What is the structure of DNA in terms of nucleotides?

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). Mitochondria are structures within cells that convert the energy from food into a form that cells can use.

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 sequence, 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 helix 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.

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1. A molecule of DNA consists of two strands that form a double helix structure.

DNA is a macromolecule consisting of two strands that twist around a common axis in a shape called a double helix. The double helix looks like a twisted ladder—the rungs of the ladder are composed of pairs of nitrogenous bases (base pairs), and the sides of the ladder are made up of alternating sugar molecules and phosphate groups.

Molecules of DNA range in length from hundreds of thousands to millions of base pairs. The smallest chromosome in the human genome, Chromosome 21, has around 48 million base pairs.

2. Each DNA strand is composed of nucleotides—units made up of a sugar (deoxyribose), a phosphate group, and a nitrogenous base.

Each strand of DNA is a polynucleotide composed of units called nucleotides. A nucleotide has three components: a sugar molecule, a phosphate group, and a nitrogenous base.

The sugar in DNA’s nucleotides is called deoxyribose—DNA is an abbreviation for deoxyribonucleic acid. RNA molecules use a different sugar, called ribose. Covalent bonds join the sugar of one nucleotide to the phosphate group of the next nucleotide, forming the DNA strand’s sugar-phosphate backbone.

A nitrogenous base is an organic molecule that contains nitrogen and has the chemical properties of a base. There are four nitrogenous bases that occur in DNA molecules: cytosine, guanine, adenine, and thymine (abbreviated as C, G, A, and T). RNA molecules contain cytosine, guanine, and adenine, but they have a different nitrogenous base, uracil (U) instead of thymine.

3. The sequences of nitrogenous bases on the two strands of a DNA molecule are complementary.

The sequence of nitrogenous bases on one strand of a DNA molecule’s double helix matches up in a particular way with the sequence on the other strand. Adenine pairs with thymine and cytosine pairs with guanine.

Why do the nitrogenous bases pair in this specific way? The bases on each strand are joined to the bases on the other strand with hydrogen bonds, but different bases have different chemical structures. Cytosine and thymine (and uracil in RNA) are pyrimidines, containing one ring. Adenine and guanine are purines, containing two rings. The pyrimidines pair with the purines: cytosine and guanine form three hydrogen bonds, and adenine and thymine form two.

4. Specific sequences of nitrogenous bases that code for particular proteins or regulatory RNA molecules are called genes.

Each strand of DNA is like a recipe book for synthesizing proteins. Certain sequences of nitrogenous bases along the strand encode particular RNA molecules. These sequences are called genes. mRNA molecules transcribed from genes are translated into proteins later.

Chromosomes can vary widely in their number of base pairs and genes. The longest chromosome in human cells, Chromosome 1, is around 249 million base pairs long and has between 2000 and 2100 distinct genes. Chromosome 21, the shortest human chromosome, consists of 48 million base pairs and contains between 200 and 300 genes. Overall, prokaryotic cells have shorter chromosomes with fewer genes. For example, the bacterium Carsonella rudii has only 159,662 base pairs and 182 genes in its entire genome.

Although genes get most of the credit for what DNA does, they make up only about 1% of DNA (in humans). Genes are separated from one another by sequences of nitrogenous bases that don’t provide instructions for RNA synthesis. These are called intergenic regions. Even within genes, there are regions of noncoding DNA called introns.

Noncoding regions of DNA are important because they provide binding sites for proteins that help activate or deactivate the process of transcription. They can also provide protection for the coding regions. For instance, telomeres consist of repetitive sequences that protect the genetic information on each DNA molecule from being damaged during cell division.

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