Chapter 3_ M Phase – Introductory Biology I

Learning Objectives

  • Understand chromosome replication requirements before mitosis (interphase).

  • Differentiate between somatic cells and germ cells.

  • Describe outcomes of cell division for both cell types.

  • Analyze micrographs of cells undergoing mitosis to explain chromosome behavior.

Introduction

  • The cell cycle: orderly sequence from a single parent cell to two identical daughter cells.

  • M phase: includes mitosis (alignment, separation of chromosomes) and cytokinesis.

  • Understanding chromosome structure and organization is critical for studying mitosis.

Prokaryotic vs Eukaryotic Genomes

  • Prokaryotic genome:

    • Single double-stranded DNA molecule in a loop (nucleoid, not membrane-bound).

  • Eukaryotic genome:

    • Multiple linear double-stranded DNA molecules stored in the nucleus.

  • Chromosomes: condensed, coiled structures of DNA, essential for efficient cell division.

  • Ploidy: number of copies of each type of chromosome; humans are diploid (2 copies of each of 23 chromosomes).

Homologous Chromosomes

  • Chromosomes that code for the same gene sets; humans have 23 pairs.

  • Total chromosome count in humans: 46.

Chromosome Structure and Packing

  • DNA must be tightly compacted to fit within the cellular nucleus (2 meters of DNA, 2 nm diameter).

  • First level of DNA compaction involves wrapping around histone proteins to form chromatin.

  • Nucleosome: histone-DNA complex; linker DNA connects nucleosomes.

  • Sister chromatids are formed after DNA replication in the S phase of interphase.

  • Sister chromatids are connected at the centromere.

Mitotic Phase

  • Includes mitosis (nuclear division) and cytokinesis (cytoplasm division).

  • Mitosis phases:

    • Prophase: nuclear envelope dissolves; organelles fragment.

    • Prometaphase: nuclear envelope further fragments; spindle microtubules develop, binding to kinetochores.

    • Metaphase: chromosomes aligned at the metaphase plate; cohesin proteins hold sister chromatids together.

    • Anaphase: cohesin proteins degrade; sister chromatids separate and are pulled to opposite poles.

    • Telophase: chromosomes reach poles, decondense; new nuclear envelopes form around each set of chromosomes.

Cytokinesis

  • Physical separation of cell components into daughter cells.

  • In animal cells:

    • Contractile ring of actin forms at the equator, creating a cleavage furrow that deepens until division is complete.

  • In plant cells:

    • Formation of a cell plate from vesicles (Golgi apparatus) that fuse at the metaphase plate, building a new cell wall.

Evolutionary Connections

  • Mitotic spindle critical for eukaryotic division; no mitosis in prokaryotes.

  • FtsZ protein in prokaryotes resembles tubulin, crucial for cell division.

  • Recent findings of Asgard archaea providing insights into the evolutionary transition from prokaryotes to eukaryotes.

Key Questions and Review

  • Review genome organization in prokaryotes vs eukaryotes.

  • Discuss principal role of histone proteins in chromatin packing.

  • Identify stages of mitosis where significant processes occur (e.g., sister chromatids separate during anaphase).

  • Understand differences in cytokinesis in animal vs plant cells.