Lecture 22

Question 1

Three steps are involved in making a polypeptide

Answer 1

1) initiation
2) elongation
3) termination or translation

Question 2

1) Initiation

Answer 2

1) ribosome small subunit binds to Shine-Dalgarno sequence of mRNA, just upstream of the start codon. Initiator tRNA hydrogen bonds to mRNA.
2) initiation factors (proteins) are required to bring all translation components together. GTP hydrolysis provides the energy for assembly.

Question 3

Elongation

Answer 3

1) codon recognition
2) peptide bonds formation
3) translocation

Question 4

2) Elongation steps

Answer 4

1) codon recognition: anticodon of incoming amino acetyl tRNA pairs with complementary mRNA codon.
2) peptide bond formation: an rRNA of the large subunit catalyzes peptide bond between amino end of a.a. In A site and caboxyl end of the growing chain in the P site. Polypeptide is removed form tRNA in the P site and attached to the new amino acid in the A site
3) translocation: ribosome translocates the tRNA in the A site to the P site. Empty tRNA in the P site is moved to the E site for release

Question 5

Peptide bond formation occurs via

Answer 5

Dehydration reaction which is catalyze by an rRNA molecule

Question 6

The peptide transferase activity of the ribosome is

Answer 6

• NOT due to enzyme
• is due to large subunit rRNA

( 23S rRNA in prokaryotes; and 28S rRNA in eukaryotes)

Question 7

Ribozymes

Answer 7

Catalytically active RNA molecules

Question 8

3) Termination of translation

Answer 8

1) ribosomes reaches a STOP codon. A protein shaped like a tRNA (release factor) enters the A site.
2) the release factor promotes hydrolysis if the bond between they tRNA in the P site and last amino acid of the polypeptide
3) the two ribosomal subunits and the other components dissociate. 2 GTPs required

Question 9

Polyribosome is polysome

Answer 9

Many ribosomes can translate a single mRNA simultaneously.

• more efficient

Question 10

Bacteria can simultaneously

Answer 10

transcribe DNA and translate mRNA

Question 11

Can eukaryotes transcribe and translate at the Same time ? Why if so or not

Answer 11

NO because those processes happen in different places

Question 12

Post-translational modification

Answer 12

Some proteins are modified AFTER the polypeptide is made

Question 13

Examples of post-translational modification

Answer 13

• phosphorylation
• Glycoslyation
• proteolytic processing

Question 14

Phosphorylation

Answer 14

Requires protein kinase and ATP

Changes activity/shape of protein

Question 15

Glycosylation

Answer 15

Carbohydrate added

Makes glycoproteins

Question 16

Proteolytic processing

Answer 16

Protease cuts at a specific site in protein

Question 17

Polypeptide synthesis always begins in the

Answer 17

Cytosol

Question 18

When organelles require other signal peptides to function, the entire peptide is made

Answer 18

In the cytosol first, and then moved to the location specified by its signal peptide sequence.

Question 19

Nuclear pores allow transport of

Answer 19

MRNA and ribosomes subunits out, and some proteins such as DNA polymerase and lamina into the nucleus.