Why Do Purines Pair With Pyrimidines In The DNA Ladder?

Two complementary strands of the DNA can accommodate only three nitrogen rings between them. More or fewer nitrogen rings will not fit because of space issues. Thus, purine-purine cannot pair with each other due to the unavailability of the required space to bond together in the DNA double helix.

Is the pairing of purine to a pyrimidine?

Because purines always bind with pyrimidines – known as complementary pairing – the ratio of the two will always be constant within a DNA molecule. … There are two main types of purine: Adenine and Guanine. Both of these occur in both DNA and RNA.

Why does guanine pair with cytosine?

Guanine and cytosine make up a nitrogenous base pair because their available hydrogen bond donors and hydrogen bond acceptors pair with each other in space. Guanine and cytosine are said to be complementary to each other.

Why it is important that each complementary base pair have one purine and one pyrimidine nitrogenous base?

Notice that each base pair consists of a purine and a pyrimidine. The nucleotides in a base pair are complementary which means their shape allows them to bond together with hydrogen bonds. The A-T pair forms two hydrogen bonds. … The hydrogen bonding between complementary bases holds the two strands of DNA together.

Do pyrimidines form covalent bonds with purines?

Purines form covalent bonds with pyrimidines. Purines consist of a two-ring structure. Purines form hydrogen bonds with pyrimidines.

Is cytosine a purine?

Because of their structural similarity, we usually refer the nine-member double rings adenine and guanine as purines, and six-member single-ring thymine, uracil, and cytosine are pyrimidines. Fig. 1.5. (A) Chemical structure of pyrimidines and purines nitrogenous bases in DNA and RNA.

What do purines and pyrimidines have in common?

Purine vs Pyrimidine

Pyrimidine is a heterocyclic aromatic organic compound that is composed of carbon and hydrogen. It comprises adenine and guanine as nucleobases. Both purine and pyrimidine have same functions. They are vital for the production of DNA and RNA, starch and proteins.

What would happen if two pyrimidines bonded?

Two purines and two pyrimidines together would simply take up too much space to be able to fit in the space between the two strands. … The only pairs that can create hydrogen bonds in that space are adenine with thymine and cytosine with guanine. A and T form two hydrogen bonds while C and G form three.

How are purines and pyrimidines linked in a double helix?

The nitrogenous bases on the two strands of DNA pair up, purine with pyrimidine (A with T, G with C), and are held together by weak hydrogen bonds. Watson and Crick discovered that DNA had two sides, or strands, and that these strands were twisted together like a twisted ladder — the double helix.

What is the difference between the purines and the pyrimidines?

Adenine and guanine are the two purines and cytosine, thymine and uracil are the three pyrimidines. The main difference between purines and pyrimidines is that purines contain a sixmembered nitrogencontaining ring fused to an imidazole ring whereas pyrimidines contain only a sixmembered nitrogencontaining ring.

Why purine and pyrimidines are called nitrogenous bases?

The nitrogen bases are also called nucleobases because they play a major role as building blocks of the nucleic acids deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). There are two major classes of nitrogenous bases: purines and pyrimidines. … Like pyridine, each pyrimidine is a single heterocyclic organic ring.

Why is purine important in DNA synthesis?

Purine nucleotides are involved in many cellular functions as components of DNA and RNA, as sources of energy, as enzyme cofactors in metabolic pathways, and as components of signal transduction. … PRPP is also used in pyrimidine and pyridine nucleotide synthesis and in salvage of preformed purine bases.

How are purines and pyrimidines metabolized?


Following their degradation in the intestinal tract, the resulting mononucleotides may be absorbed or converted to purine and pyrimidine bases. The purine bases are then oxidized to uric acid, which may be absorbed and excreted in the urine.

What is between guanine and cytosine?

Guanine pairs with cytosine with 3 hydrogen bonds. This creates a difference in strength between the two sets of Watson and Crick bases. Guanine and cytosine bonded base pairs are stronger then thymine and adenine bonded base pairs in DNA.

What is the function of cytosine?

Cytosine is an important part of DNA and RNA, where it is one of the nitrogenous bases coding the genetic information these molecules carry. Cytosine can even be modified into different bases to carry epigenetic information. Cytosine has other roles in the cell, too, as the energy carrier and cofactor CTP.

Which bases pairs with cytosine?

In base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine.

How are purines and pyrimidines numbered?

The nucleotides are shown with standard numbering convention. The aromatic base atoms are numbered 1 through 9 for purines and 1 through 6 for pyrimidines. The ribose sugar is numbered 1′ through 5′.

What is the difference between a purine and a pyrimidine quizlet?

A nucleoside is a nucleobase with an added sugar (ribose). … in Purines, the purine ring is built on the sugar. In pyrimidines, the pyrimidine ring is assembled without the sugar.

Why does adenine always pair with thymine and guanine with cytosine in DNA?

Adenine and Guanine are purines, and Thymine and Cytosine are pyrimidines. So, for a uniform diameter, a purine must always bond with a pyrimidine. Adenine pairs with only Thymine because they form two hyrdrogen bonds while Guanine forms three hydrogen bonds and can only bond with Cytosine.

Why does adenine always pair with thymine and cytosine always pair with guanine in DNA?

The chemical structures of Thymine and Cytosine are smaller, while those of Adenine and Guanine are larger. Size and structure of the specific nucleotides cause Adenine and Thymine to always pair together while Cytosine and Guanine always pair together. Therefore the two strands of DNA are considered complimentary.

What would happen if adenine paired with guanine?

How would the shape of a DNA molecule change if adenine paired with guanine and cytosine paired with thymine? The DNA molecule would have irregular widths along its length. The DNA molecule would be longer.

What is the purpose of nitrogenous bases?

Not only is a nitrogenous base the building blocks for genetic information carrying molecules like DNA and RNA, but different forms of the nitrogenous base serve in various cellular roles from signal transduction to growing microtubules.

What is the relationship between adenine and thymine and between cytosine and guanine?

Adenine always binds to thymine, while cytosine and guanine always bind to one another. This relationship is called complementary base paring. These complementary bases are bonded together via hydrogen bonds, which can be easily broken apart when the DNA needs to unzip and duplicate itself.