Skills: Cell Division (DP IB Biology: SL)

• Cells undergoing different stages of the cell cycle can be identified using photomicrographs taken from microscope slides
• Cells undergoing certain stages of the cell cycle have distinctive appearances

Interphase

• As cells spend the majority of the cell cycle in this stage then most cells will be in this stage
• The chromatin is visible so the nuclei have a dark appearance

Prophase

• Chromosomes are visible
• The nuclear envelope is breaking down

Metaphase

• Chromosomes are lined up along the middle of the cell

Anaphase

• Chromosomes are moving away from the middle of the cell, towards opposite poles

Telophase

• Chromosomes have arrived at opposite poles of the cell
• Chromosomes begin to uncoil (are no longer condensed)
• The nuclear envelope is reforming

Cytokinesis

• Animal cells: a cleavage furrow forms and separates the daughter cells
• Plant cells: a cell plate forms at the site of the metaphase plate and expands towards the cell wall of the parent cell, separating the daughter cells

Identification of phases

Micrograph showing a cell undergoing prophase (P)

Micrograph showing cells undergoing metaphase (M) and anaphase (A)

Micrograph showing cells undergoing metaphase (M) and anaphase (A)

Micrograph showing a cell undergoing anaphase (A)

Investigating mitosis in root tissue

• Growth in plants occurs in specific regions called meristems
• The root tip meristem can be used to study mitosis
• The root tip meristem can be found just behind the protective root cap
• In the root tip meristem, there is a zone of cell division that contains cells undergoing mitosis
• Pre-prepared slides of root tips can be studied or temporary slides can be prepared using the squash technique (root tips are stained and then gently squashed, spreading the cells out into a thin sheet and allowing individual cells undergoing mitosis to be clearly seen)

Micrograph showing a stained root tip

Analysis

• Cells undergoing mitosis (similar to those in the images below) can be seen and drawn
• Annotations can then be added to these drawings to show the different stages of mitosis
• The mitotic index can be calculated

The mitotic index

• The mitotic index is the proportion of cells (in a group of cells or a sample of tissue) that are undergoing mitosis
• The mitotic index can be calculated using the formula below:

• You can multiply the answer by 100 if you need to give the mitotic index as a percentage

mitotic index = 0.76

mitotic index = 0.44

Step 1: Identify the cells undergoing mitosis

Step 2: Count the total number of cells

Total number of cells (green + red) = 20 + 55 = 75

Step 3: Substitute numbers into the equation

mitotic index = 0.27