (Focus Fig 3.4)

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical sets.

  • Prophase: Chromatin condenses into visible chromosomes; nuclear envelope breaks down; spindle fibers form.

  • Metaphase: Chromosomes align at the cell's equatorial plate.

  • Anaphase: Sister chromatids are pulled apart to opposite poles.

  • Telophase: Nuclear envelopes reform around each set of chromosomes; chromosomes decondense.

  • Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Example: Mitosis ensures that each daughter cell receives an identical set of chromosomes.

Transcription

Formation of mRNA

Transcription is the process by which the information in a DNA sequence is copied into a complementary RNA sequence.

  • Initiation: RNA polymerase binds to the promoter region of DNA.

  • Elongation: RNA polymerase synthesizes the RNA strand by adding nucleotides complementary to the DNA template.

  • Termination: RNA polymerase reaches a terminator sequence and releases the newly formed mRNA.

Three Main Events of Transcription (Fig 3.30)

  • 1. Initiation: RNA polymerase and transcription factors assemble at the promoter.

  • 2. Elongation: RNA polymerase moves along the DNA, unwinding the helix and synthesizing RNA.

  • 3. Termination: RNA polymerase detaches from DNA, and the pre-mRNA is released.

Additional info: In eukaryotes, the pre-mRNA undergoes processing (capping, polyadenylation, and splicing) before becoming mature mRNA.

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