Half the DNA in each

The idea

Germ-cells, formed by meiosis in specialised organs, have half the chromosomes and DNA of body cells, so their fusion re-establishes the chromosome number and DNA content instead of doubling it.

Doubling problem

Suppose a new individual got a full set of DNA from each of its two parents.

How much DNA would the new individual have, compared with a parent?

• Twice as much
• The same amount
• Half as much

Keep the DNA steady

Press Next step. In this picture, an ordinary body cell has 4 units of DNA. This number is only an example.

Half plus half is whole

Germ-cells are made in specialised organs by a cell division called meiosis. They have half the chromosomes and half the DNA of body cells. When two germ-cells fuse, the full number returns, so it does not double.

Germ-cells, formed by meiosis in specialised organs, have half the chromosomes and DNA of body cells, so their fusion re-establishes the chromosome number and DNA content instead of doubling it.

Notes

Germ-cells (made by meiosis) have half the chromosomes and DNA of body cells, so fusion restores the number instead of doubling it.

Check yourself

Germ-cells are formed by a cell division called …

Compared with body cells, germ-cells have …

In an example, a body cell has 4 units of DNA. How many units does a germ-cell have?

Answer: 2 units

• fragmentation. Fragmentation is a multicellular organism breaking up into pieces.
• meiosis — correct. Yes!
• regeneration. Regeneration is pieces of a cut organism growing into complete organisms.
• twice the chromosomes and DNA. If each germ-cell had a full set, the new generation would have twice the DNA, so germ-cells have only half.
• the same chromosomes and DNA. Germ-cells have only half the number of chromosomes and half the DNA of non-reproductive body cells.
• half the chromosomes and half the DNA — correct. Yes!