Name two enzymes directly involved in synthesizing and joining DNA segments on the lagging strand during semi-conservative DNA replication.

When DNA double-strand breaks occur, an enzyme called ATM kinase phosphorylates a histone protein variant called H2AX. This phosphorylation enables H2AX to bind to a mediator protein called MDC1, which recruits DNA repair machinery.

Use your knowledge of protein structure to suggest how the phosphorylation of H2AX enables it to bind to MDC1.

The synthesis of a new DNA strand requires nucleotides to be in their activated triphosphate form (such as dGTP) rather than their monophosphate form (dGMP).

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

Single-stranded DNA damage during replication is recognized by a ring-shaped protein complex called the 9-1-1 complex (comprising Rad9, Rad1, and Hus1). When bound to damaged DNA, the 9-1-1 complex recruits the ATR kinase, which halts the cell cycle at the S-phase checkpoint to allow time for DNA repair before replication continues. A mutation can result in a person producing a non-functional version of the Rad9 protein.

Predict the potential consequences of having a non-functional Rad9 protein on both the cellular level and the whole organism.