DNA replication occurs in the synthesis or S phase of the Cell Cycle. Every chromosome is copied with high fidelity in a process that involves a large number of enzymes. In this process, the double-stranded DNA is unwound and each individual strand is used as a template for the production of the complementary strand. The end result is the production of two identical copies of the genetic material. This process is depicted in the animation below.

The replicated chromosomes contain two identical strands of DNA that remain attached until they become separated toward the end of mitosis (in anaphase). Since this is the form of chromosomes that is easiest to isolate and visualize, this is the structure with which most people are familiar. The process is depicted in schematic form below.

x-shaped chromosome

Remember that the X-shaped molecule is really composed of two copies of one chromosome.

Errors may occur during replication that lead to changes in the nucleotide sequence of the chromosomes. If these changes occur within genes, they can alter the function of the cell. Human cells have evolved several mechanisms to correct errors of this type but they are not perfect. Mistakes that occur during DNA replication can lead to the generation of cells with mutated genes. Accumulations of mutations can lead to the development of cancer. There are several cancer types that are associated specifically with the breakdown of the repair processes that normally function during DNA replication. The processes by which mutations are generated will be dealt with in the 'Causes of Mutation' section.

All dividing cells must go through the process of DNA replication. Since cancer cells are often rapidly dividing, this phase of the cell cycle is the target of many of the chemotherapy agents that will be described in the 'Cancer Treatments' section. Some examples include doxorubicin, cyclophosphamide, carboplatin, cisplatin, topotecan and etoposide (VP-16).

A Closer Look at Chromosomes and Genes

The bulk of the DNA in cells is located in the cell's nucleus in the form of chromosomes. Humans have 46 chromosomes in all, comprised of two sets of twenty-three. Each parent contributes 23 chromosomes to their offspring via the gametes they contribute; egg or sperm. Each parent contributes one of each type of chromosome, i.e. one chromosome #1, one #2, one #3, etc. That means that each person has twenty three pairs of chromosomes. Each chromosome is comprised of a single piece of DNA containing millions of nucleotides bound to several different proteins. The genes are spread out along the chromosomes along with large amounts of DNA that has no known function.

Any particular gene is always found at the same position on the same chromosome. For example, if a gene controlling eye color is located on chromosome 1 in one individual, the same gene would be at the same position in every other person examined. Since we have two copies of each chromosome, that means that we have two copies of each gene. This relationship is depicted below. The chromosome marked with the male (arrow) symbol represents the one contributed by the father and the chromosome marked with the female (cross) symbol represents the one contributed by the mother. A very important thing to know is that the version of the gene present on the two chromosomes does not have to be the same. Continuing the example from above, the father's gene for eye color might lead to the production of blue eyes whereas the mother's version of the gene might lead to the production of brown eyes. The color seen in the eyes of the child is a result of the activity of both copies of the gene.

In the diagram below, the bands of color represent genes. For some genes, the versions inherited from both parents are the same and for some they are slightly different. The different versions, or alleles are indicated by slightly different colored stripes. The pair of chromosomes below represent two versions of the SAME chromosome (i.e. 2 forms of chromosome 1, 2 or 3, etc.) that would be contributed by the parents.

male and female chromosome

The Cell Cycle: DNA Replication

A Closer Look at Chromosomes and Genes