Parasitology-3

Question 1

What is unique about the eukaryotic phylogeny of parasites?

Answer 1

It is uncertain when divergence of these parasites occurred as it could be early on although this could be due to highly divergent sequences forming long branches with a few long branches in a phylogenetic tree tend to attract each other at the bottom of the tree

Question 2

What occurred to the genome of parasites through evolution?

Answer 2

There has been genome compaction with regards to the number and length of introns with shorter genes and proteins
There is genome elimination with fewer numbers of accessory factors and isoforms there is also loss of enzymes on biosynthetic pathways
In some cases it demanded expansion of gene families mostly related to virulence and immune evasion or the function of novel organelles allowing parasites to specialize in their particular niche at the cost of self-sufficiency

Question 3

What is the origin of hydogenosomes and mitosomes?

Answer 3

They are divergent or reduced mitochondria derived from a single primary endosymbiotic event which is a defining characteristic of all eukaryotes determined due to their double membrane
DNA evidence found in 1998 confirmed this
Only one type is found in an organism

Question 4

What are the commonalities between hydrogenosomes and mitosomes?

Answer 4

They both have a double membrane but only have some cardiolipin in the inner membrane
There is no cristae, cytochromes or respiration
More commonly found in anaerobic or microaerophilic protozoans (e.g. trichromonas vaginalis, giardia lamblia)

Question 5

What is the function of mitosomes and hydrogenosomes?

Answer 5

The hydrogenosome (but not mitosomes) oxidises pyruvate/malate from glycolysis to produce ATP. H2 is a by-product.
There are a few carriers of ATP,ADP,NADH and enzymes such as NADH dehydrogenase
There is also translocation machinery at some level and chaperones

Question 6

What do hydrogenosomes, mitosomes and the mitochondria have in commone?

Answer 6

There is a unifying pathway for all is Fe-S cluster assembly
Fe-S clusters are found in several metalloproteins, most important in enzymes involved in oxidation-reduction reactions for energy metabolism

Question 7

What are enzymes which contain Fe-S clusters?

Answer 7

IscS is a cysteine desulferase

IscU is a scaffold protein where Fe-S cluster is formed and then transferred to a Fe-S containing protein

Question 8

Do protozoans contain peroxisomes?

Answer 8

No

Question 9

What drives secretion in giardia and entamoeba?

Answer 9

These species have an absence of golgi but do contain secretory vesicles which are relices of golgi

Question 10

How does entamoeba organise its genome?

Answer 10

Lacks a nucleolus instead has rRNA genes organised apart from the genome as extrachromosomal circular DNA

Question 11

Why is giardia considered eccentric?

Answer 11

It has a symmetric shape, two nuclei, 4 symmetric pairs of flagella and a ventral attachment disc

Question 12

What are the features of apicomplexan parasites?

Answer 12

They are all obligatory intracellular parasites
Most (except cryptosporidium) contain a plastid-like organelle termed an apicoplast through a secondary endosymbiotic event

Question 13

What is the origin of the apicoblast?

Answer 13

There was a photosynthetic eukaryote which was engulfed by a secondary eukaryote, this is evidenced by the 4 membranes around the apicoblast
This eukaryote lost many genes and had other transferred to the nucleus as well as mediating the evolution of new nuclear genes

Question 14

What is the function of the apicoblast for apicomplexan parasites?

Answer 14

It is essential to the parasites with 4 key biosynthetic pathways occurring in this organelle including fatty acid synthesis, Isoprenoid synthesis, Fe-S cluster biosynthesis, Haem biosynthesis (in close association with mitochondria
It also plays a role in the formation of the paristophosphorus vacuole

Question 15

What is the structure of apicomplexan parasites?

Answer 15

Have a rigid crescent shape due to an underlying cortex of microtubules connected to a barrel like structure (conoid).
The spiral organisation of the microtubules which covers 2/3 of the cell body can combine to form a motor membrane complex allowing for gliding motility

Question 16

What is the importance of the apical organelles in apicomplexan parasites?

Answer 16

The apical secretory organelles including the rhoptry, microneme and dense granules along with the gliding motility are essential for host cell invasion

Question 17

What are kinetoplastids?

Answer 17

Parasites which contain the kinetoplast organelle such as Tryrpanosoma spp and Leishmania spp

Question 18

What is the kinetoplast?

Answer 18

A large, highly branched and very divergent mitochondrion

Question 19

What is unique about the mRNAs of the kinetoplast?

Answer 19

There is pan-editing of its mRNA in the 3’ to 5’ direction mediated by the editosome with the sites for editing indicated through gRNAs which precisely controls either the insertion or deletion of Uridines through mismatch in the base pairng of the premRNA and the gRNA

Question 20

What is the genome structure of the kinetoplastid?

Answer 20

It is a DNA network of 50 copies of maxicircles which code premRNAs and roughly 10,000 minicircles which code guide RNAs

Question 21

What is the glycosome?

Answer 21

A single membrane organelle of kinetoplastids which compartmentalizes glycolytic enzymes down to the level of phosphoglycerate kinase

Question 22

How does the glycosome enhance glycolysis?

Answer 22

Through compartmentalisation of glycolytic enzymes and co-factors, the sacrifice of NADH as instead of investing a pyruvate into the TCA and NADH into the electron transport chain NAD is reoxidised
It uses a membrane bound glycerol-phosphate oxidising system (kinetoplast) which allows continuous reoxidation of NADH into the glycosome making glycolysis less efficient but faster at production of ATP

Question 23

What occurs to metabolism in the bloodstream from of kinetoplastids?

Answer 23

In this form there is plenty of glucose available and glycolysis is enhanced while the TCA cycle and electron transport chain are greatly reduced and there is continuous reoxidation of NADH allowing for faster, but less efficient production of ATP

Question 24

What occurs to the kinetoplast in the insect stage of kinetoplastids?

Answer 24

The kinetoplast enlarges and branches, kinetoplast develops fully functional TCA and ETC

Question 25

What is unique about mRNA transcription in kinetoplastids?

Answer 25

Some trypanosomes have mRNAs transcribed by RNA polymerase I, this is a much stronger polymerase
This useful for things such as VSG and procyclin are variable surface antigens in T. Brucei which are blood-stage and insect stage respectively these require a high level of production as there is an extremely dense VSG surface coat which is the major defense against the host immune system

Question 26

What is unique about the promoters for kinetoplastid gene expression?

Answer 26

Genes in kinetoplastids have no core promoter of RNA pol. II
Transcription initiates randomly at the strand switch region 1-13 kb long
Cluster of genes are in an inverted orientation from STS
Transcription irradiates bidirectionally from STS
Gene regulation is mostly post-transcriptional level through control of mRNA stability and translation

Question 27

Why is the structure of mRNAs of kinetoplastids unique?

Answer 27

Most protein coding genes are transcribed as polycistronic mRNAs by RNA polymerase II
A polycistronic transcript is individualized into single protein coding mRNA as a result of capping and polyadenylation
Capping and polyadenylation are post-transcriptional processes the polyadenylation of the first mRNA occurs after the capping of the second mRNA

Question 28

What is unique about splicing of mRNAs in kinetoplastids?

Answer 28

Most RNAs will only spice out section of the same strand (in cis) however in kinetoplastids RNA can undergo trans splicing where two different strands of mRNA end u being joined together this process adds a 39 nt splice leader to each mRNA and is essential for gene expression this SL brings the Guanosine cap to the mRNAs

Question 29

What is unique about the guanosine cap in kinetoplastids?

Answer 29

There is random initiation of transcription and transcription of polycistronic genes, post-translational trans-splicing and capping
Post-translational 3’ polyadenylation