DNA structure

🧬 DNA Structure – 9th Grade Advanced Biology Notes

1. What is DNA?

• DNA stands for Deoxyribonucleic Acid.

• DNA stores genetic information and instructions for making proteins.

• DNA is found mainly in the nucleus of eukaryotic cells.

• DNA passes hereditary traits from parents to offspring.

2. DNA Structure

• DNA has a double helix shape (twisted ladder).

• DNA consists of two strands running in opposite directions (antiparallel).

• DNA is made of repeating units called nucleotides.

3. Nucleotides

Each nucleotide has 3 parts:

• Phosphate group – Helps form the DNA backbone.

• Deoxyribose sugar – A 5-carbon sugar.

• Nitrogen base – Contains genetic information through its sequence.

4. Nitrogen Bases

DNA contains four nitrogen bases:

• A = Adenine

• T = Thymine

• C = Cytosine

• G = Guanine

5. Complementary Base Pairing

• A pairs with T (2 hydrogen bonds).

• C pairs with G (3 hydrogen bonds).

• Complementary means bases match with specific partners.

• Hydrogen bonds hold the base pairs together.

6. DNA Backbone

• The sides of DNA consist of alternating sugars and phosphates.

• The nitrogen bases form the rungs (steps) of the ladder.

• The sequence of nitrogen bases stores genetic instructions.

7. Important Vocabulary

• Double helix: DNA’s twisted ladder shape.

• Nucleotide: Basic building block of DNA.

• Complementary: Two bases that pair together.

• Hydrogen bonds: Weak bonds holding complementary bases together.

• Antiparallel: DNA strands run in opposite directions.

• Sugar-phosphate backbone: The structural sides of DNA.

⭐ Key Facts to Memorize

• DNA = Double helix.

• Nucleotide = Sugar + Phosphate + Base.

• A ↔ T (2 hydrogen bonds).

• C ↔ G (3 hydrogen bonds).

• DNA stores genetic information.

• The order of bases determines genetic instructions.

• DNA strands are complementary and antiparallel.

Practice Question: If one DNA strand is A T G C C A, what is its complementary strand?

Answer: T A C G G T