7.2 Transcription And Gene Expression

Question 1

what is the promotor

Answer 1

the non-coding sequence responsible for the initiation of transcription

Question 2

what is the promotor’s function?

Answer 2

the binding site for RNA polymerase

Question 3

How is the binding of the RNA polymerase to the promotor mediated and controlled? and how?(2)

Answer 3

• an array of transcription factors (eukaryotes)
• transcription factors bind to either proximal control element (near the promotor) or distant control elements (at distance)

Question 4

what occurs after the RNA polymerase has bound to the promotor?

Answer 4

it causes DNA strands to unwind and separate

Question 5

what is the region of DNA which is transcribed by RNA polymerase called?

Answer 5

coding sequence

Question 6

when will RNA polymerase stop transcribing the DNA?

Answer 6

when it reaches a terminator sequence

Question 7

how many of the two polynucleotide strands is transcribed into RNA?

Answer 7

1

Question 8

what is the antisense strand?

Answer 8

the strand which is transcribed into RNA

Question 9

what the antisense strand (template strand) complementary to?

Answer 9

• complementary to RNA sequence (will be ‘DNA version’ of tRNA anticodon sequence)

Question 10

what is the sense strand?

Answer 10

the strand that is not transcribed into RNA

Question 11

what would the sense strand / coding strand be?

Answer 11

• be the ‘DNA version’ of the RNA sequence (eg. T instead of U)
  (it is the DNA copy of the RNA sequence)

Question 12

why is the determination of the sense or antisense strand gene specific?

Answer 12

as either one of the polynucleotide strands may contain a gene

Question 13

what is the process of transcription? (8 steps)

Answer 13

Initiation
- RNA polymerase bind to the promotor region
- separated DNA by breaking H-bonds
- exposing antisense strand as template
Elongation
- RNA polymerase joins adjacent nucleotide base pairings from the free RNA nucleotides (nucleoside triphosphate)
- RNA polymerase joins them together using phosphodiester bonds in a 5’->3’ direction
Termination
- once the RNA polymerase reached the terminator region it will cause the RNA polymerase to disassociate and the DNA to wind up and join back together

Question 14

what are the three post-transcriptional event that occur in order to form mature messenger RNA?

Answer 14

• capping
• polyadenylation
• splicing

Question 15

what is capping? (3 things)

Answer 15

• addition of a methyl group to the 5’ end of the transcribed RNA

Question 16

what does capping do? (2)

Answer 16

• methylated cap provides protection against degradation by exonucleases
• allows transcript to be recognized by the cell’s translational machinery

Question 17

what is polyadenylation?

Answer 17

the addition of along chain of adenine nucleotides (poly-A tail) to the 3’ end of the transcript

Question 18

what does polyadenylation do?

Answer 18

the poly-A tail improves the stability of the RNA transcript and facilitated its export from the nucleus

Question 19

what is splicing? (3 point explanation)

Answer 19

• non-coding sequences/ introns which must be removed prior to forming mature RNA
• coding region/exons which are fused when introns are removed to from a continuous sequence
  (introns are intruding sequences, exons are expressing sequences)
• process of removing introns = splicing

Question 20

what is it called when exons are also removed as a result of splicing?

Answer 20

alternative splicing

Question 21

what will the selective removal of exons result in?

Answer 21

the formation of different polypeptides from a single gene
eg. a particular protein may be membrane bound / cytosolic depending on the presence of an anchor motif)

Question 22

what are the two groups of proteins which mediate the binding of RNA polymerase to the promotor?

Answer 22

transcription factors
regulatory proteins

Question 23

what do transcription factors do? (2)

Answer 23

• transcription factors form a complex with RNA polymerase at the promotor
• RNA polymerase cannot initiate transcription without these factors (so they regulate gene expression)

Question 24

what do regulatory proteins do?

Answer 24

-bind to DNA sequence outside of the promotor and interact with the transcription factors

Question 25

what are the two regulatory proteins that bind to the DNA sequence?

Answer 25

• activator protein
• repressor protein

Question 26

what do activator protein do?

Answer 26

bind to enhancer site and increase the rate of transcription (mediating complex formation)

Question 27

what to repressor proteins do?

Answer 27

bind to silencer sequence and decrease the rate of transcription (preventing complex formation)

Question 28

what can the presence of transcription factors or regulatory proteins be?

Answer 28

tissue specific
(chemical signals (hormones) can moderate protein levels and mediate change in gene expression)

Question 29

what are the DNA sequences that regulatory proteins bind to?

Answer 29

control elements

Question 30

what are three characteristics of control elements?

Answer 30

• come are located close to the promotor (proximal elements) and other are distant (distal elements)
• most genes have multiple control elements and gene expression is tightly controlled and coordinated process

Question 31

where do transcription factors or regulatory proteins usually bind to? (2)

Answer 31

• regulatory proteins usually bind to distal control elements
• transcription factors usually bind to proximal elements

Question 32

what is eukaryotic DNA wrapped around?and what does this form? (2)

Answer 32

histone proteins to form compact nucleosomes

Question 33

what do histone proteins’ protruding tails do?

Answer 33

determine how tightly the DNA is packaged

Question 34

what kind of charge do histone proteins typically have?

Answer 34

positive charge
(associate with negatively charged DNA)

Question 35

what does acetylation do? (2)

Answer 35

• adding an acetyl group to the histone tail neutralizing the charge
• making DNA less tightly coiled and increased transcription

Question 36

what does methylation do? (2)

Answer 36

• adding methyl group to the tail maintains positive charge
• making DNA more coiled and reducing transcription

Question 37

what is DNA like when it exists as heterochromatin?(3)

Answer 37

• condensed
• DNA is supercoiled
• not accessible for transcription

Question 38

what is DNA like when it exists as euchromatin?(2)

Answer 38

• loosely packed
• accessible for transcription machinery

Question 39

How is euchromatin and heterochromatin found in different cells? (2)

Answer 39

different cells will have varying segments of DNA packaged as euchromatin and heterochromatin
- some segments of DNA may be permanently supercoiled while other segments may change over the life cycle of the cell

Question 40

what is the other form of methylation?

Answer 40

direct methylation (as opposed to the methylation of histone tails)

Question 41

what does increased methylation of DNA do?

Answer 41

• decreases gene expression by preventing the binding of transcription factors
• genes that are not transcribed tend to exhibit more DNA methylation than genes that are actively transcribed

Question 42

what is epigenics?

Answer 42

study of changes in phenotype as a result of variations in gene expression

Question 43

what does epigenic analysis show? (4)

Answer 43

DNA methylation patterns may change over the course of a lifetime
- influenced by heritability bur not genetically predetermined
- different cell types in same organisms may have different methylation patterns
- environmental factors influence levels of methylation in cells (eg diet, pathogen)