DNA Replication

• Complementary base pairing allows prediction of one DNA strand's sequence if the other is known
• It enables cells to reproduce DNA strands using the information in the complementary strand.

Replication Process Steps

• The double helix unwinds from histones.
• DNA helicase opens short segments of the helix, exposing nitrogenous bases at the replication fork
• DNA polymerase moves along each strand, reads exposed bases, and arranges complementary nucleotides: If the polymerase finds the sequence TCG, for example, it assembles AGC across from it
• One polymerase moves toward the replication fork, creating a continuous DNA strand.
• Another polymerase moves away from the replication fork, copying DNA in short segments.
• DNA ligase joins the short DNA segments together.
• Semiconservative replication produces two DNA molecules, each with one new and one old helix.
• New histones are synthesized in the cytoplasm and transported to the nucleus to form new nucleosomes around the DNA.
• DNA polymerase replicates about 100 base pairs per second
• Thousands of polymerase molecules work simultaneously to replicate all 46 chromosomes in 6-8 hours

DNA Damage Response (DDR)

• DNA polymerase proofreads new base pairs and replaces incorrect pairs with correct ones.
• This results in one mistake per billion base pairs replicated.

Mutations

• Mutations are changes in DNA structure resulting from replication errors or environmental factors.
• Mutations can be passed to daughter cells and may have no adverse effect or kill a cell, turn it cancerous, or cause genetic defects.
• Some mutations don't alter protein structure because new base sequences code for the same amino acid
• Changes in protein structure aren't always critical to function
• Most mutations don't affect protein structure since 98% of DNA doesn't code for proteins.
• Some amino acid substitutions are more critical than others, affecting the severity of a mutation.
• Defects in the DNA damage response can cause diseases like anemia, cancer, immune deficiency, and brain defects.

Cell Cycle Phases

• The cell cycle extends from one cell division to the next, with four main phases: G1, S, G2, and M.
• G1 (first gap phase): Cell synthesizes proteins, grows, and carries out its functions
• Cells accumulate materials needed for DNA replication: Lasts 8-10 hours in cultured fibroblasts.
• S (synthesis phase): Cell duplicates centrioles and nuclear DNA through semiconservative replication
• The two identical sets of DNA will be divided between daughter cells: Takes 6-8 hours in cultured fibroblasts.
• G2 (second gap phase): Cell grows further, makes more organelles, finishes centriole replication, and synthesizes enzymes for cell division
• Cell checks DNA replication fidelity and repairs errors: Lasts 4-6 hours in fibroblasts.
• M (mitotic phase): Cell replicates its nucleus and divides into two daughter cells
• Takes 1-2 hours in cultured fibroblasts.
• G1, S, and G2 phases are collectively called interphase (the time between M phases).
• The cell cycle length varies among cell types
• Some cells enter Go phase and cease dividing for extended periods, while an inability to stop cycling can lead to cancer.

Cell Division: Mitosis

• Mitosis functions include development, growth, cell replacement, and tissue repair.
• Mitosis has four recognizable phases: prophase, metaphase, anaphase, and telophase.
  • Prophase
  • Chromosomes shorten and thicken.
  • There are 46 chromosomes, each with two chromatids and one DNA molecule per chromatid.
  • The nuclear envelope disintegrates, releasing chromosomes into the cytosol.
  • Centrioles sprout spindle fibres
  • A pair of centrioles lies at each pole of the cell.
  • Some spindle fibers attach to the kinetochore on each side of the centromere and tug the chromosomes to the midline of the cell
  • Metaphase: Chromosomes align on the cell equator.
  • Spindle fibers form the mitotic spindle.
  • Microtubules extend from centrioles to chromosomes, and shorter microtubules form an aster to anchor the assembly to the plasma membrane.
  • Anaphase: Sister chromatids separate at the centromere, becoming daughter chromosomes.
  • Daughter chromosomes migrate to opposite poles, with centromeres leading and arms trailing.
  • Motor proteins in the kinetochore crawl along the spindle fiber
  • Telophase: Daughter chromosomes cluster on each side of the cell.
  • The rough ER produces a new nuclear envelope around each cluster.
  • Chromosomes uncoil and return to chromatin form.
  • The mitotic spindle breaks up, and nucleoli form in each new nucleus.
• Telophase overlaps with cytokinesis (division of the cytoplasm).
• Myosin pulls on actin microfilaments, creating a cleavage furrow that pinches the cell in two.
• Interphase begins for the new cells.
• Mitosis (nuclear division) can occur without cytokinesis (cellular division), resulting in multinucleated cells.
• Somatic cells are genetically variable due to DNA replication errors and new mutations, creating genetic mosaicism.
• Identical twins aren't genetically identical due to spontaneous mutations acquired independently in the womb.

Cell Division Control

• Cells divide when they grow large enough, replicate DNA, receive nutrients, are stimulated by growth factors, or neighboring cells die.
• Cells stop dividing when they contact neighboring cells or nutrients/growth factors are withdrawn, called contact inhibition.
• Absence of contact inhibition leads to uncontrolled cell division, a characteristic of cancer.

Cell Cycle Regulation

• The cell cycle is regulated by a molecular timer and checkpoints.
• Cyclins and cyclin-dependent kinases (Cdks) are key proteins in the timer
• Kinases add phosphate to proteins, activating or suppressing their function.
• Cdks are dormant without cyclins, but cyclin levels rise and fall through the cell cycle, activating Cdks
• Proteasomes degrade cyclins at the end of mitosis, deactivating Cdks until the next cycle.
• Cyclin-Cdk complexes trigger biochemical reactions at checkpoints, preparing the cell to move to the next cycle phase.
• G1 checkpoint determines whether the cell proceeds to the S phase or enters the Go phase
• G2/M checkpoint determines whether the cell proceeds to mitosis
• A checkpoint at the metaphase-anaphase transition determines whether the cell proceeds to anaphase.
• Malfunctions in the process can cause failure of tissue maintenance/repair or uncontrolled tissue growth and cancer.
• Cyclin-Cdk complexes control DNA and centriole replication in the S phase, chromosome condensation, nuclear envelope breakdown, mitotic spindle formation, and chromosome attachment in prophase, and centromere splitting and sister chromatid separation at anaphase.

Test your knowledge of DNA replication. This quiz covers base pairing, enzyme functions like DNA polymerase and ligase, and the semiconservative nature of replication.