Post Transcriptional processing and Translation

Question 1

post transcriptional processing of RNA occurs in …

Answer 1

both eukaryote and prokaryotic cells.
in prokaryotic cells, rRNA and tRNA go through it , but mRNA is directly translated into protein.
in eukaryotes each RNA type undergoes post transcriptional processing , which allows for further gene regulation
in eukaryotes , this only happens in the nucleus.

Question 2

what is the primary transcript?

Answer 2

initial mRNA nucleotide sequence. also called the pre-mRNA or heterogenous nucleur RNA.

Question 3

how is the primary transcript processed?

Answer 3

its processed in three ways,

1) addition of nucleotides,
2) deletion of nucleotides
3) modification of nitrogenous bases.

Question 4

what is the job of the 5’ cap?

Answer 4

serves as an attachment site in protein synthesis and as protection against degradation

Question 5

what is the 3’ end?

Answer 5

poly adenylated tail (poly A tail) - also protects against exonucleases.

Question 6

before leaving the nucleus the primary transcript is cleaved into what?

Answer 6

introns and axons by snRNPs (snurps) - several snurps associate with proteins and they form a complex called spliceosome. inside the splicesome, introns are looped bringing the axons together. introns are excised and exons are cut together to form a single mRNA that will code for protein.

Question 7

introns are degraded where?

Answer 7

nucleus.

exons exit the nucleus to be translated.

Question 8

What is the reason that there are more proteins than genes?

Answer 8

due to diff splicing patterns of the same gene

Question 9

translation is directed by what?

Answer 9

mRNA- carries the code form the nucleus to the cytosol in form of codons.

Question 10

what about tRNA?

Answer 10

has a set of nucleotides that are complementary to the codon , called the anticodon. tRNA sequesters the aa that corresponds to the anticodon.

Question 11

what about rRNA?

Answer 11

WITH protein makes up the ribosome, which provides a site for translation to take place.

Question 12

what is the ribosome made up of?

Answer 12

small subunit and large subunit made from rRNA and many proteins.
prokaryotic ribosomes (30s and 50s, total 70s) are smaller then eukaryotic (40s and 60s= 80s)
ribosomes are made in the nucleolus- which is not in prokaryotes. the ribosome is assembled in the nucleolus , small and large subunit are exported separately to the cytoplasm.

Question 13

what happens after post transcriptional processing in a eukaryote?

Answer 13

mRNA leaves the nucleus though the pores and enters the cytosol. with the help of initiation factors, the 5’ end attatches to the small subunit of ribosome. a tRNA with anticodon CAU, sequesters aa methionine and settles at the P site . this is a signal for the large subunit to join and form the initiation complex . this process is called initiation.

Question 14

how does elongation of the polypeptide begin?

Answer 14

a tRNA with a corresponding aa attaches to the A site (aminoacyl site) at the expense of 2 GTPs . the carboxyl terminus of the methionine attaches the N terminus of the aa at the A site in a dehydration run catalyzed by peptide transferase (enz of the ribosome). then translocation occurs.

Question 15

what happens in translocation?

Answer 15

tRNA with the methionine moves to the E site where it can exit the ribosome. a tRNA with a newly formed di peptide moves to the P site, clearing the A site for the next tRNA. translocation requires another GTP

Question 16

How long is elongation continued?

Answer 16

until a stop codon reaches the P site. - that’s when termination occurs. when a stop codon reaches the A site, proteins known as the release factors will find to the A site - allowing for water molecule to add to the polypeptide chain. the poplypeptide is free from the tRNA and ribosome , the ribosome breaks up into subunits .

Question 17

what does the aa sequence determine?

Answer 17

the folding conformation ! even as the polypeptide is being translated , it begins to fold. the folding process is assisted by proteins called chaperones.

Question 18

What is added in post translational modifications?

Answer 18

sugars, lipids or phosphate groups may be added to amino acids.

Question 19

Proteins injected into the ER lumen are destined to become membrane bound proteins of ?

Answer 19

nucleus envelope, ER, golgi, lysosomes, plasma membrane or to be secreted from the cell.

Question 20

what is the signal peptide?

Answer 20

20 amino acid sequence , near the front of the polypeptide. its recognized by protein -RNA signal recognition particle, which will carry the entire ribosome complex to a receptor protein on the ER. there the protein grows across the mem, where its either released into the lumen or remains partially attached to the ER.

Question 21

Summary of translation

Answer 21

translation begins on a free floating ribosome. a signal peptide at the beg go the translated polypeptide may direct the ribosome to attach to the ER , in which case the polypeptide is injected into the lumen ( may be secreted from the cell via the golgi or may remain partially attatched to the membrane).

Question 22

The genetic code

Answer 22

mRNA nucleotides are strung togetherto form a genetic code which translates DNA nucleotide sequence into an aa sequence and ultimately to a protein.

Question 23

the genetic code is degenerative

Answer 23

more than 1 series of three nucleotides may code for any amino acid.

Question 24

unambiguous

Answer 24

any single series of three nucleotides will code for one and only one aa

Question 25

almost universal

Answer 25

nearly every living organism uses the same code.

Question 26

what is a codon?

Answer 26

three consecutive nucleotides on a strand of mRNA

Question 27

what are the stop codons?

Answer 27

UAA
UGA
UAG
These signal an end to protein synthesis .

Question 28

what is the start codon?

Answer 28

AUG, it also acts as a codon for the aa methionine.