4. GENOME STRUCTURE Flashcards
1
Q
- What are histones & describe their structure?
A
- Histones are basic/positively charged which interact with negatively charged DNA
- There are 8 histones: 2A, 2B, 3 & 4, which form the octamer core which wraps around DNA
- Histone 1 joins the linker DNA to the core octamer
- The octamer core binds around 146 bp of DNA & forms the nulceosome
2
Q
What’s the difference between acrocentric, submetacentric & metacentric chromosomes?
A
- Metacentric = centromere is in the centre & there’s a short arm & long arm
- Submetacentric = centromere is slightly higher, very short arm & long arm
- Acrocentric = centromere is much higher than centre, satellite & long arm
3
Q
Define the genome
A
- The genome is the primary DNA sequence containing all the gene products for a human including the regulatory signals
4
Q
Define the exome
A
- The exome is the all the coding segments
5
Q
- Define a gene
A
- A gene is a segment of DNA including all the control regions which are involved in tissue specific expression & the regulation of the protein
6
Q
Give two properties of genes
A
- Gene vary in size, largest is dystrophin
- Gene can form clusters
- -> these gene clusters can allow for co-ordinated gene regulation
7
Q
- What is the intergenic region?
A
- The intergenic region is the region in between the genes (exome) and makes up 98% of the genome
- It contains genes with no known function such as repetitive DNA, endogenous retroviruses & pseudogenes
8
Q
What do enhancers & silencers do?
A
- Enhancers (activators) upregulate gen expression, they can be located in the gene or distant from it.
- Silencers (repressors) downregulate gene expression
- Both silencers & enhancers are position independent, meaning they can still carry out their function regardless of location
9
Q
What do insulators do?
A
- Insulators are located in between enhancers & silencers to prevent them from influencing or affecting other genes
10
Q
What are the steps of transcription?
A
- RNA Pol II & transcription factors are recruited to the gene forming a closed complex
- DNA double helix unwinds to form an open complex
- RNA Pol II moves along the strand, elongating the mRNA (elongation)
- RNA Pol II encounters a stop codon (termination)
- RNA Pol II dissociates
11
Q
What are three post-transcriptional modifications?
A
- Capping at 5’
- Splicing
- Polyadenylation at 3’
12
Q
*What is capping?
A
- After 25-35 nucleotides have been added a methylated guanosine cap is added to the 5’ end
- This helps prevent degradation by nucleases
13
Q
What three enzymes are involved in adding the 5’ cap?
A
- RNA 5’triphosphatase - catalyses the first step, produces a disphosphate end
- RNA guanyltransferase - adds the guanosine
- N7G - methytransferase - methylates the guanosine at position 7
- The first two activities are carried out by a bifunctional capping enzyme (CE)
14
Q
Describe the process of splicing
A
- Splicing involves the removal of introns & the ligation/joining together of exons
- Splicing is carried out by a SPLICEOSOME (approx 150 proteins)
- The hydroxyl of the previous exon bonds with the phosphate of the next exon to produce a 2’5 linkage
- Spicing also involves the formation of a lariat like intermediate
15
Q
Describe the addition of the 3’ poly A tail
A
- A chain of adenine bases (250) will be added to the 3’ end to prevent degradation
- 4 enzymes are involved in polyadenylation: CPSF, CSTF, PAP & PAB
