Cellular Form and Function: Nuclear Structure, Nucleic Acids, Protein Synthesis, and Cell Cycle

Structure and Function of the Nucleus

  • General Overview and Dimensions

    • The nucleus is typically the largest cellular organelle, measuring approximately 5 μm5\,\mu\text{m} in diameter.
    • It houses the cell's genetic material and serves as the primary control center for cellular activity.

  • Variations in Nuclear Quantity

    • Uninucleate: Most cells in the human body possess a single nucleus.
    • Anuclear: Certain cells lack a nucleus entirely, such as mature red blood cells (erythrocytes).
    • Multinuclear: Some cells contain multiple nuclei, such as skeletal muscle cells.

  • Nuclear Envelope and Porous Machinery

    • Nuclear Envelope: A double-membrane structure surrounding the nucleus.
    • Nuclear Pores: Perforations through the envelope formed by a ring-shaped protein complex known as the nuclear pore complex.
    • Regulates molecular traffic entering and exiting the nucleus.
    • Holds the inner and outer layers of the nuclear envelope together.
    • Nuclear Lamina: A web-like scaffold of protein filaments lining the inner surface of the nuclear envelope, providing structural support.

  • Nucleoplasm and Internal Components

    • Nucleoplasm: The fluid environment contained within the nuclear envelope.
    • Chromatin: Fine, threadlike structures composed of DNA complexed with proteins.
    • Nucleolus (plural: nucleoli):
    • A prominent, dark-staining region within the nucleus composed of RNA, enzymes, and histones.
    • Highly developed in cells with elevated rates of protein synthesis, such as liver cells, nerve cells, and muscle cells.
    • Serves as the synthesis site for ribosomal RNA (rRNA) and ribosomal subunits.

Structure of the Nucleus

Nucleic Acid Chemistry: DNA and RNA Structure

  • Deoxyribonucleic Acid (DNA) Overview

    • DNA is a long, threadlike polymer with a uniform diameter of 2 nm2\,\text{nm}, though individual molecules vary in length.
    • The nucleus of a typical human cell contains 46 DNA molecules (chromosomes), divided into 23 chromosomes inherited maternally (via the oocyte) and 23 inherited paternally (via the sperm).
    • Uncoiled, an average human DNA molecule spans approximately 2 inches2\,\text{inches} in length.

  • Nucleotide Subunit Structure

    • DNA and other nucleic acids are polymers constructed from monomeric units called nucleotides.
    • Each nucleotide consists of three integral components:
    1. A five-carbon sugar: Deoxyribose (in DNA) or Ribose (in RNA).
    2. A phosphate group.
    3. A nitrogenous base.

  • Classification of Nitrogenous Bases

    • Purines: Double-ringed nitrogenous bases.
    • Adenine (A): Chemical structure C5H5N5\text{C}_5\text{H}_5\text{N}_5
    • Guanine (G): Chemical structure C5H5N5O\text{C}_5\text{H}_5\text{N}_5\text{O}
    • Pyrimidines: Single-ringed nitrogenous bases.
    • Cytosine (C): Chemical structure C4H5N3O\text{C}_4\text{H}_5\text{N}_3\text{O}
    • Thymine (T): Chemical structure C5H6N2O2\text{C}_5\text{H}_6\text{N}_2\text{O}_2 (found exclusively in DNA)
    • Uracil (U): Chemical structure C4H4N2O2\text{C}_4\text{H}_4\text{N}_2\text{O}_2 (found exclusively in RNA)

Five Nitrogenous Bases found in DNA and RNA

  • The DNA Double Helix Architecture
    • DNA consists of two polynucleotide chains wrapped into a double-helix geometry resembling a spiral staircase.
    • Sugar-Phosphate Backbone: The outer sidepieces consist of alternating deoxyribose sugar units and phosphate groups.
    • Base Pairs: The internal rungs are formed by nitrogenous bases joined together by weak hydrogen bonds.
    • Law of Complementary Base Pairing:
    • Adenine (A) pairs strictly with Thymine (T) via 2 hydrogen bonds.
    • Guanine (G) pairs strictly with Cytosine (C) via 3 hydrogen bonds.
    • Hydrogen bonds are weak interactions that allow the two DNA strands to be unzipped during replication and transcription.

