Transcription and Translation

Question 1

What is transcription

Answer 1

1) Information carried by DNA is transcribed into messenger RNA (mRNA)
2) DNA strands acts as a template for new RNA strand synthesis by RNA polymerase II
RNA polymerase II synthesises a single stranded mRNA in the 5’ → 3’ direction

Question 2

3 steps of transcription

Answer 2

Initiation
Elongation
Termination

Question 3

Differences between RNA and DNA

Answer 3

1. DNA= double stranded, RNA= single stranded
2. DNA= deoxyribose, RNA= ribose
3. DNA= thymine, RNA= uracil

Question 4

types of RNA involved in protein synthesis

Answer 4

mRNA
tRNA
rRNA

Question 5

Transcription- Initiation

Answer 5

1. RNA polymerase binds to promoter region on DNA (TATA box= TATAAA)
2. RNA polymerase can open up and unwind DNA
3. Transcription factors (TF II) bind to promoters for RNA polymerase to DNA
4. Initiation complex= TF, RNA polymerase and DNA
5. Enhancers involved, mediators link TFs and initiation complex to RNA pol II
6. Transcription bubble formed
7. 5’ end of mRNA is methylated- stabilises mRNA

Question 6

What are promoters?

Answer 6

Sites on DNA that RNA polymerase bind to to initiate transcription

Question 7

Transcription- Elongation

Answer 7

RNA Pol II copies the DNA template – complementary base pairing except U (uracil) replaces T (thymine) in RNA
5’ → 3’ direction, adding bases to the 3’ end of the growing mRNA
Template strand= antisense
Non template strand= sense strand
RNA pol II adds nucleotides to antisense strand from 5’ to 3’ but RNA pol II reads DNA in 3’ to 5’ direction

Question 8

Transcription- Termination

Answer 8

eukaryotes-unknown
But in bacteria (prokaryotes):
GC rich hairpin lopped formed following followed by a string of uracil residues
Formation of a Poly-A tail (Adenine)- AAAA I added to 3’ end of mRNA (by poly-A polymerase)
mRNA detaches due facilitated by weak A-U pairing
transcript leaves polymerase complex

Question 9

What happens between transcription and translation?

Answer 9

mRNA processing- splicing- removing introns and joining exons

Question 10

Prokaryotic ribosome structure

Answer 10

2 subunits= 50S (Large), 30S (small)= 70s ribosome
50S= 23 S rRNA
30S= 16 S rRNA

Question 11

Eukaryotic ribosome structure

Answer 11

2 subunits= 60S (large), 40S (small)= 80s ribosome

60S has 3 active sites for amino acid= E, P, A

Question 12

What is tRNA?

Answer 12

Carries anticodon
Codon is specific for anticodon
Anticodon= 3 bases complementary to codon

Question 13

3 steps of translation

Answer 13

Initiation, elongation, termination

Question 14

Translation- Initiation

Answer 14

1. Free 30S small subunit
2. Initiation complex= 30S subunit, start codon attaches to 30S (AUG) mRNA, tRNA, Initiation factors (IF-1,2,3)
3. 50S large subunit joins initiation complex
4. Initiator= aminoacyl tRNA binds to start codon
5. tRNA start codon(UAC) enters P site

Question 15

Translation- Elongation

Answer 15

1. First tRNA attaches to A site (acceptor site) on large subunit
2. Assisted by elongation factors (EF1α)
3. Initiating tRNA occupies the P site with its anticodon positioned at the mRNA codon
4. A second aminoacyl tRNA anticodon enters A site with its anticodon positioned at the mRNA codon
5. Peptide bond formed between 2 amino acids at P and A site
6. Following elongation polypeptide of translocated from A site to P site
7. Then moves to E site
8. Translocation= mRNA moving along ribosome= facilitated by EF protein
9. Peptidyl transferase catalyses the formation of a peptide bond between the 2 amino acids

Question 16

Translation- termination

Answer 16

1. Ribosome reaches a stop codon- no tRNA can bind to this
2. Release factors (eRF1) at A site recognise stop codon and release attached polypeptide from final tRNA and ribosome subunits dissociate
   Stop codons=UAA, UAG, UGA

Question 17

Functions of proteins

Answer 17

Protein functions
1) Catalysis:
Almost all chemical reactions in a living
cell are catalyzed by protein enzymes e.g alcohol dehydrogenase
2) Transport:
Some proteins transports various
substances, such as oxygen, ions e.g haemoglobin
3) Information transfer: For example, hormones.