Lecture 6: Gene to protein Tranlsation

Question 1

Genetic information flows from _____ through the process of translation

Answer 1

mRNA to protein

Question 2

A cell translates an mRNA message into a protein with the help of

Answer 2

transfer RNA (tRNA)

Question 3

What do tRNAs do?

Answer 3

Transfer amino acids to the growing polypeptide in a ribosome

Question 4

The tRNA molecule:

Answer 4

• A cell has all 20 amino acids available in its cytoplasm.
• Molecules of tRNA are not identical.
• Each carries a specific amino acid on one end.
• Each has an anticodon on the other end that base-pairs with a complementary codon on mRNA

Question 5

What does the anticodon bind to? and the other end?

Answer 5

base pairs with with a complementary codon on mRNA

- an amino acid binds to the other end

Question 6

The structure of tRNA:

a tRNA molecules consists of about ___ ____ folded back on itself

Answer 6

80 nucleotides

Question 7

The structure of tRNA: includes a loop containing …

Answer 7

the anticodon and an attachment site at the 3’ end for an amino acid

Question 8

The structure of tRNA: hydrogen bonds cause the molecule to

Answer 8

twist and fold into a three-dimensional molecule

Question 9

Shape of tRNA

Answer 9

is roughly L shaped

Question 10

Like other types of RNA, tRNA molecules are transcribed from

Answer 10

DNA templates

Question 11

In both eukaryotic and bacterial cells are tRNA used repeatedly?

Answer 11

YES:
• It picks up its designated amino acid in the cytosol, and deposits it at the ribosome.
• The tRNA then returns to the cytosol to pick up another copy of that same amino acid.

Question 12

Accurate translation requires two recognition steps:

Answer 12

1. A correct match between a tRNA and an amino acid

2. A correct match between the tRNA anticodon and an mRNA codon

Question 13

recognition step 1: 1. A correct match between a tRNA and an amino acid

Answer 13

1. Each amino acid is joined to the correct tRNA by aminoacyl- tRNA synthetase

Question 14

Aminoacyl-tRNA synthesise work:

Answer 14

• There are 20 different synthetases that match the 20 different amino acids.
• Each has an active site specific for a particular tRNA–amino acid combination.
• The synthetase catalyzes a covalent bond between them in a process driven by ATP hydrolysis.
• The result is an aminoacyl- tRNA or charged amino acid

Question 15

Recognition step 2:

2. A correct match between the tRNA anticodon and an mRNA codon

Answer 15

• If each anticodon had to be a perfect match to each codon, we would expect to find 61 types of tRNA
• The actual number is about 45, because the anticodons of some tRNAs recognize more than one codon.
• Rules for base pairing between the third base of the codon and the anticodon are relaxed.
• This flexible base pairing is called wobble.

Question 16

Site for translation:

Answer 16

RIBOSOME

Question 17

Ribosomes facilitate

Answer 17

specific coupling of tRNA anticodons with mRNA codons in protein synthesis

Question 18

A ribosome consists of

Answer 18

a large and a small subunit, each made up of proteins and ribosomal RNA (rRNA).

Question 19

Why is rRNA the most abundant type of cellular RNA

Answer 19

because most cells contain thousands of ribosomes

Question 20

A ribosome has 3 binding sites for tRNA::

Answer 20

– The P site holds the tRNA that carries the growing polypeptide
chain
– The A site holds the tRNA that carries the next amino acid to be added to the chain
– The E site is the exit site, where discharged tRNAs leave the ribosome

Question 21

P site =

Answer 21

Peptidyl-tRNA binding site

Question 22

A site -

Answer 22

Aminoacyl-tRNA binding site

Question 23

E site -

Answer 23

Exit site

Question 24

Ribosomes work in translation:

Answer 24

• The ribosome holds the tRNA and mRNA in close proximity and positions the new amino acid for addition to the carboxyl end of the growing polypeptide.
• It then catalyzes the formation of the peptide bond.
• As the polypeptide becomes longer, it passes through an exit tunnel and is eventually released.

