A2 Biology OCR June 2015 Revision Thread

For bases what do RNA Contain? Is that just AUG? Wb MRNA Aswell?

mRNA and tRNA have the bases A, U, C, G

Posted from TSR Mobile

On 2014 paper it said which are the common nuetiode bases in DNA and mRNA and didn't mention uracil??

Can't find it! Page 176 is about genetic engineering and bacteria? You must have a different book, it's not even on our specification is it?

There is no Uracil in DNA ! During transcription Thymine is swapped for uracil to make mRNA

Credit goes to some other guy who posted this before

I just downloaded it and saved it fam
Posted from TSR Mobile

Can someone please list all the things we need for answering:

"outline the steps involved in sequencing the genome of an organism"


Posted from TSR Mobile

Can any 1 help me out for the june 2014 hardy Weinberg Eq it says dominant has brown colour and 45/60 in population are brown and says work out dominant frequency allele so why do you do 15/60 instead of 45/60???

Heyy! Sorry if this is in the wrong place :3
Has anyone got some good resources on the lac operon? Or can someone explain it to me in really easy terms?
Also, for homeobox sequences do we need to know about a specific organism because it's HSW?
Thank you
xx

I know people have probably already asked this so sorry in advance but how much detail do we need to know for sequencing a genome? My school notes are totally different to the OCR text book (they don't mention BAC and mention primers and nucleotides instead) is this as there's several methods to sequence a genome or this one in my notes really simplified?

Shear the genome sample into 100,000bp sections. Cut open BACs with restriction endonuclease and insert the sections, then insert the BACs into different E.coli (sticky ends & DNA ligase used). Grow these bacteria in nutrients so they'll divide & replicate DNA to produce a clone library.

Identify the E.coli containing a BAC with the section you need to sequence, and cut it out with restriction endonuclease. Amplify it with PCR and use many different restriction endonucleases to create overlapping fragments (since each restriction enzyme has a diff restriction site). Overlapping fragments must not exceed 750bp; this is the max length that can be sequenced at once to ensure accuracy and manageability.

Order the fragments by size using gel electrophoresis: place two electrodes in agarose gel and put the fragments in wells cut at the end nearest the negative electrode. Immersion in buffer solution neutralises pH and also provides free electrons to carry the current. DNA's phosphate group is negatively charged so they're attracted to & will diffuse towards the positive electrode. Shorter fragments move fastest and therefore further than longer ones, and their positions can be revealed using a dye that stains DNA & the southern blotting technique.

Perform PCR again but this time include modified DNA nucleotides in addition to normal ones. These will have a different colour for each base and don't bind to further nucleotides, meaning they throw off DNA/Taq polymerase. This is called chain termination and eventually it'll produce fragments of all lengths each ending in a modified nucleotide. Run them through gel electrophoresis to order them by size: the base sequence can now be read by observing the colours and correlating them to the correct base. It's read starting at the positive end to ensure the bases are read in the correct order, and remember it's the complementary sequence that you're reading because these modified bases were added in PCR, not part of the original molecule. (Nowadays this is read using a computer.)

Use the sequences of overlapping fragments to construct the sequence of the 100,000bp section, and use microsatellites (repetitive sequences 3 to 4bps in length, found at specific chromosome loci) as reference points to see where & on which chromosome this section was sampled from to assemble the sequence of the whole genome.


Posted from TSR Mobile

I shall tell you a tale about the lac operon...

So E coli needs to respire, right? Sometimes glucose isn't available for this, so they respire lactose instead.

In order to respire lactose, certain enzymes are needed:
Lactose permease - transports lactose into the cell
Beta galactosidase - catalyses hydrolysis of lactose to glucose and galactose

Now on the lac operon, you've got 5 regions in total, and they look a lil' like this:

I --------P-O-Z-Y

I = represents the regulatory gene. Technically it's not part of the operon but its codes for the repressor protein which I'll mention in a bit.

P - Promoter region. An enzyme called RNA polymerase binds to this region so it can move along the operon and transcribe the structural genes Z and Y, which I'll get to in a bit.

O - Operator region. Can switch the operon on and off. How, I hear you ask? Well, you know that repressor protein I mentioned earlier? It binds to this operator region, and as the operator region is right next to the promoter region, it covers part of the promoter region too, so that RNA polymermase cannot attach to it since the repressor is in the way.

Z and Y - these are the structural genes those code for Lactose permease and beta galactosidase. Y = lactose permease, Z = beta galactosidase.

Now when lactose isn't present, bacteria cannot respire it. That means those structural genes cannot be transcribed. Here's how:
With no lactose being present, the regulator gene is expressed to synthesise the repressor protein. This protein binds to the operator region, and prevents RNA polymerase from binding to the promoter region, so those structural genes I was talking about can't be synthesised. Sad times!

Buuuuuut when you've got lactose...
Lactose binds to the repressor protein, causing it to change shape so it can no longer bind to the operator region. This means there's space for RNA polyermase to bind to the promoter region, YAY! RNA polymerase can move along the operon and transcribe those structural genes to form lactose permease and beta galactosidase! Good times

How's that?