# DAT question

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#### stpele01

##### Full Member
5+ Year Member
Hello,

Can someone explain this to me please:

As single strand of dna undergoes 4 rounds of replication. What percentage of the total dna pressent after these replications is comprised of nucleotides from the originial dna molecule?
It says the answer is 6.25%.

Do we start counting from 50 down?

#### nnal

##### Full Member
7+ Year Member
Start with 100% so after 1 round it is-->50-->25-->12.5-->6.25, hope that makes sense.

#### stpele01

##### Full Member
5+ Year Member
Start with 100% so after 1 round it is-->50-->25-->12.5-->6.25, hope that makes sense.
I thought that since we're starting with a single-stranded DNA, we have half the amount of DNA so we start from 50%. Can you explain why this is wrong?

#### aznnytezx

##### Membership Revoked
Removed
It has been a while since I have taken genetics. Also I haven't even done any DAT studying for biology yet, but I will try my best. So you have one strand of DNA being replicated meaning it'll make a copy. So you start off with ONLY ONE. Then you replicate it for first round which will result in TWO DNA Strand, second round will result in FOUR, third replication will be 4 x 2 = EIGHT, and fourth replication will be 8 x 2 = SIXTEEN. you have a total of 16 dna strand now. There is only ONE original, the rests are replicated. So 16 total, 1 original? 1/16 = .0625 x 100% = 6.25%.

#### aznnytezx

##### Membership Revoked
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I may be wrong, so don't take my word for it.

#### aqz

##### Full Member
7+ Year Member
Pretty sure this is an english thing. No offense if your first language is actually english lol.

A single strand of DNA does NOT equal single-stranded DNA. DNA is always a double helix with each strand running anti-parallel to the other.

In this case, you're starting out with 2 strands of the same DNA...so after one round:

II = II II

Two rounds, then three, then 4:

II II = II II II II

II II II II = II II II II II II II II

II II II II II II II II = II II II II II II II II II II II II II II II II (16 double helices, containing 32 strands total)

Since you started out with 2 strands, 2/32 = 1/16 = 6.25

#### nnal

##### Full Member
7+ Year Member
That's how I thought of it as well when I first started studying, lol. We don't start with 50% because that would mean we have already gone through a round of replication. You START with the original strand of DNA (unless they say otherwise), therefore, you have a 100%.

#### nnal

##### Full Member
7+ Year Member
Pretty sure this is an english thing. No offense if your first language is actually english lol.

A single strand of DNA does NOT equal single-stranded DNA. DNA is always a double helix with each strand running anti-parallel to the other.

In this case, you're starting out with 2 strands of the same DNA...so after one round:

II = II II

Two rounds, then three, then 4:

II II = II II II II

II II II II = II II II II II II II II

II II II II II II II II = II II II II II II II II II II II II II II II II (16 double helices, containing 32 strands total)

Since you started out with 2 strands, 2/32 = 1/16 = 6.25

I thought viruses could possibly have ssDNA?

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#### stpele01

##### Full Member
5+ Year Member
I thought viruses could possibly have ssDNA?
We could do it this way as well. I feel like the question isnt written correctly. Is it just me? Regardless if its double stranded or single strand, can we still doing it this way?

I = II

Two rounds, then three, then 4:

II = II II

II II = II II II II

II II II II = II II II II II II II II (8 double helices, containing 16 strands total)

Since you started out with 1 strand, 1/16 = 6.25

#### aqz

##### Full Member
7+ Year Member
Yes it really doesn't matter.

All that matters is that you understand how it works. This usually comes up in PCR (polymerase chain reaction) which is a lab technique to amplify the number of DNA.

#### two thirty

##### Full Member
5+ Year Member
7+ Year Member
Aqz - you're good to go. Can't wait to read your DAT breakdown.
Seriously. No pressure, but I'm expecting 24+'s across the board. He seems like he knows everything front to back.

#### aqz

##### Full Member
7+ Year Member
I promise you that I will never get 24s across the board.

I don't even know everything left to right. Just seems like it because I answered a lot of people's questions in the past couple weeks by searching wikipedia and google....

Alright off to sleep.

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