27. RIBOZYMES

Question 1

1. What are Ribozymes?

Answer 1

• they are Catalytic RNA molecules
• they function as enzymes

Question 2

2. Where are Ribozymes found?

Answer 2

• they are found in Prokaryotes
• they are found in lower eukaryotes

Question 3

3. What role do Riboenzymes have?

Answer 3

• they have a role in RNA splicing in some organisms
• EG: Protozon Tetrachymena (in the intron part)

Question 4

4. What ends does an intron have?

Answer 4

• it has a 5’ end splice site
• this is the GU
• this is the donor
• it has a 3’ end splice site
• this is the AG
• this is the acceptor

Question 5

5. What is one of the 2 essential Transesterification reactions regarding introns?

Answer 5

• there is 2 ‘ OH of a specific branch point nucleotide
• it is found within the intron (A)
• this performs a nucleophilic attack
• it attacks the first nucleotide of the intron
• it does this at the 5’ splice site (GU-donor)
• this forms the Lariat Intermediate

Question 6

6. What is the 2nd essential Transesterification reaction regarding introns?

Answer 6

• the 3 ‘ OH of the released 5’ exon
• will perform a nucleophilic attack
• it attacks the first nucleotide
• that follows the last nucleotide of the intron
• at the 3’ splice site (AG- Acceptor)
• it joins the exons
• it releases the Lariat Intermediate intron

Question 7

7. Describe RNA Splicing in 3 steps?

Answer 7

1. THE PRE-MRNA:
   - is cut at the 5’ splice site
   - this is the junction of the 5’ exon and the intron GU
2. THE 5’ END OF THE INTRON:
   
   • is then joined to the branch point (A) within the
     intron
   • this generates the lariat-shaped molecule
     characteristic
3. THE 3’ SPLICE SITE IS CUT:
   
   • this is the AG acceptor
   • the two exons are joined together
   • the intron is released

Question 8

8. What role do introns have in RNA Alternative Splicing?

Answer 8

• they produce different proteins from the same pre-
  mRNA transcript

Question 9

9. What is Tropomyosin?

Answer 9

• this is an essential thin filament protein
• it regulates muscle contractions and relaxation
• it reacts with the actin, myosin and troponin complex

Question 10

10. List the functional and evolutionary role of Alternative Splicing?

Answer 10

1. Some genes can encode more than one polypeptide
   - this depends on which segments are treated as exons
   - this happens during RNA splicing
2. The number of proteins an organism can produce
   
   • is much greater than the number of genes
3. Different exons may code for different domains in a
   protein
4. Exon shuffling may result in the evolution of new
   proteins

Question 11

11. What kind of architecture do proteins usually have?

Answer 11

• they have a modular architecture
• this consists of different domains

Question 12

12. What is Translation?

Answer 12

• this is the synthesis of a polypeptide chain
• it is directed by DNA

Question 13

13. What happens during Translation?

Answer 13

• the genetic information is passed from the mRNA to
  the protein

Question 14

14. Which components are present during Translation?

Answer 14

1. mRNA
2. tRNA
   
   • this is the Transfer RNA
   • it transfers the amino acids to their position along
     the polypeptide sequence
   • it transfers these amino acids from the cytoplasmic
     pool to the ribosome
3. RIBOSOMES
   - this is a complex structure
   - it consists of a coupled mRNA and tRNA
   - it also has an amino acid added to it

Question 15

15. How does the tRNA act as a translator?

Answer 15

• it reads the nucleic acid
• this nucleic acid is the mRNA codon
• it interprets it as an amino acid
• this makes it a protein

Question 16

16. What is the structure of the tRNA?

Answer 16

1. IT IS A SINGLE STRAND
   
   • it is about 80 nucleotides long
2. IT LOOKS LIKE A FOUR LEAF CLOVER (2D Shape)
   
   • it has a L shape when it is 3D
3. IT CARRIES A SPECIFIC AMINO ACID
   
   • on one end
4. IT HAS AN ANTICODON ON THE OTHER END
   
   • this is a type of nucleotide triplet
   • this anticodon pairs with a complementary codon
   • this complementary codon is on the mRNA
   • they bond using hydrogen bonding

Question 17

17. In which direction does translation proceed along the mRNA strand?

Answer 17

• it proceeds in a 5’-3’ direction
• the ribosomes read the mRNA in a 5’-3 direction

Question 18

18. In which direction does Polypeptide synthesis occur?

Answer 18

• it goes in an N terminal to a C terminal direction

Question 19

19. Which enzyme works with tRNA?

Answer 19

• Aminoacyl tRNA synthetase
• it joins the amino acid to the tRNA
• there are 20 different kinds of Aminoacyl-tRNA
  synthetases
• one for each type of amino acid