DNA Replication

CREDITS Animation & Design: Bing Rijper Narration: Dale Bennett Script: Gemma Young It might be hard to believe, but at the very start of your life you were a single, microscopic cell called a zygote. Your body now contains millions of cells, which all came about by the process of cell division. Cell division happens when one cell divides to form two cells and it is the basis of growth and repair in humans. It is also used for asexual reproduction in organisms like bacteria. But, a single cell cannot just split itself in half to form two whole new cells - they would essentially only be half a cell. Before the cell divides everything inside needs to be copied - this includes all the parts of the cell like the mitochondria, as well as the chromosomes inside the nucleus. A human body cell contains 23 pairs of chromosomes - 46 altogether. During cell division, the DNA in the chromosomes is copied, or replicated to form 46 pairs (92), double the number of chromosomes. These pairs split apart when the cell divides to form two new daughter cells, each having the correct number of chromosomes. The splitting apart of the chromosome pairs happens during a stage in cell division called mitosis. [insert link to ‘mitosis’ video] Let’s take a closer look at DNA replication. DNA is made up of many nucleotides, each containing a base represented by the letters A, T, C and G. A piece of DNA contains two strands of nucleotides, twisted together to form a double helix. The strands are complementary, this means that whenever there is an A in one strand, it will be joined to a T in the opposite strand and whenever there is a C it will be joined to a G. If you are not familiar with the structure of DNA then take a look at the video What is DNA? During DNA replication, the double helix unwinds. Then, an enzyme called DNA helicase unzips the DNA so the two strands are separated. This happens at several points along the DNA. An enzyme called DNA polymerase attaches itself to the DNA strands and is used to add complementary free nucleotides to the now exposed bases on both strands. A pairs with T, and C pairs with G. This forms two DNA molecules, each of which have one brand new strand, and one from the original DNA. These two strands twist to form a double helix. The two new DNA molecules are both identical to the original DNA molecule. In some ways, DNA replication is similar to transcription, a process that happens during protein synthesis.They both use enzymes to join free nucleotides together and they both take place in the nucleus. But there are important differences: In DNA replication, each DNA strand is used as a template to make two new identical DNA molecules. In transcription, the coding strand of the DNA molecule is used to produce a single strand of RNA. So, to recap. In this video you have learnt why DNA needs to be replicated before a cell divides and how this takes place.