DNA replication & Cell processes - Increased complexity of Eukaryotes Flashcards
Describe eukaryote chromosome structure
Consists of histones Histone + DNA = Nucleosome
6 nucleosomes per turn
Histones are Octamers (made from 8 proteins) Rich in BASIC amino acids
Basic amino acids have +ve charge at physiological pH that binds to phosphate on DNA
Condensed form of chromosomes are only visible during metaphase
Describe the eukaryotic cell cycle
G0 – Cells not dividing (e.g. Neurones + Heart muscle cells)
G1 – Growth phase – Up regulation biosynthetic activities, proteins and organelles formed & enzymes required in the S phase
S – DNA replication takes place (only takes a few hours)
G2 – Spindle proteins produced (required during anaphase)
What are telomeres?
DNA in eukaryotes is non linear in the lagging strand there is a problem in replicating the end of the DNA strand.
These ends are called telomeres
Problem replicating end of strand due to primers needing to attach
Telomerases are used to extend the 3’ end of the existing DNA strand so a base sequence is created that is complimentary to the primer this allows for a free 3’ hydroxyl group to be present, to which nucleotides can add primase, DNA polymerase + telomerase is used to replicate the strand in the 5’ to 3’ direction
Telomerase is only found in certain cells (Cancer cells/ Foetal cells these rapidly divide)
Describe eukaryotic gene expression
No purine rich base sequence (binding site of 16s ribosome in 30 ribosome) is present *this sequence is only present in prokaryotes
Translation is initiated by translational start signals (AUG) near 5’ end
What types of RNA polymerases are present in eukaryotes?
Types are present in eukaryotes:
Type 1 – rRNA production – insesnsetive to alpha amanitin
Type 2 – mRNA & snRNA – Strongly inhibited by alpha amanitin
Type 3 – tRNA & rRNA – Inhibited by high conc. of alpha amanitin
What is the role of alpha amanitin?
alpha amanitin binds to DNA polymerase II. more than 100 deaths a year from ingesting amanita phalloides.
Describe transcription initiation
Eukaryotes
- Promoter sequences - RNA polymerase recognise these sequences
Prokaryotes
- Purine rich base sequence
- Sigma factors present - RNA polymerase to promoter sequences
Describe transcription factors
Transcription factor (complexes) are formed from many basal factors joined together
The basal factors can increase/decrease rate of transcription activation
Activators (molecules) bind to enhancers on part of the DNA (may be many bases away) Enhancers attach to RNA polymerase activation occurs inc rate of transcription
What are the three transcription initiation sequences?
TATA box (-25=35 of region)
CAAT box
GC box
How is gene regulation expressed?
Chromatin remodelling Histone acetyl transferase Histone deacetylase Enhancers Modification
How does Histone acetyl transferase
regulate gene expression?
weakens histone + chromatin association
How does Histone deacetylase regulate gene expression?
strengthens histone + chromatin association
How do enhancers regulate gene expression?
Sequence of DNA that activators bind to bind then to RNA polymerase leading to RNA replication may be silencers next to enhancer sequence, when a repressor binds to silencer, the enhancer sequence is distorted activator can no longer bind
How does modification regulate gene expression?
Methylation of 5’ carbon on cytosine residue represses gene expression
What is splicing?
Eukaryotic DNA contains introns + exons mRNA that is produced must be processed
Processing involves removing the mRNA that has been coded by the introns
Interferons do not contain introns (exception to rule)
Pre-mRNA to mature mRNA (involves snurps) to several snurps= splicosome)
