Polytene chromosomes

Question 1

What are the 4 reasons why Drosophila are weird?

Answer 1

No recombination in males
No recombination on chromosome 4 in either males or females
Females can store sperm for several days
Polytene chromosomes

Question 2

When do Drosophila use polytene chromosomes? Why?

Answer 2

During larval stages in certain cells. Might be an adaptation to increase gene expression

Question 3

What process creates polytene chromosomes?

Answer 3

Endoreduplication

Question 4

What is endoreduplication?

Answer 4

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

Question 5

What also happens during endoreduplication that normally only happens in meiosis?

Answer 5

Homologous chromosomes synapse

Question 6

What are polytene chromosomes?

Answer 6

Giant synapsed chromosomes

Question 7

Which two chromosomes will not synapse during endoreduplication?

Answer 7

X and Y

Question 8

What happens near the centromeres to form polytene chromosomes?

Answer 8

Centromeres and pericentrometric regions of every single chromosome gets pulled together into the chromocentre

Question 9

What is the DNA around the centromere like?

Answer 9

Tightly packed heterochromatin

Question 10

What is the centromere lump in polytene chromosomes called?

Answer 10

Chromocentre

Question 11

Is Drosophila chromosome 1 telocentric, metacentric, or acrocentric?

Answer 11

Telocentric

Question 12

Is Drosophila chromosome 2 telocentric, metacentric, or acrocentric?

Answer 12

Metacentric

Question 13

Is Drosophila chromosome 3 telocentric, metacentric, or acrocentric?

Answer 13

Metacentric

Question 14

Is Drosophila chromosome 4 telocentric, metacentric, or acrocentric?

Answer 14

Telocentric

Question 15

How many arms of chromosome 1 stick out of a polytene chromosome?

Answer 15

1

Question 16

How many arms of chromosome 2 stick out of a polytene chromosome?

Answer 16

2, the left arm (2L) and right arm (2R)

Question 17

How many arms of chromosome 3 stick out of a polytene chromosome?

Answer 17

2, the left arm (3L) and right arm (3R)

Question 18

How many arms of chromosome 4 stick out of a polytene chromosome?

Answer 18

Just 1, but chromosome 4 is really tiny so unless we did a really good job with our prep, it won’t show up

Question 19

How is each arm coming out of a polytene chromosome distinguishable from each other?

Answer 19

The chromatids stack on each other and each arm has a distinct banding pattern

Question 20

What types of mutations can we easily see with polytene chromosomes?

Answer 20

Deletions, translocations, and inversions. Namely large scale chromosome changes

Question 21

How many arms of the Y chromosome stick out of a polytene chromosome?

Answer 21

None, it’s mostly heterochromatin and gets lumped into the chromacentre

Question 22

What types of mutations are not very good for looking for with polytene chromosomes?

Answer 22

Indels and base pair substitutions

Question 23

How many segments is the Drosophila genome divided into?

Answer 23

102

Question 24

How many segments are on the X chromosome? Which ones? How are they arranged?

Answer 24

Contains segments 1 to 20. Segment 1 is at the telomere and segment 20 is at the heterochromatin near the centromere

Question 25

How many segments are on chromosome 2? Which ones? How are they arranged?

Answer 25

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

Question 26

How many segments are on chromosome 3? Which ones? How are they arranged?

Answer 26

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

Question 27

How many segments are on chromosome 4? Which ones?

Answer 27

Contains only segments 101 and 102

Question 28

How many segments are on the Y chromosome?

Answer 28

None, it's pretty much all heterochromatin and is in the chromocentre

Question 29

How is each segment divided?

Answer 29

Into 6 regions labelled A to F

Question 30

How is each lettered region divided?

Answer 30

Each band in the region is numbered

Question 31

How does the systematic nomenclature of polytene chromosomes work?

Answer 31

First number is the segment, second is the region letter, third number is the band in the region

Question 32

If we have the name of a specific band on a polytene chromosome, how do we figure out which chromosome it's on?

Answer 32

We can tell the chromosome based on the segment

Question 33

What can you do to find a specific gene sequence on a polytene chromosome?

Answer 33

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