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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, each containing 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 Structure and Nuclear Envelope

Nuclear Envelope

The nuclear envelope is a double-membrane structure that surrounds the nucleus in eukaryotic cells, 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.

  • Histones: Proteins that help package DNA into a compact, organized structure, and play a role in gene regulation.

Additional info: Chromatin can exist as euchromatin (less condensed, transcriptionally active) or heterochromatin (more condensed, transcriptionally inactive).

Cell Cycle

Phases of the Cell Cycle

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 is replicated.

  • G2 Phase (Gap 2): Cell prepares for mitosis; checks for DNA damage and repairs if necessary.

G1 and G2 Checkpoints

  • G1 Checkpoint: Ensures the cell is ready for DNA synthesis; checks for DNA damage and sufficient resources.

  • G2 Checkpoint: Ensures DNA replication is complete and checks for DNA damage before mitosis.

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 and prevent reannealing.

  • Ligase: Joins Okazaki fragments on the lagging strand.

Mitosis

Phases of Mitosis (Focus Fig 3.4)

Mitosis is the process by which a eukaryotic cell divides its nucleus and contents to produce two genetically identical daughter cells.

  • 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 of the cell.

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

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

Transcription

Formation of mRNA

Transcription is the process by which a segment of DNA is used as a template to synthesize messenger RNA (mRNA).

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

  • Elongation: RNA polymerase synthesizes the mRNA strand by adding complementary RNA nucleotides.

  • 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 mRNA.

  • 3. Termination: Transcription ends when RNA polymerase encounters a stop signal.

Example: The gene for hemoglobin is transcribed into mRNA, which is then translated into the hemoglobin protein.

Additional info: In eukaryotes, the primary mRNA transcript (pre-mRNA) undergoes processing, including addition of a 5' cap, poly-A tail, and splicing to remove introns before becoming mature mRNA.

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