Asked by: Bassou Prata
science genetics

Why do purines bond with pyrimidines?

Last Updated: 27th June, 2020

These nucleotides are complementary —theirshapeallows them to bond together with hydrogen bonds. Inthe C-Gpair, the purine (guanine) has three binding sites,and sodoes the pyrimidine (cytosine). Thehydrogenbonding between complementary bases is what holdsthe twostrands of DNA together.

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Besides, why do purines and pyrimidines bond together?

The molecular structure of both pyrimidinesandpurines allow them to only be able to bond witheachother and not within the group. Thymine(pyrimidine)andadenine (purine) both have two atomsthat can either providea H bond or receive it. Cytosine(pyr.) and guanine (pur.)can stablish three Hbonds.

Also Know, do pyrimidines form covalent bonds with purines? Pyrimidine-pyrimidine pairingsdonot occur because these relatively small moleculesdo notget close enough to form hydrogenbonds.Purine-purine links do notform becausethese bases are too large to fit in the spacebetween thepolynucleotide strands.

Likewise, what would happen if purines bond with purines?

Therefore, during pairing in DNA, twopurinescannot pair up together because there is not muchspace enoughbetween the two DNA helical strands to accommodatetwopurine groups, and thereby FOUR RINGS. Hence whileDNApairing, a purine always pairs up withapyrimidine.

What do purines pair with pyrimidines?

They pair together throughcomplementarypairing based on Chargaff's Rule (A::T andG::C). Thepurines in DNA are adenine and guanine, the sameas in RNA.The pyrimidines in DNA are cytosine and thymine;in RNA,they are cytosine and uracil.

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What is the base pairing rule for RNA?

The base pairing of guanine (G) and cytosine(C)is just the same in DNA and RNA. So in RNAtheimportant base pairs are: adenine (A) pairs with uracil(U);guanine (G) pairs with cytosine (C).

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What is the smallest unit of DNA called?

The smallest chromosome, chromosome 21,containsabout 300 genes. (Chromosome 22 should be thesmallest, butthe scientists made a mistake when they firstnumbered them!). TheDNA that contains your genes is storedin your cells in astructure called the nucleus.

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Can you find out why the distance between two polynucleotide chains in DNA remains almost constant?

As a result of these bonds, thedistancebetween two polynucleotide chains in DNA remainsalmostconstant. The bases in two strands of DNAarepaired through hydrogen bond (H-bonds) and form base pairs . Asaresult of these bonds, the distance betweentwopolynucleotide chains in DNA remainsalmostconstant.

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What does uracil pair with?

During the synthesis of an RNA strand from a DNAtemplate(transcription), uracil pairs only with adenine,and guaninepairs only with cytosine. Uracil is alsoa componentof several coenzymes that act in conjunction withenzymes in severalprocesses of carbohydratemetabolism.

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What is complementary base pairing?

Complementary base pairing is the phenomenonwherein DNA guanine always hydrogen bonds to cytosine and adeninealwaysbinds to thymine.

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What are the two bases of purines?

There are many naturally occurring purines.Theyinclude the nucleobases adenine (2) and guanine (3). In DNA,thesebases form hydrogen bonds with theircomplementarypyrimidines, thymine and cytosine,respectively.

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Why does a bond with T and C with G?

Complementary Base Pairing
You see, cytosine can form three hydrogenbondswith guanine, and adenine can form two hydrogenbonds withthymine. Or, more simply, C bonds withG and Abonds with T. It's called complementary basepairing becauseeach base can only bond with a specificbasepartner.

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Why does adenine have to pair with thymine?

Adenine is always paired with thymine,andguanine is always paired with cytosine. These are known asbasepairs. Adenine forms two hydrogen bondswiththymine in DNA and two hydrogen bonds with uracil inRNA,while three hydrogen bonds are formed between guanineandcytosine.

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How many hydrogen bonds are in A and T?

Cytosine and Guanine are held together bythreehydrogen bonds. The pairing of adenine and thymineshare twohydrogen bonds, thus the bond is slightly weakerand slightlylonger.

Haroun Poole


What are the three parts of a nucleotide?

Both deoxyribonucleic acid (DNA) and ribonucleic acid(RNA)are made up of nucleotides which consist of threeparts:
  • Nitrogenous Base. Purines and pyrimidines are the twocategoriesof nitrogenous bases.
  • Pentose Sugar. In DNA, the sugar is 2'-deoxyribose.
  • Phosphate Group. A single phosphate groupisPO43-.

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Are hydrogen bonds strong?

Greater electronegativity of the hydrogenbondacceptor will lead to an increase inhydrogen-bondstrength. The hydrogen bond isone of the strongestintermolecular attractions, but weaker than acovalent or an ionicbond. Hydrogen bonds areresponsible for holdingtogether DNA, proteins, and othermacromolecules.

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Do purines equal pyrimidines?

Chargaff's rules state that DNA from any cell ofanyorganisms should have a 1:1 ratio (base Pair Rule)ofpyrimidine and purine bases and, morespecifically,that the amount of guanine should be equal tocytosine andthe amount of adenine should be equal tothymine. Thispattern is found in both strands of theDNA.

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Is uracil a purine?

The other type is called a purine.Uracil,a nitrogenous base found in RNA, is a pyrimidine. Twootherpyrimidines are cytosine and thymine. Thymine is only foundinDNA.

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What are the two pyrimidines?

Pyrimidines contains one carbon-nitrogen ringandtwo nitrogen atoms.They have low melting point. Bluespheresare Nitrogen atoms and Pyrimidines is a one ringmolecule.Two Purines are Adenine and Guanine. TwoPyrimidinesare Thymine and Uracil.

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Can adenine pair with itself?

Chargaff's rule, also known as the complementarybasepairing rule, states that DNA base pairs arealwaysadenine with thymine (A-T) and cytosine with guanine(C-G).A purine always pairs with a pyrimidine and viceversa.However, A doesn't pair with C, despite that being apurineand a pyrimidine.

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What type of bonds hold the 2 strands of DNA together?

The hydrogen bonding between complementarybasesholds the two strands of DNA together. Hydrogenbondsare not chemical bonds. They can be easilydisrupted. Thispermits the DNA strands to separate fortranscription(copying DNA to RNA) and replication (copyingDNA toDNA).

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What happens if adenine and guanine bond?

The base pairs are adenine-thymineandguanine-cytosine in DNA, and adenine-uracilandguanine-cytosine in RNA or in hybrid DNA-RNA pairing.Ifadenine formed a hydrogen bond to adeninethestructure would not be as stable and not form a structuresuitableto build a helix.

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What type of bonds connect the bases to each other?

Covalent bonds occur within each linear strandandstrongly bond the bases, sugars, and phosphate groups (bothwithineach component and between components). Hydrogen bondsoccurbetween the two strands and involve a base from one strandwith abase from the second in complementary pairing.

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What bonds exist between DNA subunits and nitrogenous bases?

The nitrogen bases are held together byhydrogenbonds: adenine and thymine form two hydrogenbonds;cytosine and guanine form three hydrogenbonds.