3B1+Bio+DNA+essay+by+Xue+Chen

DNA is deoxyribonucleic acid. It is a molecule that carries genetic information which is important for all cellular functions, such as cell divisions and cell differentiation. Each DNA has **two parallel strands** twisted around each other to form a **double helix**. A molecule of DNA is wrapped around in proteins to form a single chromatin thread. A basic unit of DNA is a **nucleotide**. Each nucleotide is made up of **deoxyribose** ( a sugar ), a **phosphate group** and a **nitrogen-containing base**: adenine, guanine, thymine and cytosine. Let's look at the sugar first. In DNA, the sugar is a **pentose** ( five-carbon ) **sugar** and it has **one oxygen atom less**. The sugars are joined together by phosphate groups that form phosphodiester bonds between the third and fifth carbon atoms of adjacent sugar rings. These asymmetric bonds mean a strand of DNA has a **direction**. In a double helix the direction of the nucleotides in one strand is opposite to their direction in the other strand. This arrangement of DNA strands is called **antiparallel**.

The bases bases are classified into two types; adenine and guanine are fused five- and six-membered heterocyclic compounds called purines, while cytosine and thymine are six-membered rings called pyrimidines. A fifth pyrimidine base, called uracil (U), usually takes the place of thymine in RNA and differs from thymine by lacking a methyl group on its ring. Uracil is not usually found in DNA, occurring only as a breakdown product of cytosine.

In between the two backbones are the nitrogen-containing bases. The backbone and the base are held together by hydrogen bonds and the double helix is stablized thus. Each type of base on one strand forms a bond with just one type of base on the other strand. This is called complementary base pairing. Here, purines form **hydrogen bonds** to **pyrimidines**, with A bonding only to T, and C bonding only to G. This arrangement of two nucleotides binding together across the double helix is called a **base pair**. As hydrogen bonds are not covalent, they can be broken and rejoined relatively easily. The two strands of DNA in a double helix can therefore be pulled apart like a zipper, either by a mechanical force or high temperature. As a result of this complementarity, all the information in the double-stranded sequence of a DNA helix is duplicated on each strand, which is vital in DNA replication. Indeed, this reversible and specific interaction between complementary base pairs is critical for all the functions of DNA in living organisms. Reference: GCE 'O' Level Biology Matters []