Cell Biology Chapter 19

Question 1

For some genes, the RNA transcript is what? For many other genes, the ultimate product is what? mRNAs encode instructions for what, the process of what?

Answer 1

the final product; protein; for translation, the process of assembling amino acids into a polypeptide

Question 2

Ribosomes carry out the process of what? tRNA molecules align what? Aminoacyl-tRNA synthetases attach amino acids to what?

Answer 2

polypeptide synthesis; the amino acids in the correct order; to their appropriate tRNA molecules

Question 3

Ribosomes are are built from what? rRNA performs many of the what?

Answer 3

dissociable subunits, the large and small subunits; of the key functions of ribosomes

Question 4

What are ribosomes four important sites?

Answer 4

the mRNA binding site, the A (aminacyl) site, the P (peptidyl) site, the E (exit) site

Question 5

A tRNA molecule is an adaptor that binds both a what? tRNAs attached to an amino acid are said to be what?

Answer 5

a specific amino acid and the mRNA sequences that specify the amino acid; charged

Question 6

Each tRNA recognizes codons in what because of what? Cells typically have 20 different what? How many aminoacyl-tRNA synthetase for each amino acid?

Answer 6

in mRNA because of their complementarity to the anticodon; aminoacyl-tRNA synthetases to attach each amino acid to the appropriate tRNA; 1

Question 7

The sequence of codons in mRNA directs the order of what? mRNA must first be exported from the what to the what? The 5’ end of the messages is a what type of sequence?

Answer 7

of amino acids in the polypeptide; from the nucleus to the cytoplasm; is an untranslated sequence that aids in mRNA interaction to the ribosome

Question 8

The 5’UTR precedes what? An untranslated region at the 3’ end (3’UTR) of the mRNA is located where?

Answer 8

the start codon which is the first to be translated (usually AUG); after the stop codon

Question 9

The stop codon signals what? What are the stop codons? Eukaryote mRNAs also have what?

Answer 9

the end of translation; UAG, UAA, UGA; 5’ cap and 3’ poly (A) tail

Question 10

Most mRNAs in eukaryotes are what meaning what? In bacteria and archaea some are what? What are operons?

Answer 10

are monocistronic, meaning they encode just one polypeptide; some are polycistronic, encoding several polypetides usually with related functions; polycistronic transcription units

Question 11

The mRNA is read by what and how? Translation begins where?

Answer 11

by ribosome in the 5’ to 3’ direction; at the N-terminus of the polypeptide and adds amino acids to the growing chain until the C-terminus is reached

Question 12

What are translation 3 stages?

Answer 12

Initiation, Elongation, Termination

Question 13

How does initiation of translation happen in bacteria?

Answer 13

The mRNA binds first to the small subunit. A sequence in the mRNA binds an RNA sequence in the ribosome and position the start codon in the P-site. A modified tRNA methionine (fMet) binds to the P site. The large subunit joins to form the ribosome and all the closed sites: A,P,E

Question 14

How does initiation of translation happen in eukaryotes

Answer 14

The tRNA with methionine binds first to the small subunit in the P-site; the mRNA is scanned from the 5’ until the AUG codon matches the tRNA; The large subunit joins to form the ribosome and all the closed sites: A,P, and E

Question 15

What are the 3 repetitive steps of Elongation?

Answer 15

Binding of aminoacyl-tRNA (charged tRNA) to the ribosome A site brings a new amino acid into position; Peptide bond formation links this amino acid to the growing polypeptide. The polypeptide is transferred from the P-site to the A-site where peptidyl transferase (rRNA) make the peptide bond; Translocation- the Ribosome moves toward the 3’ of the mRNA advancing 3 nucleotides through. The A-site is bound by factors that assist on the translocation; the tRNA with the polypeptide moves to P; the uncharged tRNA moves to E to exit

Question 16

What are polyribosome or polysome? These maximize the efficiency of what?

Answer 16

a cluster of such ribosomes; of mRNA utilization

Question 17

Codons are read on the what? Stop codons are recognized by what?

Answer 17

the mRNA one after the other, until a stop codon arrives at the A site; protein release factors rather than tRNAs

Question 18

Once release factors bind to the stop codons, translation is what?

Answer 18

terminated through release of the completed polypeptide

Question 19

Translation converts information in what? The ribosome reads the mRNA how?

Answer 19

mRNAs into a chain of amino acids linked by peptide bonds; codon by codon in the 5’ to 3’ direction as the ribosome moves towards the 3’

Question 20

RNA molecules play important roles in what?