DNA Structure and Hydrogen Bonding

  • Ribonucleic Acid (RNA) Characteristics
    • RNA exists as a single nucleotide chain rather than a double helix.
    • Contains the sugar ribose instead of deoxyribose.
    • Utilizes uracil (U) in place of thymine (T), pairing with adenine (A).
    • Significantly smaller than DNA, ranging from under 100 to just over 10,000 bases per molecule.
    • Primarily functions in the cytoplasm to execute protein synthesis.
    • Key RNA Subtypes:
    • Messenger RNA (mRNA): Transcribes and carries genetic codes from the nucleus to cytoplasm.
    • Ribosomal RNA (rRNA): Structural and catalytic constituent of ribosomes.
    • Transfer RNA (tRNA): Delivers specific amino acids to the ribosome during translation.

Structure of Transfer RNA

Organization of Chromatin and Chromosomes

  • Hierarchical Levels of DNA Packaging
    • If fully extended, the total DNA in a single human cell would measure approximately 6 feet6\,\text{feet} in length, packaged into a nucleus 5 μm5\,\mu\text{m} in diameter.
    • Packaging Hierarchy:
    1. Naked DNA Double Helix: Width of 2 nm2\,\text{nm}.
    2. Nucleosome Formation: DNA winds around octamers of core proteins called histones (clusters of 8 histone molecules). The resulting DNA-protein complex is called a nucleosome core particle, measuring 11 nm11\,\text{nm} in diameter.
    3. Zigzag Chromatin Fiber: Nucleosomes coil into a condensed fiber measuring 30 nm30\,\text{nm} in width.
    4. Irregular Loop Domains: The 30 nm30\,\text{nm} fiber folds into extended loop structures with an overall thickness of 300 nm300\,\text{nm}.
    5. Condensed Chromatids: In dividing cells, loops fold further to form thick chromatid fibers measuring 700 nm700\,\text{nm} in width.

Chromatin Structural Hierarchy

  • Metaphase Chromosome Structure
    • Before cell division, DNA replicates to yield two identical parallel filaments called sister chromatids.
    • Sister chromatids are joined together at a narrowed region called the centromere.
    • Kinetochores: Protein plaques situated on each side of the centromere that serve as attachment points for spindle fibers during mitosis.

Chromosome Structure at Metaphase

Mechanisms of Protein Synthesis

  • Definition of the Gene and Expression

    • Classical Definition: A segment of DNA that codes for a specific protein.
    • Modern Definition: An information-containing segment of DNA that codes for the production of a molecule of RNA, which plays a functional role in protein synthesis.
    • The human genome contains approximately 20,000 genes, accounting for only about 2% of total genomic DNA. The remaining 98% consists of non-coding DNA involved in structural organization and gene regulation.
    • Basic flow of genetic information (Central Dogma):     DNA→TranscriptionmRNA→TranslationProtein\text{DNA} \xrightarrow{\text{Transcription}} \text{mRNA} \xrightarrow{\text{Translation}} \text{Protein}

  • Transcription: DNA to mRNA

    • Occurs inside the nucleus and is catalyzed by the enzyme RNA polymerase.
    • Mechanism:
    1. RNA polymerase binds to a promoter region near the start of a gene and unzips the DNA double helix.
    2. RNA polymerase reads nitrogenous bases along one DNA template strand and synthesizes a complementary strand of pre-mRNA:
      • DNA Cytosine (C) →\rightarrow mRNA Guanine (G)
      • DNA Guanine (G) →\rightarrow mRNA Cytosine (C)
      • DNA Thymine (T) →\rightarrow mRNA Adenine (A)
      • DNA Adenine (A) →\rightarrow mRNA Uracil (U)
    3. The enzyme rewinds the DNA helix behind it as it moves.
    4. Transcription stops when RNA polymerase encounters a terminator sequence.
    • Post-Transcriptional Nuclear Processing:
    • Nuclear enzymes process raw pre-mRNA before export.
    • Introns: Non-coding intervening sequences within pre-mRNA that are excised.
    • Exons: Coding sequences that remain and are spliced together to form functional mRNA.

