The big idea: DNA replication copies one DNA molecule to make two. Both have exactly the same base sequence as the original.
The points to remember
- Replication turns one DNA molecule into two molecules.
- Both new molecules have the same base sequence as the original: they are exact copies.
- The base sequence carries the genetic information, so both copies carry the same genes.
- Replication happens before a cell divides, so each new cell can get a complete copy.
Remember it as: One molecule in, two out, the same sequence in both.
Real example: E. coli. In the best conditions the gut bacterium E. coli divides about every 20 minutes. Before each division it copies its whole chromosome, about 4.6 million base pairs, so both new cells carry the same genes.
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Why copy at all?: Every time a cell divides, each new cell needs a complete, exact copy of the DNA. Cells divide for three reasons.
Reproduction
- A bacterium copies its DNA, then splits into two new organisms.
- Cells that make sex cells copy their DNA first too.
Growth
- A fertilized egg divides again and again.
- A human body has trillions of cells, all from one.
Tissue replacement
- Worn-out or damaged cells are replaced.
- Skin, gut lining and blood are renewed all the time.
Real example: a grazed knee. When you graze your knee, skin cells at the edge of the wound divide to fill the gap. Each new cell gets an exact copy of the DNA, so it makes the same proteins as the skin cells it replaces.
Do not mix them up: Replication copies the DNA. Cell division comes after it and shares the two copies between the two new cells.
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How it is asked: A short 'outline why DNA is replicated' or 'state what is the same about the copies', often as the first part of a longer replication question.
Outline why DNA replication is needed in a multicellular organism such as a human.
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