Why does a cell need to make a copy of its dna? (explain about replication and mrna transcription)

A cell needs to make a copy of its DNA for cell division.

A DNA replication is the process by which a single DNA produces another double stranded DNA by means of different enzymes.

In case of DNA, the structure is double stranded, which needs to open up before replication for the enzymes to work. Thus the enzyme helicase removes the DNA helix and the enzyme gyrase releases the helix. Then, enzymes like DNA dependent DNA polymerase comes into site with three different subtypes of the enzyme which does several jobs. It replicates from the 5'-3' direction. It brings different nucleotide triphosphates from the cytoplasm and breaks the triphosphate bond which releases energy. This energy is used by the enzyme to join the nucleotide with the previously formed chain. The enzyme DNA Ligase thereby joins the DNA strand with the previous batch and the final new double stranded DNA is produced. This is DNA replication which helps in cell division, because without the presence of second set of DNA, the cell division isn't possible.

DNA replication:

Cells duplicate their DNA when they decide they need to divide. DNA is the manual for building and operating a cell. DNA replication needs to occur because existing cells divide to produce new cells.

mRNA transcription:

The information in a strand of DNA is copied into a new molecule of messenger RNA (mRNA). This process is known as mRNA transcription.

substitution is the answer in this case