Unit 7_DNA Structure, Replication & Cell Cycle

History of DNA Discovery

  • Rosalind Franklin (1952)

    • She utilized X-ray crystallography to take an image of the DNA molecule to visualize its patterns.

    • The resulting X-rays demonstrated that DNA is twisted around each other, resembling a helix, and consists of two strands.

    • The scientific community learned the double helix structure specifically from the X-ray work performed by Rosalind Franklin.

  • Watson & Crick (1953)

    • They utilized findings from Rosalind Franklin’s X-ray image (often described as stealing her ideas) and claimed the discovery as their own.

    • They proposed the formal model that DNA is composed of two chains of nucleotides that are twisted to form a double helix DNA.

    • They were recognized as the first scientists to create a physical model of DNA.

Chemical Structure of DNA

  • Definition and General Structure

    • DNA (Deoxyribonucleic acid) is a nucleic acid that stores genetic information inside organisms.

    • It features a double helix structure, often compared to the shape of a twisted ladder.

    • The strands of DNA are built from smaller subunits known as nucleotides.

  • The Nucleotide Unit

    • Every single nucleotide consists of three distinct parts:

      1. Phosphate Group

      2. Sugar (Deoxyribose)

      3. Nitrogen Base

  • Phosphate & Sugar Backbone

    • The phosphate and sugar components combine to form the "backbone" of the DNA.

    • This backbone provides the physical structure for the molecule.

    • Crucially, the backbone does not code for individual traits; every organism possesses the same backbone structure.

  • Nitrogen Bases

    • There are four types of nitrogenous bases included in DNA nucleotides:

      • Adenine (A)

      • Thymine (T)

      • Cytosine (C)

      • Guanine (G)

    • The specific order of these nitrogen bases determines the genetic traits of an individual, making every person unique.

    • Hydrogen Bonds: Nitrogen bases are held together in the center of the helix by hydrogen bonds.

The Genetic Code and Complementary Base Pairing

  • Complimentary Base Pairing Rules

    • Nitrogen bases follow specific pairing rules:

      • Adenine (A) always pairs with Thymine (T). A helpful mnemonic is "Apples in Trees."

      • Cytosine (C) always pairs with Guanine (G). A helpful mnemonic is "Cars in Garages."

  • The Nature of the Genetic Code

    • Sequencing: The order or sequencing of the nucleotides (specifically the nitrogenous bases) is what constitutes the "genetic code."

    • Stored Information: This sequence forms the blueprints for genetic information.

    • Trait Determination: Differences in the trait of an organism are determined by differences in the nucleotide base sequence. For example, hair and eye color differences (e.g., Molly having green eyes vs. Stacey having brown eyes) are due to the specific order of their nucleotides.

    • Elemental Composition: Nucleotides contain five primary elements: Carbon, Hydrogen, Oxygen, Nitrogen, and Phosphorus.

    • Protein Synthesis: DNA determines traits by providing the code for manufacturing proteins.

  • Universality of the Genetic Code

    • The genetic code is universal, meaning it is the same for all living things.

    • All organisms, including both prokaryotes and eukaryotes, utilize DNA as their genetic material.

    • Because the code (ATCG) is universal, scientists can compare nucleotide sequences between different species, such as humans and chimpanzees.

The Cell Cycle: Overview and Purpose

  • Definition

    • The cell cycle refers to the process of growth and division of the cell.

  • Applicability

    • This cycle occurs in eukaryotic cells. Prokaryotes do not go through the cell cycle; instead, they divide through a process called binary fission.

  • Reasons for Cell Division

    • Repair: Necessary for fixing damage, such as when you cut your finger and cells must heal the wound.

    • Replace: Cells become old and die over time, requiring replacement.

    • Growth: Increasing the size of the organism by adding more cells (e.g., growing from a baby to an adult).

  • Major Stages of the Cell Cycle

    • The cycle consists of three main parts:

      1. Interphase

      2. Mitosis

      3. Cytokinesis

Interphase: The Preparation Phase

  • Interphase Characteristics

    • Known as the "Doubling Phase" or the preparation phase for division.

    • This is the longest phase of the cell cycle; the cell spends approximately 90%90\% of its life in this stage.

  • Stages of Interphase

    1. Gap 1 (G1):

      • The cell focuses on growth, increasing in size and storing nutrients.

      • New proteins (enzymes) and organelles are produced.

      • The cell hits a checkpoint to ensure it is prepared for DNA replication.

    2. Synthesis (S-Phase):

      • This is the stage of DNA synthesis or replication.

      • The cell creates two identical copies of chromosomes.

      • The copying process is "semiconservative."

