1. The Double Helix Structure:
* DNA is a double helix, meaning it consists of two strands twisted around each other.
* Each strand is made of nucleotides, which consist of a sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C).
2. Base Pairing Rules:
* The two strands of DNA are held together by hydrogen bonds between the bases. These bonds follow specific rules:
* Adenine (A) always pairs with thymine (T)
* Guanine (G) always pairs with cytosine (C)
* This complementary base pairing is essential for accurate replication.
3. DNA Replication:
* When a cell divides, its DNA must be copied so that each daughter cell receives a complete set of chromosomes.
* Step 1: The double helix is unwound by enzymes called helicases.
* Step 2: Each single strand acts as a template for building a new complementary strand.
* Step 3: Enzymes called DNA polymerases move along the template strand, adding nucleotides to the new strand. They do this following the base pairing rules:
* If they encounter a thymine (T) on the template strand, they add an adenine (A) to the new strand.
* If they encounter a guanine (G) on the template strand, they add a cytosine (C) to the new strand.
* Step 4: Two identical DNA molecules are produced, each with one original strand and one newly synthesized strand. This is known as semi-conservative replication.
In summary, the specific pairing of bases in DNA serves as a code for producing exact chromosome copies because it ensures that each new strand is an exact replica of the original strand. This is crucial for maintaining the genetic integrity of the organism.
