Lecture 6: Transcription

Question 1

Compare the overview of transcription vs translation

Answer 1

Transcription is transfer of information from DNA into mRNA. vs Translation is nucleotide sequence of mRNA translated into amino acid sequence of a protein.

Question 2

What is the difference between transcription in eukaryotes vs prokaryotes

Answer 2

The mRNA molecule doesn’t need to undergo any RNA processing in transcription before translating in prokaryotic cells

Question 3

Describe 3 steps of Initiation- transcription

Answer 3

RNA polymerase binds to a nucleotide sequence called the promoter. This also determines which strand will be the template.

2. DNA strands unwind
3. RNA polymerase initiates RNA synthesis at start point within promoter of template strand

Question 4

Compare initiation prokaryote vs eukaryote

Answer 4

P: RNA polymerase can bind directly to promoter itself
E: transcription factor proteins facilitate the binding of RNA polymerase to the promoter and initate transcription

Question 5

Describe elongation- transcription

Answer 5

RNA polymerase catalyses RNa nucleotides being added complementary to DNA template to the 3’ end.
The New RNA peels away from the template strand, dissociating from the DNA strands which can come back together to wind back.

Question 6

Describe Termination- transcription

Answer 6

Complete RNA transcript is released. RNA polymerase detaches from DNA

Question 7

Compare Termination in prokaryotes vs eukaryotes

Answer 7

P: transcription happens because of termination sequence in transcribed RNA. This makes a signal causing polymerase to release DNA and transcript.
E: When the polyadenylation signal sequence is transcribed into RNA, proteins in the nucleus bind to it and these cut the RNA from the polymerase.

Question 8

In DNA A-T, in RNA

Answer 8

A-U

Question 9

Give an example of some genes may overlap

Answer 9

HIV-1 genomes consists of nine genes, some of which are overlapping.

Question 10

What are the modifications done to premRNA before it leaves in Eukaryotes

Answer 10

Enzymes add 5’ cap (has guanine base with triP linkage to 5’UnTranslatedRegion. 3’ poly A tail to pre mRNA.
Splicing: Introns (non coding regions) are cut out, and exons are ligated together to form the mature mRNA - UTR coding segmentUTR .

Question 11

What is the function of 5’ cap and 3’ poly A tail.

Answer 11

Promotes the export of mRNA from nucleus through nuclear pores and protects mRNA from degradation from hydrolytic enyzmes . 5’ cap facilitates ribosome attachment at the start of translation

Question 12

What splices the premRNA

Answer 12

snRNP: small nuclear ribonucleoproteins (snRNA ribozyme and protein) + other proteins make the Spliceosome which splices pre mRNA

Question 13

How does the spliceosome splice

Answer 13

snRNA in the spliceosome pairs with bases in the pre mRNA intron (splice sites)
The spliceosome cuts the premRNA, releasing the intron for degradation and splices the exons together. Then releasing the mRNA

Question 14

How does each exon get expressed in the protein

Answer 14

Different exons code for specific domains of proteins which have distinct functions.

Question 15

What catalyses the splicing

Answer 15

snRNA

Question 16

Describe initiation (translation) eukaryote

Answer 16

1. initiator tRNA binds to small ribosomal subunit
2. 5’ cap mRNA binds to the small ribosomal subunit
3. initiator tRNA H-binds to start codon AUG- methionine on mRNA .
4. Large ribosomal subunit the binds and intiator tRNA is positioned into P site

Question 17

Describe elongation (translation)

Answer 17

1. tRNA pairs complementary to mRNA codon in A site
2. An rRNA molecule of the large ribosomal unit (ribozyme) catalyses the formation of peptide bond between the new amino acid and the growing peptide
3. The ribosome moves tRNAs into neighbouring sites which moves the mRNA along and expose the next codon into A site

Question 18

Describe Termination (translation)

Answer 18

When a stop codon is pulled into A site, it binds to a release factor- a protein shaped like tRNA. This promotes release of polypeptide from tRNA in P site by hydrolysis. Polypeptide released through exit tunnel and the mRNA + ribosome complex dissociate

Question 19

What is the signal mechanism for targetting proteins to the ER

Answer 19

Signal peptides on the newly generated polypeptides can be met by SRP which docks with the receptor protein on the ER and lets the protein be released into the ER where are they modified- later golgi.

Question 20

What are polyribosomes - for efficiency

Answer 20

A single mRNA strand along which many ribosomes are travelling. Each of the ribosomes is synthesising growing polypeptide chains

Question 21

How are specific amino acids joined to specific tRNA

Answer 21

Enzymes (1 for each amino acid) called aminoacyl tRNA synthetases catalyse the covalent bonding of correct amino acid to tRNA molecule that binds to active site, using ATP.

Question 22

Where does the 5’ cap bind at the start of translation

Answer 22

Small subunit of ribosome under E site

Question 23

Compare P site, A site and E site of tRNA binding sites on ribsome

Answer 23

P site: holds the tRNA carrying the growing polypeptide chain

A site: holds the tRNA carrying the next amino acid to be added

E site: where discharged tRNAs leave the ribosome.

Question 24

Where is the energy for translation come from

Answer 24

GTP

Question 25

Why can coupled translation happen in prokaryotes not in Eurkaryotes

Answer 25

In Eukaryotic cells transcription and translation separated by physical barrier of the nucleus + pre mRNA modification needs to happen

Question 26

What is coupled transcription

Answer 26

translation begins while mRNA is still be transcripted