Cell Division and DNA Replication

Introduction

Cell division and DNA replication are fundamental processes in all living organisms, ensuring growth, development, tissue repair, and the transmission of genetic information. This guide covers the purposes of cell division, the cell cycle, mitosis, cytokinesis, binary fission, and the molecular mechanisms of DNA replication.

Purposes of Cellular Division

Why Cells Divide

• Reproduction: In unicellular organisms, cell division is a means of reproduction (e.g., binary fission in bacteria).

• Growth and Development: Multicellular organisms begin as a single cell (zygote) and undergo numerous cell divisions to form tissues, organs, and the entire body. An adult human has approximately 37 trillion cells.

• Replacement of Lost or Damaged Cells: Cells lost through programmed cell death (apoptosis) or injury are replaced by new cells produced through cell division. For example, bone marrow continuously produces new blood cells.

• Differentiation: Stem cells divide to produce daughter cells that differentiate into specialized cell types.

The Cell Cycle

Overview and Categories of Cell Division

The cell cycle is the ordered sequence of events that a cell undergoes from its formation to its own division. Cells can be grouped based on their activity in the cell cycle:

• Non-dividing (G0 phase): Cells such as neurons, muscle cells, and red blood cells exit the cell cycle and remain in a resting state until they die.

• Quiescent (can divide if needed): Some cells, like certain stem cells and immune cells, do not normally divide but can re-enter the cell cycle if required.

• Regularly dividing: Cells that frequently divide, such as epithelial cells lining body surfaces and blood stem cells in bone marrow.

Phases of the Cell Cycle

• Interphase (90% of the cycle):

  • G1 phase: Cell grows and performs its normal functions.

  • S phase: DNA is replicated in the nucleus.

  • G2 phase: Cell prepares for division.

• M phase (Mitosis and Cytokinesis, 10%):

  • Mitosis: Division of the nucleus and genetic material.

  • Cytokinesis: Division of the cytoplasm and cellular contents.

Chromosomes, Chromatin, and Chromatids

Definitions and Relationships

• Chromosome: A DNA molecule packaged with proteins (histones). Each chromosome contains many genes.

• Chromatin: The less condensed form of DNA and proteins found in the nucleus during interphase; actively expressed genetic material.

• Chromatid: Each of the two identical halves of a replicated chromosome, joined at the centromere.

• Centromere: The constricted region of a chromosome where sister chromatids are attached.

In humans, somatic cells have 46 chromosomes (23 pairs). Before division, each chromosome is replicated to form two sister chromatids.

Mitosis: Phases and Events

Overview

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical nuclei. It is followed by cytokinesis, which divides the cytoplasm.

Phases of Mitosis

1. Prophase:

   • Chromosomes condense and become visible as sister chromatids.

   • Mitotic spindle (microtubules) begins to form in the cytoplasm.

   • Nucleolus disappears.

2. Prometaphase:

   • Nuclear envelope fragments.

   • Spindle microtubules attach to kinetochores (protein complexes at centromeres).

3. Metaphase:

   • Chromosomes align at the metaphase plate (cell equator).

   • Each chromatid is attached to spindle fibers from opposite poles.

4. Anaphase:

   • Cohesion proteins are cleaved, separating sister chromatids.

   • Chromatids (now individual chromosomes) move toward opposite poles.

5. Telophase:

   • Chromosomes de-condense.

   • Nuclear envelope reforms around each set of chromosomes.

   • Nucleolus reappears.

6. Cytokinesis:

   • Division of the cytoplasm, resulting in two genetically identical daughter cells.

Cytokinesis in Animal and Plant Cells

• Animal Cells: A contractile ring of actin filaments forms a cleavage furrow, pinching the cell in two.

• Plant Cells: Vesicles deliver cell wall materials to the center of the cell, forming a cell plate that develops into a new cell wall, separating the two daughter cells.

What Happens if Cytokinesis Does Not Occur?

• The cell may become multinucleated (e.g., muscle fibers, some plant cells), resulting in a single cell with multiple nuclei.

Control of the Cell Cycle

Checkpoints

• The cell cycle is regulated by three main checkpoints:

• G1 checkpoint: Determines if the cell will proceed to DNA synthesis.

• G2 checkpoint: Ensures DNA replication is complete and undamaged before mitosis.

• M checkpoint (Spindle checkpoint): Ensures all chromosomes are properly attached to the spindle before anaphase.

Binary Fission in Prokaryotes

Mechanism

• Binary fission is the process by which prokaryotic cells (e.g., bacteria) reproduce asexually.

• The single, circular chromosome is replicated, and each copy attaches to different parts of the cell membrane.

• The cell elongates and divides into two genetically identical daughter cells.

DNA Structure and Replication

Structure of the DNA Double Helix

• DNA is composed of two antiparallel strands forming a double helix.

• Each strand consists of nucleotides with a sugar-phosphate backbone and nitrogenous bases (adenine, thymine, cytosine, guanine).

• Bases pair specifically: adenine with thymine, cytosine with guanine.

Semiconservative Model of DNA Replication

• Each new DNA molecule consists of one parental (old) strand and one newly synthesized strand.

Replication Bubble and Enzymes Involved

• Replication begins at origins of replication, forming a replication bubble with two replication forks.

• Key enzymes and proteins:

  • Helicase: Unwinds the DNA double helix.

  • Single-strand binding proteins: Stabilize unwound DNA.

  • Topoisomerase: Relieves tension ahead of the replication fork.

  • Primase: Synthesizes RNA primers.

  • DNA polymerase: Adds nucleotides to the growing DNA strand.

  • Ligase: Joins Okazaki fragments on the lagging strand.

Synthesis of Leading and Lagging Strands

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

• Lagging strand: Synthesized discontinuously as Okazaki fragments, which are later joined by DNA ligase.

Role of Telomeres

• Telomeres are repetitive DNA sequences at the ends of eukaryotic chromosomes that protect genetic information during replication.

• They prevent the loss of important DNA and protect chromosome ends from deterioration or fusion with other chromosomes.

Comparison Table: Mitosis vs. Binary Fission

Key Terms

• Apoptosis: Programmed cell death.

• Centromere: Region where sister chromatids are joined.

• Chromatin: DNA-protein complex in non-dividing cells.

• Chromosome: DNA molecule with associated proteins.

• Interphase: Period of the cell cycle between divisions.

• Mitotic spindle: Structure made of microtubules that separates chromosomes during mitosis.

• Okazaki fragments: Short DNA fragments synthesized on the lagging strand.

• Replication fork: Y-shaped region where DNA is being unwound and replicated.

• Telomere: Protective DNA sequence at chromosome ends.

Equations and Models

• Semiconservative DNA Replication:

• Cell Cycle Timing:

Additional info: Some context and definitions have been expanded for clarity and completeness.