DNA

Early Studies As early as 1900, scientists knew that chromosomes were located in the nucleus of a cell. They also knew that chromosomes carried hereditary information. But it wasn’t until 1952 that it became clear that the genetic material was DNA! Understanding the function of DNA requires some knowledge of its structure and organization.

DNA Double Helix The molecule of DNA looks like a twisted ladder. The sides of the ladder are made up of alternating molecules of deoxyribose, a sugar (D) and a phosphate (P).

DNA Double Helix (continued) The rungs of the ladder are made up of nitrogen bases: G=guanine C=cytosine T=thymine A=adenine Each base forms half a rung of the ladder, pairing with only its partner base: G pairs only with C and T pairs only with A. Organisms have different traits because they have different DNA base pair sequences.

DNA Double Helix (continued) If unwound and tied together, the strands of DNA would stretch more than 5 feet but would be only 50 trillionths of an inch wide. DNA molecules are among the largest molecules now known.

Genes The DNA sequence of base pairs specifies the exact genetic instructions required to create a particular organism with its own unique traits. A gene is a specific DNA sequence in the DNA molecule (ranging from fewer than 1 thousand bases to several million) that holds the information about a specific trait; making it the basic physical and functional unit of heredity.

Genes (continued) The human genome is estimated to comprise more than 100,000 genes. A gene is located in a particular position on a specific chromosome. The light and dark bands on the picture below are the genes on a chromosome.

Chromosomes You have to have a place to keep all the DNA that contains the genes. What better place than a chromosome! The 3 billion base pairs in the human genome are organized into 23 distinct pairs of microscopic units called chromosomes. Apart from reproductive cells and mature red blood cells, every cell that has a nucleus contains chromosomes made of DNA. The nucleus of most human cells contains two sets of 23 chromosomes – one set given by each parent. This includes an XX pair for a female or an XY pair for a male.

Chromosomes (continued) Chromosomes can be seen under a light microscope and, when stained with certain dyes, reveal a pattern of light and dark bands reflecting regional variations in the amounts of A and T vs. G and C. A karyotype is an analysis that distinguishes the chromosomes from each other based on their differences in size and banding pattern. Upon doing a karyotype, some types of chromosomal abnormalities may be detected, including missing or extra copies of a chromosome or breaks that rejoin upside down (translocations). Most changes in DNA, however, are too subtle to be detected by this technique and require detailed molecular analysis. These DNA abnormalities (mutations) are responsible for many inherited diseases such as cystic fibrosis and sickle cell anemia or may predispose an individual to cancer, major psychiatric illnesses, and other complex diseases.

Karyotype Microscopic examination of chromosome size and banding patterns allows medical laboratories to identify and arrange each of the 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes) into a karyotype, that then serves as a tool in the diagnosis of genetic diseases. The extra copy of chromosome 21 in this karyotype identifies this individual as having Down's syndrome.

DNA Replication The bonds between the nucleotide bases on each DNA strand are weak, allowing the bases to come apart when duplication of the strand is needed. During mitosis, chromosomes make exact copies of themselves. The DNA molecules must also make exact copies of themselves. The DNA molecule comes apart like a zipper being unzipped. The two halves of the DNA separate between the base pairs forming the rungs. Each half acts as a pattern for a new half to form. As a result of the DNA molecules making exact copies of themselves, two identical chromosomes are made.

DNA Makes Proteins DNA controls many activities in the cell. One of these activities is to make proteins. A gene’s sequence can carry information required for constructing proteins (needed for cell and tissues maintenance). Humans can make at least 100,000 different kinds. Human genes vary widely in length, often extending over thousands of bases, but only about 10% of the genome is known to include the protein-coding sequences of genes. Proteins are large, complex molecules made of long chains of subunits called amino acids. Twenty different kinds of amino acids are usually found in proteins.

DNA Makes Proteins (continued) Within a gene, each specific sequence of three DNA bases, called codons, directs the cell’s protein- making machinery to make specific amino acids. For example, the base sequence “ATG” codes for the amino acid methionine. Since 3 bases code for 1 amino acid, the protein coded by an average-sized gene (3000 bp) will contain 1000 amino acids. The genetic code is thus a series of codons that specify which amino acids are required to make up specific proteins. What does protein-making have to do with genetic traits? The arrangement of base pairs to make proteins also codes for genetic traits.