AQA GCSE FOUNDATION Biology: DNA and Genetic Inheritance

AQA GCSE Foundation Biology: DNA and Genetic Inheritance

The Structure of DNA

DNA, or deoxyribonucleic acid, is the molecule that carries the genetic instructions for all living organisms. It's found in the nucleus of every cell and is organized into structures called chromosomes.

Here's the key structure of DNA:

• Nucleotides: DNA is made up of repeating units called nucleotides. Each nucleotide has three parts:

  • Sugar: A deoxyribose sugar molecule.
  • Phosphate group: A group containing phosphorus and oxygen.
  • Nitrogenous base: One of four nitrogen-containing molecules: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).

• Double helix: Two strands of nucleotides are linked together by weak hydrogen bonds between the bases. The two strands twist around each other to form a double helix, similar to a spiral staircase.

• Base pairing: The bases on one strand of DNA pair specifically with the bases on the other strand:

  • Adenine (A) always pairs with Thymine (T).
  • Cytosine (C) always pairs with Guanine (G).

This specific base pairing is crucial for DNA's function as it allows the molecule to replicate accurately, ensuring that genetic information is passed on correctly from one generation to the next.

Gene Transmission

Genes: Segments of DNA that carry the instructions for specific traits are called genes.

Chromosomes: DNA is organized into long, thread-like structures called chromosomes. Humans have 23 pairs of chromosomes, with one chromosome from each pair coming from the mother and the other from the father.

Gametes: Reproductive cells (egg and sperm) contain only half the number of chromosomes (23) as other cells in the body. This is because during sexual reproduction, each parent contributes one chromosome from each pair to their offspring.

Fertilization: When an egg and a sperm fuse during fertilization, the offspring receives a full set of 23 pairs of chromosomes, half from each parent. This is how genetic information is passed on from one generation to the next.

The process of gene transmission can be summarized as follows:

1. Replication: Before a cell divides, the DNA molecule makes an exact copy of itself. This ensures that each new cell receives a complete set of genetic instructions.
2. Meiosis: During the formation of gametes (egg and sperm), the number of chromosomes is halved. This is achieved through a process called meiosis, which involves two cell divisions.
3. Fertilization: The fusion of a sperm and an egg cell combines the genetic information from both parents, creating a unique combination of genes in the offspring.

Understanding the structure of DNA and the process of gene transmission is essential for understanding how traits are inherited from parents to offspring.