Determining Chromosome Number During Mitosis

In order to better understand the concept of 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

The Cell Cycle

cell cycle

G1 - initial growth cycle

S - DNA synthesis (replication)

G2 - second growth stage

M - mitosis

Mitosis in a Nutshell


  • 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

  • Microtubules grow from the centrioles and attach to the centromere
  • The chromosomes line up in the middle of the cell ("the equator")
Metaphase Metaphase

Anaphase Anaphase Anaphase
  • The centromeres break and the sister chromosomes separate
  • The chromosomes migrate toward opposite poles
  • Cytokinesis begins


  • The chromosomes decondense
  • The nuclear envelope forms
  • Cytokinesis reaches completion, creating two daughter cells
Telophase Telophase

To down load a simply great Quick Time video (.mov file), click here. Warning: the file is 4 Megs.

If you have any questions or comments, please feel free to e-mail me.

Image of the cell cycle taken from All other drawings by Michael Muller