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DNA carries the central instructions for protein synthesis in the cell. DNA carries genes that are encoded and transferred to RNA molecules. These RNA molecules leave the nucleus (in animal cells) and are translated into functional proteins. Although the DNA found in chromosomes (type B) is the most popular type of double-helix structure, there are several other forms of DNA.
DNA types are differentiated by their formation and helix structure. The components of the double helix are specific for all DNA. DNA consists of a sugar-phosphate backbone with an internal nitrogenous base. The nitrogenous base's hydrogen bond holds the double helix structure by combining two complementary strands of DNA. The external backbone is negatively charged, providing interaction with other molecules.
B-DNA is the form commonly observed in chromosomes. B-DNA is a right-handed helix with 10 base pairs per turn. B-DNA is replicated and used in the transcription and translation of RNA, which is the molecule used for protein synthesis. B-DNA can be denatured, which means the hydrogen bonds are removed. This is essentially the first step in replicating DNA in the cell.
A-DNA is also a right-handed helix. However, there are more base pairs per turn. A-DNA has 11 base pairs per turn. Other than the more compact structure, A-DNA is similar to B-DNA. It is biologically active in the cell, and it forms crystallized structures in lab experiments.
Z-DNA is the type of DNA that is a left-handed helix. It is also known to be biologically active in zigzag formations of repeating base pair sequences. Z-DNA has 12 base pairs per turn, so it carries the most genes between each turn. Z-DNA plays a role in RNA transcription, which is the protein synthesis process of creating mRNA from a strand of DNA. mRNA (message RNA) is the molecule that carries transcribed genes to ribosomes where proteins are synthesized.
cDNA (complementary or clonal DNA) is a type of DNA used to describe libraries of genetic information. cDNA is used in testing for pharmaceuticals and research of diseases. cDNA is a complementary strand that is transcribed in laboratories to create genes. Genetic engineering also uses these DNA libraries to create modified versions of genomic information.