Chromosomes, DNA and genes
You have been told two things in two different chapters, and never told how they fit together. This page is that join.
The two claims
Look back at what you already know:
- In the chapter on cell division you learnt that chromosomes in the nucleus carry the genetic material from one generation to the next.
- In the chapter on the chemical basis of life you learnt that DNA stores and transmits genetic information, and that its four bases are adenine, thymine, cytosine and guanine.
Both are true, because they are statements about the same object at different scales. A chromosome is made of DNA.
One thing inside another
Chromosome
A thread in the nucleus, visible under a microscope when the cell is dividing. Humans have 46 of them in each body cell.
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DNA
The chromosome is a single very long DNA molecule, coiled and wound tightly around proteins so it fits inside the nucleus.
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The sequence of bases
Along the DNA lie the four bases in a particular order. That order is the information.
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Gene
A gene is a length of DNA — a stretch of that base sequence — which controls one characteristic. Mendel's factor, made of chemistry.
Figure to be added
Which base pairs with which
DNA has two strands, and they are held together by the bases reaching across to each other. The pairing is not free: each base has exactly one partner.
Adenine with thymine
Wherever one strand carries A, the other carries T at that position.
Cytosine with guanine
Wherever one strand carries C, the other carries G at that position.
Why fixed pairing matters
Because each base has only one partner, either strand is enough to rebuild the other. That is how a cell copies its DNA before it divides, and it is why the two daughter cells of mitosis carry identical genetic information — the thing you were told happens, without being told how.
Chromosome, DNA, gene
A chromosome is a long molecule of DNA. A gene is a length of that DNA controlling one characteristic. The bases pair A with T and C with G.