Cellular Control - Gene Expression

Question 1

why is gene expression necessary

Answer 1

• nearly all our body cells contain all the genes necessary to code for all proteins
• we do not want all proteins to be made in all cells as cells are specialised to perform different functions
• some proteins will only be needed at certain times

Question 2

what is gene expression

Answer 2

mechanisms for switching genes on and off in different cells and at different times

Question 3

what are the different levels at which gene expression can be controlled

Answer 3

• transcriptional level
• post-transcriptional level (mRNA modified)
• translational level
• post-translational level (protein modified)

Question 4

what is the transcriptional level regulation of gene expression

Answer 4

• genes may be switched on or off
• amount of mRNA produced through transcription can be regulated

Question 5

what are the three methods of transcriptional level regulation

Answer 5

• histone modification (eukaryotes)
• DNA methylation
• transcription factors

Question 6

why is histone modification a method of regulating gene expression

Answer 6

when DNA is tightly wound around histone proteins e.g. during mitosis or meiosis it is not able to be used for protein synthesis

Question 7

what are the two methods of histone modification

Answer 7

• acetyl groups
• methyl groups

Question 8

describe how acetyl groups can modify histones

Answer 8

• acetyl groups can bind to the histone proteins making them less positive so that they bind less tightly to the negatively charged DNA
• this means transcription can occur

Question 9

describe how methyl groups can modify histones

Answer 9

• methyl groups can bind to histone proteins making them more hydrophobic to they bind more tightly to DNA
• this means that transcription cannot occur

Question 10

describe DNA methylation

Answer 10

• methyl groups may attach to DNA altering gene expression
• when methyl groups are added to DNA in a gene promoter they typically prevent transcription

Question 11

what are transcription factors

Answer 11

proteins that can bind to DNA and regulate the transcription of a gene

Question 12

where must RNA polymerase bind

Answer 12

• must bind to DNA near the start of a gene
• this region of DNA is called the promoter

Question 13

what is an operon

Answer 13

• structural genes that code for proteins that work together are sometimes found in groups
• these groups of genes can have a single promoter for the RNA polymerase to bind to

Question 14

what is the Lac Operon

Answer 14

a collection of genes responsible for lactose digestion and their regulation in E.coli (prokaryote)

Question 15

why is the Lac Operon and its regulation necessary

Answer 15

• E.coli normally respires glucose
• however it is also able to absorb and hydrolyse lactose if glucose is not available
• it only produces the enzymes to do so when lactose is available
• this saves resources

Question 16

briefly what happens in E.coli in the presence of lactose

Answer 16

transcribes the genes to produce lactose permease and Beta-galactosidase

Question 17

what is lactose permease

Answer 17

acts as a carrier protein, transporting lactose into the cell

Question 18

what is Beta-galactosidase

Answer 18

hydrolyses lactose into glucose and galactose

Question 19

what is the promoter region

Answer 19

region where RNA polymerase binds

Question 20

what is the operator region

Answer 20

region where repressor protein binds

Question 21

describe what happens in E.coli when lactose is not present

Answer 21

• regulator gene codes for repressor protein
• repressor protein binds to operator site and blocks transcription of structural genes by RNA polymerase
• therefore the proteins are not made

Question 22

describe what happens in E.coli when lactose is present

Answer 22

• regulator gene codes for repressor protein
• lactose enters the cell by diffusion and binds to the repressor, causing its tertiary structure to change
• the repressor is no longer complementary to the operator region, so RNA polymerase is able to bind to the promoter and start transcribing genes for the two enzymes

Question 23

what do eukaryotic genes contain (regions)

Answer 23

• exons
• introns

Question 24

what are exons

Answer 24

DNA that codes for amino acids

Question 25

what are introns

Answer 25

non-coding DNA within a gene

Question 26

what must happen in the nucleus to mRNA before it can leave (post-transcriptional regulation)

Answer 26

- primary mRNA must be edited in the nucleus to remove the introns (splicing) - this produces mature RNA corresponding to the DNA exons - endonuclease enzymes may be used

Question 27

what is the post-translational level regulation of gene expression

Answer 27

proteins that have been transcribed and translated may need to be activated

Question 28

what are the ways in which a protein can be altered

Answer 28

- proteins may be modified in the Golgi apparatus - cyclic AMP, synthesised from ATP by adenylyl cyclase, activates many proteins including protein kinases - kinases can phosphorylate enzymes. this alters the protein charge, which can then alter the tertiary structure and hence function

Question 29

describe the process of post-translational control by cyclic AMP

Answer 29

- signalling molecule binds to complementary receptor site on cell surface membrane - transmembrane (G) protein activated - adenylyl cyclase activated - cAMP formed from ATP (second messenger) - protein kinases activated - many enzymes activated by adding a phosphate - transcription factors enter the nucleus - causing other genes to be transcribed

Question 30

describe transcription factors in eukaryotes

Answer 30

- in eukaryotes, many proteins act as transcription factors either allowing RNA polymerase to bind to the promoter (activators) or preventing it binding (repressors) - also sometimes short non-coding pieces of RNA can bind to DNA controlling transcription

Question 31

explain why some regions of DNA can be described as non-coding

Answer 31

- not present in mature mRNA - not translated - editing of mRNA (introns removed) - regulatory genes

Question 32

suggest why non-coding regions of DNA show more variation

Answer 32

not selected against in natural selection as they don't affect survival