DNA strands.. why are there 2?

[spudpeel]

I have been told that there are 2 strands of DNA in the helix because the complementary strand prevents mutation to the structure and makes it more stable. this makes sense, but my problem is that this suggests that only one strand codes for anything. My teachers said the DNA is read from the 5' end to the 3' end (or vice versa?), and so it cannot read the DNA on the other side, but surely, if the whole molecule is turned through 180 degrees, the complementary bases can be read? And if not, is there a marker which signifies the strand to be read?

They also said that DNA can only be read from a promoter sequence, but surely these can exist in the other strand?

I'd really appreiciate it if someone could answer this. It's probably badly written, so i hope you know what i mean!

[SkepticLance]

DNA is a double helix, but it is a single molecule. Thus it would be more accurate to say it has two half strands.

 

The reason it needs both is for chemical stability. Half a strand has all these reactive sites 'hanging off' it. They will pick up nucleotides, but are also capable of picking up other materials that we could call organic chemical trash.

 

To keep itself complete, and preserve the genetic code, DNA needs both half strands.

[Yggdrasil]

Different genes can be on different strands of DNA. So you are correct when you say that promoters and genes can be on the "other" strand of DNA.

 

For example, when you look at the Homo sapiens X chromosome you can see arrows pointing up and down in the column marked O. The arrows pointing up represent genes on one strand and the arrows pointing down represent arrows on the other strand.

[Dr. Dalek]

DNA is stable for the fact that it has double strands as opposed to RNA which often has only one strand and undergoes far more frequent mutations and is more prone to hydrolysis.

Check out this for more details: http://en.wikipedia.org/wiki/RNA#Comparison_with_DNA

[Dak]

My teachers said the DNA is read from the 5' end to the 3' end (or vice versa?), and so it cannot read the DNA on the other side, but surely, if the whole molecule is turned through 180 degrees, the complementary bases can be read?

 

this is only really true when looking at just one gene.

 

a gene has a promoter region and a coding region. without going into too much biomolecular detail, the promoter region will 'collect' certain protiens and assemble them around the strand in a way that will initiate transcription (ie, the creation of messenger RNA from the DNA coding region).

 

the other strand, being complimentary, will look like this:

 

---promoter----coding region----

---anti-pro-----anti-coding-----

 

Note that, on the lower strand, the 'anti-pro(moter)' and 'anti-coding (region)' will not make sence; also, the lower strand has no active promoter region: so, looking at just one gene, it's true that only one strand gets transcribed*.

 

As you and Yggdrasil said, tho, when looking at the entire genome it's entirely possible to have genes on both strands, like this:

 

....------------------------>

---promoter----coding region------anti-codeing-----anti-pro---

---anti-pro-----anti-coding--------coding region----promoter-----

................................................<------------------------

 

(blue arrow = gene)

 

*actually, this isn't entirely 100% true. sometimes, genes can overlap like this:

 

.....----------------------------------------------->

---promoter-------------------(AntS)------------(stop)---anti-prom

---anti-prom------------------(stop)------------(AntS)---promoter

............................................<------------------------------

 

AntS = the complimentary strand to the stop codon.

 

in this case, there is a region of the double-stranded helix where both strands are transcribed becase they both belong to different genes.

[Bluenoise]

To be entirley fair dak there isn't such thing as an anti-promoter.

A promoter isn't so much a code as it is a topographical region of DNA that interacts with proteins in a sort of "lock and key" mechanism. Both strands have equal importance in it's function overall as proteins interacting with the groove inbetween the strands for the most part.

The idea of identifying a promoter by the sequence writen on the same strand containing the gene in the same direction is really a convention to simplify notation.

[Dak]

Ah, yes, my bad -- cheers.

[KAZU]

is there a marker which signifies the strand to be read?

They also said that DNA can only be read from a promoter sequence' date=' but surely these can exist in the other strand?

[/quote']

 

I believe you just answered your last question. The marker you speak of =promoter or initiation seq.

[Noam123]

Another reason for double stranded DNA is because in this way each strand has a backup strand, so if one strand is damaged, the other holds the information to correct it.

 

Try to run your Hard Drive without a backup and we'll see how far you'll reach...

[futureless]

I believe there are two strands, beacuse this structure gives DNA stability and complementary base pairing means each strand is a mirror image of the other. There are two strands, but only one is used in protein synthesis- the sense strand, which has promoter codons for transcription.

Also if you think about it, this way every time DNA replication occurs the new DNA strand will be the same as the original DNA!