Why does DNA polymerase only build 5' to 3'?

In IB Biology terms, the chemistry of nucleotide addition requires a free 3' hydroxyl group on the growing strand. DNA polymerase catalyzes the bond formation by attaching the next nucleotide to that 3' end, which naturally extends the strand in the 5' to 3' direction. If polymerase tried to build 3' to 5', the energy and reaction setup would not work the same way, and proofreading would become far less reliable. This is why the direction rule is not optional, it is a constraint of molecular chemistry. Once you accept that rule, leading vs lagging strands becomes a design problem the cell has to solve. In IB Biology, naming the direction clearly is often the difference between full marks and partial credit.

What exactly are Okazaki fragments, and what do they prove?

Okazaki fragments are short DNA segments synthesized on the lagging strand because continuous replication is impossible there. Each fragment begins at an RNA primer laid down by primase, then DNA polymerase extends the fragment until it reaches the previous one. They prove that the lagging strand is synthesized discontinuously, even though the overall replication process is continuous at the fork. In IB Biology, you should connect Okazaki fragments to enzyme teamwork: primers start, polymerase extends, enzymes replace RNA, and ligase seals. Students sometimes define fragments but forget ligase, which costs marks. A strong answer treats fragments as steps in a cycle, not a standalone term.

How are RNA primers removed and replaced with DNA?

IB Biology expects you to know the principle: primers are temporary RNA starting points that must be removed so the final molecule is all DNA. In prokaryotes, DNA polymerase I can remove RNA primers with exonuclease activity and replace them with DNA nucleotides. In eukaryotes, enzymes such as RNase H help remove the RNA, and a DNA polymerase fills the gap. Then DNA ligase seals the remaining nick in the sugar-phosphate backbone. The key logic is that polymerase cannot start from nothing, so primers are necessary, but RNA cannot remain in the finished chromosome. When you explain this clearly, you show the examiner you understand process, not just vocabulary.