Prokaryotic & Eukaryotic Genome Flashcards
(39 cards)
Compare eukaryotic and prokaryotic cells
Size Nucleus Organelles Ribosomes Cell wall Plasma membrane (S)
Compare the prokaryotic and eukaryotic genomes (exclude organisation)
Size & no. of genes Appearance Molecule Association w proteins Location Extrachromosomal DNA Level of DNA packing/coiling
Why is the packing of DNA important?
- compact–> fit in nucleus (eu)
- regulate gene expression (eu)
- prevent DNA entanglement and breakage
Describe how DNA is packed in eukaryotes
- -vely charged DNA double helix winds arnd octamers of 8 +vely charged histone proteins via electrostatic interactions. → nucleosome (10nm fibre). Linker DNA joins adjacent nucleosomes.
- 10nm fibre coils around itself→ 30nm chromatin fiber/solenoid (interphase DNA)
- 30nm fibre form looped domains when associated w scaffold proteins→ 300nm fibre
- Supercoiling→ metaphase chromosome
Describe how DNA is packed in prokaryotes
DNA double helix folds into looped domains by protein-DNA associations→ supercoiling w the help of DNA gyrase & topoisomerase
Compare the organisation of eukaryotic and prokaryotic genome
Non-coding regions - introns - promoters (s) - enhancers/silencers - repeated seq Operon Ori
Explain why the end replication problem occurs
DNA polymerase needs free 3’ OH grp to add free nt to growing DNA strand→ RNA primer synthesised to provide the free 3’ OH end for the addition of nt
⇒ 5’ end of newly synthesised strand will have RNA primer removed wout replacement of DNA due to no 3’ end available for DNA polymerase→ 3’ overhang → chromosomes shorten every replication ⇒ shortening of telomeres
Describe the role of telomeres
Definition: non-coding DNA made up of a series of tandem repeat seq, found at both ends of eu chromosomes
- Non-coding telomeres ensure vital genetic information are not lost due to end replication problem, where daughter chromosomes shorten at the telomeres w each round of DNA replication, as DNA polymerase can’t replace RNA primers w DNA
- Telomeres protect and stabilise terminal ends of chromosomes, by forming a loop w 3’ overhang, preventing ends of chromosome from fusing w those of other chromosomes→ prevent DNA repair machinery from recognising ends of chromosomes as DNA damage, protecting the chromosome, prevent apoptosis
- Telomeres allow for own extension, as 3’ overhang providing an attachment point for the correct positioning of telomerase enzyme
Describe the function of telomerase
- AS complementary in conformation and charge to a specific telomeric DNA sequence
- Catalyses the formation of phosphodiester bonds
translocate in 5’ to 3’ direction→ series of tandem repeats of GGTTAG→ elongate telomere - Telomerase RNA
> Anneal via cbp w ss 3’ overhang of telomere→ align telomerase in correct orientation wrt DNA
> Template for elongation of 3’ overhang, synthesise complementary DNA seq via cbp, where A pairs w U, T w A, C w G→ tandem repeat seq
Describe the role of centromeres
Structure: seq of non-coding DNA region of chromosome made up of tandem repeats
- Allowing sister chromatids to adhere to each other
- Allow kinetochore proteins & subsequently kinetochore microtubules to attach→ bivalents can be aligned along metaphase plate during MI of meiosis + chromosomes aligned singly along metaphase plate during M/MII of meiosis→ kinetochore microtubules shorten and separate them to opposite poles during A/AI/AII
⇒ allows proper alignment and segregation of chromosomes
Describe splicing
introns excised, exons joined tgt by spliceosome, a snRNA (small nuclear RNA)-protein complex, which recognises seq at intron-exon boundary→ produce functional proteins
Describe alternative RNA splicing
different exons of a single pre-mRNA joined to produce different mature mRNAs → one gene can code for >1 polypeptides; diff proteins can be produced
What is the function of promoters?
Recognition & binding site for GTFs, which recruits RNA polymerase→ transcription initiation complex→ initiate transcription
- Critical elements: TATA box determines precise location of transcription start site
What is the function of enhancers ?
