Which dna bases are purines

Watch this animation of DNA packaging. The DNA molecule is a polymer of nucleotides. Each nucleotide is composed of a nitrogenous base, a five-carbon sugar deoxyribose , and a phosphate group. There are four nitrogenous bases in DNA, two purines adenine and guanine and two pyrimidines cytosine and thymine.

A DNA molecule is composed of two strands. Each strand is composed of nucleotides bonded together covalently between the phosphate group of one and the deoxyribose sugar of the next.

From this backbone extend the bases. The bases of one strand bond to the bases of the second strand with hydrogen bonds. Adenine always bonds with thymine, and cytosine always bonds with guanine. The bonding causes the two strands to spiral around each other in a shape called a double helix. Ribonucleic acid RNA is a second nucleic acid found in cells. RNA is a single-stranded polymer of nucleotides.

It also differs from DNA in that it contains the sugar ribose, rather than deoxyribose, and the nucleotide uracil rather than thymine. Prokaryotes contain a single, double-stranded circular chromosome.

Eukaryotes contain double-stranded linear DNA molecules packaged into chromosomes. The DNA helix is wrapped around proteins to form nucleosomes. The protein coils are further coiled, and during mitosis and meiosis, the chromosomes become even more greatly coiled to facilitate their movement. Chromosomes have two distinct regions which can be distinguished by staining, reflecting different degrees of packaging and determined by whether the DNA in a region is being expressed euchromatin or not heterochromatin.

Skip to content Chapter 9: Introduction to Molecular Biology. Previous: Chapter 9: Introduction to Molecular Biology. Next: 9. Share This Book Share on Twitter. The RNA copy of the instructions is then used in the cell to produce amino acids and the required proteins. When a living cell is ready to divide into two new cells, the two sides of the DNA molecule separate by neutralizing the hydrogen bonds linking the purines and the pyrimidines.

Instead of using RNA on a section of the DNA ladder, the whole ladder separates and new nitrogenous bases are added to each side. Because each base will only accept one partner, each side becomes a full and exact duplicate of the other. For example, if a DNA bond was an adenine-thymine link, one side has the adenine molecule and the other side has the thymine molecule.

The adenine attracts another thymine molecule and the thymine attracts an adenine molecule. The result is two identical adenine-thymine bonds in two new strands of DNA. The two purine nitrogenous bases of DNA are essential for all cell protein production and for cell division.

The cell division made possible by the DNA copying mechanism forms the basis for all growth and for all forms of reproduction of living organisms. Bert Markgraf is a freelance writer with a strong science and engineering background.

Online he has written extensively on science-related topics in math, physics, chemistry and biology and has been published on sites such as Digital Landing and Reference. Jun 28, In DNA, they pair with their complementary pyrimidine bases, thymine and cytosine, respectively. In RNA, they pair with their complementary pyrimidine bases, uracil and cytosine, respectively. Adenine A and guanine G look like this: Thymine T , uracil U , and cytosine C look like this: When we have a base pair, a purine base and its complementary pyrimidine base hydrogen-bond.

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