How are the new strands of DNA lengthened?

New strands of DNA are lengthened through a process called DNA replication, which occurs during the S phase of the cell cycle. During this process, the enzyme DNA polymerase adds nucleotide units to the growing DNA strand, using the original strand as a template. This addition occurs in a 5' to 3' direction, as the enzyme can only attach nucleotides to the 3' end of the growing strand. Additionally, RNA primers are initially laid down to provide a starting point for DNA polymerase to begin synthesis.