Chapter 2 - Lecture 2: Mitosis and Cytokinesis: The M Phase of the Cell Cycle

Mitosis and Cytokinesis

Overview of M Phase

The M phase is a critical stage of the cell cycle during which a cell divides its duplicated genetic material. It consists of two main processes: mitosis (nuclear division) and cytokinesis (cytoplasmic division). Together, these processes ensure that each new cell receives a complete diploid (2n) set of chromosomes.

• Karyokinesis: Division of the nucleus, partitioning chromosomes into two identical daughter nuclei.

• Cytokinesis: Division of the cytoplasm, producing two genetically identical daughter cells.

Stages of M Phase

M phase is divided into a sequence of stages:

• Prophase (1st)

• Prometaphase (2nd)

• Metaphase (3rd)

• Anaphase (4th)

• Telophase (5th)

Main Processes During M Phase

• DNA replication and transcription

• Chromosome alignment and segregation

• Regulation by checkpoints

What is Mitosis?

Definition and Purpose

Mitosis produces two genetically identical daughter cells, each with a complete set of chromosomes. It is essential for growth, tissue repair, and asexual reproduction in eukaryotic organisms.

• Humans: Somatic cells contain 46 chromosomes (23 pairs).

• Result: Each daughter cell receives an identical set of chromosomes.

Why is Mitosis Necessary?

• Maintains the diploid (2n) number in somatic cells

• Essential for growth, tissue repair, and asexual reproduction

Mitotic Spindle

Structure and Function

The mitotic spindle is a structure that separates duplicated chromosomes during mitosis. It is made of microtubules (green fibers) and is produced at the microtubule-organizing center (MTOC).

• Kinetic core microtubules (pulling): Connect to chromosomes at the kinetochore and pull sister chromatids apart.

• Polar microtubules (pushing): Connect across the equator of the cell, pushing the poles apart.

Mitotic Terminology

• Centrosome: Organelle near the nucleus, contains centrioles (in animal cells), organizes spindle fibers.

• Centromere: Region of a chromosome where spindle fibers attach via the kinetochore.

• Kinetochore: Protein complex at the centromere, attaches chromosomes to spindle fibers.

• Centriole: Cylindrical organelle in the centrosome, involved in spindle fiber development.

Stages of Mitosis

Prophase (1st Stage)

Chromosomes condense and become visible. The centrosomes move to opposite poles, and spindle microtubules begin to form. The nuclear envelope starts to break down.

Prometaphase (2nd Stage)

• Nuclear envelope completely breaks down

• Spindle fibers attach to chromosomes at their kinetochores

• Chromosomes begin moving toward the cell's equator

Metaphase (3rd Stage)

• Chromosomes align along the metaphase plate (cell's equator)

• Each chromosome is attached to spindle fibers from both poles

• Spindle checkpoint ensures proper attachment before separation

Anaphase (4th Stage)

• Sister chromatids separate at the centromere, becoming daughter chromosomes

• Kinetochore microtubules shorten, pulling chromosomes to opposite poles

• Polar microtubules elongate, pushing the poles farther apart

Telophase (5th Stage) and Cytokinesis

• Chromosomes reach opposite poles and begin to decondense into chromatin

• Spindle fibers disassemble

• Nuclear envelopes reform around each chromosome set

• Cytokinesis: Contractile ring forms and tightens at the cell's midpoint, dividing the cytoplasm and producing two genetically identical diploid (2n) daughter cells

Key Terms and Concepts

• Spindle Checkpoint: Ensures chromosomes are properly attached before separation

• DNA Replication: Occurs prior to mitosis, ensuring each chromosome consists of two sister chromatids

• Chromosome Segregation: Accurate distribution of chromosomes to daughter cells

Example: Human Somatic Cell Division

In humans, a somatic cell with 46 chromosomes undergoes mitosis to produce two daughter cells, each with 46 chromosomes. This process is essential for tissue growth and repair.

Summary Table: Stages of Mitosis

Stage	Main Events
Prophase	Chromosomes condense, spindle forms, nuclear envelope breaks down
Prometaphase	Spindle fibers attach to kinetochores, chromosomes move toward equator
Metaphase	Chromosomes align at metaphase plate, spindle checkpoint
Anaphase	Sister chromatids separate, move to opposite poles
Telophase	Chromosomes decondense, nuclear envelopes reform
Cytokinesis	Cytoplasm divides, two identical daughter cells produced

Relevant Equations

• Chromosome Number After Mitosis: $2n \rightarrow 2n$ (Diploid to diploid)

Additional info:

• Checkpoint regulation is crucial for preventing errors in chromosome segregation, which can lead to genetic disorders.

• Spindle fiber dynamics are essential for accurate chromosome movement and cell division.