Campbell and Reece 8th edition

[JackBean]

it will also be on the inside of the ER membrane, it will also be on the inside of the vesicle and Golgi membrane, but it will be on the outside of the plasma membrane after exocytosis occurs.

Think about how are the membranes oriented

State where signal receptors may be located in target cells

Where would you expect receptors for extracellular molecules?

[Stephen1993]

thank you

About that question I know that the signal receptors would be on the outside of the plasma membrane but I wonder why it would be on the INSIDE of the ER membrane, vesicle and Golgi body- I mean how would the signal reach the receptor if they are on the INSIDE???
Or are the ER membrane, vesicle and Golgi body something to do with exocytosis???

thank you

[JackBean]

yes, they are the part of protein transport machinery.

And BTW, the receptors do not have to be on the outside of the cell

[Stephen1993]

thank you

quickly :
"And BTW, the receptors do not have to be on the outside of the cell"
don't you mean organelles instead of cell?

ok this is from pg 80 of the 8th edition biology book
"Sanger and his co-workers were able, after years of effort, to reconstruct the complete amino acid sequence of insulin. Since then, most of the steps involved in sequencing a polypeptide have been automated."
what does the last sentence mean?

thank you

[JackBean]

no, I meant on the outside of cells. Even you wrote that. You need to understand, which side is which on plasma membrane and organelle membrane

The last sentence means, that we have machines, which are able to sequence proteins by themselves. You just add the sample (protein), solutions and turn it on

[Stephen1993]

ok thank you

i want to clear up some things

So a gene is a length of sequence of the DNA molecule that codes for a particular trait.
DNA consists of a double helix which has a one strand and a complementary strand.
so a gene between the sequence of DNA strand as well as its complementary strand?

because in protein synthesis if DNA template strand number 1 is transcribed to mRNA then translated to a polypeptide number 1 from DNA template strand number 1
but what if you choose the other DNA complementary strand so the DNA template number 2 then transcribed to mRNA then translated to a polypeptide then wouldn't this polypeptide different to the polypeptide number 1???

if they are different then how the definition of the gene works?

and also:
And you know we have alleles for a gene
If one allele is dominant to the other then how does it know which chromosome out of the homologous chromosome to transcribe and translate to a particular polypeptide?? because the dominant allele is express
what about the recessive allele? is it redundant i.e. not transcribed and translated to form a polypeptide?

thank you

[JackBean]

So a gene is a part of the DNA molecule that codes for a particular protein or RNA. (trait can be codšed by several genes).

Sure, DNA has two strand and both of them could and can be template for transcription. The promoter determines, where (AKA on which strand) will bind DNA Pol and what way will it go. And of course, if one piece of DNA was transcribed from both strand, different polypeptide would form.

Regarding the dominancy, that's not that simple question. There can be several reasons for dominancy/recesivity. Either the gene can be silenced or mutated, so that the protein is not functional anymore. Or it is only partly functional (e.g. if you mutated one AA in the active side, so that the substrate would bind less efficiently)

[Stephen1993]

thank you

so for a DNA molecule you got two strands
e.g. one strand might contain
AGGTCGTA

then the other complementary strand must be

TCCAGCAT

so if you transcribe and then translate each of them separately then the first strand will give a different polypeptide to the one made from the second strand

so polypeptide one may code for hair colour and the other different polypeptide may code for skin colour or something like that

is that right or is there something wrong because i though a gene is a sequence of DNA so that include both strands of the DNA sequence?? but both strands gives different polypeptides and so how does the definition of a gene works????????????????????????????

a bit confused

and about dominance and recessiveness of allele
were you telling me before that a recessive allele will give a non-functional protein and the dominant allele will give a functional protein and so is that why the dominant is expressed?

if an allele gives a partially functional protein just like what you have said before then do we have a situation of codominance and incomplete dominance???

thank you again

[Stephen1993]

hello
i am wondering if there is anyone to help?

thank you
Stephen

[canalon]

No, in general*, genes are not overlapping. Both strands can be transcribed because they carry genes, but for a given sequence only one of the strand will be transcribed. The other strand is there only to allow the replication of the DNA.
One of the obvious clue that it does not work like that is that the start codon is not just a stop codon in reverse. And conversely that stop codons in reverse code for perfectly legitimate aa



*In prokaryotes and viruses some genes can overlap on part of the sequence. But this is more an exception than the rule.

[Stephen1993]

thank you for that

Pg 81 of the textbook
Thus, the function of a protein- for instance, the ability of a receptor protein to bind to a particular pain-relieving signalling molecule- is an emergent property resulting from exquisite molecular order.
What does the last bit of the sentence mean especially ‘exquisite’.

thank you

[JackBean]

I would guess that the authors wanted to explicit, how beauty and complicated this order is. But I'm not native speaker