The Increased Complexity Associated with Eukaryotes Flashcards
what is chromatin and where is it wound around
Chromatin is DNA wound around octamer cores of histone proteins (lysine and arginine) to form nucleosomes
how does chromatin appear and becomes known as chromatin fibre
‘beads on a string’
organised into helical array- becomes known as chromatin fibre
6 nucleosomes per turn of its helical array
Nucleosomes attach to protein scaffold which condense into larger fold and eventually form chromosomes.
Describe properties of histone protein
- basic
- hence positively charged
- bind to DNA at physiological PH
Describe the role of the cell cycle
4 stage process
Gap 1
- Metabolic changes prepare the cell for division
-The cell becomes committed to division after restriction point (R)
Synthesis
- DNA replication produces a copy of the genetic material
- Each chromosome now consist of two sister chromatid
Gap 2
- Metabolic changes assemble cytoplasmic materials necessary for mitosis and cytokinesis
Mitosis
- nuclear division followed by cell division
G,S,G2 - interphase
how is the cell cycle regulated
Restriction point (R) - located at G1 phase, ensures cell doesn’t replicate damaged DNA
G2- checkpoint - validates correct DNA replication
what is the biological significance in DNA replication
- extreme accuracy is necessary to preserve integrity of the genome in successive generations
-High fidelity in eukaryotes due to slower replication rates - Eukaryotic chromosomes are larger and more complex than prokaryotic material, there they are replicated from multiple origins of replication
-The enzymes involved in eukaryotic DNA replication are more complex that hose in prokaryotic DNA replication
Summarise DNA replication
Helicase using energy from ATP hydrolysis opens up DNA helix.
replication forks are formed at each replication origin as the DNA unwinds.
2. the opening of the double helix causes over winding - resolved with topoisomerases
3. Primers are formed by the enzyme PRIMASE.
Using the primer DNA polymerase can start synthesis
4. 3 major DNA polymerases are then involved:
α, δ and ε.
a. DNA pol α adds a short (20 to 30 nucleotides) DNA fragment to the RNA primer on
both strands, and then hands off to a second polymerase
b. While the leading strand is continuously synthesized by the enzyme pol δ, the lagging
strand is synthesized by pol ε
- A sliding clamp protein known as PCNA holds the polymerase in place so it doesn’t slide of DNA
- At the end of DNA replication, the RNA primers are replaced by DNA- 5’-3’ exonuclease and polymerase activity of DNA polymerase ε
- The okazaki fragments in the lagging strand are joined after the replacement of the primers with the DNA
- The gaps that remain are sealed by DNA ligase which froms the phophsdiester bond.
what is the difference between prokaryotic and eukaryotic replication
Prokaryotes
- single origin of replication
- replicate 1000 nucelotides/s
- 5 DNA polymerase types
- no telomerase
- use DNA pol I to remove RNA primer
- DNA pol III to elongate strand
Eukaryotes
- multiple origin of replication
-50-100 nucelotides rate of replication
- 14 DNA polymerase type
- Telomerase needed
- RNase H- RNA primer removal
-Pol alpha,delta,epsilon
what is telomeres and what is the problem in eukaryotic DNA
Telomers - ends of chromosomes (protective caps)
Eukaryotic DNA has issue with replicating telomers
-synthesis is difficult in lagging strand bc there is no primer possible for the end of the strand. Therefore this DNA would remain unpaired and chromosmes get shorter when cell divides
what is telomerase
- enzyme that produces telomeres
- Contains RNA region so can be used as template
- extend 3’ lagging stand to allow replication of chromosomes to complete
- occurs as binding-polymerisation translocation
what sequence is repeated in telomere regions
TTAGGG
TTGGGG
what does telomerase contain and how does it work
- Catalytic part and built in RNA template
- attaches to end of chromosome
- DNA nucleotides complementary to the RNA template
are added to the 3’ end of the lagging DNA strand - Once the 3’ end is long enough, DNA polymerase adds the complementary nucleotides
what does telomerase contain and how does it work
- Catalytic part and built in RNA template
- attaches to end of chromosome
- DNA nucleotides complementary to the RNA template
are added to the 3’ end of the lagging DNA strand - Once the 3’ end is long enough, DNA polymerase adds the complementary nucleotides
what does telomerase contain and how does it work
- Catalytic part and built in RNA template
- attaches to end of chromosome
- DNA nucleotides complementary to the RNA template
are added to the 3’ end of the lagging DNA strand - Once the 3’ end is long enough, DNA polymerase adds the complementary nucleotides
when does telomeres operate
Foetal development – rapid cell division
* Cancer – immortal cells
* Cellular aging
* (Not in adult cells)
what are the 3 RNA polymerases in eukaryotic cells
RNA polymerase I – rRNA (5.8S, 18S and 28S)
2. RNA polymerase II – protein-coding genes, snoRNA, miRNA, siRNA and most snRNA
3. RNA polymerase III – tRNA, rRNA (5S), some snRNA
what happens in initation of transcription of genes
RNA polymerase binds to the DNA of the gene at a promoter
* The promoter comes before (and slightly overlaps with) the transcribed region whose
transcription it specifies. It contains recognition sites for RNA polymerase or transcription
factors (TFs) to bind to
* The DNA opens up in the promoter so that RNA polymerase can begin transcription
* Once the transcription bubble has formed, the polymerase can start transcribing
what is TATA box
The TATA box occurs between -30 and -25 elements of many eukaryotic promoters
o It is recognised by a general TF, allowing others and RNA polymerase II to bind
o The high frequency of A and T makes it easy to separate DNA strands in this region
What are transcription factors
Basal/general TFs are proteins that are essential for transcription initiation. They bind to the promoter,
helping RNA polymerase to bind. Many promoters have a TATA box.
How is gene expression regulated
Chromatin remodelling
- Enhance and silencer element
- methylation modification
What does chromatin remodelling involve
Rearrangement of chromatin from condensed state to transceiptionall assessable state
Allow TF to bind and DNA binding proteins to access DNA and control gene expression
What can alter structure of chromatic
Epigenetic modifications to histone proteins such as methylation,demethylation, acteylation and deacetylation can alter chromatin
- transcriptional activation/ repression
Epigenetics is covalent modification to DNA which impacts gene expression without affecting the underlying genetic sequence
What do enhancer and silencer elements do
Regulate transcription through binding of multitude of TFs that activate or repress transcription
SILENCERS are antagonist of enhancers and repress transcription of genes when bound to TFs
What is methylation
Occurs at Cytosine based of eukaryotic DNA
Common epigenetic signal that cells use to lock genes in the “off” position
