Theme 2C

Question 1

What is translation?

Answer 1

The assembly of amino acids into polypeptides on ribosomes.

Question 2

Where does translation take place?

Answer 2

In the cytoplasm for eukaryotes and everywhere in prokaryotes

Question 3

What is tRNA’s role in translation?

Answer 3

It brings amino acids to the complex to be added to the polypeptide chain.

Question 4

How is the sequence of amino acids determined?

Answer 4

By the sequence of codons in the mRNA. It is read from the 5’-3’ direction.

Question 5

What direction is the polypeptide assembled?

Answer 5

From the N-terminal end to the C-terminal end

Question 6

Non-polar amino acids

Answer 6

R groups usually contain -CH2 or -CH3

Question 7

Uncharged polar amino acids

Answer 7

R groups usually contain -OH

Question 8

Charged polar amino acids

Answer 8

R groups that contain acids or bases that can ionize

Question 9

Aromatic amino acids

Answer 9

R groups contain a carbon ring with alternating single and double bonds

Question 10

What are the special functional amino acids?

Answer 10

1. Methionine: first amino acid in a polypeptide
2. Proline: causes a kink in polypeptide chains
3. Cysteine (S-S): disulfide bridge contributes to the structure of amino acids

Question 11

What is the shape of tRNA and what causes it?

Answer 11

Winds into four double-helical segments = four-leaf clover. Caused by tRNA base-pairing with itself.

Question 12

What is the anticodon and where is it?

Answer 12

A three-nucleotide segment that base-pairs with a codon in mRNAs. Located at the tip of one of the double-helical segments.

Question 13

What is opposite from the anticodon?

Answer 13

A free 3’ end that links to the amino acid corresponding to the anticodon. Known as the acceptor stem.

Question 14

What is charging?

Answer 14

When aminoacyl-tRNA adds the amino acid to the acceptor stem of the correct tRNA.
Charging rxn: amino acid + tRNA + ATP -> aminoacetyl-tRNA + AMP + PPi

Question 15

How many aminoacyl-tRNAs are there?

Answer 15

20 for 20 different codons

Question 16

How many sense codons are there?

Answer 16

61, written from 5’-3’ as they appear in mRNA

Question 17

What are the rules of the genetic code?

Answer 17

1. Codons are read from 5’-3’
2. Codons are non-overlapping and the msg contains no gaps
3. Msg is translated in a fixed reading frame set by the start codon

Question 18

Are there 61 different tRNAs for the 61 codons?

Answer 18

No, because some tRNAs can read more than one codon. This is due to the base at the 5’ end of the anticodon, it can form H-bonds with more than one type of base located at the 3’ end = wobble.

Question 19

What has no wobble?

Answer 19

The first two nucleotides in the anticodon are fixed.

Question 20

What are ribosomes?

Answer 20

Ribonucleoprotein particles carry out protein synthesis by translating mRNA into chains of amino acids. Composed of the large subunit and small subunit.

Question 21

What is the large subunit?

Answer 21

Contains the peptidyl-transferase center for the formation of peptide bonds.

Question 22

What is the small subunit?

Answer 22

Contains the decoding center where charged tRNAs read and decode the codon of mRNA

Question 23

What is the S (svedberg unit)?

Answer 23

The measure of sedimentation velocity and therefore mass.

Question 24

Where are the subunits located?

Answer 24

They exist separately in the cytoplasm but join together on the mRNA.

Question 25

What is the P site?

Answer 25

The peptidyl site, binds to the tRNA attached to the growing peptide chain.

Question 26

What is the A site?

Answer 26

The aminoacyl site, binds to the tRNA carrying the next amino acid to be added.

Question 27

What is the E site?

Answer 27

The exit site, binds the tRNA that carried the previous amino acid added.

Question 28

What are the translation initiation steps?

Answer 28

1. Met is brought to the P site of small ribosome subunit (requires GTP energy) 2. Complex of initiator tRNA is brought to small ribosome subunit of the 5' capped end. Then it scans along 5'-3' until it reaches the start codon. 3. Complementary base pairing occurs between anticodon of initiator tRNA and start codon. 4. Large ribosome subunit binds to the small to form the initiation complex. Can now accept tRNA in the A site. 5. GTP is hydrolyzed to GDP and translation begins

Question 29

What are the translation elongation steps?

Answer 29

1. Correct aminoacyl tRNA is loaded into the A site by EF (elongation factor) 2. Peptidyl transferase in the large subunit forms a peptide bond between the carboxyl group of the growing chain and the amino group of amino acid in the A site. 3. Ribosome translocates in which tRNA with the polypeptide shifts from A to P site and uncharged tRNA goes from P to E and is ejected. 4. Next codon of mRNA is now in the A site and the next aminoacyl tRNA can be loaded in by EF-GTP

Question 30

What are the translation termination steps?

Answer 30

1. Stop codons are recognized by protein release factors 2. The release factor binds to the A site and stimulates peptidyl transferase to cleave the polypeptide from the P site 3. Ribosome subunits separate and detach from mRNA and empty tRNA and release factors also separate

Question 31

What are the types of post-translational regulation of proteins?

Answer 31

1. Phosphorylation 2. Ubiquitination 3. Proteolysis

Question 32

What is phosphorylation?

Answer 32

The addition of phosphate to proteins by kinases can activate or inhibit their activity (example: phosphorylates activate Microtubule Associated Protein (MAP) to promote spindle assembly and mitotic entry)

Question 33

What is ubiquitination?

Answer 33

The addition of ubiquitin molecules to proteins target them for destruction by the proteasome

Question 34

What is proteolysis?

Answer 34

The specific cleavage of a protein can induce activity.

Question 35

What is epigenetics?

Answer 35

Changes in gene transcription without changes in the DNA sequence. Modifications to lysines on histone tails affect transcription of genes (histone code).

Question 36

What does histone acetyltransferase do?

Answer 36

Adds acetyl groups to histone tails to increase gene transcription by loosening DNA binding. Loosely bound caused by repulsion, thus makes it easier to access everything needed for transcription.

Question 37

What does methylation of histone tails do?

Answer 37

Activate and repress transcription of genes. DNA can be methylated closer to the promoter to repress transcription.

Question 38

What does chromatin remodelling complex do?

Answer 38

Displaces the nucleosomes from the promoter regions activating transcription. It gives more access to everything needed for transcription.

Question 39

What does the removal of the stop codon in mRNA cause?

Answer 39

No increase/decrease in gene expression, longer polypeptide, regulated translationally

Question 40

What does the increased ubiquitination of proteins cause?

Answer 40

Decreased gene expression, enhanced protein degradation, regulated post-translationally

Question 41

What does complete inhibition of charging cause?

Answer 41

Decreased gene expression, no translation occurring, regulated translationally

Question 42

What does a decrease in histone acetylation in promoter cause?

Answer 42

Decreased gene expression, DNA wound more tightly histone preventing access to transcriptional initiation complex, regulated post-translationally

Question 43

What does the change to the amino acid attached to tRNA from serine to lysine cause?

Answer 43

No increase/decrease in gene expression, changes the amino acid sequence of the polypeptide, regulated translationally