Name two enzymes involved in joining nucleotides or DNA fragments during DNA replication and repair.

Sometimes, DNA damage activates a signaling pathway. One enzyme involved in this response is called Chk2. Chk2 works as follows:

• Chk2 phosphorylates a protein called p53.
• When Chk2 phosphorylates p53, p53 becomes able to bind to specific promoter regions on DNA to initiate repair gene expression.

Use your knowledge of protein structure to suggest why phosphorylation of p53 allows it to bind to DNA.

During DNA repair, new nucleotides must be joined together. This reaction requires the nucleotides to be in a triphosphate (phosphorylated) form (like dATP) rather than a monophosphate form.

Use your knowledge of energy changes in chemical reactions to suggest why the nucleotides must be phosphorylated.

Sometimes, double-strand breaks in DNA are detected by a protein complex called MRN. MRN binds to the broken DNA ends and recruits a kinase called ATR, which ultimately stops the cell cycle at the $G_2/M$ checkpoint to allow time for repair. A mutation could result in a person having non-functional forms of the gene that codes for one of the MRN proteins.

What can you predict about the possible effects of having a non-functional form of the MRN complex?