WEEK 2 - ribosomes, protein synthesis & endoplasmic reticulum

Question 1

(L3) 2 main subunits of ribosomes:

Answer 1

1. Small subunit
2. Large subunit

• the2 subunits join on mRNA near 5’ end

Question 2

Ribosomes:

Answer 2

• very small structures
  -found in ALL living cells that play a crucial role in protein synthesis
• intercellular structure made of RNA and protein
• site of protein synthesis in the cell
  -2 main subunits, comprised of RNA and proteins – read genetic code and assemble amino acids into polypeptide chains

Question 3

Role of small subunit in ribosome:

Answer 3

decodes genetic info stored in the mRNA

• framework where tRNA matched to mRNA

Question 4

Role of the large subunit of a ribosome:

Answer 4

catalyses formation of peptide bonds between amino acids to form a polypeptide chain.

Question 5

How is the ribosome highly ordered?

Answer 5

with the rRNA molecules forming a scaffold that positions the proteins in the correct orientation to carry out their functions.

Question 6

What do ribosomes do…

Answer 6

Reads the messenger RNA sequence and translates that genetic code into a specified string of amino acids, which grow into long chains that fold to form proteins.

Question 7

How long (approx) does it take to synthesise a protein?

Answer 7

1 min

Question 8

What makes up ribosomal mass?

Answer 8

• approx 50% ribosomal mass = a few rRNA molecules
• remaining mass consists of over 80 different proteins

Question 9

How many binding sites does a ribosome have for RNA and what are they?

Answer 9

4 binding sites in total

• 1 for mRNA
• 3 (A,P,E) for tRNA

Question 10

Where does the translation of mRNA into protein happen?

Answer 10

in cytosol on ribosome

Question 11

What is the reaction for translation driven by?

Answer 11

elongation factors, using GTP hydrolysis

Question 12

How does process stop…

Answer 12

• Process continues until it reaches a stop codon
• Release factor binds to the ribosome – terminates translation and polypeptide is released

Question 13

Role of chaperones:

Answer 13

Folding of newly synthesised proteins assisted by chaperones

Question 14

Transcription=

Answer 14

the genetic info from a strand of DNA is copied into a strand of mRNA

Question 15

Translation=

Answer 15

formation of a chain of amino acids based on the info contained on the mRNA

Question 16

3 classes of RNA molecules:

Answer 16

• messenger RNA
• ribosomal RNA
• transfer RNA

Question 17

RNA molecules:

Answer 17

• single stranded
• sugar = ribose
• contain Uracil not Thymine
• dont form helices - fold into compplex structures that are stabilised by internal complementary base-pairing

Question 18

STEPS IN TRANSCRIPTION:

Answer 18

• DNA unzips: enzymes split apart base pairs and unwind the DNA double helix
• Bases pair up: free nucleotides attach to complementary bases along the new strands using RNA polymerase
• New backbone formed: The sugar-phosphate backbone is assembled to complete the RNA strand.

Question 19

What happens once termination is complete?

Answer 19

Once termination is complete, mRNA molecule peels away from DNA template

A nucleotide is added to 5’ end – capping (an N7 – methylated guanosine)

Question 20

Splicing=

Answer 20

Noncoding nucleotide sequences (introns), removed from mRNA strand

Question 21

Polyadenylation =

Answer 21

Sequence of adenine nucleotides called a poly – A tail added to the 3’ end of the mRNA molecule

Question 22

What does the poly - A molecule do?

Answer 22

Poly – A tail signals to the cell that the mRNA molecule is ready to leave the nucleus and enter the cytoplasm

Question 23

mRNA:

Answer 23

• Most variable class of RNA
• RNA polymerase II synthesises mRNA – requires transcription factors to initiate transcription
• Many different mRNA molecules in a cell at any given time
• Some mRNA abundant, others rare
• Variable life-span e.g. transcripts for signalling proteins degraded in <10 mins, transcripts for structural proteins may remain intact for >10 hrs
• Cells can be characterised by spectrum of mRNA molecules present – the transcriptome

Question 24

How is transcription regulated?

Answer 24

1. Initiation
   - RNA polymerase and transcription factors bind to the DNA strand at a specific area that facilitates transcription – promoter region
2. Elongation
   - RNA polymerase begins moving down the DNA template strand in the 3’ to 5’ direction, adding complementary nucleotides

(Remember – complimentary base pairing
Progressively longer chain of nucleotides)

3. Termination and edititing
   Elongation process needs to end and mRNA to separate from DNA template – termination
   Termination can occur as soon as the polymerase reaches the termination sequence, but in some cases a terminator factor (protein) is also needed.

Question 25

tRNA

Answer 25

transfer DNA - adaptor molecules that recognise and bind codon and amino acid