  • Translation: mRNA to Polypeptide

    • Occurs in the cytoplasm where mRNA is decoded to build an amino acid chain.
    • Key Components:
    • mRNA: Contains a 7-methylguanosine protein cap acting as a ribosomal recognition site and codons (triplets of nucleotides).
    • tRNA: L-shaped molecule possessing an anticodon loop (3 bases complementary to an mRNA codon) and a 3' amino acid-accepting end (sequence ACC).
    • Ribosomes: Complex structures of rRNA and proteins, comprising a small subunit and a large subunit. Contain three binding pockets:
      • A site (Aminoacyl site): Receives incoming tRNA.
      • P site (Peptidyl site): Holds the tRNA carrying the growing polypeptide chain.
      • E site (Exit site): Discharges uncharged tRNAs.
    • Steps of Translation:
    1. Initiation: The small ribosomal subunit binds the mRNA cap, scans for the start codon (AUG), and positions the initiator tRNA carrying methionine (anticodon UAC) into the P site. The large subunit then docks onto the complex.
    2. Elongation: The next tRNA carrying an amino acid enters the A site. A peptide bond is formed between the adjacent amino acids. The ribosome translocates 3 nucleotides down the mRNA. The uncharged tRNA shifts to the E site and exits, while the tRNA holding the polypeptide moves to the P site.
    3. Termination: The ribosome reaches a stop codon (UAG, UAA, or UGA). A protein called a release factor binds to the A site, causing the completed polypeptide to be cleaved from the tRNA and releasing the ribosomal subunits.
    • Polyribosomes: Clusters of multiple ribosomes simultaneously translating a single mRNA molecule to rapidly produce many protein copies.

  • Post-Translational Processing and Secretion

    • Polypeptides destined for membrane insertion, lysosomes, or exocytosis are directed into the rough endoplasmic reticulum (ER).
    • Processing Steps:
    1. The signal peptide at the leading end of the protein guides the ribosome to dock on the rough ER surface. The polypeptide threads through a pore into the ER cistern.
    2. Enzymes inside the rough ER modify the protein by removing signal sequences, folding the tertiary structure, introducing disulfide bridges, and attaching carbohydrate groups (glycosylation).
    3. Formed proteins pinch off from the ER inside membrane-bound transport vesicles.
    4. Transport vesicles fuse together to form new cis cisternae of the Golgi complex.
    5. The protein matures as cisternae progress through the Golgi stack to the trans face.
    6. At the trans face, the cisternae break apart into Golgi vesicles (which mature into lysosomes or secretory vesicles).
    7. Secretory vesicles undergo exocytosis to release mature proteins outside the cell.

Protein Processing and Secretion Pathway

Gene Regulation and Non-Protein Synthesis

  • Mechanisms of Gene Induction
    • Genes can be selectively turned on or off based on cellular requirements, developmental signals, or hormonal stimulation.
    • Case Study: Casein Synthesis Induced by Prolactin:
    1. Prolactin binds to specific cell-surface receptors on mammary gland epithelial cells.
    2. Receptor binding triggers an intracellular signal cascade using energy from ATP hydrolysis:        ATP→ADP+Pi\text{ATP} \rightarrow \text{ADP} + \text{P}_i
    3. A regulatory protein (transcription activator) becomes phosphorylated and activated.
    4. The activated transcription activator translocates into the nucleus and binds to the promoter region of the casein gene.
    5. Binding facilitates RNA polymerase docking and transcription of casein mRNA.
    6. Casein mRNA enters the cytoplasm, attaches to ribosomes on the rough ER, undergoes translation, processing in the Golgi complex, and secretion into milk via exocytosis.

Mechanism of Gene Activation by Prolactin

  • Indirect Genetic Control of Non-Protein Molecules
    • Molecules such as glycogen, triacylglycerols, phospholipids, steroids (e.g., testosterone), and pigments do not have genes encoding them directly.
    • Synthesis of these non-protein compounds is under indirect genetic control: genes encode the enzymes responsible for catalyzing their metabolic production.

DNA Replication and Cell Division

  • Semiconservative DNA Replication
    • Prior to cell division, nuclear DNA must be accurately duplicated.
    • Enzymatic Process:
    1. Unwinding: The DNA double helix unwinds from histones.
    2. Unzipping: The enzyme DNA helicase breaks hydrogen bonds between nitrogenous bases, opening up a replication fork.
    3. Synthesis: Enzymes called DNA polymerase move along each exposed strand, read nitrogenous bases, and synthesize matching strands. Leading strands are synthesized continuously, while lagging strands are assembled discontinuously.
    4. Ligation: On discontinuous strands, DNA segments are joined together by DNA ligase.
    5. Semiconservative Assembly: Each new DNA molecule contains one original parent strand and one newly synthesized daughter strand.
    6. Histones synthesized in the cytoplasm associate with the new DNA molecules to construct fresh nucleosomes.

