Lecture 14: Translation and protein synthesis

Question 1

Ribosomes can be as little as […] apart on mRNAs.

Answer 1

80 nt

Question 2

What is the function of tRNAs?

Answer 2

To bring amino acids to the ribosome and to read the mRNA codons using their anticodons.

Question 3

Ribosomes synthesize proteins from the […] to the […]

Answer 3

N-terminus (NH2), C-terminus (COOH)

Question 4

What are the 3 steps in translation?

Answer 4

Initiation, elongation, and termination

Question 5

tRNA has a […] structure

Answer 5

Clover leaf secondary

Question 6

Amino acids are attached to their corresponding tRNA at the […] end, at the […].

Answer 6

3’ end, acceptor stem

Question 7

What attaches amino acids to the acceptor stem?

Answer 7

The aminoacyl-tRNA-synthetase

Question 8

Describe the structure of the E. Coli ribosome, including size and major components.

Answer 8

The ribosome is 70S overall, but is made up of a small 30S portion and a large 50S portion. The 30S portion is made of 16S rRNA and 21 polypeptide chains. The 50S portion is made of 5S rRNA an 23S rRNA along with 34 polypeptide chains.

Question 9

The site of translation is […]

Answer 9

At the ribosome.

Question 10

Describe what happens in the initiation step of protein synthesis.

Answer 10

The 30S recognizes the Shine-Dalgarno sequence first and then scans for the AUG to start. It recruits the 50S and then tRNAs so that translation can begin.

Question 11

Describe what happens in the elongation step of protein synthesis.

Answer 11

The ribosome moves down the mRNA moelcule, producing a growing chain of amino acids. The amino acid from the tRNA in the A site gets transferred to the chain of amino acids from the tRNA in the P site, and then they shift over by one codon and the process repeats.

Question 12

Describe what happens in the termination step of protein synthesis.

Answer 12

The ribosome hits a stop codon and is released from the mRNA along with the completed protein.

Question 13

The reaction that allows the transfer of the amino acid chain from the tRNA at the P site to the tRNA at the A site is called […]

Answer 13

The transpeptidation reaction

Question 14

Explain how the transpeptidation reaction occurs.

Answer 14

The NH2 of the amino acid attached to the A site tRNA performs a nucleophilic attack on the carboxyl group of the closest amino acid to the P-site. As a result, the chain gets added on top of the initial amino acid.

Question 15

What are the 3 major forms of regulation of mRNA synthesis (protein synthesis)?

Answer 15

Negative control, positive control, and attenuation.

Question 16

What is negative control regulation mRNA synthesis?

Answer 16

It involves the use of repressor proteins to turn off a genes/a set of genes.

Question 17

What are the two types of negative control of mRNA synthesis? Explain the difference between them

Answer 17

Induction and repression. Induction involves turning on a gene when the enzyme is needed according to whether its substrate is present, which serves as the inducer. Repression involves turning off a gene when the end product accumulates, which serves as the corepressor.

Question 18

Using the lac operon as an example, explain how induction works.

Answer 18

These genes code for the metabolism of lactose, which is not necessary if there is no lactose in the cell. If lactose is absent, a repressor will bind to the operator of the gene to prevent a ribosome from binding there. If lactose is present, it will bind to the repressor and deactivate it. The promoter is now recognizable by the RNA polymerase and the genes can be expressed. Once enzymes have consumed all the lactose and there’s none left, the repressor will return and gene expression will be shut down again.

Question 19

Explain how gene repression works.

Answer 19

Gene repression takes place when the end product of a given reaction accumulates such that the enzymes are no longer needed. When there is no buildup, the repressor is inactive, so the ribosome can access the promoter and synthesize proteins. When there is a buildup of end product, this serves as a corepressor, which binds to the repressor to deactivate it. This blocks access to the promoter and the gene is shut down.

Question 20

What is positive control regulation of mRNA synthesis?

Answer 20

It makes use of an activator protein to turn on a gene only in the presence of a controlling factor that has nothing to do with the enzyme itself or its substrate. It relies on an environmental factor.

Question 21

Explain how positive control works, using the Lac operon as an example.

Answer 21

It uses glucose concentration as the controlling factor, as when glucose it low the cell can use lactose as an energy source instead. When glucose concentration is low, this produces a high concentration of cyclic AMP (cAMP), which induces the Lac genes. The cAMP binds to the CAP protein, which activates it to bind to the promoter (NOT the operator). This recruits the RNA polymerase and allows more synthesis of the genes than would occur otherwise.

Question 22

What is attenuation?

Answer 22

It is a fine-tune control of the initial and continuation of transcription.

Question 23

Where does attenuation take place on the gene?

Answer 23

In the leader region of the mRNA (trpL in the case of the trytophan operon).

Question 24

What are the two possible structures in the tryptophan operon leader?

Answer 24

Genes 1 and 2 can base pair, forming a stem loop, and genes 3 and 4 can base pair, forming another stem loop and a Rho-independent terminator. Alternatively, 2 and 3 can base pair, forming a stem loop, and no Rho-independent terminator is formed.

Question 25

Explain how attenuation in the tryptophan operon leader works.

Answer 25

It controls whether cells need to produce more tryptophan by synthesizing all the way to the end of the gene or not. To sense which to do, the ribosome reads the leader sequence before it starts synthesis, which includes 2 trp codons. If there is enough trp for those, the ribosome will proceed through the Rho-independent terminator form will emerge, cutting off synthesis after gene 4. If there is not enough trp, the ribosome will stall and prevent gene 1 from being synthesized, so gene 2 will base pair with gene 3 and transcription will continue through the whole gene.