Question: How do cells duplicate dna?

Ciarán O'Brien answered on 21 Nov 2014:

DNA is copied out by tiny little factories made of protein called DNA polymerase that float about inside the cell.

DNA is a long, complicated molecule, and parts of it have unique shapes and compositions that proteins can bind to under the right conditions, like if if the cell senses a load of food outside the cell, the sensors will change the chemistry inside the cell so that certain proteins will bind to the parts of DNA that make the chemicals that can break that food down to be absorbed.

When the cell is ready to divide, there’s plenty of food stocked inside the cell, conditions outside are safe, and proteins called initiator proteins can bind to DNA to unzip it (DNA is made of two strands and often tightly wound together for safety). The initiator proteins also separate the two strands that make up DNA.

DNA strands are made of a long string of nucleotides which bind to each other, and each stand is “complimentary”, which means that if you know the composition of one strand, you can always know what the other strand was made of because it’s kind of a mirror image. Once the strands are separated, proteins called DNA polymerase has space to bind to the single strand on either side, and start “repairing” it by attaching mirror image nucleotides where the other strand used to be. In this way, each strand of the original DNA can be turned into a copy of the original double-stranded DNA. DNA has two different ends, called the 5′ (five prime) and 3′ (three prime) ends, and DNA polymerase can only extend DNA in the direction of 5′ -> 3′, so that means one strand can be copied as it’s unravelled by the initiator proteins, while the other side has to be elongated in bits as the start point changes every time the DNA is unraveled a little more. This picture might give you a clearer idea of what I mean:

The strand that replicates in a stop/start manner need another protein called DNA ligase to join up each bit once the polymerase has extended it and moved back to start another section.

Our cells don’t just duplicate DNA when they divide, but it’s the only time they duplicate the whole lot in one go. At other times, your cells will need to build proteins or respond to some issue the comes up, and then it will duplicate just the bits of DNA that can help solve those problems. The DNA is duplicated just like above, only it makes copies of a couple of genes. Those copies are then shipped off to other proteins called ribosomes, which use the DNA as a blueprint to build proteins. Or sometimes the DNA is used to signal other parts of DNA to be copied. It’s a very complicated network that signals itself as often as it signals anything else in the cell, and it’s not fully understood yet.