In this animation, we’ll see the remarkable way our DNA is tightly packed up, so that 6 feet of this long molecule fits into the microscopic nucleous of every cell. The process starts when DNA is wrapped around special proteins molecules called “Histones”. The combined loop of DNA and protein is called a “Nucleosome”. Next, the nucleosomes are packaged into a thread. The end result is a fiber known as “Chromatin”. This fiber is then looped and coiled yet again. Leading finally to the familiar shapes known as “Chromosomes”, which can be seen in the nucleus of dividing cells. Chromosomes are not always present. They form around the time cells divide when the two copies of the cell’s DNA need to be separated. When DNA replicates, its strands are separated by the enzyme Helicase. Single stranded DNA binding proteins keep the strands from reanealing One DNA strand encodes the leading strand, which forms from its 5′ to its 3′ end. Using DNA polymerase III. No problem here, but the lagging strand presents problems. It has to form from 5′ to 3′ too. It forms in pieces called the “Okazaki fragments”. First, an RNA primase lays down an RNA primer. Then, DNA polymerase III lays down a new DNA. The process repeats again and again. DNA polymerase I replaces the RNA primers with DNA. Finally, DNA ligase links the Okazaki fragments.