Gene Expression And Regulation In Prokaryores

Question 1

What is Gene Expression?

Answer 1

Gene (DNA) undergoes transcription into m-RNA that can translate the encoded genetic information into proteins

Question 2

What is transcription ?

Answer 2

Synthesis of RNA from DNA as a template by an enzyme called DNA dependant RNA polymerase ( RNAP )

Question 3

Steps of transcription in eukaryotes

Answer 3

1. Initiation
2. Elongation
3. Termination

Question 4

Initiation of transcription in prokaryotes( Mainly and detailed ) :

Answer 4

Mainly : It involves the binding of RNAP with the promotor.
Detailed : 1) Segma factor ( specific protein factor ) recognises and binds to the promotor region at the TATA box
2) RNAP starts transcription at the start point (+1)

Question 5

What is the promoter region :

Answer 5

Nucleotides sequence , in dna , to which RNAP binds to begin transcription

Question 6

The nucleotides sequence in The promoter region in prokaryotes include :

Answer 6

1. TATA Box
2. TTGACA Box

Question 7

About TATA box :

Answer 7

1. Formed of six nucleotides (TATAAT)
   2.located 10 base pairs upstream to the start point (around base number -10 )
2. It determines where transcription starts

Question 8

About TTGACA Box :

Answer 8

1. About 35 bases upstream to the start point ( located at -35 base)
2. Determines the frequency of transcription

Question 9

What is the start point ?

Answer 9

1. Numbered as (+1)
2. The first base transcribed as RNA (initial base of mRNA )
3. The nucleotide at the 3’ end of the transcription region
4. Adjacent nucleotides as we go upstream the promotor , they are given negative numbers
5. Adjacent nucleotides as we go downstream the promotor are given positive numbers

Question 10

RNA polymerase Function mainly :

Answer 10

1) Synthesis of all types of RNA except for the RNA primers for DNA replication that are synthesised by Primase.
2) unwinds the DNA because it has intrinsic helicase activity
3) Guides the nucleotides into position
4) facilitates attachment and elongation
5) has intrinsic proofreading activity and replacement capabilities
6) termination recognition capability

Question 11

Prokaryotic RNAP structure :

Answer 11

1. It’s formed of Core enzyme and sigma factor (Multi subunit enzyme ) (( HOLOENZYME ))
2. Core enzyme is two identical alpha subunits , non-identical beta subunits ( B and B’ ) and omega subunit
   Alpha subunits : regulatory B subunit : binds to the DNA. B’ : responsible for the formation of phosphodiester bond
3. Sigma factor : enables the enzyme to recognise promotor region on the DNA as this enzyme lacks the ability to recognise the promotor region on the DNA template , different sigma factors recognise different group of genes and it is released after the process of initiation ( after applying its function )

Question 12

The antibiotic “rifampicin “

Answer 12

• It binds with the B’ subunit of RNAP and inhibits RNA synthesis as it interferes with the formation of first phosphodiester bond
• it is useful in treating TB

Question 13

The process of elongation :

Answer 13

1) binding of RNAP with the template strands produce local unwinding of the double helix and expose bases
2)After the release of sigma factor , the core enzyme moves along template and synthesizes RNA in the direction of 3’- 5’ with the base sequence complementary to to that of the template using ribonucleoside triphosphate and releases pyrophosphate.

Question 14

The process of termination is :

Answer 14

The process of transcription ending and releasing of RNA molecule by RNAP that recognises the termination signal

Question 15

Mechanisms for transcription termination :

Answer 15

1) Rho factor dependent termination = ATP dependent
2) Rho factor independent termination = intrinsic termination

Question 16

Rho factor dependent termination :

Answer 16

1. Rho factor ( termination factor ) binds at a Rho utilisation site (Cytosine rich sequences )
2. This factor run along the mRNA toward RNAP (in a 5’ - 3’ end )
3. When the factor reaches the polymerase , it it causes RNAP to dissociate from the DNA.

Question 17

Rho factor independent termination

Answer 17

It depends on the termination sequence that is complementary sequence rich in GC that are present at the 3’end of mRNA

These complementary bases join each other forming hairpin like structure that leads to dissociation of RNA from the DNA and release the RNA polymerase

Question 18

What is termination signal ?

Question 19

There are two types of genes according to their expression :

Answer 19

1) Constitutive Gene = housekeeping genes
2) inducible Genes

Question 20

Constitutive Genes :

Answer 20

They are NOT regulated Genes = they are continuously expressed at a low rate = they are always coding their proteins products which are required for the basic cellular functions

Question 21

Inducible Genes

Answer 21

They are negatively regulated genes by repressors , they express their proteins product only in the presence of an inducer or derepressor

Question 22

What is operon ?

Answer 22

Linear array of the genes that are involved in a metabolic pathway

Question 23

An example of Operon :

Answer 23

Lactose operon of E.coli

Question 24

The Operon is grouped with ………….. that ………

Answer 24

Regulatory genes , determine the operon transcription as a single long piece of mRNA

Question 25

What is lac Operon :

Answer 25

Operon that contains the genes responsible for lactose metabolism by E.coli bacteria when lactose is available to the cell but glucose is not

Question 26

Genes of lac operon of E.coli :

Answer 26

1.Structural Genes as Lac z, Lac Y and Lac A gene
2. Regulatory Gene OR lac 1gene
3. Operator region
4.A single common Promotor

Question 27

Lac Z gene :

Answer 27

encodes the B- galactosidase
( hydrolysis of lactose to glucose and galactose )

Question 28

Lac Y gene :

Answer 28

Encodes permease enzyme
( allows lactose transport into the cells )

Question 29

Lac A gene

Answer 29

Encodes thiogalactoside transacetylase
( rids the cell of toxic thiogalactosides that also get transported in by LacY )

Question 30

The three genes transcription :

Answer 30

They are transcribed into ONE LARGE POLYCISTRONIC mRNA MOLECULE

Question 31

What is the cistrons ?

Answer 31

The part of mRNA that gets translated into polypeptide, begins with start codon , ends with a stop codon

Question 32

Lac 1 gene OR regulatory Gene :

Answer 32

• constitutive Gene
• Codes for the regulatory proteins ( Lac depressor )

Question 33

Operator region :

Answer 33

Where the Lac repress or binds and inhibits gene expression

Question 34

Single common promoter

Answer 34

The site where RNA polymerase binds to it and start transcription of the structural gene

Question 35

The process of regulation of Lac Operon :

Answer 35

When Glucose is the only sugar available —> gene repression ( Negative regulation )

When lactose is the only sugar available ——-> gene induction ( Positive regulation )

Question 36

Gene repression of Lac operon in E.Coli :

Answer 36

1. The repressor protein binds to the operator site
2. This binding interferes with the progress of RNA polymerase
3. Blocking the transcription of the structural genes

الـ ( structural genes ) هي الي رح تعطي الأنزيمات يلي رح تعمل metabolism لل lactose فا بالتالي ما رح يصير عنّا metabolism لل Lactose

Question 37

Gene induction of Lac operon :

Answer 37

1. Conversion of small amount of lactose to an allolactose ( lactose isomer, Gene inducer)
2. Allolactose binds to the repressor proteins causing conformational change
3. Can’t bind to the operator
4. Allowing RNA polymerase to enter at the promoter region and initiate transcription of the structural genes

Question 38

Stopping Gene induction :

Answer 38

Occurs after the removal of the inducer because the lac mRNA is unstable and decays within minutes