This third H-bond in the G:C base pair is between the additional exocyclic amino group on G and the C2 keto group on C. The pyrimidine C2 keto group is not involved in hydrogen bonding in the A:T base pair. These are the complementary base pairs. This will be encountered during recombination in Chapter 8.
Rather the two strands are coiled around the same helical axis and are intertwined with themselves which is referred to as a plectonemic coil. One consequence of this intertwining is that the two strands cannot be separated without the DNA rotating, one turn of the DNA for every "untwisting" of the two strands.
The N7 and C6 groups of purines and the C4 and C5 groups of pyrimidines face into the major groove, thus they can make specific contacts with amino acids in DNA-binding proteins. Base pairs stack, with some rotation between them. Three different forms of duplex nucleic acid have been described. The most common form, present in most DNA at neutral pH and physiological salt concentrations, is B-form. That is the classic, right-handed double helical structure we have been discussing.
This is called A-form nucleic acid. Nucleotides are arranged in two long strands that form a spiral called a double helix. An important property of DNA is that it can replicate, or make copies of itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases.
This is critical when cells divide because each new cell needs to have an exact copy of the DNA present in the old cell. Other chapters in Help Me Understand Genetics. Genetics Home Reference has merged with MedlinePlus. Learn more. Franklin documented this structure in what became known as Photo Armed with the information that DNA was a double helix and previous reports that the bases adenine and thymine occurred in equal amounts within DNA, as did guanine and cytosine, Watson and Crick published a landmark paper in the journal Nature.
In that paper, they proposed a model of DNA as we now know it: a double helical ladder with sugar-phosphate sides and rungs made up of A-T and G-C base pairs. They also suggested that, based on their proposed structure, DNA could be copied — and, therefore, passed on. Watson, Crick and Wilkins were awarded the Nobel Prize in medicine in "for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material.
Genes encode proteins that perform all sorts of functions for humans and other living beings. The human gene HBA1, for example, contains instructions for building the protein alpha globin, which is a component of hemoglobin, the oxygen-carrying protein in red blood cells, according to the NLM.
To take another example, the gene OR6A2 encodes an olfactory receptor, a protein that detects odors in the nose, scientists reported in in the journal Gene. Depending on which version of OR6A2 you have, you may love cilantro or think it tastes like soap, according to a study published in in the journal Flavour. DNA sequencing involves technology that allows researchers to determine the order of bases in a DNA sequence.
The technology can be used to determine the order of bases in genes, chromosomes or an entire genome. In , researchers completed a "working draft" sequence of the human genome, according to the National Human Genome Research Institute , and finished the project in A person's DNA contains information about their heritage, and it can sometimes reveal whether they are at an elevated risk for certain diseases.
DNA tests, or genetic tests, are used for a variety of reasons, including to diagnose genetic disorders, to determine whether a person is a carrier of a genetic mutation that they could pass on to their children and to examine whether a person is at risk for a genetic disease.
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