Lecture 8 Cell Cycle and Mitosis

Introduction to Cell Cycle and Mitosis

  • Course: Biol 103: Introductory Biology I Lecture 8
  • Lecturer: Dr. Michael D. Preston, Assistant Professor, Ecosystem Science and Management
  • Contact: michael.preston@unbc.ca
  • Office Hours: 12:20-1:00 pm Mon/Fri 7-238 or by appointment

Learning Objectives

  • Describe the bacterial and eukaryotic cell cycle.
  • Outline the stages of mitosis and cytokinesis.
  • Describe the mechanisms that control mitosis.
  • Summarize cell cycle regulation.
  • Recommended Reading: Chapter 7 – Cell Cycles.

Prokaryotic Cell Division

  • Prokaryotic cells, like Escherichia coli, undergo a process called binary fission, which involves:
    • Coordinated cytoplasmic growth.
    • DNA replication.
    • Cell division, resulting in two daughter cells from a single parent cell.

The Bacterial Cell Cycle

  • Initiated by the origin of replication (ori).
  • Sequential steps include:
    1. A bacterial cell before DNA replication.
    2. Replication begins at ori, progressing in both directions.
    3. The two origins migrate to the poles while replication continues.
    4. Replication is completed.
    5. Cell division starts with the inward growth of the plasma membrane and synthesis of a new cell wall, resulting in two daughter cells.

Evolution of Mitosis

  • It is theorized that binary fission is the ancestral division process from which mitosis evolved.
  • Variations in the mitotic apparatus across modern organisms suggest evolutionary intermediates.
  • Eukaryotes, having multiple chromosomes, derive advantages through this complexity, especially in managing DNA segregation during cell division.

Eukaryotic Chromosomes

  • Eukaryotic cells possess chromosomes that are units of genetic information, allowing for precise distribution during mitosis.
  • Key characteristics:
    • Chromosomes are nuclear units of genetic materials composed of chromatin.
    • The term "chromosome" comes from "chroma" (color) and "soma" (body).

Chromosomes and Ploidy

  • The number of chromosomes is species-specific:
    • Humans: 46 chromosomes (2 sets of 23 homologous chromosomes).
    • Homologous chromosomes share the same genes but can have different alleles.
  • Ploidy refers to the number of chromosome sets in a cell:
    • Diploid (2n), examples include Homo sapiens.
    • Haploid (n), examples include E. coli.

Composition of Chromosomes

  • Chromosomes are composed of:
    • Histones: proteins around which DNA winds, forming nucleosomes.
    • Nucleosomes are coiled into a chromatin structure that leads to the formation of the 30 nm chromatin fiber.
    • The mature chromosome form is seen in metaphase.

Sister Chromatids

  • Result of DNA replication during Interphase, producing two identical strands called sister chromatids.
  • They are connected by a centromere and bound by cohesin proteins.
  • Kinetochores attach on either side of the centromere, serving as attachment points for spindle fibers during mitosis.

Chromosome Segregation During Mitosis

  • During mitosis, each sister chromatid is segregated into the two daughter cells, ensuring equal distribution of genetic material.

Phases of the Cell Cycle

  • 1. G1 phase: Cell performs its function and may grow.
  • 2. S phase: DNA replication and chromosome duplication occur.
  • 3. G2 phase: Brief gap for continued growth and preparation for mitosis and cytokinesis.

The S Phase

  • Different stages include:
    • G₁: Chromosomes are unreplicated, consisting of one double helix.
    • After S phase: Each replicated chromosome comprises two sister chromatids held together.
    • During M phase, sister chromatids separate into daughter chromosomes.

Mitotic Cell Cycle Stages

  • After interphase, mitosis progresses through five stages:
    • Prophase (Before)
    • Prometaphase (Before-Middle)
    • Metaphase (Middle)
    • Anaphase (Back)
    • Telophase (End)

Cytokinesis

  • Cytokinesis involves dividing the cytoplasm following mitosis:
    • Furrowing: Contractile rings of microfilaments lead to indentations around the cell, tightening until the cells separate.
    • Cell Plate Formation: In plant cells, vesicles containing cell wall material collect at the spindle midpoint, fusing to form a new wall between daughter cells.

Mitotic Regulation

  • The cell cycle is tightly regulated by:
    • Cyclins and Cyclin-dependent Kinases (CDKs).
    • Activation occurs through cyclin binding to CDKs, forming cyclin-CDK complexes.
    • Fluctuations in cyclin concentrations allow checkpoint regulation at three critical points during the cycle.

Checkpoints and Regulators

  • G₁ checkpoint: Monitors DNA damage, with p53 as a crucial checkpoint protein that can trigger DNA repair or apoptosis.
  • G₂ checkpoint: Ensures DNA replication is complete and that the cell is ready to divide.
  • M checkpoint: Ensures proper spindle attachment before anaphase initiation.

Cancer and Cell Division

  • Cancer results from the loss of control in cell division, leading to uncontrolled proliferation and tumor formation.
  • Cancer cells can metastasize, forming new tumors elsewhere.
  • Oncogenes are mutated versions of genes that typically regulate the cell cycle such as those involved in the cyclin-CDK system.

Apoptosis

  • Apoptosis is a regulated process of programmed cell death that is integral to organism development and homeostasis. It is triggered by internal or external signals, contributing to tissue balance in multicellular eukaryotes.