# Since the mRNA codons correspond to DNA codons and tRNA codons correspond to mRNA codons, is there any difference between a DNA sequence and a tRNA sequence other than the substitution of Thymine with Uracil?

## Mostly, I am confused about the translation from mRNA to tRNA and want confirmation that mRNA is complementary to DNA. For example, is this correct?: ATC GTA (DNA) $\rightarrow$ UAG CAU (mRNA) $\rightarrow$ AUG GUA (tRNA)

May 9, 2018

I'll try to work you through it below - it'll be kinda long.

#### Explanation:

The entire "DNA gets turned into mRNA" is a little bit more complicated because we have to consider the 5$to 3$ direction of DNA.

DNA has a top strand that runs 5$-3$..and a complementary bottom strand that also runs 5'-3', but it runs in the opposite direction (like it got flipped around), so it is oriented in its 3$-5$ direction.

5$-ATGCGTAGT-3$: This is the top strand

The complementary bottom strand is:
3$-TACGCATCA-5$

So we see the double strand as:
5$-ATGCGTAGT-3$
3$-TACGCATCA-5$

Ok, so that's cool. Now, the reason we are talking about this is because the mRNA is transcribed using the bottom strand as a template!! So, the above piece of DNA would result in an mRNA (lowercase) sequence that looks something like this:
5$-aug cgu agu -3$
3$-TACGCATCA-5$

The mRNA sequence is: 5$-aug cgu agu -3$
If we look at the top strand DNA sequence, we see it is: 5$-ATGCGTAGT-3$....so the mRNA that gets made is the SAME sequence at the top strand!!(U instead of T) This is because the mRNA is made as a compliment of the bottom strand, and the top strand is the compliment to the bottom strand.

Now lets look at the tRNA. tRNA molecules are individual molecules of mRNA. They have really 2 important parts (don't tell an RNA researcher I said this! - i did my phd in RNA structure and they'd take it back if they read that)....the anticodon loop and the 3$hydroxyl. An AMINO ACID gets put on the 3$ hydroxyl group by an enzyme, and this becomes a CHARGED tRNA.

Now off to the ribosome and those codons on the mRNA. Ribosome threads the mRNA through it and it has these spaces that allow tRNA to fit in. The Space will stop over the top of a 3 nucleotide portion of the mRNA - this is the codon. The mRNA will have the sequence:
5$-aug cgu agu -3$, and each of those 3 nucleotides are codons. The anticodon loop of the tRNA has a 3 nucleotide section that can basepair with the condons on the mRNA.

3$uac5' - this is the anticodon of the tRNA 5$-aug cgu agu -3$When the base pairing occurs, the ribosome plucks off the amino acid that is on the tRNA and moves onto the next codon. 3$gca5$5$-aug cgu agu -3`...and the next tRNA binds and transfers its amino acid onto the previous amino acid.

On and on this goes and the amino acid chain gets longer and longer as each successive tRNA transfers its amino acid onto the growing chain.