Cell Division, Mitosis, and Meiosis


Cell Division Functions in Reproduction, Growth, and Repair

Cell division involves the distribution of identical genetic material, DNA, to two daughters cells. What is most remarkable is the fidelity with which the DNA is passed along, without dilution or error, from one generation to the next.


Core Concepts:


In order to better understand the concept of cell division and genetics, some basic definitions are in order:


Chromosome structure

chromosome structure
  • composed of DNA and protein (histones) all tightly wrapped up in one package
  • duplicated chromosomes are connected by a centromere

 


2n=4 Example - an organism is 2n = 4. 
  • Chromosomes 1 & 2 are homologous chromosomes
  • Chromosomes 3 & 4 are homologous chromosomes
  • Chromosomes 1 & 3 came from the mother
  • Chromosomes 2 & 4 came from the father

 


Typical Animal Life Cycle

animal life cycle

The Cell Cycle

cell cycle G1 - first gap

S - DNA synthesis (replication)

G2 - second gap

M - mitosis


Mitosis in a Nutshell

Interphase 
  • is the "resting" or non-mitotic portion of the cell cycle. 
  • It is comprised of G1, S, and G2 stages of the cell cycle. 
  • DNA is replicated during the S phase of Interphase
Prophase Prophase Prophase - the first stage of mitosis.
  • The chromosomes condense and become visible
  • The centrioles form and move toward opposite ends of the cell ("the poles")
  • The nuclear membrane dissolves
  • The mitotic spindle forms (from the centrioles in animal cells)
  • Spindle fibers from each centriole attach to each sister chromatid at the kinetochore
Compare Prophase to the Prophase I and to the Prophase II stages of mitosis.

Metaphase
  • The Centrioles complete their migration to the poles
  • The chromosomes line up in the middle of the cell ("the equator")
Compare Metaphase to the Metaphase I and to the Metaphase II stages of mitosis. 
Metaphase Metaphase
Anaphase Anaphase Anaphase
  • Spindles attached to kinetochores begin to shorten.
  • This exerts a force on the sister chromatids that pulls them apart.
  • Spindle fibers continue to shorten, pulling chromatids to opposite poles.
  • This ensures that each daughter cell gets identical sets of chromosomes

Compare Anaphase to the Anaphase I and to the Anaphase II stages of mitosis.

Telophase
  • The chromosomes decondense
  • The nuclear envelope forms
  • Cytokinesis reaches completion, creating two daughter cells
Compare Telophase to the Telophase I and to the Telophase II stages of mitosis.
Telophase Telophase

 


Cytokinesis Divides the Cytoplasm

In animal cells, cytokinesis occurs by a process known as cleavage

Chromosome Separation Is the Key Event of Mitosis


Regulation of the Cell Cycle

The cell cycle is controlled by a cyclically operating set of reaction sequences that both trigger and coordinate key events in the cell cycle Cyclins and Cyclin-Dependent Kinases - The Cell-Cycle Clock

Rhythmic fluctuations in the abundance and activity of cell-cycle control molecules pace the events of the cell cycle.

MPF - Maturation Promoting Factor (M-phase promoting factor) PDGF - Platelet-Derived Growth Factors - An Example of an External Signal for Cell Division

PDGF is required for the division of fibroblasts which are essential in wound healing

Density Dependent Inhibition Anchorage Dependence Cells Which No Longer Respond to Cell-Cycle Controls - Cancer Cells

Meiosis

More definitions:


Meiosis in a Nutshell


The stages of meiosis can be broken down into two main stages, Meiosis I and Meiosis II


Meiosis I

Prophase 1 Prophase I - most of the significant processes of Meiosis occur during Prophase I
  • The chromosomes condense and become visible
  • The centrioles form and move toward the poles
  • The nuclear membrane begins to dissolve
  • The homologs pair up, forming a tetrad
    • Each tetrad is comprised of four chromotids - the two homologs, each with their sister chromatid
  • Homologous chromosomes will swap genetic material in a process known as crossing over (abbreviated as XO)
    • Crossing over serves to increase genetic diversity by creating four unique chromatids
Compare Prophase I to Prophase II and to the Prophase stage of mitosis.

 


Crossing Over

  • Genetic material from the homologous chromosomes is randomly swapped
  • This creates four unique chromatids
  • Since each chromatid is unique, the overall genetic diversity of the gametes is greatly increased

 


Metaphase I
  • Microtubules grow from the centrioles and attach to the centromeres
  • The tetrads line up along the cell equator
Compare Metaphase I to Metaphase II and to the Metaphase stage of mitosis.
Metaphase 1
Anaphase 1 Anaphase I
  • The centromeres break and homologous chromosomes separate (note that the sister chromatids are still attached)
  • Cytokinesis begins
Compare Anaphase I to Anaphase II and to the Anaphase stage of mitosis.

Telophase I

  • The chromosomes may decondense (depends on species)
  • Cytokinesis reaches completion, creating two haploid daughter cells
Compare Telophase I to Telophase II and to the Telophase stage of mitosis.
Telophase 1

 


Meiosis II

Anaphase 2 Prophase II
  • Centrioles form and move toward the poles
  • The nuclear membrane dissolves
Compare Prophase II to Prophase I and to the Prophase stage of mitosis.

Metaphase II

  • Microtubules grow from the centrioles and attach to the centromeres
  • The sister chromatids line up along the cell equator
Compare Metaphase II to Metaphase I and to the Metaphase stage of mitosis.
Metaphase 2
Anaphase 2 Anaphase II
  • The centromeres break and sister chromatids separate 
  • Cytokinesis begins
Compare Anaphase II to Anaphase I and to the Anaphase stage of mitosis.

Telophase II

  • The chromosomes may decondense (depends on species)
  • Cytokinesis reaches completion, creating four haploid daughter cells
Compare Telophase II to Telophase I and to the Telophase stage of mitosis.
Telophase 2

 


A Comparison between Mitosis and Meiosis

Mitosis vs Meiosis

 


Some questions to ponder

cockroach

The Consequences of Meiotic Mistakes

Nondisjunctions occur when homologous chromosomes fail to separate at meiosis I or when chromatids fail to separate at meiosis II.  Polyploidy can occur when whole sets of chromosomes fail to separate at meiosis I or II.