Question 25

3 stages of translation:

Answer 25

-initiation -elongation -termination • All three stages require protein “factors” that aid in the translation process

Question 26

Ribosome association and initiation of translation

Answer 26

-A small ribosomal subunit binds with mRNA and a special initiator tRNA which carries methionine binds to the start codon. - Attachment of a large ribosomal subunit, forming the translation initiation complex then occurs. • Energy in the form of a GTP molecule is invested in the formation of the initiation complex. GTP --> GDP + Pi • Initiator tRNA is in the P site.

Question 27

Ribosome association and initiation of translation: In BACTERIA

Answer 27

In bacteria, the binding occurs at a specific RNA sequence, just upstream of the start codon, AUG.

Question 28

Ribosome association and initiation of translation: In EUKARYOTE

Answer 28

In eukaryotes, the small subunit, with the initiator tRNA already bound to the mRNA, binds to the 5ʹ cap of the mRNA and then moves downstream along the mRNA until it reaches the start codon AUG.

Question 29

Elongation stage:

Answer 29

During the elongation stage, amino acids are added one by one to the preceding amino acid at the C-terminus of the growing chain • Each addition involves proteins called elongation factors and occurs in three steps: – codon recognition – peptide bond formation – translocation • Translation proceeds along the mRNA in a 5′to3′ direction

Question 30

Elongation:

Answer 30

``` 1. Codon recognition: Incoming tRNA base pairs with 3ʹ complementary mRNA anti-codon in A site ``` 2. Peptide bond formation: Polypeptide from tRNA in the P site is attached to the amino acid in the A site via a peptide bond. 3. Translocation: The mRNA with its bound tRNAs moves along bring the next codon to be translated to the A site. The tRNA from the A site moves to the P. The empty tRNA in the P site moves to the E site and is released.

Question 31

Termination of trnalstion: Termination occurs when

Answer 31

when one of the three stop codons reaches the A site of the ribosome. - A release factor binds to the stop codon and causes hydrolysis of the bond between the polypeptide and its tRNA in the P site. - This frees the polypeptide, which is released through the exit tunnel of the ribosome’s large subunit. - The translation complex disassembles. - Break down of the translation assembly requires the hydrolysis of two more GTP molecules

Question 32

Polyribosomes: A single mRNA may be used to make

Answer 32

many copies of a polypeptide simultaneously as multiple ribosomes, polyribosomes or polysomes, trail along the same mRNA.

Question 33

Often translation is not sufficient enough to make ___ | SO..

Answer 33

- Often translation is not sufficient to make a functional protein. - Polypeptide chains are modified after translation or targeted to specific sites in the cell

Question 34

During and after synthesis, a polypeptide chain spontaneously

Answer 34

could and folds into its 3D shape

Question 35

Proteins may all require post-translational modification before doing their job

Answer 35

– Some polypeptides are activated by enzymes that cleave them – Other polypeptides come together to form the subunits of a protein

Question 36

Protein folding:

Answer 36

- During and after synthesis, a polypeptide chain spontaneously coils and folds into its three-dimensional shape - Amino acids interact with each other to produce a well-defined three-dimensional structure - Some neurogenerative diseases result from accumulation of amyloid fibrils formed when proteins fold incorrectly. - Some allergies may result from misfolded proteins.

Question 37

two populations of ribosomes are evident in cells:

Answer 37

- free ribosomes (in the cytosol) | - bound ribosomes (attached to the ER)

Question 38

Free ribosomes mostly

Answer 38

synthesise proteins that function in the cytosol

Question 39

Bound ribosomes make

Answer 39

proteins of the endomembrane system and proteins that are secreted from the cell

Question 40

ribosomes are identical and can

Answer 40

switch from free to bound

Question 41

polypeptides destined for the ER or for secretion are marked by

Answer 41

a signal peptide

Question 42

Targeting polypeptides to specific locations:

Answer 42

1) Polypeptide synthesis always begins in the cytosol 2) A signal recognition particle (SRP) binds to the signal peptide of a polypeptide momentarily stopping translation 3) The SRP binds to a receptor protein in the ER membrane 4) The SRP leaves & polypeptide synthesis resumes with simultaneous translocation of the polypeptide across the membrane 5) a Signal leaving enzyme cuts off the signal peptide 6) The completed polypeptide is released