Original post by AortaStudyMoreHey, good choice of university subject :P I can't remember quite how much detail you need at AS regarding these things, because I've been taught a lot about protein synthesis and immunology (nothing about fish though haha), so I'll go over what I think should be adequate for AS.
So, protein synthesis, let's start with DNA. DNA is made of 2 strands, one of them is called the sense strand and one of them is called the antisense strand. The sense strand contains the same base sequence as the mRNA that is transcribed, however, all the T's are replaced with U's. Now, as we know, bases bond with complementary bases, i.e. A pairs with T or U and G pairs with C. This means that the mRNA must be formed using the antisense strand if it is going to get the same base sequence as the sense strand. Let me show this visually
DNA
TCGATATTC (Antisense strand)
AGCTATAAG (Sense strand)
Transcription
TCGATATTC (Antisense strand)
AGAUAUAAG (mRNA)
What we can see here is that the sequence of bases in mRNA = the sequence of bases in the sense strand (with T's replaced with U's). We can also see that the mRNA has 3 codons, AGA, UAU and AAG. Codons technically only refer to the triplets on mRNA rather than the triplets on the sense DNA strand.
Anyway, that's the process of transcription, which is carried out by RNA polymerase II. It is worth noting that the mRNA formed is not translated directly, it needs to undergo splicing. This is because a gene on DNA is made up of exons (which code for protein) and introns (which don't code for protein). These introns must be cut out or else you won't get a functioning protein. If you're interested in learning some more details stuff, then here are some extra cheeky facts, introns are cut out by snRNPs, which stands for small nuclear ribonucleoproteins, and also, before mRNA can be translated, it must have a couple of things added to it, these include a 7-methylguanlyate cap and a poly-adenosine tail. Anyway, you don't need to know that, I thought you might just be interested.
Right, so the spliced mRNA enters the cytoplasm, and comes across a ribosome, the ribosome slides down the mRNA until it comes in contact with AUG, the start codon. This is where tRNA gets involved, tRNA is a clover-shaped RNA molecule, on one end it has an anticodon, which contains the complementary bases to the codon, and on the other end is an amino acid. In the case of AUG, the anticodon would be UAC and the amino acid would be methionine. Once the methionine tRNA is bound, protein translation begins, so what happens is another tRNA molecule carrying an amino acid comes a long and binds to the next codon after AUG, peptidyl transferase then forms a peptide bond between the 2 amino acids, once it has done this, the first tRNA molecule leaves, and the ribosome slides down, once it has done this, another tRNA molecule comes a long with a amino acid, and the process repeats until you come across a stop codon. Once a stop codon is reached, the ribosomes dissociate and the protein goes off to wherever it needs to go.
Phew, I'll try not to write as much for the next 2 answers...
Okay so I could literally write an essay on T cells and B cells and their interaction, but I'll try and keep it short. Basically, you have an antigen in tissue, this can either be on a host cell or on a bacteria. What happens is phagocytes endocytose the antigen and present it on their surface. A T-cell with a specific complementary receptor then comes a long and binds to the antigen, thus activating it into a helper T cell. The helper T cell will then look for a B cell that has also endocytosed the same antigen, this B cell will also be presenting the antigen on its surface. The T cell binds to this antigen and release interleukins too. Interleukins are a type of cytokine, cytokines are just molecules released by immune cells that have some kind of effect on another immune cell, in this case, the combination of the B cell binding to the T cell and the release of interleukins leads to the activation of the B cell. The B cells then undergo clonal expansion, where they form memory B cells and plasma cells by mitosis. B cells then release antibodies, which can agglutinate, opsonise and neutralise pathogens, while T helper cells activate other cells of the immune system and T killer cells specifically kill cells infected by viruses or cancer cells. The immune response is a huge thing to try and explain, that was a very very brief overview, but it should hopefully give you a slightly better understanding of what is going on.
Right, finally, the thing i know least, basically, in fish, the blood flow through the gills is in the opposite direction to the flow of water over the gills. There is a fundamental reason for this. If the blood and water flowed in the same direction, then eventually the oxygen concentrations in both would equilibriate, and this isn't very efficient. I'll show you what I mean:
---------> Flow of water + blood
10-9-8-7-6-5-5-5-5-5 (Oxygen conc in water)
0-1-2-3-4-5-5-5-5-5 (Oxygen conc in blood)
As we can see, this is only going to saturate the blood by 50% = Not good
But when you have countercurrent flow, you get a concentration gradient for the entire time that blood and water are flowing through the gills, i.e.
---------> Flow of water
10-9-8-7-6-5-4-3-2-1 (Oxygen conc in water)
9-8-7-6-5-4-3-2-1-0 (Oxygen conc in blood)
<---------- Flow of blood
This is much more efficient. The reason this works is because you want the regions of the blood and water that are low in oxygen to be next to eachother, so that you don't get that equilibrium establishing. You may think that if the low oxygen water meets the low oxygen blood then there still won't be much exchange, but because the water has a higher oxygen concentration then the blood, then there is always going to be a diffusion gradient. .
I don't know how well I explained that last one, it's a tricky concept, but I'm tired haha. Enjoy reading all that, I'm off to bed!