Written by tutor Kathie Z.
A copy of DNA must be replicated before the cell can reproduce itself. The DNA molecule has to replicated, then transcribed into Ribonucleic Acid (RNA), and translated by protein synthesis in three very complex processes in order to duplicate itself. This lesson will discuss the first step: DNA Replication.
Deoxyribonucleic Acid (DNA) is the tiny molecule inside every cell that carries all our genes in a code (genetic code). Most of the time, DNA is coiled up tightly inside of our chromosomes, but when needed, it unravels its double helix shape, and looks like a twisted rope ladder. Four chemical bases make up the rungs of the ladder. Each base pairs with only one other, so the sequence of bases along one strand of the DNA is a perfect mirror image of the sequence on the other side. When the strand divides down the middle, each half of the strand can be used like a template to make a copy.
It has been proven that adenine always pairs with thymine and cytosine always pairs with guanine in DNA. The backbone of the double helix are alternating molecules of deoxyribose (5 sided sugar) and phosphate groups. The bars of DNA are the four special chemicals called bases: guanine (G), adenine (A), cytosine ( C) and thymine (T). These bases in DNA are set in groups of three called codons, and the order of the bases in each codon varies to provide a chemical code for the cell to make a particular amino acid.
The original DNA double helix is called the parent strand. Replication is making a copy of DNA which when replicated in a complimentary strand .
Eukaryotic cells start the unzipping and start the replication process in the nucleus. On the other hand, prokaryotic cells (which have no nucleus) unzip and start the replication process in the cytoplasm. But in both cases the replication happens during the S phase of mitosis in the cell cycle.
- DNA gyrase (an enzyme) makes a cut in the double helix of and each side separates like unzipping a zipper.
- Helicase (an enzyme) causes the the double strand of DNA to unwind into its rope ladder appearance.
- Single Strand Binding proteins (SSB - several small proteins) temporarily bind to each side and keep them separate.
- DNA polymerase (an enzyme complex) "walks" down the strand and adds new nucleotides to each strand. These make a complementary strand because the complementary nucleotides pair with the A with T, G with C on the existing strand.
- A sub-unit of the DNA polymerase "proofreads" the new DNA.
- DNA ligase (an enzyme) seals up the fragments into one continuous strand.
- The new copies automatically re-zip themselves into the double helix.
The consequence of all these steps is that two exact duplicate strands of DNA are produced: each of them consisting of one of the original parent strands and a new daughter strand. The cell can now undergo cell division and provide each daughter cell with a complete copy of the DNA molecule.
- The Biology Coloring Book, by Robert D. Griffin
- Visual Factfinder Science, by John Farndon