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Genetics - DNA Structure and Function

Grade 9IB

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

🔑Concepts

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DNA stands for Deoxyribonucleic Acid and is the molecule that carries genetic information for the development and functioning of an organism.

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The basic building block of DNA is the nucleotide, which consists of three parts: a phosphate group, a deoxyribose sugar (C5H10O4C_5H_{10}O_4), and one of four nitrogenous bases.

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There are four nitrogenous bases categorized into Purines (AA and GG) and Pyrimidines (CC and TT).

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Complementary Base Pairing: Adenine (AA) always pairs with Thymine (TT) via 22 hydrogen bonds, and Cytosine (CC) always pairs with Guanine (GG) via 33 hydrogen bonds.

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The DNA molecule is a double helix consisting of two antiparallel strands, meaning they run in opposite directions (5′5' to 3′3' and 3′3' to 5′5').

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A gene is a specific sequence of nucleotides in DNA that codes for a functional product, usually a protein. The sequence of bases determines the sequence of amino acids.

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DNA is organized into structures called chromosomes. Humans typically have 2323 pairs of chromosomes (2n=462n = 46).

📐Formulae

%A=%T\%A = \%T

%G=%C\%G = \%C

%A+%T+%G+%C=100%\%A + \%T + \%G + \%C = 100\%

Number of Purines=Number of Pyrimidines\text{Number of Purines} = \text{Number of Pyrimidines}

💡Examples

Problem 1:

A sample of double-stranded DNA is analyzed and found to contain 22%22\% Cytosine (CC). Calculate the percentage of Adenine (AA) in this DNA sample.

Solution:

If C=22%C = 22\%, then by Chargaff's rule, G=22%G = 22\%. Together, C+G=44%C + G = 44\%. The remaining 56%56\% (100%−44%100\% - 44\%) must be A+TA + T. Since A=TA = T, the percentage of Adenine is 56%÷2=28%56\% \div 2 = 28\%.

Explanation:

According to Chargaff's rules, in any double-stranded DNA, the amount of Guanine equals Cytosine, and the amount of Adenine equals Thymine.

Problem 2:

Given the following DNA template strand sequence: 5′−A−T−G−C−C−G−A−T−3′5'-A-T-G-C-C-G-A-T-3', determine the sequence of the complementary DNA strand.

Solution:

3′−T−A−C−G−G−C−T−A−5′3'-T-A-C-G-G-C-T-A-5'

Explanation:

To find the complementary strand, you must apply base-pairing rules (A↔TA \leftrightarrow T and G↔CG \leftrightarrow C) and ensure the orientation is antiparallel (5′5' pairs with 3′3').