Transcription and gene expression Flashcards
1
Q
What is a coding sequence?
A
- The region of DNA that is transcribed by RNA polymerase
2
Q
What are the three stages of transcription and in what direction does it occur?
A
- Initiation, elongation and termination
- Transcription occurs in the 5’ to 3’ direction at the coding region
3
Q
Explain the initiation stage.
A
- RNA polymerase unwinds the double helix at the gene (DNA) called promoter (close to the 5’ end of the coding region)
- The DNA is used as a template to make a copy in the form of mRNA
- The DNA is separated into two strands. RNA polymerase will make a complementary strand of mRNA.
- The mRNA itself starts on the 5’ and ends on the 3’. The original DNA strand (that was separated) will therefore be 3’ to 5’.
4
Q
What is the sense and antisense strand and on which one is the mRNA strand made during the initiation stage?
A
- Transcription occurs on the antisense strand. It is the template for transcription.
- The mRNA strand that is built ‘reads’ the antisense strand which is the strand (3’ to 5’). By reading it creates its own complementary RNA strand from free nucleoside triphosphates.
- The mRNA makes bases that are complementary to the ones on the antisense strand.
- Therefore the bases of the mRNA are identical (except for the T and the U) to the sense strand. The sense strand is the other DNA strand that is not transcribed (5’ to 3’). Called coding strand.
- The codons on the newly synthesized mRNA will later code for a protein
5
Q
What happens during elongation?
A
- The bases of the antisense strand are complementary copied into the mRNA. The mRNA becomes longer.
- The RNA nucleotides (the bases) contain the sugar ribose. These have the hydroxyl group (OH)
- The DNA nucleotide contain the sugar dioxyribose which lacks the hydroxyl group (OH).
- During elongation the RNA polymerase forms covalent bonds between two mRNA bases.
6
Q
What happens during termination?
A
- The last stage, the mRNA synthesis is complete.
- RNA polymerase will transcribe until it reaches a sequence of DNA (terminator)
- The RNA polymerase detaches from the DNA.
- During transcription the DNA double helix rewinds as the RNA polymerase moves through the gene of interest. And energy is generated
- The transcript is released from the template and the DNA rewinds completely
- The RNA polymerase detaches and moves to other parts of the DNA (other promoters)
7
Q
How does post-transcriptional modification link to introns and exons?
A
- This only occurs in eukaryotes because prokaryotes have no nucleus, the mRNA immediately begins translation
- This modification in eukaryotes needs to occur because of the introns and exons
- Introns: DNA sequences in genes that contain no coding information and they control sequences that regulate transcription (5′- and 3′)
- Exons: DNA sequences that code for a polypeptide (5′- and 3′)
8
Q
What are the three steps of post-transcriptional modification?
A
- Transcription (production of the mRNA which is the modification of RNA)
- Capping: A methyl group is added to the 5’ end of the transcribed RNA. It provides protection against degradation
- Polyadenylation: when a poly-A tail is added to the 3’ end. This improves the stability of the RNA transcript
- Splicing: removing the introns and joining the exons to from mature mRNA
9
Q
How does splicing work?
A
- Only in eukaryotes
- It involves a splicesome which acts as an enzyme
- The exons are tied together and the spliceosome detaches leaving the exons to form mature mRNA. During this, the intron and the splicesome are cut off.
- The exons form a continuous sequence. Splicing makes it possible for multiple proteins to be produced from the same gene. (Alternative splicing)
- This is because different exons are tied together, giving rise to different forms of proteins. Exons can be spliced in alternative patterns.
- The old theorem ‘one gene - one polypeptide’ is therefore no longer valid. Splicing of different exons is a form of alternative gene expression.
10
Q
How and why is transcription regulated?
A
- The non-coding regions of DNA (e.g. promoters), enhancers and silencers help
- Promoters affect transcription by controlling if RNA polymerase can access the gene
- A lot of energy is required to produce proteins, because not all proteins are needed all the time, the process must be controlled
11
Q
What is are promoters and operators?
A
- Promoter: a short DNA sequence placed before a gene, which acts as a binding point for the RNA polymerase enzyme
- It is an example of non-coding DNA with a function
- Operator: allows a cell to regulate whether a gene is transcribed or not
12
Q
How is gene expression regulated by proteins?
A
- Promoters regulate transcription. When a certain food source (lactose) is absent, the active repressor protein binds to the operator next to the promoter
- This means the RNA polymerase cannot bind with the promoter anymore (transcription is stopped)
- When the food source (lactose) attaches to the active repressor, RNA polymerase can bind to the promoter and transcription occurs
13
Q
How is transcription regulated in eukaryotes?
A
- Some proteins such as enhancers and silencers bind to specific non-coding DNA base sequences to regulate transcription
- Activator proteins bind to enhancers to increase the rate of transcription. Repressors bind to silencers to decrease the rate of transcription
- Enhancers and silencers are also non-coding regions of DNA with specific functions
14
Q
Which structures are non-coding regions with a specific function?
A
- Enhancers, silencer and promoter
- Repressors are proteins to stop transcription. It is not a DNA region
- Gene expression (genes to be transcribed) is regulated by proteins (e.g. repressors) that bind to specific base sequences of DNA
15
Q
What is another method to regulate transcription?
A
- Histones can be acetylated or methylated. DNA can also be methylated.
