Seminar 13: The flow of information from genes to proteins

Question 1

what are the 3 major steps in transcription

Answer 1

1. initiation
2. elongation
3. termination

Question 2

differentiate b/w initiation in prokaryotes & eukaryotes

Answer 2

PRO: Sigma factors work allow RNA poly to bind to promotor & initiate transcription

EU: General transcription factors, recruit RNA poly to promotor

Question 3

HOW does RNA poly recog WHERE to start transcription?

Answer 3

Recognise promoters
- specific seq in DNA that define transcriptional start site of genes

Question 4

Elongation occurs in the __ to __ direction

Answer 4

5’ to 3’

Question 5

what happens in elongation?

Answer 5

• RNA poly UNWINDS DNA
• complementary nucleotides introduced to template strand

Question 6

func of the termination seq?

Answer 6

defines the END of gene
so transcription STOPS

Question 7

RNA is in the same 5’ to 3’ seq as _____ DNA strand, but has __ instead of __

Answer 7

• COMPLEMENTARY
• Uracil no Thymine

Question 8

why is there no RNA splicing in prokary

Answer 8

no introns

Question 9

how many pp subunits in bacterial RNA enzyme, desc them

Answer 9

• 5 pp subunits
• 2 identical alpha
• Beta & Beta prime
• omega

Question 10

there are __ RNA poly in eukaryotes

Answer 10

3

Question 11

func of RNA poly 1

Answer 11

transcribes most of rRNA genes

Question 12

func of RNA poly 2

Answer 12

transcribes majority of mRNA & some SMALL RNA

Question 13

func of RNA poly 3

Answer 13

transcribes tRNA genes, some snRNA & other small RNA genes

Question 14

what happens in RNA processing?

Answer 14

• splicing of introns
• add 5’ methyl cap
• add 3’ poly A tail

Question 15

what cuts introns out

Answer 15

spliceosome
- attach remaining exons together

Question 16

func of poly A tail

Answer 16

signals to transport molecules that mRNA transcript is ready to leave nucleus

Question 17

func of 5’ methyl cap

Answer 17

5’ cap helps ribosome attach to transcript for translation

Question 18

modifications to mRNA help to ___ the ___ of transcript from ____

Answer 18

PROTECT the ENDS of transcript from DEGRADATION

Question 19

what is an open reading frame?

Answer 19

Region of DNA seq that corresponds to a mRNA transcript that can be translated by a ribosome > continuous pp seq. Begins w/ start codon & ends w/ stop codon (intervening seq = multiple of 3 nucleotides)

Question 20

What happens if the translation is in an incorrect reading frame?

Answer 20

Non functional polypeptide chain, shorter, w/ incorrect A.A & in wrong sequence. Cannot form correct protein as it wont fold into tertiary structure.

Question 21

desc degeneracy of genetic code

Answer 21

• Some amino acids are coded by more than 1 codon
• Degeneracy is only in 3rd codon

Question 22

START codon

Answer 22

AUG

Question 23

STOP codons

Answer 23

UAA, UAG, UGA

Question 24

Advantage of degeneracy of genetic code?

Answer 24

Reduces impact of genetic mutations

Question 25

mRNA func

Answer 25

protein synthesis, template for ribosome to build protein

Question 26

tRNA func

Answer 26

recognise codons via anticodon (complementary to mRNA codon) & carries A.A to mRNA strand

Question 27

rRNA func

Answer 27

ribosome synthesis, translate genetic info

Question 28

desc INITIATION in EUKARYOTES - translation

Answer 28

- Small subunit of ribosome in complex w/ methionine tRNA & initiation factors bind to 5' end of mRNA - scans the strand until finds AUG codon = translation initiation site

Question 29

desc INITIATION in PROKARYOTES - translation

Answer 29

- Bacteria don't have 5' caps to initiate translation, have LEADER SEQ (upstream of 1st AUG codon) - Shine-Dalgarno seq Serves as ribosomal binding site (by base seq of seq)

Question 30

desc elongation in translation (4)

Answer 30

- tRNA enters A site by recog complementary codon to its anticodon - Ribosome catalyses peptide bond formation b/w 2 A.A in A & P site - Then, ribosome repositions over NEXT codon & uncharged tRNA is released from E site - Repeat process until pp chain formed

Question 31

how is TERMINATION initiated in translation?

Answer 31

- When stop codon positioned on A site of large subunit, recog by proteins (release factors: RF1 or RF2) - Binding forces enzyme (Peptidyl transferase) in ribosome to catalyse addition of H2O instead of A.A to peptidyl tRNA - P site A.A will detach from tRNA - Releases new PP chain - RF3 (bound to GDP) joins to ribosome - RF3 replace GDP w/ GTP - Brings conformational change in RF3 - Triggers RF1 & RF2 to UNBIND from ribosome - RF3 catalyses GTP hydrolysis - Allows ribosomal subunits to dissociate from one another & from mRNA

Question 32

what are the 3 sites of the large ribosomal subunit ?

Answer 32

- A site = Amino Acetyl (where tRNA arrives) - P site = Peptidyl (tRNA moves here, peptide bond forms b/w A.A in tRNA at A site & P site) - E site = Exit site