the origin of microbial life

Question 1

discuss the evidence that suggets RNA is thought to be the precursor to life (evidence to suggest RNA existed before DNA); 4 marks

discuss a critisicm against this theory

discuss an explanation for the fact that RNA can not replicate itself

if RNA can replicate itself then why do DNA and proteins exist

Answer 1

1) both proteins and DNA first need RNA to exist
2) RNA can catalyse synthesis of other RNA molecules
3) RNA enzymes (ribozyme) are needed to form peptide onds between RNA molecules
4) RNA can cleave other RNA molecules

RNA has not been shown to be able to replicate itself

RNA doesnt need to replicate itself directly in order to replciate itself;
RNA enzymes synthesise other RNA enzymes using free nucleotides, and there RNA enzymes than create another RNZ enzyme which then creates the original enzyme (auto-catalytic set)
ribozymes are transferred to new environments so different autocatalytic sets emerge which are in competion with eachother

RNA is very unstable so there was selection for sets of RNA which store their information as DNA which is more stable due to being ds, the catalytic function was then passed to proteins.

Question 2

discuss LUCA properties; 5

discuss evdience that LUCA emerged in a hydrothermal pore; 5

Answer 2

• last universal common ancestor of all life on earth
• existed 3.5 billion years ago
• archaea and bacteria are very different in terms of DNA replication, biochemical pathways, cell wall properties etc so its hard to predict what LUCA was like
• the fact that archaea and bacteria are so different may suggest that life emerged twice and that LUCA was a structure with no cell wall; perhaps it was just a collection of self replicating particles in a non living compartment like a pore in a hydrothermal vent. once a phosphatase enzyme developed which could extract energy from the proton gradient in pores and a fatty acid compartment emerged the system could break away from the pore
• breaking away occured twice; one for bacteria, once for archaea

i) those pores have been shown to lead to life like entities in the lab
ii) pores contain iron sulphur compounds which have catalytic properties and can therefore act as catalytic vessels
iii) phosphorus in pores leads to phosphate
iiii) temperature gradient in pores is theorised to concentration nucleotides at one end which may help them join together, as well as cause fatty acids to form vesicles
v) chemistry of pores allows a proton gradient to form

Question 3

discuss how the ancient symbiotic event solved the size limitation problem microbes had and knock on effects; 4 marks

Answer 3

• single celled organism could not grow substantially large in size because this would decrease the SA:V ratio and therefore the oxygen gaining and energy gaining ability (needs proton gradient)
• because of this size limitation, the size of an organisms the genome is limited and therefore so is the organisms complexity
• engulfing of bacteria with mitochondria like function solved the energy barrier because cells can have multiple mitochondria and therefore get bigger; larger cell volyme means more information stored and more complex pathways exist
• a bigger genome evolved with comp;ex genes which are expressed as proteins which link cells together into a mass of cells; the mass of cells has different part differentiated for different functions

Question 4

after the mitochondria like bacteria were engulfed, what happened to most of the

Answer 4

• most were transferred to the host genome
• bacterial genomes contain parasitic DNA this meant pieces of the DNA started moving around the host genome and disrupting the host genome causing introns to emerge
• there was still high selection for these newly emerged eukaryotes though despite these disrupted genes

Question 5

why dont bacteria have introns in the same way that early eukaryotes did

Answer 5

there are such large numbers of bacterial organisms that any event of parasitic DNA movement resulting in an intron is selected against and removed

Question 6

in early eukaryotes which 5 solutions came about to deal with disrupted genes (introns)

what are these 5 methods now also used for in cells

Answer 6

sex; DNA recombination and variance between offspring; those with less parasitic DNA survive

sliposomes; RNA-protein complexes which remove introns from the primary transcript mRNA

emergence of the lipid bilayer around the nucleus (nuclear envelope) and seperaton of transcription and translation; no chance of mRNA being translated without having its introns removed inside the nucleus

methylation; eventually the ability to stop parasitic DNA being expressed arose (methylated genes arent expressed)

methods of destroying RNA; RNAi destroys mRNA before or during translation

these same methods are now also used to control gene expression

Question 7

why wasnt there selection for no introns

Answer 7

there are benefits to introns; differential splicing to produce multiple proteins from a single primary transcript

Question 8

how many introns does the average gene have

Answer 8

8