DNA

What is DNA?

Inside cells lies a long string of genes, each of which encodes a particular protein. This strand is called DNA, and is a kind of “recipe” for which proteins should be formed by the cell’s ribosomes. The DNA consists of four units called A, G, T and C, which sit in a specific order depending on which protein they encode. The ribosomes read the order of these units and make sure that amino acids are put together to form the right protein.

DNA strands are built of nucleotides, which consist of a deoxyribose, a phosphate group, and a nitrogen base.

What are nucleotides?

The nucleotides in a DNA strand are bound together by a phosphodiester bond between the 5′-carbon atom in one nucleotide and the 3′-carbon atom in the next nucleotide. This means that the two ends of the DNA strand are not the same, and therefore DNA strands have polarity and a direction. One end is given as the 5′ end, that is the end where the 5′ carbon atom is bonded to a phosphate group that is not bonded to a 3′ carbon atom. The other end is indicated as the 3′ end, since at that end there is a 3′ carbon atom that is not bonded to a phosphate group.

There are 4 different nitrogen bases that are part of DNA strands, they can be divided into two groups called purines and pyrimidines and there are two bases in each group.

The two purines, adenine (A) and guanine (G), consist of two nitrogenous rings, and the two pyrimidines, thymine (T) and cytosine (C), consist of a single nitrogenous ring.

DNA molecules are made up of 2 DNA strands, which form a double helix structure. Through the nitrogen bases, the two strands bind to each other with hydrogen bonds, this is called base pairing. Each strand has a backbone (a “backbone”) consisting of the deoxyribose-phosphate portion of the nucleotides, and the nitrogen bases, facing each other between the two backbones so that they can base-pair. An adenine base pairs always with a thymine and a guanine base pairs always with a cytosine. This means that all base pairs consist of a purine and pyrimidine, which means that the base pairs take up roughly the same space between the two backbones.

The two DNA strands in a DNA molecule are antiparallel, that is, the 5′ end of one strand pairs with the 3′ end of the other strand. Therefore, one should be aware that when describing a direction on DNA (e.g. from the 5′ end to the 3′ end), it is the direction of one of the strands that you describe, and not the direction of the entire double helix. Two strands of DNA that bind to each other are described as complementary to each other.

What is genetic information?

DNA molecules are the units that carry our genetic information and ensure that it can be passed on during cell division. The genetic information lies in the form of the sequence of the 4 nitrogen bases. The order of the 4 bases on a DNA strand can be read a bit like a text in a book, and the “text” on our DNA contains all our genetic information.

The property of DNA that allows the genetic information a strand contains to be passed on is that DNA is double-stranded and that the two DNA strands are complementary to each other. The two strings can act as a template for each other precisely because they are complementary. When the genetic information is to be passed on, the two strands can be separated from each other and a new strand can be built from each of the two original strands. The result is that you have two identical DNA molecules, which can be distributed in separate cells by cell division. The mechanism that copies DNA is called replication.