Illustration of a helicase enzyme separating two strands of deoxyribonucleic acid (DNA). The process of unraveling a double helix of nucleic acid strands is important to many cellular functions, such as DNA replication, RNA synthesis, RNA transcription, DNA recombination, and DNA repair.

Illustration showing a reverse transcriptase enzyme transcribing a single strand of RNA into double-stranded DNA. Reverse transcriptase is a polymerase enzyme commonly found in retroviruses that catalyses reverse transcription, the process of synthesizing DNA from an RNA template.

Illustration of cell duplication (top to bottom): organelles inside the cell, the cell nucleus with chromosomes undergoing mitosis, and a DNA molecule representing the genetic material of the cell.

Illustration showing a reverse transcriptase enzyme transcribing a single strand of RNA into double-stranded DNA. Reverse transcriptase is a polymerase enzyme commonly found in retroviruses that catalyses reverse transcription, the process of synthesizing DNA from an RNA template.

Illustration showing a reverse transcriptase enzyme transcribing a single strand of RNA into double-stranded DNA. Reverse transcriptase is a polymerase enzyme commonly found in retroviruses that catalyses reverse transcription, the process of synthesizing DNA from an RNA template.

Illustration of DNA testing results, showing how a single strand of DNA (top) can be analyzed and used as evidence in a criminal case. DNA is extracted from a sample and treated, and the pattern is transferred to a nylon sheet (bottom left). Analysis can compare DNA from a crime scene to a sample from a suspect, as shown in the patterns at right.

Cartoon of a cow in therapy, "You're having more calves now but you're enjoying it less?"

Cartoon of cloned horses, "Who's number 1?"

Computer graphics showing sperms swimming toward the egg.

Molecular structure of DNA double helix.

A-DNA (end) composed of red bases and yellow backbone.

Space-fill model of A-DNA. The bases appear in red and the backbone in yellow.

Space-fill model of B-DNA (left), A-DNA (middle), Z-DNA (right). The bases appear in red and the backbone in yellow.

Molecular model of a monoclonal antibody bound to an antigen (red). Monoclonal antibodies are produced from clones of hybridoma cells, making only antibody molecules with a singular specificity.

DNA sequence analysis: the samples are loaded onto a gel plate and then into a sequencer, which automatically analyzes the DNA molecules. The results are shown here on an electropherogram chart, also known as a chromatogram.

Gene splicing.

DNA replication initiation by an enzyme in a prokaryote (E. coli), based on a Kornberg model. The dnaA protein enzyme binds to DNA at the replication origin in a core complex (upper left), beginning the "melting," or opening, of the DNA double strand. Not shown, the helicase enzyme, dnaB, loaded in a complex with dnaC, will then continue unzipping the DNA.

Medical illustration of genetic or birth defects.

Illustration showing DNA double helix. DNA is made of four nitrogenous bases: thymine, cytosine, adenine, and guanine. They form two polynucleotide chains held together by hydrogen bonds between their adjacent bases, thereby creating a double helix.

Illustration showing DNA double helix with genetic damage.