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DNA is a double helix formed by base pairs attached to a sugar-phosphate backbone.


People have known for many years that living things inherit traits from their 1) ___. That common-sense observation led to agriculture, centuries ago, the purposeful breeding and cultivation of animals and plants for desirable characteristics. Firming up the details took quite some time, though. Researchers did not understand exactly how traits were passed to the next 2) ___ until the middle of the 20th century. Now it is clear that genes are what carry our traits through generations and that genes are made of deoxyribonucleic acid (DNA). But genes themselves don’t do the actual work. Rather, they serve as instruction books for making functional molecules such as RNA 3) ___ ___ and proteins, which perform the chemical reactions in our bodies. Occasionally, there is a kind of typographical error in a 4) ___ DNA sequence. This mistake – which can be a change, gap or duplication – is called a mutation. A mutation can cause a gene to encode a protein that works incorrectly or that doesn’t work at all. Sometimes, the error means that no protein is made. Not all DNA changes are harmful. Some mutations have no effect, and others produce new versions of proteins that may give a survival advantage to the organisms that have them. Over time, 5) ___ supply the raw material from which new life forms evolve.


Our modern understanding of DNA’s role in heredity has led to a variety of practical applications, including forensic analysis, paternity testing, and genetic screening. Thanks to these wide-ranging uses, today many people have at least a basic awareness of DNA. All living things are made of cells. The 6) ___ in the human body have 23 pairs of chromosomes, which are made of DNA, and which reveal a lot about each individual.

DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA). The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The order, or 7) ___, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences. DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule. Together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. The structure of the double 8) ___ is somewhat like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder.


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. It may be surprising, then, to realize that less than a century ago, even the best-educated members of the scientific community did not know that DNA was the hereditary material! The work of Gregor Mendel showed that traits (such as flower colors in pea plants) were not inherited directly, but rather, were specified by genes passed on from parents to 9) ___. The work of additional scientists around the turn of the 20th century, including Theodor Boveri, Walter Sutton, and Thomas Hunt Morgan, established that Mendel’s heritable factors were most likely carried on chromosomes. Scientists first thought that proteins, which are found in chromosomes along with DNA, would turn out to be the sought-after genetic material. Proteins were known to have diverse amino acid sequences, while DNA was thought to be simply a repetitive polymer, due in part to an incorrect (but popular) model of its structure and composition. Today, we know that DNA is not actually repetitive and can carry large amounts of information, and that DNA itself is the actual 10) ___ material


Punnett Square animation, explaining basic genetic inheritance


Test your DNA


The 23andMe PGS test uses qualitative genotyping to detect clinically relevant variants in the genomic DNA of adults from saliva collected using an FDA-cleared collection device

Sources: https://ghr.nlm.nih.gov/primer/basics/dna; KhanAcademy.com; Wikipedia


ANSWERS: 1) parents; 2) generation; 3) ribonucleic acid; 4) gene’s; 5) mutations; 6) cells; 7) sequence; 8) helix; 9) offspring; 10) genetic


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