7.2 Transcription and gene expression (HL) Flashcards
What are promoters?
Non-coding sequence/ region of DNA responsible for initiation of transcription
How do promoters help production of polypeptides?
They act as binding sites for RNA polymerase, which is responsible for formation of covalent bonds between nucleotides that synthesize RNA/mRNA for polypeptides.
How is binding of RNA polymerase to promoter mediated?
Controlled by multiple transcription factors in eukaryotes
What are operators?
They also regulate transcription however usually through inhibition
What are repressors?
Proteins that bind to operators to prevent RNA polymerase from transcribing
In a lac operon, does the lactose need to be broken down or made and why?
When lactose is present, enzymes need to be made to break it down to provide energy for the prokaryote.
How does a lac operon work?
- Lactose binds to the repressor
- Repressor cannot bind to the operator
- RNA polymerase binds to the promoter allowing genes that produce proteins to be transcribed
- mRNA is used to make enzyme to break down lactose after which repressor is free and it will bind to the operator again
Why does transcription need to be inhibited in a lac operon?
If lactose is not present, there is no need to transcribe enzymes for it. Hence, it is more energy efficient for the prokaryote
Similarities and differences between gene expression in prokaryotes and eukaryotes
They both have promoters however eukaryotes don’t have operators. Instead they have enhancers and silencers
What is an enhancer?
Coding sequence that binds to activator proteins to increase rate of transcription
What is a silencer?
Coding sequence that can bind to repressor proteins to block or reduce rate of transcription
How can environment of an organism impact gene expression?
Exposure to pathogens, chemicals, toxic materials. Hair and skin color can be affected by sunlight and temperatures.
What is a morphogen?
Chemical that affects gene expression by regulating transcription factors
How does environment affect fur pigmentation in Himalayan rabbits?
Gene C controls pigmentation. It is active when temp is between 15-25C. At higher temp, it is inactive so fur will be white as no pigment is produced. In colder weather, the rabbit’s colder extremities (feet, nose, ears) produce a black pigment
What are specific examples of environmental impact on gene expression?
- Changing of flower color due to pH of soil
- Animals changing sex in response to social cues
What are the 3 regions of the gene
The promoter (where transcription is initiated), the coding region (region transcribed) and the terminator (where transcription ends).
How would you describe the concentration of morphogens in embryonic cells?
They contain uneven concentrations which results in activation and inhibition of different genes in different cells and hence why we are unique
What is a nucleosome made of?
- Histone H1
- DNA coils
- Octamer of histones
What part of nucleosome is exposed and what does this allow?
Tail of histone is usually exposed which allows it to be easily modified
2 chemical groups that affect transcription regulation
Acetyl and methyl group
Does methylation of histones promote or inhibit transcription?
Direct methylation usually inhibits however it can either promote or inhibit depending on location in genome
What can DNA methylation be affected by?
They can be associated with age, diet, environmental conditions or exposure to infectious agents
How are euchromatin and heterochromatin made?
- Heterochromatin is when DNA is bound more tightly to the histone to inhibit transcription making it less accessible
- Euchromatin is when DNA is loosened to make it more accessible to transcription factors
What are the types of methylation?
Hypermethylation is when genes are unable to express completely while hypomethylation is when a lot of genes are being transcribed and expressed
Why will twins show different levels of methylation?
They will have different experiences and environment stimuli which will cause different levels of methylation in different chromosomes
Why do cells become more specialized as they age?
As cells age, they have gone through acetylation and methylation so there is more inhibited and promoted DNA
What is acetylation?
It is addition or removal of acetyl groups from the histone
Outline the process of acetylation on lysine of histone tails
- Lysine usually bears a positive charge which binds to negatively charged DNA to form condensed structure, inhibiting transcription
- Histone acetylation neutralizes positive charge so structure is less condensed= more transcription
What is epigenetics?
It is the study of heritable changes in organisms that are brought about by changes in gene expression as opposed to modification of genetic code
How can modified histones get inherited by offspring?
As not all epigenetics get erased (usually 1% remains), some can get passed down to offspring during cell division
What are epigenetic tags?
Markers that acetylation and methylation used to mark DNA to control transcription.
Epigenome
Sum of epigenetic tags
Does methylation of DNA change in replication or get copied?
It gets copied
Why do new zygotes not inherit all epigenetic tags?
They get erased because babies need unmarked DNA so cells can differentiate into specialized cell types
Give examples of how inheritance and creation of epigenetic tags can be influenced at different stages in life
- Fertilization: Some imprinted genes are not erased
- Pregnancy: Maternal diet
- Infancy: Early exposure to bacterial microbes
- Young adult: Diet, lifestyle
- Senior: Age related changes
Why dont prokaryotes have pre-mRNA as opposed to eukaryotes?
Prokaryotes don’t have a nucleus so they combine transcription and translation. Eukaryotes require post-transcription modification (removal of introns) so immediate product is pre-mRNA and after splicing, it is mature mRNA `
What were introns referred to as previously?
‘Junk’ DNA
What direction does transcription occur in and through which parts?
5’ to 3’ direction. 5’ end/ phosphate of RNA nucleotide is added to 3’ end of growing mRNA
What is covalently bonded in transcription?
Ribonucleoside triphosphates
What are introns?
Non-coding region of the gene that doesn’t express but plays a role in transcription initiation
What are exons?
Part of the mRNA which expresses genes
What is a spliceosome?
Complex made of smaller mRNA and proteins that removes introns and causes it to form a loop while allowing exons to be joined
What can introns do after being removed?
They will be broken down and used to form nucleotides
What is mature mRNA?
It contains only exons and leaves nucleus to be translated into polypeptides
What is alternative splicing?
Removal of exons from the mRNA
What does mature mRNA consist of on both ends?
It will have a poly A tail (100-200 adenines) on 3’ end and it will be capped on the 5’ end by a methyl group
Purpose of capping mature mRNA
Prevents degradation of mRNA and allows it to be recognized
Purpose of poly A tail
Improves stability and facilitates export
What does alternative splicing imply for a gene?
They can have varying biological functions as different proteins can be made
What is a protein family that is commonly alternatively spliced?
Immunoglobulin
What happens after enhancer or silencer is activated?
DNA will start to bend using DNA bending protein and enhancer/silencer will be on top of promoter. According to their function, this can either attract or prevent transcription factors and hence affect the calling for RNA polymerase
What is a distal control element?
Includes enhancers and silencers but are distant from promoter
What is a proximal control element?
Nearer to promoter and binding is necessary to initiate transcription
