FAQs about DNA

Interesting stuff that you don't need to know.

Q. How many base pairs are there in human DNA?

A. There are about 3 billion base pairs in the human genome. And you have two copies of each gene in every cell. so every nucleus has 6 billion DNA bases pairs.

Q. Do lower life forms have less?

A. Yes. Algae, for instance, have less than a million. Interestingly, some amphibians (like frogs) have more base pairs than humans! Because it is unlikely that more genes are needed to make a frog, there must be a lot of noncoding DNA in frog cells.

Q. How many different genes are there in the human genome?

A. In the summer of 2000, the Human Genome Project finished the sequencing of the 3 billion bases in human DNA. This means we now know the order of the adenine/thymine/guanine/cytosine bases in human DNA. Nevertheless, the exact number and function of the genes represented may not be completely known for some time. But it seems like every week a new gene is localized (at a given spot on a certain chromosome) and its function determined. Estimates today are that there may be about 25,000 functional genes on the 23 human chromosomes. Every cell of each person has two copies of each gene (one from their mother, one from their father) that appear in the diploid set of 46 chromosomes.

Q. Are all of our 25,000 human genes essential for life?

A. No. In other organisms only 60% of the genes (or less) are essential. In human cells, many genes are never or seldom expressed (induced) and so their value in health is dubious. Interestingly, the 23,000 genes account for only 3% of the DNA in every cell. For this and other reasons, random DNA mutations do not always produce devastating effects.

Q. What is the typical length of a human gene?

A. The smallest genes are 10-20,000 bases long. The largest gene known (coding for a muscle protein) is 100-times longer at 2 million bases! Remember though, most genes are composed of many noncoding regions (introns).

Q. I know a nucleotide is a small molecule and a base even smaller. Still, 3 billion of these strung out would seem to create a pretty long helix. How long would all the DNA in a human cell measure?

A. If all the DNA in a human cell nucleus were laid out it would measure about six feet in length. If the DNA in all of our cells were laid end-to-end, it could be stretch between the earth and the sun 200 times!

Q. Why does DNA have so much "garbage" (noncoding regions) that have to be spliced out in the mRNA?

A. Introns (noncoding regions) may have always been an integral part of the most primitive of genes from ancestral single-cell organisms that first formed on earth. Cutting out introns allows splicing of exons in different ways to produce different versions of a protein with just one "gene". Having a single gene code for several variants of a protein is an efficient use of DNA and may have hastened the evolution of new and potentially useful proteins. Why these introns remain and have become more numerous in higher organisms today is not known.

Q What's the difference between a DNA mutation and a noncoding ("garbage") region in DNA?

A. A mutation is any alteration of the native DNA base sequence caused by physical, chemical or biological means. Mutations may disrupt normal transcription of a gene and hence produce a different, sometimes faulty protein. Noncoding regions of DNA are not thought to be the result of mutation and have no direct effect on transcription or protein synthesis. There are, however, other noncoding regions in the DNA called promoter regions which control the timing of gene transcription. These appear to make up the bulk of DNA.

Q. Do mutations that might occur in introns (noncoding regions of DNA) have any impact on the individual or offspring?

A. No. These regions are subject to mutation but because they are largely uninvolved in protein synthesis, their alteration has no known effect on health.

Q. Why does RNA contain uracil instead of thymine?

A. Until recently, this was not known. Both bases pair only with adenine, so who cares? Because uracil is an easier molecule for cells to make than thymine, the real question is why doesn't DNA use uracil too? It turns out that the cytosine in DNA often spontaneously converts to uracil. Luckily there is a DNA repair enzyme (uracil-DNA glycoylase) that routinely checks DNA for such conversions and changes these uracils back to cytosine. If uracil were a normal component of DNA, this enzyme would not be able to tell native uracil from converted cytosine and DNA repair would suffer.