DNA Replication (Leaving Cert Biology): Revision Notes

DNA Replication

• At the end of mitosis, each new cell has single-stranded chromosomes.
• Before these cells can divide again, the DNA in each chromosome must make an exact copy of itself.
• This process is called DNA replication, and it ensures that the single-stranded chromosomes become double-stranded.
• DNA replication occurs during interphase in the nucleus.

Mechanism of DNA Replication

1. The double helix unwinds, exposing the two strands of DNA.
2. An enzyme breaks the hydrogen bonds between the complementary bases, causing the strands to open like a zip.
3. The complementary strands move apart.
4. Free DNA nucleotides move into the nucleus from the cytoplasm.
5. These nucleotides attach to their complementary bases on the exposed DNA strands:

• A pairs with T.
• C pairs with G.

1. The enzyme DNA polymerase assembles the new strands by joining the nucleotides.
2. Each new DNA strand has the exact same sequence of bases as the original.

6. The new strands are half old DNA and half new DNA. The two new DNA molecules are identical to the original DNA and to each other.
7. Each new stretch of DNA rewinds to form a double helix.

Importance of DNA Replication

• Allows for cell continuity: Ensures that all cells in the body contain the same genetic information.
• Variation: DNA replication is a very accurate process, but occasional mistakes (mutations) in the sequence of bases allow for variation, which is essential for evolution.