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 loci)

• 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 homologous chromosomes

• haploid (n) - cellular condition where each chromosome type is represented by only one chromosome

• 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

• 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

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 mitosis, 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

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

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

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

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

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