• Explain how DNA accounts for the transmission of heredity characteristics.
  • Summarize the main scientific investigations that led to the modern understanding of DNA and genes.
  • Research genetic technologies.



DNA is made up of a series of nucleotides which are composed of a deoxyribose sugar, a phosphate, and a nitrogen base. There are four possible nitrogen bases: adenine (A), cytosine (C), guanine (G), and thymine (T). Adenine and guanine have a double-ring structure and are classified as purines while cytosine and thymine have a single-ring structure and are classified as pyrimidines.

Nucleotides are joined together to form long chains. The phosphate group of one nucleotide bonds to the sugar of an adjacent nucleotide.

Discovering the Double HelixEdit

Rosalind Franklin and Maurice Wilkins used X-ray diffraction to capture an image of DNA in which a coil structure can be identified. Using this information James Watson and Francis Crick proposed the double helix model of DNA. Two chains of nucleotides coil together with the sugar and phosphate making the sides and the nitrogen groups of complementary nucleotides in both strands bond to form the "rungs".

Nucleotide SequencesEdit

Unique genetic information is determined by the sequence of nucleotides. The closer the relationship between two organisms the more similar their genetic code will be.

The sequence of one strand in a DNA molecule is complementary to the sequence of the other strand.

Genes and ProteinsEdit

DNA stores information in a genetic code. The sequence of nucleotides in DNA determines the proteins synthesized in the cell. Proteins are chains of amino acids linked by peptide bonds, or polypeptides. Proteins act as structural components, hormones, and enzymes.

The Genetic CodeEdit

The genetic code contains instructions on how to build proteins. Sequences of three bases, or codons, in the genetic code correspond to one of 20 types of amino acid. A sequence of codons tells the cell which amino acids to string together and thus which proteins to build.

RNA Genetic Code
First base Second base Third Base
U phenylalanine serine tyrosine cysteine U
phenylalanine serine tyrosine cysteine C
leucine serine stop stop A
leucine serine stop tryptophan G
C leucine proline histidine arginine U
leucine proline histidine arginine C
leucine proline glutamine arginine A
leucine proline glutamine arginine G
A isoleucine threonine asparagine serine U
isoleucine threonine asparagine serine C
isoleucine threonine lysine arginine A
methionine threonine lysine arginine G
G valine alanine aspartate glycine U
valine alanine aspartate glycine C
valine alanine glutamate glycine A
valine alanine glutamate glycine G

DNA FingerprintingEdit

Courts sometimes use DNA evidence to convict or acquit criminals. To obtain this evidence scientists isolate DNA from blood, hair, skin, or other biological materials found at the crime scene. They also take a DNA sample from the suspect. The samples are cut into fragments with enzymes at specific locations and the fragments are compared. These fragments form distinct patterns which can be used to identify a person or other organism since no two individuals have the same DNA. This process is called DNA fingerprinting.

DNA fingerprinting can also determine relatedness among people by comparing their DNA, which can be used to determine paternity or family pedigrees. It is also used by conservation biologists who study endangered wildlife species. By determining species type, maternity, paternity, siblings, and gender, researchers can answer questions about how individuals move within a population, and the genetic variability within those populations. Such information is important in managing populations of endangered or threatened species.