Lecture 21 (mRNA processing)

Question 1

define exons

Answer 1

Exons - sequence which leaves nucleus as mRNA. Coding sequence as translated into amino acid sequence

Question 2

define introns

Answer 2

nonreading RNA that lies between exons

During splicing, introns are cut out and exons are ligated together to form mature mRNA molecules

Question 3

what do the 5’ cap and poly-A tail do?

Answer 3

Many promote export of mRNA from nucleus

Protects mRNA from degradation

5’ cap facilitates ribosome attachment

Question 4

what makes up the spliceosomes ?

Answer 4

snRNPs - small nuclear ribonuclearproteins. Along with other proteins, these create the spliceosomes that form on pre-mRNA.

Question 5

explain splicing:

Answer 5

In spliceosome, snRNA pairs with bases in premRNA intron (splice sites)

Spliceosomes cut pre-mRNA, releasing introns to degrade and splices exons together; catalysed by snRNA (small nuclear RNA)

Spliceosomes release mRNA

Question 6

define genes

Answer 6

Genes - regions of DNA which code for functional products, either polypeptides or RNA molecules

Question 7

define a codon

Answer 7

Codons - sequence of 3 nucleotides that translate into amino acids

Question 8

briefly explain the steps of the set up for translation (do NOT explain initiation)

Answer 8

A small ribosomal unit attaches to mRNA. tRNA attaches next

Larger subunit added as tRNA binds to p-site

Another tRNA for next codon binds to A-site as previous tRNA kicked out

Question 9

what does each tRNA molecule have?

Answer 9

amino acid attachment site,
and antidocon loop to pair with mRNa codon

Question 10

whcih enzyme helps bind the amino acid to tRNA molecules

Answer 10

Enzymes called “amino acyl tRNA synthetase” attach the correct amino acid to each tRNa molecule (with the corresponding anticodon).

This occurs in cytosol
tRNA covalently bound to amino acid

Question 11

explain the steps for tRNA to bind to it’s amino acid:

Answer 11

*Amino acid and tRNA enter active site of aminoacyl-tRNA synthetase

*Using ATP, synthase catalyzes covalent bonding

*Aminoacyl tRNA released

Question 12

define the P site, A site, and E site

Answer 12

P site - holds tRNA carrying growing polypeptide chain

A site - holds tRNA carrying next amino acid to be added

E site - where discharged tRNAs leave ribosome

Question 13

what does every mRNA molecule contain to help with tranlation?

Answer 13

ribosomal binding site at 5’ end

Question 14

What are the 3 stages of translation?

Answer 14

initiation
elongation
termination

Question 15

explain the steps of initiation:

Answer 15

Eukaryotes - small ribosomal subunit binds to initiator tRNA

5’ cap of mRNa binds to small ribosomal subunit

Initiator tRNA scans mRNa molecule for start codon (AUG)

Anticodon of initiator tRNA hydrogen bonds to start codon

Larger ribosomal subunit binds, and initiator tRNA positioned in P-site

Translation begin

Question 16

Explain the steps for elongation:

Answer 16

Anticodon of aminoacyl tRNA pairs with complementary mRNA codon in A-site

rRNA molecule of large ribosomal subunit catalyses formation of peptide bond between new amino acid and growing peptide

Ribosome moves tRNA into neighboring sites. mRNA moves along with bound tRNAs, bringing next codon into A-site

Question 17

Expalin teh steps for termination:

Answer 17

When ribosome reaches STOP codon (UAA, UAG, UGA), A site accepts release factor

Release factor promotes release of polypeptide from tRNA in P site by hydrolysis

Peptide exits through exit tunnel

mRNa and ribosomal subunits dissociate

Question 18

explain the steps for polypeptide modification in ER - how the complex arrives at ER:

Answer 18

Polypeptide synthesis begins on a free ribosome in cytosol

SRP binds to signal peptide halting synthesis momentarily

SRP binds to ER receptor protein, and part of a protein complex that forms a poor

SRP leaves, polypeptide synthesis resumes with translocation across membrane

Signal peptide cleaved by enzyme in receptor protein complex

Polypeptide leaves the ribosome and folds into final conformation inside the ER membrane.

Question 19

define polyribosome

Answer 19

Polyribosome - single mRNa strand where many ribosomes travel, each one synthesising a growing polypeptide chain