endoplasmic reticulum

what are the key points of the endoplasmic reticulum?

• its a continuous membrane system

• the cell’s assembly and modification site

• amount of ER membranes varies depending on the type of cell 

• abundant in protein and lipid synthesizing cells

do mature red blood cells have an endoplasmic reticulum?

no - they don’t make lipids and proteins anymore so there’s no need

what is the difference between the rough and smooth endoplasmic reticulum? Describe what the rough endoplasmic has

• has parallel flattened sacs (cisterns)

• has ribosomes

• it’s outer membrane is continuous with the nuclear envelope’s outer membrane and its lumen is continuous with the perinuclear space of nucleus

what is the difference between the rough and smooth endoplasmic reticulum? describe the sER

• no ribosomes 

• usually smaller than the rER 

• system of irregular tubes 

a ribosome consists of a large and small subunit, what is the small subunit and large subunit responsible for? 

small subunit → mRNA decoding (reads the mRNA sequence and matches it with tRNA)

large subunit → catalysis of peptide bond formation (speeds up the formation of peptide bonds) 

what are the ribosome subunits made out of? proteins and rRNA

proteins and ribosomal RNA

we say rRNA is a ribozyme, what is meant by this?

the rRNA of the larger subunit has catalytic ability as in it cataylses the formation of peptide bonds between the amino acids 

what is svedberg (s) units?

unit based on sedimentation time, e.g. 40s called 40s as that’s how long it takes the molecule to sediment in a centrifuge

what is the larger subunit called by svedberg units? 

60s 

what is the smaller subunit called by svedberg units?

40S

what molecules by svedberg units or not make up the 40s subunit?

30 proteins (30 types not 30 physically), and 18S rRNA

what molecules by svedberg units or not make up the 60s subunit?

40 proteins (40 types not physically 40), 5S rRNA, 28S rRNA, 5.8S rRNA

outline the steps of initiation 

• small ribosomal subunit binds to mRNA’s cap and moves to the start codon AUG (initiation site)

• the first tRNA molecule arrives carrying methionine, the tRNA molecule binds its anticodon with the codon AUG 

• large ribosomal subunit arrives and binds with the small ribosomal subunit forming the complete ribosome 80S 

OUTLINE the steps of elongation

• new tRNAs bring in more amino acids one by one

• the methionine of the first tRNA is transferred to the A-site amino acid 

• ribosome (rRNA of the large subunit’s catalytic ability) catalyse the formation of peptide bonds between amino acids 

• the ribosome moves along the mRNA catalysing the formation of peptide bonds between amino acids and growing the polypeptide chain 

outline the steps of termination

• the ribosome reaches the stop codon, and no more tRNAs can bind to the polypeptide chain

• instead a release factor binds to the polypeptide chain via entering the A site, triggering the release of the polypeptide chain

• once the ribosome is detached from the polypeptide chain the large and small subunits dissociate from each other → cycle restarts

what is the role of the codons

they encode for specific amino acids

which site of the ribosome does the first tRNA molecule occupy?

the P site

which site of the ribosome does the second tRNA molecule occupy?

the A site 

as the polypeptide chain continues to grow, where is the amino acid from the P site transferred to?

it is transferred to the A site amino acid

what happens in the A site 

new tRNA molecule arrives carrying the next amino acid 

what happens in the P site

holds the tRNA with the growing peptide chain and is where the peptide bond is formed

on which type of ribosomes are all proteins first synthesized at?

free ribosomes → ribosomes floating in the cytosol

which types of proteins reach completion at free ribosomes? 

proteins of the cytosol, nucleus/nucleolus, mitochondria, and peroxisome 

which types of proteins finish completion on the ribosomes of the rER?

Those destined for secretion, plasma membrane or lysosomes

what is the signal sequence?

a short continuous code that is usually found near the N-terminus of the polypeptide 

it acts like a postal code - tells the cell where the protein needs to go 

the signal sequence is still visible when the protein is folded, what is it now called?

the signal polypeptide

what is the signal patch 

instead of one short continuous stretch of amino acids, a signal patch consists of multiple separate amino acid regions that come together only after the protein has folded into its 3D shape

• once folded these regions form a signal path - a recogniseable shape that directs the protein to its correct destination

all proteins start synthesis on the free ribosomes (the ones floating in the cytosol) but some are transported to ribosomes on the rER to finish synthesis, what signals them to go there?

the signal sequence of the protein - is what determines if the polypeptide is transported to the er to complete translation or if it is free to go to their end destination

what is co-translational transport? 

the protein being transported to the er while the protein is still being synthesized to complete translation

what is an SRP molecule?

a signal recognition particle that is a type of ribonucleoprotein particle

outline the steps of co-translational transportation

• initiation of protein translation occurs at free ribosome in cytosol 

• if polypeptide has an er signal sequence, an SRP molecule binds to it halting translation temporarily 

• SRP brings ribosome to the er membrane where the SRP binds to a SRP receptor anchoring the ribosome 

• ribosome is then bound to a protein translocator and SRP is displaced and released for reuse

• and translation continues with the growing polypeptide being threaded through the protein translocator into the lumen of the ER 

what is the next steps of co-translational transport once the polypeptide has been threaded through the protein translocator into the lumen of the er? and what is the final product?

• once enough of the chain has entered the lumen (pulled by the signal sequence) an enzyme called signal peptidase cleaves the signal sequence

• rest of polypeptide continues to be synthesized and fully threaded into the lumen

• the final product is a mature soluble protein in the lumen of er

what happens to the Signal sequence when cleaved?

it remains in the membrane of the er or is degraded

need to make transmembrane protein cards

what are the processes of the lumen in the rER? (7) 

• removal of signal peptide

• orientation of transmembrane proteins

• N-glycolysation

• folding of proteins (aided by chaperones)

• quality control; misfolded proteins are transported to the cytosol where they are degraded by the multienzyme complex proteasome

• retention of ER resident proteins (some not all) 

• proteins are transported to the golgi by transport vesicles