Lecture 27 - Chromosomes Flashcards
How errors of homologous recombination can happen
Unequal crossing over when many copies of certain DNA elements (like genes) are present in the genome
2 possible consequences of homologous recombination at gene level
1) Gene deletion
2) Gene duplication
Goal of chromosomes in metaphase
Highly condensed for transmission to daughters
Goal of chromosomes in interphase
Decondense (regulated) for transcription and regulation
What is though to be the structure of chromosomes during interphase method that proved it
Loops that always come back to a scaffold.
Genes localization by in situ hybridization
method that proved evidence for loops in interphase chromosome
Genes localization by in situ hybridization
To what extent are loops of chromatin dependent (2 levels)
Each one is independent in terms of chromatin condensation and enhancer action
Name for the scaffold of chromatin loops
SAR : Scaffold-associated region
MAR: Matrix-attachment region
Function of scaffold region (2)
1) Barrier to spreading chromatin condensation (boundary elements)
2) Barrier to action of enhancers
Length of a chromatin loop
1-4 Mbp
where lampbrush chromosomes are found + their size
In amphibian mature oocytes (1 mm diameter cells)
Charact. of lampbrush chromosomes (2)
Highly transcriptionally active for a long time + spread because cell is very large
What’s the name of giant chromosomes found in drosophila’s salivary gland + name of phenomenon observed
Polytene chromosomes -> cellular giantism
What causes cellular giantism
10 cycles of DNA replication without cell divison (1024 daughter chromatids)
Light bands vs Dark bands on polytene chromosome
Dark = condensed chromatin
What are polytene chromosome puffs associated to
Decondensed chromatin and transcription
How puffs visualized with CTD of Pol II
Red associated w/ phosphorylated CTD, green unphosph. Use antibody for CTD of Pol II
3 main regions found on a (replicated) chromosome
Sister chromatids, centromere, telomeres
Chromosome state when showing karyotype
Metaphase
Chromosome painting explanation (how karyotype revealed)
Fluorescence in situ hybridization w/ panel of probes of distributed sequences
Where chromosome breaks/translocations can occur
In somatic cell division in an organism
What are chromosome breaks/translocations
2 chromosomes break (double strand break) and each ‘‘half’’ goes w/ ‘‘half’’ of other chromosome
Example of chromosome translocation and special name of resulting chromosome and condition
Chromosomes 9 and 22. Small resulting chromosome = philadelphia chromosome. Chronic myelogenous leukemia
What can cause chromosome translocations (2)
1) Nonhomologous end joining between the 2 (both were broken)
2) Errors in homologous recombination because of identical repeats
Problem of chromosome translocations in germ line (cells leading to gametes)
Lead to inperfect alignement of chromosomes during meiosis and reduced number of gametes produced that contain all genes
Why imperfect alignement of chromosomes during meiosis affects number of gametes produced
Perfect alignement of chromosome (disrupted by translocations )during meiosis is required to produce functional gametes
What happens to organisms carrying variant chromosomes (received translocated chromosomes from mom or dad’s gamete)
Sterile/greatly reduced fecundity -> Can’t produce functional gametes : Chromosomes won’t be aligned perfectly during meiosis
What explains fact that hybrids are sterile
If receive 2 gametes from different species, chromosomes won’t align perfectly during meiosis so they can’t produce functional gametes
To what extent/on what scale the karyotype of a species changes and due to what
Changes in evolutionary time due to chromosome variants succesfully passes between generations
3 elements absolutely required to replication and stable inheritance of chromosomes
1) ORI (at least 1)
2) Centromere
3) 2 telomeres (ends)
Example of method used to check which regions in DNA are necessary for replication and stable inheritance of chromosomes
Grow yeast leu- (w/o leucine gene) with plasmids containing LEU and random DNA fragments, in a medium without leucine
What is ARS + length
Autonomously replicating sequence (100 bp)
Effect of putting LEU gene in plasmid w/ only ARS
Replicaton occurs but mitotic segregation is faulty (large proportion of cells don’t receive plasmid)
Effect of putting LEU gene in plasmid w/ ARS and centromeric sequence
Replication occurs and mitotic segregation is successful
Role of centromeric sequences
Equal partitioning/distribution of plasmid to daughter cells during mitosis
Effect of putting LEU gene w/ ARS and centromeric sequence in a linear plasmid
No progeny to the daughter cell
Effect of putting LEU gene w/ ARS, centromeric sequence AND 2 telomeric sequences in a linear plasmid
Linear plasmid behaves like normal chromosome (replication and mitotic segregation are successful
Role of telomeric sequence
Promotes stability of chromosome so it can be passed for a long period of time
Structure of centromeric sequence
Region 1 conserved
Region 2 AT rich
Region 3 conserved
2 differences between yeast and higher eukaryotes about centromeric sequence
1) Higher eukaryotes : Longer region II (264 bp instead of 78-86 in yeast)
2) Yeast -> One or several repeats of CEN
Higher euk -> several repeats of CEN
Special thing about centromeric nucleosomes
Contain special variant of histone H3 called CENP-A
Utility of variant CENP-A in centromeric nucleosomes
Drives kinetochore assembly, which is what spindle microtubules will bind to
Problem of the lagging strand mechanism
5’ end of daughter strands cannot be completed because no DNA put at the end after primer removal
Solution to lagging strand mechanism
Telomerase, a DNA polymerase that extends telomeres to complete 5’ end of daughter strand
In what cells of the body telomerase is active
Germ cells and stem cells (+ reactivated in cancer cells)
What happens to mice missing telomerase gene
Are OK for 3 generations, and then fecundity declines
Telomeric repeat in human telomeres
TTAGGG
