Explain why dna must be replicated before mitosis and the role of helicase in dna replication

What is the compliment dna strand to the following sequence: ttgactaggcta

Ihave no idea how to solve these. plus there's another question that is 2) a 10 kilogram meteorite is collected in space and brought back to earth. it is composed of 1.25 kg of uranium-238 and 8.75 kg of lead-206. how many u-238 half lives have passed? what is the approximate age of the meteorite?

Which to produce involved from a symbiotic relationship of organisms which resulted in eukaryotic organisms contain chloroplast