Determining Chromosome Number During Mitosis
In order to better understand the concept of genetics, some basic definitions are in order:
- gene - basic unit of heredity; codes for a specific trait
- locus - the specific location of a gene on a chromosome (locus - plural
- chromosome - elongate cellular structure composed of DNA and protein
- they are the vehicles which carry DNA in cells
- diploid (2n) - cellular condition where each chromosome type is
represented by two
- haploid (n) - cellular condition where each chromosome type is
represented by only one
- homologous chromosome - chromosome of the same size and shape
which carry the same
type of genes
- chromatid - one of two duplicated chromosomes connected at the
- centromere - region of chromosome where microtubules attach during
mitosis and meiosis
- composed of DNA and protein (histones) all tightly wrapped up in one package
- duplicated chromosomes are connected by a centromere
||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
G1 - initial growth cycle
S - DNA synthesis (replication)
G2 - second growth stage
M - mitosis
- mitosis - nuclear/chemical events resulting in two daughter nuclei which
have identical genetic material to each other and to the mother cell
- cytokinesis - division of the cytoplasm. This usually occurs with
but in some
organisms this is not so
Mitosis in a Nutshell
- The stages of the cell cycle can be broken down into six stages:
- Interphase, Prophase, Metaphase, Anaphase, Telophase
- 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 - 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")
- 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
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 http://www.mcgill.ca:80/nrs/mitosis.htm.
All other drawings by Michael Muller