Which substances, that are necessary in regulating the cell cycle, are important targets for designing drugs to treat many types of cancers?

The correct option is A Oncogenes

A. Oncogenes are the genes that can transform a cell into a tumour cell. Tumours develop due to the presence of oncogenes in the body. All nucleated cells have oncogenes. Under normal conditions, these genes probably code for proteins that are necessary for the cell's growth. Oncogenes that function normally are called proto-oncogenes. Mutation in proto-oncogenes changes their activity. The cell which has these mutated genes looses the control on growth and division. It continuously divides and gives rise to a mass of cells called tumour.

B. Histones are positively charged low molecular weight proteins rich in basic amino acids lysine and arginine. They are associated with packaging of DNA in eukaryotes. There are five types of histone proteins: H1, H2A, H2B, H3 and H4.

C. Lysosomes are the single membrane bound structures that contain hydrolytic enzymes in them and due to the presence of hydrolytic enzymes, they are called the suicidal bags of the cell.

D. CDK's or Cyclin Dependent Kinases are a type of protein kinases that play a important role in regulating cell cycle. CDKs phosphorylate amino acids like serine and threonine which initiate or block activities related to cell cycle.

E. Peroxisomes are small round and oval bodies found in plants only. They are a type of microbodies and are the site of photorespiration.

Hence it's the oncogenes that are necessary in regulating cell cycle and are important targets for designing drugs to treat many type of cancers.

So, the correct answer is 'Oncogenes'.