Help with biology a/s question?

Question

Revising for a/s but can't answer 2 practice questions can anyone help/explain?

1. name the type of chemical reaction that occurs when a strand of mRNA is formed from individual nucleotides?

2. a strand of mRNA has 53 codons but only 51 amino acids. suggest 2 reasons for this difference?

help would be much appreciated

Answer 1

It’s the formation of phosphodiester bonds between the 5’ and 3’ carbon atoms on the ribose sugar backbone. In the process a pyrophosphates is formed (P-P). What type of reaction is it well it is catalysed by and enzymes so I would say that it is an enzymatic reaction.

The most probable reason for this is in translation of the mRMA you need a start codon which determines where translation will start (obviously) and also a stop codon, which needs no further explanation.

Answer 2

The answer for #1 is LIGATION not transcription. Note the question asks for the chemical reaction (there are 7 types, ligation, hydrolysis,and 5 other ones :P), and it specifies the production of a strand from nucleotides. Transcription would have mentioned the DNA template.

For #2, I'm not 100% sure why it's 53 codons and 51 amino acids. You lose 1 AA because of the stop codon, which isn't translated. The start codon (typically ATG) encodes formyl-methionine, which is an amino acid.

So here are some reasons (farfetched possibly) you could have 51 amino acids because there was a second codon after the stop, or a 1 codon exon (I don't even know if they come this short or not), post-translational modification resulting in the cleavage of an amino- or carboxy-terminus residue (the formyl-methionine could be lost in that manner), and/or there could be 3 bases of 5` untranslated mRNA (why only 3 i do not know)

Answer 3

messenger RNA is composed of nucleotides too, they just having an extra oxygen on the 2 prime carbon ie a hydroxyl group. When they are polymerised by RNA Pol, the reaction that links them together is the forming of a phosphodiester bond between the 5 prime carbon and the oxygen of the phosphate group and the 3 prime carbon of the next ribonucleotide. This is known as esterification NOT ligation. Dna Ligase is an enzyme that forms a PDB because it ligates the two ends of cut DNA - it refers to its action not the chemistry.

codons are triplets of nucleotides eg UAU will code for the amino acid tyrosine (as does UAC because the code is "redundant"). To initiate elongation of mRNA there has to be a chemical signal that says "start" and "stop" - these are codons, so "AUG" is the start codon (and also codes for methionine )- so will only code for met if it is after another AUG and before the stop codon UAA.

To answer the question, it is because 2 codons initiate and terminate the transcription of mRNA therefore a 53 codon strand will only produce a polypeptide of 51 proteins.

Answer 4

Ligation is not a proper name for a chemical reaction. Both RNA and DNA are built up from nucleotide monomers by esterification of the 3' hydroxyl group of one nucleotide with the phosphate group of another.

Obviously, one codon is used as a stop codon, but within the coding region of the mRNA that is the only codon that does not end up in the newly synthesized protein. Of course the protein may processed later to eliminate AAs.

Answer 5

1) transcription

2) the first codon is a start codon (encoding a methionine residue which is removed from the mature protein) and the last one is a stop codon (which does not encode any amino acid, but force the translation to stop).

Answer 6

1. transcription

2. two of the codons were probably start and stop codons

Answer 7

Libby, I concur... nothing is so attractive as a woman who knows her biology...