ElaborationAt this point the students should be able to describe mitosis. This next stage of the learning cycle is to further involve the students in activities that will help them to understand mitosis. It is important at this stage to identify and correct any student misconceptions.
Activity 1a: Observing Giant Chromosomes in Drosopohila ;Teacher demonstration, 5-10 minutes
It is up to the teacher's discretion as to how to present the following activity for students. It depends upon :
- the proficiency level of the class
- the ability of the class to follow directions
- availbility of compound microscopes
The teacher may want to restructure the activity based on leaner needs and resources. The teacher may:
- using a video microscope, do the entire activity as a class demonstration
- demonstrate the activity, then give students teacher prepared slides of chromosomes
- demonstrate the activity and allow students to prepare own slides in groups, pairs or single
- set up activity 1b as individual, independent learning center for students if there are not a lot of compound microscopes available
Background for Activity: The larger larvae in the vials, those which are about to pupate, have salivary gland cells which contain giant chromosomes that are actively involved in RNA synthesis. The large size of these chromosomes results because the chromosomes are partially condensed as in prophase, the homologous chromosomes lie side by side and each chromosome has been replicated a number of times and the copies are joined along their length; they have not been separated into individual nuclei (endoreduplication). These giant chromosomes make it possible to see chomosomal regions that nearly correspond to individual genes. In order to observe the chromosomes, the salivary glands must be dissected,stained, and squashed.
Directions for Teacher Demonstration for Activity 1b:
Note: It is especially important for this activity that the teacher practice ahead of time.
1. Carefully place one or two larvae crawling up the jar wall on to a dry slide.
2. Push a probe or dissecting needle through the head, just behind the black mouthparts.
3. Push a second probe through the midbody.
4. Pull the needles apart. The mouthpart should separate from the body, pulling along the salivary glands, which look shimmerry.
5. The glands should look like two elongated, grapelike, transparent clusters of tissue. Do not confuse the glands with the fat bodies of the larva. Do not be too frustrated if it takes a few larvae before you get results.
6. Wipe away excess tissue off the slide.
7. Cover the glands with a drop of acetocarmine stain and allow to stain for about 5 minutes. (alternative stain: acetoorcein) Experience will show whether more or less time is appropriate for your stain preparation.
8. Cover the stained tissue with a coverslip. Press firmly and directly down, to squash the cells and nuclei. This may be done with fingers , but first wrap the slide in paper towel to prevent cutting your fingers. Alternatively, use the eraser end of a pencil.
9. Use the medium power (10X) objective to find the tissue and the high power (40 x) to observe the chromosomes.
10. Direct students to look for: how many chromosomes there are and any banding in the chromosomes.
Activity 1b: At this point, the teacher needs to decide if :
- the activity ends here
- students will complete the activity following the directions for 1a at independent stations, or in pair, groups
- students will observe prepared slides;
- students will use particularly good preparations to try to differentiate the 4 haploid chromosomes.
Materials: prepared vials of fruit fly larvae, slides, cover slips, compound microcope, probe, stain
background exploration elaboration engagement explanation materials