Polytene chromosomes Flashcards
What are the 4 reasons why Drosophila are weird?
No recombination in males
No recombination on chromosome 4 in either males or females
Females can store sperm for several days
Polytene chromosomes
When do Drosophila use polytene chromosomes? Why?
During larval stages in certain cells. Might be an adaptation to increase gene expression
What process creates polytene chromosomes?
Endoreduplication
What is endoreduplication?
DNA gets replicated and forms two sister chromatids, but cell division doesn’t happen and the chromatids just keep pilling up until there’s thousands of copies
What also happens during endoreduplication that normally only happens in meiosis?
Homologous chromosomes synapse
What are polytene chromosomes?
Giant synapsed chromosomes
Which two chromosomes will not synapse during endoreduplication?
X and Y
What happens near the centromeres to form polytene chromosomes?
Centromeres and pericentrometric regions of every single chromosome gets pulled together into the chromocentre
What is the DNA around the centromere like?
Tightly packed heterochromatin
What is the centromere lump in polytene chromosomes called?
Chromocentre
Is Drosophila chromosome 1 telocentric, metacentric, or acrocentric?
Telocentric
Is Drosophila chromosome 2 telocentric, metacentric, or acrocentric?
Metacentric
Is Drosophila chromosome 3 telocentric, metacentric, or acrocentric?
Metacentric
Is Drosophila chromosome 4 telocentric, metacentric, or acrocentric?
Telocentric
How many arms of chromosome 1 stick out of a polytene chromosome?
1
How many arms of chromosome 2 stick out of a polytene chromosome?
2, the left arm (2L) and right arm (2R)
How many arms of chromosome 3 stick out of a polytene chromosome?
2, the left arm (3L) and right arm (3R)
How many arms of chromosome 4 stick out of a polytene chromosome?
Just 1, but chromosome 4 is really tiny so unless we did a really good job with our prep, it won’t show up
How is each arm coming out of a polytene chromosome distinguishable from each other?
The chromatids stack on each other and each arm has a distinct banding pattern
What types of mutations can we easily see with polytene chromosomes?
Deletions, translocations, and inversions. Namely large scale chromosome changes
How many arms of the Y chromosome stick out of a polytene chromosome?
None, it’s mostly heterochromatin and gets lumped into the chromacentre
What types of mutations are not very good for looking for with polytene chromosomes?
Indels and base pair substitutions
How many segments is the Drosophila genome divided into?
102
How many segments are on the X chromosome? Which ones? How are they arranged?
Contains segments 1 to 20. Segment 1 is at the telomere and segment 20 is at the heterochromatin near the centromere
How many segments are on chromosome 2? Which ones? How are they arranged?
Contains segments 21-40 and 41-60. Segments 21 - 40 are on the left arm, with segment 21 at the telomere and segment 40 at the pericentric heterochromatin. Segments 41 - 60 are on the right arm, with segment 41 at the pericentric heterochromatin and segment 60 at the telomere
How many segments are on chromosome 3? Which ones? How are they arranged?
Contains segments 61-80 and 81-100. Segments 61 - 80 are on the left arm, with segment 61 at the telomere and segment 80 at the pericentric heterochromatin. Segments 81 - 100 are on the right arm, with segment 81 at the pericentric heterochromatin and segment 100 at the telomere
How many segments are on chromosome 4? Which ones?
Contains only segments 101 and 102
How many segments are on the Y chromosome?
None, it’s pretty much all heterochromatin and is in the chromocentre
How is each segment divided?
Into 6 regions labelled A to F
How is each lettered region divided?
Each band in the region is numbered
How does the systematic nomenclature of polytene chromosomes work?
First number is the segment, second is the region letter, third number is the band in the region
If we have the name of a specific band on a polytene chromosome, how do we figure out which chromosome it’s on?
We can tell the chromosome based on the segment
What can you do to find a specific gene sequence on a polytene chromosome?
In situ hybridization with a labelled probe complementary to the sequence of interest. It will bind to the complementary sequence on the polytene chromosome on a microscope slide
