Chromosome, DNA replication and gene expression in eukaryotes Flashcards
What is chromatin?
DNA condensed and wrap around Histones
How many different types of Histones are there?
4
What is nucleosome?
complex when DNA wrapped around octamer of histones
What is structure of chromatin?
helical array of 6 nucleosomes per turn of helix
What is 4 stages of eukaryotic cell cycle?
1) S phase- DNA synthesis and replication
2) G2- synthesis of proteins for mitosis
3) Mitosis (PMAT)
3) G1- check DNA, protein/ Organelles made
What is PMAT?
1) prophase
2) Metaphase
3) Anaphase
4) Telophase
How are eukaryote chromosomes replicated?
G rich end of chromosome called Telomeres
Where is eukaryote chromosome replication observed?
In lagging strand where okazaki fragments
What is telomerase?
a reverse transcriptase, makes DNA from RNA template
What are stages of chromosome telomere replication?
1) RNA primer removed from 5’ end of lagging strand
2) Telomerase bind to overhanging DNA and adds Bases
3) telomerase moves down from 5’ to 3’ end of single strand dna
4) primase and ligase synthesise lagging strand in 5’ to 3’
What is telomerase complementary to?
G rich section of telomere
When does telomerase operate?
foetal development
sperm/egg cells
cancer cells
not found in somatic body cells
What is overall transcription/translation in eukaryotes?
Transcription in nucleus then mRNA out into cytoplasm to ribosome
What is eukaryote start signal?
AUG binded to anticodon on tRNA with Methionine
Do eukaryotes have initiation factors?
yes
how many RNA polymerases for eukaryotic transcription?
3 RNA POLYMERASES (1-3)
What are promoter sequences in eukaryotic transcript./
DNA region that initiate it e.g CAAT and GC box
What is the initiation sequence for RNA polymerase 2?
TATA box
What is needed for transcription initiation?
promoter sequence
Transcription factors
What does RNA polymerase do??
mRNA synthesis
What is the basal transcription factor
CTD= all basal factors + DNA polymerase2 TAT complex
What can basal factors do?
need to be activated to inc/decrease rate of transcription
What regulates gene expression?
Chromatin remodelling
Enhancers
Modification
What is chromatin remodelling
acylate or deacylated histones
What does histone acetyltransferase do? (HAT)
acetylate histones so make DNA more accessible to transcription factors so incr. transcription
What does Histone deacetylase (HDAC) do?
deacetylates histones so strengthen DNA with histones so down regulate transcription
What do enhancers do?
short sequence DNA that cause transcription
What do enhancers bind to?
activators then co activators attached to transcriptional factor
What is DNA modification?
DNA methylation- methyl added to DNA
How is DNA methylated ?
via DNA methyltransferase
What is gene splicing?
mRNA introns removed (non coding) , interferon proteins don’t contain introns
What does gene splicing involve?
snRNPs (Small Nuclear Ribonucleoprotein)
What do several snRNPs form?
spliceosome
Splicing mechanism steps?
- 2 snRNP bind to 5’ and 3’ site separately
- then preassembled 3rd, 4th, 5th snRNP complex bind to intron region and cleave it from exons
- spliceosome falls and exons now joined
What are splicing defects?
mutation of 1 base in intron so new 3’ splice site
What’s an example of splice defect?
Thalessemia blood disorder- red blood cell abnormlaity
What is alternative splicing?
protein diversity as protein spliced in several ways to create isoforms of protein
Is alternative splicing in prokaryotes?
no
What are post transcriptional modifications of mRNA?
5’ CAP
Poly A tail at 3’ end
What is the addition of 5’cap and 3’ poly A tail for?
protect mRNA from degradation by ribonucleases
What does 5’ cap do?
stabilise mRNA so transported out of nucleus
What does 3’ Poly A tail do?
- buffer for exonucleases
- stabilise mRNA
- enhance translation
- determines half life of mRNA
how is 3’ poly tail added?
cleavage signal
- poly A polymerase added adenine nucleotides