Semiconservative DNA Replication

  • The Cell Cycle Architecture
    • The continuous life history of a cell is partitioned into Interphase and the Mitotic Phase (M Phase).
    • Interphase Subphases:
    • First Gap Phase (G1\text{G}_1): Interval between cell division and DNA replication. The cell performs normal metabolic functions, grows, and synthesizes proteins.
    • Synthesis Phase (S\text{S}): The cell replicates its complete set of nuclear DNA and duplicates its centrioles.
    • Second Gap Phase (G2\text{G}_2): Interval between DNA replication and division. The cell repairs DNA replication errors, grows further, and synthesizes regulatory enzymes needed for mitosis.
    • G0\text{G}_0 Phase (G-Zero):
    • A non-dividing resting state entered by cells that cease cycling temporarily or permanently.
    • Examples include mature skeletal muscle cells, cardiac muscle cells, and neurons.
    • Stem cells (e.g., epithelial stem cells) never enter G0\text{G}_0 because they undergo continuous division to maintain tissue populations.

The Cell Cycle Phases

  • Stages of Mitosis
    • Mitosis is the division of the nucleus to yield two genetically identical daughter nuclei.
    • Prophase:
    • Chromatin condenses into 46 compact, visible chromosomes (each consisting of two sister chromatids).
    • The nuclear envelope disintegrates and nucleoli vanish.
    • Centrioles sprout microtubular spindle fibers and migrate to opposite poles of the cell.
    • Spindle fibers attach to kinetochores located at centromeres.
    • Metaphase:
    • Chromosomes align precisely along the equatorial plane (metaphase plate) of the cell.
    • Spindle fibers form a lemon-shaped structure called the mitotic spindle.
    • Short microtubules called asters extend from centrioles to anchor them to the inner plasma membrane surface.
    • Anaphase:
    • An enzyme cleaves the sister chromatids at their centromeres.
    • Motor proteins within kinetochores crawl along spindle fibers, pulling single-stranded daughter chromosomes to opposite poles.
    • Telophase:
    • Daughter chromosomes cluster at each pole and uncoil back into fine chromatin.
    • Rough ER constructs a new nuclear envelope around each chromosomal cluster.
    • Nucleoli reappear within each nucleus and the mitotic spindle breaks down.

Prophase and Metaphase Stages of Mitosis

Anaphase and Telophase Stages of Mitosis

  • Cytokinesis

    • Cytokinesis is the division of the cytoplasm into two independent cells.
    • Begins in late anaphase and continues through telophase.
    • Driven by myosin motor proteins pulling on actin filaments in the terminal web beneath the plasma membrane.
    • Creates an indentation called a cleavage furrow around the cell equator, pinching the cell into two genetically identical daughter cells.

  • Regulation of Cell Proliferation

    • Cells Divide When:
    • They grow large enough to provide sufficient cytoplasm for two daughter cells.
    • DNA replication is successfully completed.
    • Nutrients are plentiful.
    • They are stimulated by chemical growth factors or elevated levels of M-phase promoting factor (MPF).
    • Nearby cells die, creating open space.
    • Cells Stop Dividing When:
    • Nutrients or growth factors are depleted.
    • They encounter contact inhibition—the cessation of division when cells contact neighboring cells snugly.

Pathophysiology of Cancer and Tumor Metastasis

  • Neoplasms: Benign versus Malignant

    • Benign Tumors: Slow-growing, localized, and encapsulated masses that do not invade neighboring tissue.
    • Malignant Tumors (Cancer): Unencapsulated, rapidly growing masses that invade surrounding tissues.

  • The Metastatic Cascade

    • Metastasis is the dissemination of malignant cells from a primary tumor to form secondary tumors in distant tissues.
    • Sequential Steps:
    1. Malignant cells detach from the primary tumor and penetrate nearby blood or lymphatic vessels.
    2. Cells circulate through the vascular or lymphatic systems to distant body regions.
    3. Traveling cells exit the vessels (extravasation) into host tissue.
    4. Escaped cells proliferate and establish secondary (metastatic) tumors.
    • Tumor Angiogenesis: Malignant tumors secrete growth factors that stimulate new blood vessel growth to supply nutrients and oxygen to the growing tumor mass.

Mechanism of Cancer Metastasis

  • Lethal Effects of Cancer
    • Tumors displace and replace functional parenchymal tissue in vital organs.
    • Invasive growth erodes blood vessels, pulmonary tissue, or brain structures.
    • Expanding masses cause mechanical compression, obstructing functional organ passages.
    • Malignancies suppress immune defense, opening pathways for severe opportunistic infections.
    • High metabolic consumption by tumors deprives normal tissues of energy, resulting in severe physical wasting (cachexia).