- positive regulatory elements
- Recognition & binding site for specific transcription factors called activators→ promote assembly of transcription initiation complex by…
→ bending spacer DNA allows activators to bind & stabilise transcription initiation complex at promoter
→ recruit histone acetyl transferase & crc to decondense chromatin, ↑ accessibility of RNA pol & GTFs to promoter
⇒ upregulate/↑ freq of transcription; gene activation
What is the function of silencers?
- Negative regulatory elements
- Recognition & binding site for specific transcription factors called repressors→ prevent assembly of transcription initiation complex by…
→ interfere w action of activator by: competitive DNA binding, masking activation surface & bending spacer DNA for repressor to directly interact/bind to GTFs
→ recruit histone deacetylase & repressible crc to condense chromatin, ↓ accessibility of RNA pol & GTFs to promoter
⇒ downregulation/ ↓ freq of transcription; gene silencing/repression
Explain the importance of gene regulation
- Cellular differentiation: Regulation of gene expression→ each specialised cell hv different genes expressed→ different, tissue-specific proteins synthesised→ diff function
- Adapt to changes: proteins produced depends on circumstances & demand
- Conserve resources: transcriptional lvl, the most efficient mechanism w minimal wastage
- More varied proteome despite limited genome size: by alternative splicing
How does histone acetylation regulate gene expression?
Histone acetylation, catalysed by histone acetyltransferase: add acetyl grps to (lysine residues) histones→ removes +ve charge on histones, ↓ electrostatic interactions between -vely charged DNA and histones→ chromatin decondense
RNA pol & GTFs can access and bind to promoter, assembles transcription initiation complex→ increase/initiate transcription
How does histone deacetylation regulate gene expression?
Histone deacetylation, catalysed by histone deacetylase: remove acetyl grps→ restoring +ve charges on histones, restores tighter interaction/↑ electrostatic interactions→ chromatin condenses
RNA pol & GTFs cannot access and bind to promoter (blocked) → prevent formation of transcription initiation complex→ prevent transcription
How does DNA methylation regulate gene expression?
DNA methylases add methyl grps to selected cytosine nt in specific CG seq
→ recruit DNA-binding proteins aka transcriptional repressors, histone deacetylase, repressive chromatin remodelling complexes to condense chromatin
→ block binding of GTFs, prevent assembly of …
RNA pol & GTFs cannot access and bind to promoter (blocked) → prevent formation of transcription initiation complex→ prevent transcription
How does CRC regulate gene expression?
CRC alters struc of nucleosomes temporarily: DNA less tightly coiled to histones → chromatin decondense
RNA pol & GTFs can access and bind to promoter, assembles transcription initiation complex→ increase/initiate transcription
How does repressible CRC regulate gene expression?
alters struc of nucleosomes temporarily: DNA more tightly coiled arnd histones→ chromatin condenses
RNA pol & GTFs cannot access and bind to promoter (blocked) → prevent formation of transcription initiation complex→ prevent transcription
How does adding 5’ cap regulate gene expression?
Adding 7-methylguanosine nt to 5’ end of pre-mRNA shortly after transcription begins
- Helps the cell recognise mRNA, so subsequent steps like splicing & polyadenylation can occur
- Acts as a signal to export mRNA out of nucleus
- Stabilises and protects growing pre-mRNA from degradation by ribonucleases
- Recognised by Eu initiation factors, which bind to it→ help recruit mRNA to small ribosomal subunit→ promotes translation initiation
How does polyadenylation regulate gene expression?
3’ end of pre-mRNA cleaved downstream of polyadenylation signal (AAUAAA) → poly-A polymerase adds a long seq of adenine nt to 3’ end of pre-mRNA (poly-A tail)
- Acts as a signal to export mRNA out of nucleus
- Stabilises and protects growing pre-mRNA from degradation by ribonucleases→ more proteins made
- Interacts w Eu initiation factors and 5’ cap for initiation of translation
Describe the formation of translation initiation complex
When small ribosomal subunit, eukaryotic initiation factors & initiator tRNA form a complex, which binds to 5’ cap & poly-A tail→ mRNA circularises→ complex moves in 5’ to 3’ direction to locate start codon, AUG → binding of large ribosomal subunit completes translation initiation complex
