The nucleus of all living cells contain macromolecular species called chromosomes made up of proteins and another type of biomolecules called nucleic acids. The chromosomes are responsible for transmission of inherent characters called heredity. Nucleic acids are mainly two types: deoxyribonucleic acids (DNA) and ribonucleic acids (RNA). Nucleic acids are long chain polymers of nucleotides, so they are also known as polynucleotides. A nucleotide is composed of a base, a C₅-carbohydrate and phosphate group (orthophosphate, pyrophosphate or triphosphate), which are bonded in the sequence base–carbohydrate–phosphate. The carbohydrate molecule in DNA is \(\beta\)-D-2-deoxyribose, whereas in RNA it is \(\beta\)-D-ribose. The nucleotide act as a energy carrier and a cofactor or coenzyme for a redox enzymes. The removal of phosphate moiety from a nucleotide produces nucleoside. The N-glycoside of purine or pyrimidine bases with pentose sugars are known as nucleosides. DNA contains four bases, viz. two purine bases adenine and guanine and two pyrimidine bases cystosine and thyamine. RNA also contains four bases: the same two purine bases adenine and guanine and two pyrimidine bases cystosine and uracil (in the place of thyamine). In nucleic acids the nucleotides are joined together by phosphodiester linkage between 5′ and 3′ carbon atoms of the pentose sugar. The primary structure of nucleic acid gives the information regarding the sequence of nucleotides in the chain. The secondary structure of DNA shows that two nucleotide chains running in opposite directions and giving a right handed helix in which the two strands are held together by hydrogen bonds at definite distances.

In the secondary structure of RNA, helices are present which are only single-stranded. RNA molecules are three types: (i) messenger RNA (m–RNA) (ii) ribosomal RNA (r–RNA) and (iii) transfer RNA (t–RNA). These perform different functions. The tertiary structure gives the information of the orientation of the planar aromatic rings of the bases causing the DNA double helix to twist about the same axis. Due to twisting, a large number of hydrogen bonds are formed. The double helix of DNA is the store house of the hereditary information of the organism. The process of synthesis of RNA involving direct copying of one DNA molecule from another DNA molecule is known as replication. The process of synthesis of RNA in which a single strand DNA can act as a template is called transcription. The proteins are synthesized by various RNA molecules in the cell but the message for the synthesis of particular protein is present in DNA. This process is known as translation. In molecular biology, transcription is used as a synonym for RNA synthesis and translation is a synonym for protein synthesis. Translation is unidirectional but transcription can sometimes be reversed, i.e., RNA is copied into DNA and is known as reverse transcription.

Which of the following statements about the assembly of nucleotides in a molecule of deoxyribonucleic acid is correct?

A phosphate of one unit connects to a pentose of another

The correct answer is option 3. A phosphate of one unit connects to a pentose of another.

Let us break down the structure of a DNA molecule and how its components assemble:

Nucleotides: Nucleotides are the building blocks of DNA. Each nucleotide consists of three main components:

A phosphate group: This is a negatively charged group (-PO₄³⁻) that contains phosphorus and oxygen atoms.

A pentose sugar: In DNA, the pentose sugar is deoxyribose. Deoxyribose is a five-carbon sugar ring structure.

A nitrogenous base: There are four nitrogenous bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases contain nitrogen atoms and are responsible for encoding genetic information.

Assembly of Nucleotides: The nucleotides in DNA join together to form a single DNA strand. This process occurs through the formation of phosphodiester bonds. A phosphodiester bond forms between the phosphate group of one nucleotide and the hydroxyl group (-OH) of the pentose sugar of another nucleotide.

Backbone of DNA: As nucleotides join together, they form a backbone for the DNA molecule. This backbone consists of alternating sugar (deoxyribose) and phosphate groups connected by phosphodiester bonds. The nitrogenous bases project inward from the backbone.

So, when we consider the correct statement "A phosphate of one unit connects to a pentose of another," it accurately describes the formation of the DNA backbone. The phosphate group of one nucleotide forms a bond with the pentose sugar (deoxyribose) of another nucleotide, creating a strong and stable backbone structure for the DNA molecule.