Length of Human DNA.

Bob Bruner bbruner at uclink4.berkeley.edu
Sun Nov 23 21:32:23 EST 1997


On Sun, 23 Nov 1997 21:05:41 -0500, Binesh Bannerjee
<binesh at hex21.com> wrote:
>
>Anyhow, I have two questions (just out of curiosity...)
>
>How many nucleotides are in human DNA?

About 3 billion base pairs.

>And, is the number of nucleotides in human DNA constant across humans
>(let's say barring gender differences)...

Yes. (other than the inevitable minor differences which would occur
due to mutations)

>Is that number close? 6 billion? Or are there much more or much less?

Well, 3 billion base pairs is 6 billion bases. But the "length" is 3
billion.


>I read in a book that there were 200,000 genes in human DNA, and it said
>1000
>nucleotides per gene, which has to be an approximation, right? 

Current estimates are more like 50 to 100,000 genes. But this will be
only an estimate until we learn the complete genome sequence -- in 10
years or so.

Yes, the number of nucleotides per gene is an approximation, based on
some view of the average gene size. And of course, that ignores
non-coding sequences, such as the introns that you mention below.

>Since 3
>nucleotides
>gives you an amino acid, wouldn't the number of nucleotides in a gene
>have to be evenly divisible by 3?

Yes, again referring only to coding regions.


>But by that calculation, 200 million base pairs approximately in a human
>DNA?

You mean 200,000 genes times 1000 nucleotides per gene seems _much_
less than the genome size??? Yes, indeed. A major portion of the
genome is non-coding. This includes the introns, as well as regions
between genes. Some of this serves regulatory purposes, but some
undoubtedly has no real purpose.

For a slow growing organism, such as humans, there is little cost to
maintain unneeded DNA.

>
>How do we know that introns are non functional? 

The intron RNA generally seems to be degraded.

However, this is a generality. In some cases, the intron RNA does have
some special function. Further, the presence of the intron within the
gene may affect gene function. So we should not generalize that
introns are inert.

>Introns exist within a
>gene, right?Not junk outside of start and stop codons, right?

correct.


>Any answers appreciated...
>Including, well, if you want to know, go take a molecular bio course!

Sure, why not!  You do sound rather knowledgeable and interested.  We
do get programmers in Mol Biol courses -- and we need  them. Mol Biol
is quite dependent on computers. But this is also a good place for
questions.

bob



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