DNA AND GENES

DNA STRUCTURE DNA is found in the nucleus of cells. The structure of DNA is a double-stranded helix.

Deoxyribonucleic acid (DNA) is a polynucleotide (a molecule composed of a chain of nucleotides). Each nucleotide consists of: a nitrogen base (adenine, thymine, guanine, or cytosine) a sugar (deoxyribose) a phosphate group

In DNA, adenine always bonds with thymine, and cytosine always bonds to guanine.

James Watson and Francis Crick discovered the structure of DNA in They won the Nobel prize with Maurice Wilkins in 1962.

Rosalind Franklin’s work in x-ray crystallography was important in revealing the structure of the DNA molecule.

DNA REPLICATION The structure of DNA allows it to be easily replicated (copied). The DNA molecule “unzips” and each side serves as a template.

On each half of the molecule, a new complementary half is built. The two new DNA molecules are identical to each other. DNA replication allows cells to pass on hereditary information.

Cells divide in order to reproduce. Prior to cells dividing, the DNA is in the form of chromatin. The chromatin condenses into chromosomes so that a cell can easily divide its genetic information. chromatin chromosomes

Each human cell has 46 chromosomes.

Trisomy 21 occurs when a fertilized egg has three copies of chromosome 21.

GENES A segment of DNA is called a gene. Each chromosome contains many genes.

Genes determine the characteristics of an organism. For example: the gene that codes for the protein hemoglobin.

The higher the number of genes in an organism, the greater the chance of genetic diversity. For example, a bacterium has 470 genes; a fruit fly has 13,000 genes, a human has 42,000 genes.

MUTATIONS An alteration in the DNA sequence is known as a mutation. Mutations may be caused by: chemical agents (e.g. cigarette smoke); radiation (e.g. X-rays or UV from the Sun); or natural causes (e.g. viruses). Mutations can also occur during the process of DNA replication. Most mutations are deadly or harmful to cells, but some can be beneficial.

Example of a harmful mutation: sickle cell anemia