Answer 20

in translation mRNA, tRNA, rRNA

Question 21

mRNAs may contain what? There are several types of what?

Answer 21

mutant codons that cause errors in the polypeptide chain synthesized; mutations in which a base pair substitution in the DNA changes the mRNA

Question 22

What are the 5 types of mutation?

Answer 22

Silent, Missense, nonstop, nonsense, Frameshift mutations

Question 23

What is the silent mutation effect?

Answer 23

Changes the codon on the mRNA, but it still codes for the original amino acid due to the flexibility of the genetic code; Also applies for a substitution by a similar amino acids that does not change gene product function

Question 24

What is the missence mutation effect?

Answer 24

Changes the codon on the mRNA and now codes for different amino acid that in the wt mRNA. Effect depends on the change, it can go from no function to lower or higher function

Question 25

What is nonstop mutation effect?

Answer 25

Changes a stop codon to an amino acid codon. Longer transcript is made. Effect depends on the change, it can go from no function to lower or higher function

Question 26

What is nonsense mutation effect?

Answer 26

Changes an amino acid codon to a stop codon. A shorter transcript is made. Usually nonfunction of the new gene product

Question 27

What is the frameshift mutations effect?

Answer 27

Changes the reading frame of the message. This can arise from base-pair insertions or deletions, or combination of these. nonsense, nonstop, and missense codons can also arise from frameshift mutations

Question 28

After polypeptide chains are synthesized, they often must undergo what? This modification in the amino acids can what?

Answer 28

posttranslational modification before they can perform their functions; activate or deactivate proteins

Question 29

Modification in the amino acid can be used for recognition by?

Answer 29

organelles/targeting to specific sites in or out of the cell; signaling cascades; protein partners for protein-protein interactions

Question 30

Sometimes amino acids are removed from the what? Common processing events include what?

Answer 30

the polypetide to make the final product; chemical modification of amino acids: methylation, phosphorylation, acetylation

Question 31

As proteins are synthesized, they must be what? Polypeptides are routed to where?

Answer 31

sorted and directed to their final locations; to compartments via several mechanisms

Question 32

Each protein released to the cytosol has what? Nuclear pores are specialized channels in the what?

Answer 32

localization signals specific to the destination; nuclear envelope where inner and outer membranes are fused

Question 33

They provide direct contact between what? They are lined with what?

Answer 33

The cytosol and the nucleoplasm (interior nuclear space); a protein structure called the nuclear pore complex (NPC)

Question 34

The NPC is built from about what? The transporter is likely involved in what?

Answer 34

30 different proteins called nucleoporins; moving molecules across the nuclear envelope

Question 35

Enzymes and proteins needed in the nucleus must be imported from where? RNAs that need to be translated and components of ribosomes must be exported form where?

Answer 35

from the cytoplasm; nucleus

Question 36

Nuclear localization enable the protein to be what?

Answer 36

recognized and transported by the nuclear pore complex

Question 37

How does nuclear import via the ran/importin pathway occur?

Answer 37

A cytoplasmic protein with an NLS is recognized by a receptor called an importin which binds the NLS and mediates movement of the protein to a nuclear pore; the importin-protein complex is transported into the nucleus by the transporter at the center of the NPC; Inside the nucleus, the importin associates with a GTP-binding protein called RAN, causing importin to release the NLS containing protein; The RAn-GTP importin complex is transported back to the cytoplasm through the NPC; In the cytoplasm, the importin is released as GTP is hydrolyzed

Question 38

Export is similar to what? Nuclear export signals are recognized by what?

Answer 38

import in the general pathway; exportins which mediate transport of the complexes out of the nucleus

Question 39

RAn-GTP is maintained at what? The cytosol contains a what?

Answer 39

at high levels inside the nucleus by a guanine-nucleotide exchange factor that promotes Ran to bind GTP; a GTPase activating protein that promotes hydrolysis of GTP by Ran

Question 40

2 types of signals; and 2 types of nuclear transport receptors?

Answer 40

nuclear localization signals (NLS), nuclear export signal (NES); Importins- cytoplasm to nucleus exportins- nucleus to cytoplasm

Question 41

Ran-GTP= shuttle

Answer 41

High concentrations for Ran-GTp in nucleus; cause importins to release cargo; cause exportins to bind to cargo; GTP activating protein (GAP) high concentration in cytoplasm - hydrolyze GTP to GDP; Guanine exchange factor (GEF) high concentration in nucleus - exchange GDP for new GTP