BMS1060 - Wk6 - Transcripton & Translation

Question 1

Which enzyme catalyses phosphodiester bond formation? Which direction does it run?

Answer 1

RNA polymerase

5’–>3’

Question 2

RNA strands are opposite in _____ to DNA strands.

Answer 2

Polarity

Question 3

Where does the RNA polymerase start? What direction does it move?

Answer 3

At the transcription start site (+1) and moves downstream.

Question 4

What is the promoter region?

Answer 4

DNA sequence tha lies upstream of the start site.

Question 5

How is RNA polymerase different to DNA polymerase?

Answer 5

RNA polymerase:
- is very structurally different (although functionally similar).
-doesn’t require a primer
- doesn’t proofread
- greater error rate
- nucleotidyl transferase - catalyses linkage of ribonucleotides
- 3 diff types of RNA polymerase, producing diff types of RNA.

Question 6

Describe the initiation stage of RNA Transcription.

Answer 6

1. RNA polymerase, with help of transcription factors, finds and binds to promoter region.
2. DNA strands around start site ‘melts’.
3. Template strand enters active site where phosphodiester bonds form etc.

Question 7

Describe the Elongation and Termination Stage of RNA transcription

Answer 7

1. RNA polymerase dissociates from promoter and TFs as it moves along the template strand. (forming phosphodiester bonds)
2. Elongation complex very stable.
3. DNA sequences signal termination. Primary transcript released. Polymerase dissociates.

Question 8

What is needed for transcription?

Answer 8

DNA molecule, Transcription factors, RNA polymerase and motifs (e.g. TATA boxes in promoter region).

Initiation step requires LOTS of proteins - very complex process.

Question 9

After RNA molecules are created, they need to be processed. What 3 things occur in RNA processing? What is their purpose?

Answer 9

1. 5’ capping
2. Polyadenylation of 3’ tail
   - both protect from degradation, aid in export and translation and signify 5’ and 3’ ends of molecule.
3. Splicing of Introns - removes introns

Untranslated Regions (Cap + AAAAAA) are retained during processing but not translated.

Question 10

What are lariats?

Answer 10

lasso-shaped structures of intron RNA sequences in pre-mRNA that are removed in splicing.

Lariat sites signal the start/end of introns.

Question 11

What is alternative splicing? What does it cause?

Answer 11

A process in which exons from the same gene are joined in different combinations. Creates protein diversity.

The same genes when transcribed can produce different mRNA molecules depending on the tissue they are produced in -> affects proteins produced.

Question 12

Where does translation occur?

Answer 12

In the cytosol / cytoplasm

Question 13

What is a codon? How many possible codons are there?

Answer 13

Consecutive 3 nucleotides in RNA specifying an amino acid or ‘stop’ signal (e.g. AUG, AAC, CUG etc)

64 possible codons (4x4x4)

Question 14

There are __ amino acids. Most have more than one synonymous codon. What is this called?

Answer 14

20

Code degeneracy - occurs at 3rd position

Question 15

What is Redundancy?

Answer 15

When an amino acid has more than one tRNA molecule.

Question 16

How do you figure out the mRNA code from the tRNA anticodon?

Answer 16

Reverse order (5’->3’). Find complementary base pairs. Find associated amino acid.

5’-GUC-3’ anticodon –reverse–> CUG –complement-> GAC –> Asp

Question 17

What are the steps of translation?

Answer 17

Activation (extra step than transcription) - coupling correct AA to appropriate tRNA molecule.

Initiation - Assembly or ribosome subunits, AUG-tRNA and mRNA.

Elongation - AAs added to polypeptide chain in mini cycle. (see below)

Termination - release of polypeptide chain from tRNA and dissociation of ribosomes.

Question 18

What enzyme carries out Activation (translation)?

Answer 18

Aminoacyl-tRNA Synthetase - takes correct AA to correct anticodon.

There are around 20 different synthetase enzymes for 20 different AAs.

Question 19

What are the 3 sites on ribosome?

Answer 19

A-Site
P-Site
E-site

Question 20

Describe in greater detail the Initiation step of Translation?
- What is needed at beginning?
- What is recruited later?
- Which site do the tRNA molecules bind to?

Answer 20

Small ribosomal subunit, mRNA and Met-tRNA.

Assembles and moves from 5’->3’ until AUG codon reached.

Large ribosomal subunit recruited along with other tRNA molecules - which bind to A-site.