DNA Structure and Components

DNA (Deoxyribonucleic Acid) is a complex molecule that carries genetic information. Understanding its structure is crucial for GCSE Biology and Edexcel Combined Science students.

DNA Composition:

1. DNA strands are polymers made up of repeating units called nucleotides.
2. Each nucleotide consists of a sugar molecule, a phosphate molecule, and a base.
3. The sugar and phosphate molecules form the 'sugar-phosphate backbone' of the DNA strands, alternating along the length.
4. One of four bases attaches to each sugar: Adenine (A), Thymine (T), Cytosine (C), or Guanine (G).

Vocabulary: Nucleotide - The basic structural unit of DNA, consisting of a sugar, a phosphate, and a nitrogenous base.

DNA Structure: 5. A DNA molecule has two strands coiled together in the shape of a double helix. 6. Each base on one strand links to a complementary base on the opposite strand. 7. Complementary base pairing occurs: A always pairs with T, and C always pairs with G. 8. The complementary base pairs are joined together by weak hydrogen bonds.

Highlight: Complementary base pairing in DNA GCSE biology is a crucial concept. Remember: A-T and C-G are the only allowed pairings.

Chromosomes and Genes:

1. Chromosomes are long, coiled-up molecules of DNA found in the nucleus of eukaryotic cells.
2. A gene is a section of DNA on a chromosome that codes for a particular protein. The sequence of bases in the gene determines the protein produced.
3. All of an organism's DNA makes up its genome.

Definition: Genome - The complete set of genetic material in an organism.

DNA Extraction Practical:

This section outlines a method for DNA extraction from fruit cells, specifically strawberries, which is a common GCSE Biology experiment.

Materials needed:

• Strawberries
• Detergent
• Salt
• Ice-cold ethanol
• Beaker
• Filter
• Glass rod

Procedure:

1. Mash strawberries and place them in a beaker containing a solution of detergent and salt. Mix well.

   Example: The detergent breaks down cell membranes to release the DNA, while the salt helps the DNA stick together.

2. Filter the mixture to remove froth and large insoluble cell parts.

3. Gently add ice-cold ethanol to the filtered mixture.

   Highlight: Why is ice cold ethanol used in DNA extraction? Cold ethanol causes DNA to precipitate out of solution, making it visible.

4. Observe the DNA precipitating as a stringy, white solid. It can be carefully removed using a glass rod.

Vocabulary: Precipitate - A solid that forms and separates from a solution.

This practical demonstration of DNA extraction GCSE Biology helps students visualize the normally invisible genetic material and understand the principles behind isolating DNA from cells.