3.3- Microscope measurements and required practical 2 Flashcards
What is an eyepiece graticule?
An glass disc that is placed in the eyepiece of a microscope.
A scale is etched on the glass disc.
Why can’t the scale be directly used to measure the size of objects under an objective lens?
Because each objective lens will magnify to a different degree
How do you calibrate the eyepiece graticule?
Use a stage micrometer
Describe a method to calculate the mitotic index of cells
. Heat 1 mol dm-3 H Cl at 60°C in a water bath.
2. Cut a small sample of the root tip using a scalpel.
3. Transfer root tip to HCl and incubate for 5 minutes.
4. Remove from HCl and wash sample in cold distilled water and remove the very
tip using a scalpel.
5. Place tip on a microscope slide and add a few drops of stain
This makes the chromosomes visible and will therefore show which cells
are undergoing mitosis.
6. Lower the cover slip down carefully onto the slide. Make sure there are no air
bubbles in the slide which may distort the image, and that the coverslip doesn’t
slide sideways which could damage the chromosomes.
7. Place under a microscope and set the objective lens on the lowest
magnification.
8. Use the coarse adjustment knob to move the lens down to just above the slide.
9. Use the fine adjustment knob to carefully re-adjust the focus until the image is clear
10. To calculate mitotic index, cells undergoing mitosis must be counted (cells with chromosomes visible), as well as the total number of cells.
What is the equation for mitotic index?
Mitotic index=number of cells with visible chromosomes÷total number of cells in sample
Why do cells need to be taken from the root tip?
Because this is the region of cells undergoing mitosis
Why does the root tip need to be firmly squashed?
In order to get a thin layer of cells which light can pass through
Why was a stain used?
To make chromosomes visible
Why were the root tips placed in hydrochloric acid?
In order to soften the tissues and separate the cells
Why were the root tips heated at 60 degrees for 5 mins
To increase the kinetic energy of the molecules, increasing the rate of reaction with the acid, denaturing proteins, softening cells and stopping mitosis
