Which way is DNA read?

First, DNA polymerase builds a new strand of DNA in the 5′ to 3′ direction. Always. The new strand is built 5′ –> 3′.

Second, when people look at DNA sequences, the convention is to read it in 5″ –> 3′

With that in mind…

DNA polymerase = only synthesizes DNA in the 5′ to 3′ direction.

Because DNA  polymerase goes in the 5′ –> 3′, it has to start on the 3′ end of the old strand.

Here’s a recap of the steps…

1. — When DNA is replicated, the two complementary strands part. One strand runs 5′ –> 3′. One strand runs 3′ –> 5′
2. — DNA polymerase comes in to replicate the new strands. It does this by complementary base pairing which each of the two strands
3. — DNA polymerase can only synthesize in the 5′ –> 3′ direction. That’s the new strand. It will start at the 3′ end of the original template strand.

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