control of gene expression

Question 1

why study E.coli

Answer 1

bacteria are key human pathogens
bacterial gene expression machinery is the target for some antibiotics
E.coli is an important host for production of recombinant proteins for research and industrial/medical purposes
provides a framework for more complex organisms

Question 2

transcription in E.coli

Answer 2

1. promotor
   immediately upstream of transcribed region
   start signal
2. transcribed region
   one mrna encodes more than one protein to allow co ordinated expression of a group of genes
3. terminator = stop signal

Question 3

polycistronic

Answer 3

one mrna encodes more than one protein

Question 4

E.coli sigma 70 promotor

Answer 4

binding site for rna olymerase

40-60 bp region upstream of txn start site

Question 5

promotors

Answer 5

dictate where txn begins
strength of promotor dictates how efficiently transcription is initiated
strength dictated by sequence

Question 6

E.coli RNA polymerase

Answer 6

Magnesium dependent
multisubunit
core and sigma factor = holoenzyme

Question 7

holoenzyme

Answer 7

compound of enzyme and coenzyme

Question 8

Core

Answer 8

catalyses transcription

cant recognise/bind to promotor

Question 9

sigma factor

Answer 9

binds to core to convert it to holoenzyme

directs recognition of promotor sequences

Question 10

sigma 70

Answer 10

primary sigma factor which directs initiation from promotors of most genes in growing cells

Question 11

alternative sigma factors

Answer 11

utilized under different environmental conditions

they recognise promotors of genes appropriate to environmental conditions

Question 12

factor independent termination

Answer 12

series of 4-10 AT base pairs

a G+C region with a palindromic sequence that immediately precedes the series of A-T base pairs

Question 13

Rho dependent termination

Answer 13

rho factor = six identical subunits
rho is a helicase that unwinds RNA-DNA and RNA-RNA duplexes
powered by ATP hydrolysis

Question 14

consensus sequence

Answer 14

calculated order of most frequent nucleotides

Question 15

transcription initiation regulated by

Answer 15

negetive regulatory factor called repressors

positive acting factors called activators

Question 16

when does high transcription of lac structural genes occur

Answer 16

lactose present

glucose is absent

Question 17

lac repressor

Answer 17

key to regulation of lac operon in response to lactose
product of lacl gene is a repressor 
360 amino acid protein
forms homotetrameter
binds to lac operon

Question 18

lactose effect on the lac operon

Answer 18

lac operon induced when lactose provided

inducer is allolactose

Question 19

allolactose and the lac repressor

Answer 19

causes conformational change
binding subunits separate by 3.5A
affinity for operator reduced by 1000

Question 20

how does glucose influence transcription of lac operon

Answer 20

glucose levels influence cAMP levels
cAMP made from ATP by adenylate cyclase
glucose travel into cell inhibits adenylate cyclase and prevents cAMP accumulation
as glucose levels drop cAMP accumulates
cAMP binds to and activates CAP

Question 21

glucose present
no lactose
(lac Operon)

Answer 21

no residual txn
repressor blocks txn
CAP doesnt bind
no activation

Question 22

no glucose
high cAMP
no lactose
(lac Operon)

Answer 22

no resifual txn

repressor blocks RNA pol

Question 23

glucose present
low cAMP
lactose present
(lac Operon)

Answer 23

little txn

CAP doesnt activate

Question 24

no glucose
high cAMP
lactose present

Answer 24

high txn
no repressor
CAP activates

Question 25

genetic code features - triplet code

Answer 25

simplest with enough info to encode 20 amino acids

64 combinations

Question 26

does the code overlap

Answer 26

no

Question 27

degenerate code

Answer 27

one amino acid coded by more than 1 codon

Question 28

tRNA primary structure

Answer 28

small nucleic acids
5’ monophosphate rather than 5’ triphosphate
certain modified bases

Question 29

tRNA secondary structure

Answer 29

D loop
T loop
Variable arm
Anti codon loop
Amino acid acceptor site

Question 30

tRNA tertiary tructure

Answer 30

amino acid acceptor stem

Question 31

aminoacylation of tRNAs

Answer 31

catalysed by tRNA synthetases
1. AMP added to carboxyl group of the a.a. to give high energy intermediate
= animoacyl adenylate
2. aminoacyl adenylate reacta with appropriate uncharged tRNA
= aminoacyl tRNA and AMP

Question 32

tRNA identity elements

Answer 32

synthetase enzyme must distinguish between 40 similar but different tRNA molecules

Question 33

identity elements

Answer 33

particular parts of tRNA molecules used by the synthetase enzyme to distinguish between tRNA molecules

Question 34

acylation site

Answer 34

rejects a.a. larger than correct one