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APS 138: Molecular and cell biology > Lecture 6: Gene to protein Tranlsation > Flashcards

Lecture 6: Gene to protein Tranlsation Flashcards

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1
Q

Genetic information flows from _____ through the process of translation

A

mRNA to protein

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2
Q

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

A

transfer RNA (tRNA)

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3
Q

What do tRNAs do?

A

Transfer amino acids to the growing polypeptide in a ribosome

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4
Q

The tRNA molecule:

A
  • 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
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5
Q

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

A

base pairs with with a complementary codon on mRNA

- an amino acid binds to the other end

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6
Q

The structure of tRNA:

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

A

80 nucleotides

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7
Q

The structure of tRNA: includes a loop containing …

A

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

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8
Q

The structure of tRNA: hydrogen bonds cause the molecule to

A

twist and fold into a three-dimensional molecule

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9
Q

Shape of tRNA

A

is roughly L shaped

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10
Q

Like other types of RNA, tRNA molecules are transcribed from

A

DNA templates

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11
Q

In both eukaryotic and bacterial cells are tRNA used repeatedly?

A

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.

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12
Q

Accurate translation requires two recognition steps:

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

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

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13
Q

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

A
  1. Each amino acid is joined to the correct tRNA by aminoacyl- tRNA synthetase
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14
Q

Aminoacyl-tRNA synthesise work:

A
  • 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
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15
Q

Recognition step 2:

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

A
  • 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.
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16
Q

Site for translation:

A

RIBOSOME

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17
Q

Ribosomes facilitate

A

specific coupling of tRNA anticodons with mRNA codons in protein synthesis

18
Q

A ribosome consists of

A

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

19
Q

Why is rRNA the most abundant type of cellular RNA

A

because most cells contain thousands of ribosomes

20
Q

A ribosome has 3 binding sites for tRNA::

A

– 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

21
Q

P site =

A

Peptidyl-tRNA binding site

22
Q

A site -

A

Aminoacyl-tRNA binding site

23
Q

E site -

A

Exit site

24
Q

Ribosomes work in translation:

A
  • 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.
25
Q

3 stages of translation:

A

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

26
Q

Ribosome association and initiation of translation

A

-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.

27
Q

Ribosome association and initiation of translation: In BACTERIA

A

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

28
Q

Ribosome association and initiation of translation: In EUKARYOTE

A

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.

29
Q

Elongation stage:

A

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

30
Q

Elongation:

A 
1. Codon recognition:
Incoming tRNA base
pairs with
3ʹ complementary mRNA
anti-codon in A site
  1. Peptide bond formation:
    Polypeptide from tRNA in the P site is attached to the amino acid in the A site via a peptide bond.
  2. 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.
31
Q

Termination of trnalstion: Termination occurs when

A

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
32
Q

Polyribosomes: A single mRNA may be used to make

A

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

33
Q

Often translation is not sufficient enough to make ___

SO..

A
  • Often translation is not sufficient to make a functional protein.
  • Polypeptide chains are modified after translation or targeted to specific sites in the cell
34
Q

During and after synthesis, a polypeptide chain spontaneously

A

could and folds into its 3D shape

35
Q

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

A

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

36
Q

Protein folding:

A
  • 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.
37
Q

two populations of ribosomes are evident in cells:

A
  • free ribosomes (in the cytosol)

- bound ribosomes (attached to the ER)

38
Q

Free ribosomes mostly

A

synthesise proteins that function in the cytosol

39
Q

Bound ribosomes make

A

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

40
Q

ribosomes are identical and can

A

switch from free to bound

41
Q

polypeptides destined for the ER or for secretion are marked by

A

a signal peptide

42
Q

Targeting polypeptides to specific locations:

A

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

APS 138: Molecular and cell biology (15 decks)

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