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Biomolecules - Nucleic Acids: DNA and RNA

Grade 12ICSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Nucleic acids are long-chain polymers of nucleotides, hence they are also called polynucleotides. They are responsible for the storage and transfer of genetic information.

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A nucleotide consists of three chemical components: a pentose sugar, a nitrogenous base, and a phosphate group (derived from H3PO4H_3PO_4).

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In DNADNA, the sugar component is β-D-2-deoxyribose\beta\text{-D-2-deoxyribose} (C5H10O4C_5H_{10}O_4), whereas in RNARNA, it is β-D-ribose\beta\text{-D-ribose} (C5H10O5C_5H_{10}O_5).

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Nitrogenous bases are categorized into Purines: Adenine (AA) and Guanine (GG); and Pyrimidines: Cytosine (CC), Thymine (TT, only in DNADNA), and Uracil (UU, only in RNARNA).

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A nucleoside is formed by the linkage of a nitrogenous base to the 1′1' position of the pentose sugar. A nucleotide is formed when the 5′-OH5'\text{-OH} group of a nucleoside is esterified with phosphoric acid.

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Nucleotides are joined by phosphodiester linkages between the 5′5' carbon of one sugar and the 3′3' carbon of the next sugar.

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The Double Helix structure of DNADNA (Watson and Crick model) consists of two polynucleotide chains held together by hydrogen bonds between complementary base pairs: A=TA=T (two hydrogen bonds) and G≡CG \equiv C (three hydrogen bonds).

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Chargaff's Rule states that in a double-stranded DNADNA, the ratio of Adenine to Thymine and Guanine to Cytosine is constant and equals one, i.e., [A]+[G]=[T]+[C][A] + [G] = [T] + [C].

📐Formulae

Nucleoside=Sugar+Base\text{Nucleoside} = \text{Sugar} + \text{Base}

Nucleotide=Sugar+Base+Phosphate Group\text{Nucleotide} = \text{Sugar} + \text{Base} + \text{Phosphate Group}

Chargaff’s Rule: [A][T]=[G][C]=1\text{Chargaff's Rule: } \frac{[A]}{[T]} = \frac{[G]}{[C]} = 1

Total Purines=Total Pyrimidines  ⟹  [A]+[G]=[T]+[C]\text{Total Purines} = \text{Total Pyrimidines} \implies [A] + [G] = [T] + [C]

💡Examples

Problem 1:

A sample of double-stranded DNADNA has 20%20\% Adenine (AA). Calculate the percentage of Cytosine (CC) present in the sample.

Solution:

According to Chargaff's rule, [A]=[T][A] = [T]. Therefore, if A=20%A = 20\%, then T=20%T = 20\%. Total (A+T)=20%+20%=40%(A+T) = 20\% + 20\% = 40\%. The remaining 100%−40%=60%100\% - 40\% = 60\% consists of GG and CC. Since [G]=[C][G] = [C], the percentage of Cytosine is 60%2=30%\frac{60\%}{2} = 30\%.

Explanation:

In double-stranded DNADNA, base pairing is specific (AA with TT and GG with CC), ensuring the molar concentration of purines equals pyrimidines.

Problem 2:

If the sequence of one strand of DNADNA is 5′-TAGCATE−3′5'\text{-TAGCATE}-3', write the complementary strand sequence.

Solution:

3′-ATCGTAG−5′3'\text{-ATCGTAG}-5' or 5′-GATGCTA−3′5'\text{-GATGCTA}-3'.

Explanation:

DNADNA strands are anti-parallel. Adenine (AA) pairs with Thymine (TT), and Guanine (GG) pairs with Cytosine (CC). The 5′5' end of one strand matches the 3′3' end of the other.

Problem 3:

Explain the chemical difference between a Nucleoside and a Nucleotide.

Solution:

A nucleoside contains only the sugar and the nitrogenous base (e.g., Adenosine), while a nucleotide is a nucleoside with a phosphate group attached to the 5′5' carbon of the sugar (e.g., Adenylic acid or AMPAMP).

Explanation:

The addition of the phosphate group via an ester linkage converts a nucleoside into the acidic monomeric unit of nucleic acids.

Nucleic Acids: DNA and RNA - Revision Notes & Key Formulas | ICSE Class 12 Chemistry