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Transcription and Translation

Hi. My teacher has just taught our class about transcription but I don't get all of it. We are to do a flowchart on transcription and here's mine.

DNA double helix unwinds. (Don't know what makes it unwind....)

Enzyme DNA helicase separates the strands by breaking H bonds between nucleotides.

Enzyme RNA polymerase attaches to promoter in DNA strand and attaches complementary bases of RNA nucleotides (AUCG), forming mRNA.

mRNA (single stranded) peels away from DNA strand.

DNA molecule rewinds.

Once gene is transcribed, mRNA leaves nucleus through nucleus pores.

mRNA attaches to ribosome for translation to polypeptide.

Can anyone correct me if I'm wrong or add in additional information that I left out? Thanks.

Not sure why it's called DNA transcription because the mRNA is simply a complementary to the template strand that it was made from....

Also, can someone give me a headstart on how translation works? Thanks.

Reply 1

GregoryJL

DNA double helix unwinds. (Don't know what makes it unwind....)

I]Enzyme DNA helicase separates the strands by breaking H bonds between nucleotides.

You are getting confused between DNA replication and transcription. In the later case RNA pol will unwind the strands and hold them apart as it uses the template - if the diagram in my textbook is accurate, they reform and link up again within the RNA pol itself. DNA replication does use Helicase (as well as SSRPs and topoisomerase to hold the seperated strands apart and help fix overwinding of the helix, respectively) but they are not involved in transcription.

Enzyme RNA polymerase attaches to promoter in DNA strand and attaches complementary bases of RNA nucleotides (AUCG), forming mRNA.

True, but you've missed out quite a bit about gene regulation (which is entirely understandable) - as an example, prokaryotes often use operons which bind to operator regions just downstream of the promoter to which various things can bind to and block translation. Eukaryotic gene regulation is more complicated than this, often using proteins linking to regions of DNA potentially thousands of bases from the promoter and interacting with various transcription factors to bind onto the RNA polymerase to allow it to transcribe (if my memory serves.) You can mention more common stuff like the TATA box and other consensus sequences in the promoter for Polymerase to bind to if this stuff turns you on. :wink:

Might be worth noting that RNA pol (there are 3 ish in eukaryotes, synthesising different 'sorts' of RNA - as we are talking about coding genes, it will be Pol II) links to the coding template and reads it from 3'-->5', giving a mRNA transcript from 5' --> 3' which will obviously be the same as the non coding strand of DNA (cept replacing T's with U's.)

As a final note, it should be noted that this process has pretty big energetic and kinetic barriers - I am extrapolating from the mechanism of DNA replication, but I recall hearing that the method of adding nucleotides involves the use of triphosophate nucleotides (ATP, CTP, UTP, GTP) where the last two phosphates are cleaved off to release pyrophosphate to provide the necessary energy to drive this endergonic (endothermic, although they strictly mean different things) process forward. This is speculative, so it would be best to look it up rather than relying on the word of some stranger on the internet.

mRNA (single stranded) peels away from DNA strand.

Yes, I think so - it might be that Pol II acts to peel the RNA away from the template and reform the bonds between the two strands of DNA within the complex itself.

Once gene is transcribed, mRNA leaves nucleus through nucleus pores.

mRNA attaches to ribosome for translation to polypeptide.

If only things were simple!

The initial transcript of Pol II isn't strictly RNA - it is pre-RNA. It is modified in the Nucleus (post transcriptional modification or RNA processing) to give the finished product which is mRNA. In short, there is a poly -A tail added to the 3' end of the molecule (simply, lots of A nucleotides get added on, mostly as fodder for 3' exonucleases, I think.) Additionally, a 5' cap is added (a GTP has a strange bond to the first nucleotide to make the 5' cap look like a 3' cap, to resist attack from 5' exonucleases the cell employs to attack foreign genetic material. I think the bond is a 5'-5' phosphodiester bond (as opposed to the normal 5' - 3' bonds usually used in the sugar-phosphate backbone) but don't quote me on that. The cap is particularly important, as I believe it is used in the mechanism to expell the mRNA through the pores (GTP is also used in the mechanism, if memory serves.)

Oh, and I haven't even begun to note one hugely important piece of RNA processing - splicing. That could take up plenty of paragraphs, and I'm running low on time.

On a related note, I can't give you much on translation - the smaller ribosomal subunit binds with a few other proteins to attach to the start codon - the big subunit attaches onto this. The mechanism of translation involves the addition of amino-acyl tRNAs (a special RNA molecule specific to a given amino acid and covalently bonded to it with the aid of an ATP --> AMP hydrolysis.) This enters into the A site (as it happens, the first amino acid is always methionine in eukaryotes, as this has the 'right' tRNA for the start codon - it often gets cleaved off later on as a post translational modification. There is then a cycle of bond formation, translocation along the strand (so our tRNA is now in the P site - A stands for amino acyl, P for peptidyl. A string of peptides gets formed and is attached to the tRNA in the p site. Another 'unit' along, the tRNA breaks off from the growing peptide strand and leaves via the E site (e for exit.) Books will have diagrams understanding how this works better.

