regulation of gene expression

Question 1

what is gene expression ?

Answer 1

refers to the multistep process that ultimately results in the production of a functional gene product, either ribonucleic acid (RNA) or protein.

Question 2

kinds of genes, depending on how often they are expressed

Answer 2

1. housekeeping genes: those expressed continually, in approximately all cells, to meet basic cellular needs
2. regulated genes: Those expressed only under certain conditions in certain cells according to the organism’s need( they can be turned on and off )

Question 3

what is operon, regulatory sequence

Answer 3

operon is a cluster of genes sharing a promoter and regulatory sequence.

the regulatory sequence is the segment that is responsible for the regulation of the expression of specific genes.

Question 4

parts of operon

Answer 4

1. regulator gene: codes for DNA binding protein that acts as a repressor.
2. promoter: DNA sequence where GTFs or RNA pol bind to begin transcription process.
3. operon: is a DNA sequence where repressor bind to carry out expression regulation.
4. structural gene: is a sequence that codes for proteins and enzymes which don’t participate in gene regulation.

Question 5

role of operator in the bacteria, and where do we find the clear example in the bacteria?

Answer 5

operator regulates the expression of genes through the repressor, in the bacteria. The binding of the repressor to the operator induces negative regulation hence no expression of the genes while when the inducer molecule is bonded to the repressor its shape changes and is released from the operator hence positive regulation occurs which is an expression.

case study of role of operator is in lactose operon/ lac operon

Question 6

what is inducer as seen in the gene expression?

Answer 6

the inducer is the molecule that is responsible to bind to the repressor to change its shape and cause its release from the operator hence signaling positive regulation of a gene.

Question 7

briefly describe the lac operon gene regulation in the bacteria?

Answer 7

note: bacteria use glucose as the main source of the energy, but in a low supply of glucose then bacteria can use lactose as a source of energy and hence absorb it, inside.

negative regulation:
when bacteria have glucose the genes of lactose are repressed.
* in the absence of lactose in the system the repressor is bound to the operator to prevent the expression of the genes.

positive regulation
in absence of glucose, genes lactose are expressed.
* in presence of lactose in the bacteria, cause the expression of lactose genes. how? lactose in the bacteria acts as an inducer binding to the repressor and changes its shape hence released for operator causing RNA pol to transcribe the genes hence expressed.

Question 8

three main genes in the lac operon?

Answer 8

lacZ: This gene codes for enzymes that hydrolyzed lactose in galactose and glucose
lacY: This gene codes for enzymes that facilitate the absorption of lactose into the cell.
lacA: this codes for an enzyme whose physiological function is not known.

Question 9

what are the enzymes coded by three genes of lac operon?

Answer 9

lacZ: β-galactosidase
lacY: permease
lacA: thiogalactoside transacetylase

Question 10

role of CRP or CAP in the lac operon regulation.
CAP: Catabolite Activator protein
CRP: Catabolite Receptor Protein

Answer 10

note: lac operon regulation is not done by repressor only, but also by the use of cAMP and CAP or CRP.
how?
positive regulation
1. in the absence of Glucose enzyme is activated and causes plenty of production of cAMP that will bind to CRP/CAP, to form a cAMP-CRP complex.
2. complex bind to the CAP binding site in the DNA, causing the RNA pol to initiate transcription of operon genes.
3. the molecule bind to the repressor changing its confirmatory shape hence preventing it to bind to the operator.

negative regulation:
1. in presence of both glucose and lactose, causes enzyme adenylyl cyclase gets deactivated hence no cAMP and no cAMP-CRP.
no complex, then no initiation of RNA pol for transcription.
2. Even though the repressor may not bind to the operator, no transcription will occur.

Question 11

1. which enzyme produces cAMP in the regulation of lac operon in the bacteria?
2. which molecule that bind to the repressor to change its confirmatory shape in the regulation of lac operon using CRP?

Answer 11

1. adenylyl cyclase

2. allolactose

Question 12

what is tryptophan operon?

Answer 12

is a DNA sequence that comprises structural genes for 5 enzymes responsible for tryptophan synthesis and regulatory genes.

Question 13

how is tryptophan operon is regulated using a repressor in the bacteria?

Answer 13

1. in absence of tryptophan, the repressor is in wrong confirmatory shape which can’t bind to the operator, and transcription of 5 genes will occur.
2. presence of tryptophan, the tryptophan bind to the repressor, and the confirmatory shape change to the right one which can bind to the operator, preventing the transcription of the tryptophan.

Question 14

eukaryotic gene regulation overview!

Answer 14

1. Eukaryotic genes are not organised in the operons like in prokaryotic
2. eukaryotic regulatory genes are not linked in the genes they regulate.
3. eukaryotic DNA are enclosed in seemingly inaccessible nucleus, in multicellular organism, some particular gene are expressed in the some cell and silenced in others.
4. none expressed gene are condensed in the form called heterochromatin while expressed are in loose form called euchromatin.
5. transcription machinery gain access to the DNA packaged in nucleosomes ( 8 histone and looped DNA ), via chromatin remodelling and histone modification.
6. enzyme histone acetyltransferase ( uncoil chromatin structure ), exposes DNA from nucleosome so the transcription mechanism can transcribe leading to expression of gene.
7. enzyme histone deacetylases, ( condensed or closed structure of the chromatin ). prevent the exposure of the DNA hence preventing gene expression.