Assignment 4

November 24, 2018

This assignment is worth 5% of the course grade. Due Tue. Dec. 4

To make your job easier, a template for the report is available for download:

1. (2 points) Differences between maize lines.

Many maize lines have heterochromatic knobs which are easily distinguishable in C-banding. A karyotype of the South American line  441123 is shown in A.

The somatic karyotypes are shown for a South American maize line 441311* (B), and F1 progeny from a cross between 441311 and the North American breeding line KYS (F).  To provide landmarks useful in identifying chromosomes, three FISH probes were used:
*Lines 441123 and 441311 are very closely-related and their chromosomes can be considered comparable for our purposes.

Briefly describe the most obvious differences between lines 441311 and KYS.

2. (2 points) Use of pachytene chromosomes for measuring long arm to short arm ratios.

Figure 3 at right shows carmine-stained pachytene chromosomes from a cross of two closely-related lines, 441123 x 444331. Chromosomes are shown as follows:

1 (A)
2 (D)
3 (B)
4 (E)
5 (C)
6 (F)
7 (G)
8 (I)
9 (J)
10 (H)

Arrows - centromere

Heterochromatic knobs appear as dark spots on the chromosomes eg. at the end of the short arm of chromosome 9 (J).

The long arm/short arm ratios were measured in both in metaphase chromosomes (eg. as in part 1, panel A) or in pachytene chromosomes as in Figure3. The results are shown below:

a) What can you generalize about measuring arm ratios (AR) in metaphase, vs. pachytene chromosomes?
b) Can you think of a molecular mechanism that might account for the differences in results seen between the two methods of chromosome measurement? To answer this question, it will help to compare chromosome 7 as seen in Fig. 3G with chromosome 7 as seen in part 1.

3. (1 point) Changes in knob loci.

Most chromosomal knobs in maize consist of many tandem repeats of two satellite sequences: the 180 bp knob repeat, and the 350 bp TR-1 repeat.

The figure at right illustrates differences in knob loci that might be found in two different maize lines, one with a very large number of repeats (A) and the same chromosome with a small number of repeats (B).

Based on what you know about crossing over, briefly describe a simple molecular mechanism to account for the differences seen between lines 441311 and KYS, from the results in part 1.

Submitting your assignment

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