Chromosomes KH8 Flashcards
What is the difference between chromosome compaction during mitotic metaphase and during interphase?
Mitotic Metaphase (cell division): chromsomes ++ tightly folded to facilitate equal distribution between 2 daughter cells
No DNA transcription or replication
Interphase: less tightly coiled bc replication and transcription happen (need to have access to base pairs)
Unwinding is dynamic and controlled (never total)
What is a chromosome?
Independant linear DNA molecule in eukaryote
compaction and condensation of DNA
Made of chromatin = DNA/protein complex
(doesn’t exist as naked DNA)
human chromosome = 5 cm long when stretched, has to folded/packed/coiled at least 5,000-fold to fit in the nucleus
What is chromatin? (levels)
eukaryotic DNA and its associated proteins
By levels:
1. Chromatin (that is packed and coiled)
2. Topological domains bonded by boudary elements (interact within themselves)
3. Chromatin fibers (5-24 nm)
4. Beads on a string
5. Nucleosomes (Histone H1 octamer + DNA wrapped around)
6. DNA
What particular structure was used to better understand chromosome configuration?
Giant interphase chromosome: polythene chromosome of the fly (Drosophila) salivary glands
Same genetic material, but many parallel identical chromatids (in human normal, all different chromosomes) → repeated replication without separation (replication stops before reaching end of the telomere, 2 centromeres are at the ends, 10 cycles = 1024 copies in parallel)
How does Polytene chromosome show that the interphase chromatin organization is dynamic?
Take pictures over several hours and see some topological domains (dark bands) develop into puffs
Puffs = active transcription happening, with RNA pol II (by immunofluorescence) and unwinding
What is the structure of a chromosome during metaphase?
After replication, have 2 sister chromatids (same DNA exactly has been replicated, 46n) linked at the centromere
On each of the 4 ends, there is a Telomere
When is the Kayotype ?
All the metaphase chromosomes of a species
In humans: 46 different chromosomes, 2n from 1-22 + X, one of each is maternal and one paternal (each chromosome is an X because has been replicated)
Number, size and shape of chromosomes = species-specific
What is chromosome painting?
Fluorescence in situ hybridization (FISH) of karyotype
Panel of probes representing sequences differentially distributed among chromosomes
What does chronic myelogenous leukemia originate from?
Chromosome fusion between 9 and 22
9 end doesn’t change to much but 22 end gets much shorter → Philadelphia chromosome (22)
At the fusion of the 9 and 22 segments on the Philadelphia chromosomes, sequence codes for an oncogenic fusion protein (chimeric gene)
This protein deregulates cell growth → cancer
*Happens in blood cells
Explain chromosome rearrangement in the germ line.
Often, rearranged chromosomes have reduced fertility so not passed down → dead end → consistant karyotype across a species
Sometimes, passed onto next generation → evolution on genome
What are 4 changes that show evolution of the human karyotype?
Chromosome 11 in ancestral karyotype is unchanged but is now Chr.13
Breakage: Chr.5 (ancestor) broken → Chr. 14 and 15
Reciprocal translocation: Chr 14, 21 (ancestor) → Chr12, 22 cut and rearranged with a bit of both in each
End-to-end fusion: Chr. 9 + 11 (ancestor) → Chr. 2
Note 1 is the longest and 22 is the shortest in karyotype
What are elements that are required for replication and stable inheritance of linear chromosomes?
- Origin of replication
- Centromere
- 2 Telomeres (ends)
Which experiment permitted the discovery of elements requiered for chromosomes function?
Yeast (simple eukaryote), specifically Yeast leu-
Yeast leu- = Leucine gene inactivated by mutation, need exogenous leucine for growth
Add Leu in a plasmid and see if works with non-leucine environment: works in bacteria but not is eukaryote bc bacterial ORI doesn’t work in eukaryotes
*ARS = autonomously replicating sequence = yeast origin of DNA replication
1. Add yeast ARS to plasmid: can grow but only 5-20% of cells have plasmid…no partition between daughter
- Add CEN (DNA sequence from yeast chromosome centromere) : works for circular plasmids, not for linear plasmids
- Add telomeres → linear plasmid acts as normal chromosome (telomeres necessary for lineary DNA to survive as chromosomes)
Where to spindle microtubules attach on he sister chromatids?
on the kinetochore in the centromere (where the 2 chromatids sisters are joined)
Kinetochore is a protein complex, NOT = nucleosome
How does the attachement of the spindle microtubules to the centromer occur?
- nucleosome at centromere includes centromere-specific histone variant CENP-A (centromeric protein A)
- CNEP-A recruits CBF3 complex,
- CBF3 (centromere binding factor 3) complex recruits Ndc80 complex → attached to microtubules lateral attachment
- lateral → end-on conversion
- As Ndc80 complex advances on the spindle microtubule, the spindle microtubule dissolves.
- Ndc80 pulls its chromatid sister to the Spindle pole