    3. Gap 2 (G2):

      • Final cell growth occurs.

      • The cell checks for mistakes and prepares for nuclear division (mitosis). This is the shortest phase of Interphase.

    4. Gap 0 (G0):

      • This is the "Resting Stage." Cells that skip this stage and divide uncontrollably can lead to the formation of tumors.

The Process of DNA Replication (S-Phase)

  • Purpose

    • The end goal of the cell cycle is to create two identical daughter cells, which requires two identical sets of DNA.

  • Mechanism

    • The DNA molecule separates into two strands. Each original strand of the double helix serves as a template for producing a new complementary strand following base-pairing rules.

  • Enzymatic Steps of Replication

    1. Unzip: The enzyme Helicase breaks the hydrogen bonds holding the two strands together. This creates a "replication fork" where the strands split.

    2. Prime: An enzyme called Primase binds to the DNA at the starting point. This acts as a signal to tell the next enzyme where to begin copying.

    3. Elongation: The enzyme DNA Polymerase creates the new strand by adding nucleotides in the 5′→3′5' \rightarrow 3' direction, forming new hydrogen bonds.

      • Leading Strand: Synthesized in the same direction as helicase in one fluid motion.

      • Lagging Strand: Synthesized in the opposite direction of helicase using a "jumping" motion, which creates Okazaki fragments.

    4. Terminate: The new strand is proofread for errors by an enzyme. DNA fragments are sealed together by the enzyme Ligase.

  • Outcome

    • Two identical semi-conservative strands of DNA are produced, each consisting of one parent strand and one new strand.

Mitosis and Cytokinesis

  • Chromosome Anatomy

    • Chromosome: Package containing genetic information (DNA) passed between generations.

    • Centromere: The center point of the chromosome.

    • Chromatids: Two identical "sister" parts of the chromosome.

  • The Four Stages of Mitosis (PMAT)

    1. Prophase: Chromatin condenses into visible chromosomes. The nuclear membrane disappears, and spindles begin to form.

    2. Metaphase ("M" for Middle): Chromosomes line up in the center of the cell. Each chromosome attaches to the spindle fibers.

    3. Anaphase ("A" for Away): Chromosomes are pulled apart by their centromeres. Chromatids move toward opposite poles of the cell.

    4. Telophase ("T" for Two Nuclei): Begins when chromatids reach the poles. Two new nuclei form, and spindles disappear. The cell has not yet fully split.

  • Cytokinesis

    • This is the final stage of the cell cycle but is not considered part of mitosis.

    • The cytoplasm pinches completely in half.

    • In animal cells, the membrane pinches; in plant cells, a cell plate forms (though plant cells have rigid walls).

    • Result: Two genetically identical daughter cells are formed (exact copies).

Cell Cycle Malfunctions and Disease

  • Consequences of Disruption

    • When cell checkpoints are disrupted by hormone imbalances or mutations, the cell cycle can malfunction.

    • Common results include unregulated cell growth, tumors, and various diseases.

  • Cancer

    • Defined as uncontrolled cell growth and unregulated division.

    • Cancer cells typically skip the G0 (Gap 0) resting phase.

    • Tumors: Mass of cells produced by out-of-control growth.

    • Carcinogens: Substances that mutate DNA and cause cancer (e.g., UV light/sun exposure, tobacco, viruses like HPV).

    • Treatment: Radiation and chemotherapy are used because they interfere with the cell cycle, specifically targeting rapidly dividing cells.

  • Other Related Conditions

    • Lupus: An overactive immune system attacking healthy tissue (skin, joints, heart, lungs).

    • Rheumatoid Arthritis: An overactive immune system targeting the joints, causing inflammation and stiffness.

    • Alzheimer’s Disease: Brain cells die at an abnormal rate due to issues with senescence (aged cells that stop dividing but don't die) and apoptosis (programmed cell death/self-destruction).

    • Parkinson’s Disease: A neurodegenerative disease that reduces dopamine-producing cells, causing tremors and motor/non-motor symptoms.

Questions & Practice Data

  • Numerical Scenario: If a DNA sample contains 54%54\% Thymine, then according to base-pairing rules (A=T), Adenine must also be 54%54\%. (Note: In a standard double-stranded DNA model, (A+T)+(C+G)=100%(A+T) + (C+G) = 100\%. If A+T=108%A+T = 108\%, this implies a specialized context or single-stranded sample as presented in the prompt's practice logic).

  • Mitosis Calculation: If a cell begins mitosis with 3232 chromosomes, each of the two resulting daughter cells will also contain exactly 3232 chromosomes because they are exact copies.