Base Pair: Definition, Rules in DNA and RNA

Two nitrogen-containing bases (or nucleotides) that pair together to form the structure of DNA. The four bases in DNA are adenine (A), cytosine (C), guanine (G), and thymine (T). These bases form specific pairs (A with T, and G with C).

Base Pair Definition

A base pair is a fundamental unit of double-stranded nucleic acids, such as DNA and RNA. It consists of two complementary nucleotide bases that are bound to each other by hydrogen bonds. The four bases in DNA are adenine (A), cytosine (C), guanine (G), and thymine (T).

What is a Base Pair?

A base pair is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. DNA is made up of two linked strands that wind around each other to resemble a twisted ladder, known as a double helix.

Each strand has a backbone made of alternating sugar and phosphate groups, and the rungs of the ladder are made up of complementary pairs of nitrogenous bases.

The four bases in DNA are adenine (A), cytosine (C), guanine (G), and thymine (T). Base pairs often measure the size of an individual gene within a DNA molecule.

In single-stranded DNA/RNA, units of nucleotides are used instead of base pairs, abbreviated as nt (or knt, Mnt, Gnt) since they are not paired. To distinguish between units of computer storage and bases, kbp, Mbp, and Gbp may be used for base pairs.

A DNA nucleotide is made up of a molecule of sugar, a molecule of phosphoric acid, and one nitrogen-containing base.

Purine and Pyrimidine Groups

Purines and pyrimidines are two types of nitrogenous bases that make up the building blocks of DNA. Purines consist of a six-membered and a five-membered nitrogen-containing ring, fused together, while pyrimidines have only a six-membered nitrogen ring.

Adenine (A) and guanine (G) are purines, while cytosine (C) and thymine (T) are pyrimidines. These nitrogenous bases pair with each other through hydrogen bonds to form the base pairs that make up the rungs of the DNA ladder.

Base Pair Rules in DNA

DNA is made up of four nitrogen-containing bases: adenine (A), cytosine (C), guanine (G), and thymine (T).

The base pairs in DNA are held together by hydrogen bonds between pairs of bases, with A always pairing with T and C always pairing with G. This phenomenon is known as a complementary base pairing or Watson-Crick base pairing.

The rules of base pairing explain that the amount of adenine in the DNA of an organism is always equal to the amount of thymine, and the amount of cytosine is always equal to the amount of guanine.

The strictness of the DNA base-pairing rules means that no nucleotide pairs with any other nucleotide except for its designated partner.

RNA also has base pairs, but adenine always pairs with uracil instead of thymine. Base pairs are chemically bonded together by hydrogen bonds, which hold the two strands of the molecule together.

Watson-Crick Base Pair Hydrogen Bonds

Watson-Crick base pairing is a fundamental concept in molecular biology. It refers to the specific hydrogen bonding patterns between nucleotides on complementary DNA or RNA strands.

In DNA, adenine (A) forms a base pair with thymine (T) using two hydrogen bonds, and guanine (G) forms a base pair with cytosine (C) using three hydrogen bonds.

The purine nucleobases, adenine, and guanine are double-ringed molecules, while the pyrimidine nucleobases cytosine, uracil, and thymine are single-ringed molecules. In Watson-Crick base pairing, purines always bind with pyrimidines.

The number of hydrogen bonds between base pairs differs. G-C base pairs are bound by three hydrogen bonds while A-T/U pairs are bound by two hydrogen bonds. Contrary to popular belief, hydrogen bonds do not significantly stabilize DNA; stabilization is mainly due to stacking interactions.

DNA Base Pair Structure

In 1950, Erwin Chargaff of Columbia University showed that the molar amount of adenine (A) in DNA was always equal to that of thymine (T), and the molar amount of guanine (G) was the same as that of cytosine (C).

Chargaff’s findings clearly indicate that some type of heterocyclic complementarity exists in DNA. The Watson-Crick model explains this complementarity through specific hydrogen bonding patterns between nucleotides on complementary strands.

Base Pairs in RNA

RNA, or ribonucleic acid, is a single-stranded molecule that plays a crucial role in protein synthesis. RNA consists of four nitrogenous bases: adenine, cytosine, uracil, and guanine.

Adenine and guanine are purine bases, while cytosine and uracil are pyrimidine bases. In RNA, adenine pairs with uracil (A-U) via the same hydrogen bonding as adenine-thymine (A-T) in DNA.

RNA is transcribed from DNA and carries genetic information to ribosomes where it is translated into proteins.

The sequence of nucleotides in RNA determines the sequence of amino acids in a protein. The process begins when RNA polymerase binds to a specific region of DNA called the promoter.

The polymerase then unwinds the double helix and synthesizes an RNA molecule complementary to one strand of DNA. Once the transcription is complete, the RNA molecule undergoes post-transcriptional modifications before it can be used for translation.

Complementary Base Pairing

Complementary base pairing is the process by which bases pair up with each other in a consistent way. This process is also known as Watson-Crick base pairing.

The nitrogenous bases include adenine, thymine, cytosine, and guanine in DNA. In RNA, thymine is replaced by uracil. Adenine pairs with thymine (or uracil in RNA) and guanine pairs with cytosine.

These base pairs are chemically bonded together by hydrogen bonds that hold the two strands of the DNA molecule together.

Complementary base pairing allows DNA to accurately replicate itself. It also allows DNA to be transcribed accurately into RNA and then translated from RNA to amino acids. A complementary strand of DNA or RNA may be constructed based on nucleobase complementarity.

Each base pair takes up roughly the same space, thereby enabling a twisted DNA double helix formation without any spatial distortions. Hydrogen bonding between the nucleobases also stabilizes.

Hydrogen Bonding and Stability

Hydrogen bonding plays a crucial role in the stability of DNA. The hydrogen bonding between complementary bases sequesters the bases in the interior of the double helix, reinforcing the hydrophobic effects that stabilize DNA. The energy required for hydrogen bonding between bases is 1 to 5 kcal/mol (4 to 21 kJ/mol).

The pairing of guanine and cytosine shape and structure is very similar to that of adenine and thymine. Cytosine and guanine are held together by three hydrogen bonds, while adenine and thymine are held together by two hydrogen bonds.

Hydrogen bonds contribute to the stability of DNA but not as much as expected because DNA is stabilized by other factors such as hydrophobic interactions.

RNA has stronger hydrogen bonds than DNA, according to a study that analyzed the correlation between NMR shielding and hydrogen-bond strength using density functional theory. In RNA, adenine and uracil form two hydrogen bonds, while guanine and cytosine form three hydrogen bonds.

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