This is, I imagine, slightly over the top for what your examinations will require of you - feel free to dumb down as appropriate. :wink:

Reply 2

Catchetat

Sorry the language that you used is a bit too complicated. I am keen to learn more but I think you just need to correct my statements whether or not they are right or wrong and add in important points that I didn't add in. Thanks for the VERY detailed reply though...

Reply 3

GregoryJL

.. Okay then :biggrin:

1. True - the protein complex that does the transcription (RNA Pol II) is what unwinds the DNA.

2. False - DNA helicase (and other proteins like that) are only used in DNA replication - in transcription it is all about RNA Polymerase (well, and transcription factors et cetera.)

3. True-ish - note the direction of travel of RNA pol down the template strand, as well as noting you aren't actually making mRNA - you are making pre-mRNA, its precursor.

4 and 5. About right - although again, this isn't mRNA, but its precursor, additionally, this unwinding-transcribing-peeling-rewinding process is performed by RNA polymerase (and other stuff, if I recall.)

6. False - after transcription other things need to be done to the mRNA transcript (5' cap, poly A tail, and possibly splicing) before the RNA is pushed through the nuclear pores. See prior spiel about the various bits of RNA processing.

7. True

Translation needs diagrams - it gets complicated to explain in a body of text (no, really? :wink:

)

Your description is sufficient for Higher/A-level standard, I would think - it is strictly missing out certain things, but I guess science works by progressive oversimplification. I have no idea why it was called transcription - I suppose it suggests that the information of the sequence is just being restated by another nucleotide chain with a few cosmetic differences (U's as opposed to T's, normal ribose so a hydroxyl on the 2' position, and so on.) Unfortunately there is a tip of the iceberg thing going on here - like the whole edifice of gene regulation which occurs pretty much throughout the central dogma of cell biology (Central dogma: DNA --> RNA --> Protein.) It wasn't covered in my A-level, so I don't know if you'll ever have to bother with it at school - it is pretty cool though, so it is worth reading about.

You pick up the technical verbiage as you go along, by the way - I've only been at university for four weeks! :wink:

Reply 4

Revenged

basically...

- transcription factors bind to promotor regions (e.g. TATA box) of DNA

- RNA polymerase activates

- causes causes hnRNA to be synthesised from DNA

- hnRNA has 'introns' removed to become mRNA...

- guanine cap added

- poly-A tail added

- mRNA leaves nucleus and goes to ribosome

I doubt you'd need to know about control of gene transcription for A-level... but there are 'enhancer' regions that increase protein synthesis and 'silencer' regions that decrease it...

Reply 5

Wangers

OP, you learn in different levels, its like in chemistry you start with a strict Bohr model and progress to Heisenberg uncertainty etc. If you want to look into the topic further (which is very interesting) then I've found Alberts Molecular Biology of the Cell a brilliant read. (Don't buy it though - I think theres a new edition out in Jan)

Reply 6

Excalibur

At A-level/HL, I think what you have is basically sufficient - this is what I learnt about transcription for AS:

1. DNA double helix unwinds and H bonds break by RNA polymerase.
2. RNA polymerase attaches to a promoter region and begins transcription by attaching complementary bases of RNA nucleotides forming pre-mRNA.
3. mRNA is spliced as its introns are cut out, making a 'mature' mRNA.
4. mRNA leaves the nuclear pores.

And translation:

1. mRNA attaches to the ribosomes (made from rRNA and protein, has two subunits), either free in the cytoplasm or on the rough ER.
2. tRNA molecules (clover-shaped RNA that forms H-bonds in complementary areas and fold back in on itself; has an area with 'anticodon' 3-nucleotide bases; covalently bonds to specific amino acids) ferry amino acids around the cytoplasm.
3. As the mRNA binds to the ribosome, its first three nucleotides is read as a group (a codon). The tRNA with the complementary three nucleotide (the complementary anticodon) then comes along and H bond with the codon. This carries on down the mRNA, so that successive tRNA molecules bind to the mRNA.
4. The amino acids that had been carried to the ribosome by the tRNA are now side-by-side, so they then form peptide bonds to form a polypeptide chain.

You need diagrams for this!

Probably a gross oversimplification, but that is what is broadly required at this level I think.

Reply 7

kikilala

for better understanding,i suggest u search for the animation of them.u would find it easier to understand.

Reply 8

Wangers

Excalibur

Probably a gross oversimplification, but that is what is broadly required at this level I think.

It is, for added brownie points look up amino acyl transferases I think they're called that attach the amino acid to the tRNA binding site....

Reply 9

- skyhigh -

Wangers

I found this topic very interesting and I am considering to do something similar in uni. Do you learn this in biomedical science or biochemical science?

Reply 10

Iscariot

Revenged

Worth mentioning that some of those steps only occur in prokaryotes and some only in eukaryotes. Also I was always taught mRNA with introns primary mRNA and then post-splicing mature mRNA. Anyway it's pretty much academic.

Reply 11

Iscariot

- skyhigh

I found this topic very interesting and I am considering to do something similar in uni. Do you learn this in biomedical science or biochemical science?

You definitely do. Over and over and over and over and over again at varying degrees of detail.

Reply 12

Wangers

- skyhigh

I found this topic very interesting and I am considering to do something similar in uni. Do you learn this in biomedical science or biochemical science?