Cells: Mitosis

Question 1

Structure of Chromosomes in Mitosis

Answer 1

• As mitosis starts, the chromosomes are made of two strands joined in the middle by a centromere
• Separate strands are called chromatids
• Two strands on the same chromosome are called sister chromatids
• There are two strands because each chromosome has made an identical copy of itself during interphase
• When mitosis is over, chromatids end up as one-strand chromosomes in the new daughter cells

Question 2

Stages of Mitosis

Answer 2

• Prophase
• Metaphase
• Anaphase
• Telophase

Question 3

Prophase

Answer 3

• Chromosomes condense, getting shorter and fatter
• Centrioles move to opposite ends of the cell, forming a network of protein fibres across the cell called the spindle
• The nuclear envelope (membrane around the nucleus) breaks down, chromosomes are free in the cytoplasm

Question 4

Metaphase

Answer 4

• Chromosomes (with two chromatids) line up along the middle of the cell
• They become attached to the spindle by their centromere

Question 5

Anaphase

Answer 5

• Centromeres divide, separating each pair of sister chromatids
• Spindles contract, pulling chromatids to opposite poles of the spindle
• Centromere leads way, making chromatids appear v-shaped

Question 6

Telophase

Answer 6

• Chromatids reach opposite poles on the spindle
• They uncoil and become long and thin again
• Now called chromosomes again
• Nuclear envelope forms around each group of chromosomes so there are two nuclei
• Cytoplasm divides (cytokinesis)
• Forms two genetically identical daughter cells
• Mitosis is finished and interphase begins again

Question 7

Mitotic Index

Answer 7

• In a population of cells, the ratio of the number of cells undergoing mitosis

• The mitotic index can be calculated by counting how many cells are visible in the field of view and the number of cells visible that are in a stage of mitosis
• Then the following formula can be used: Mitotic index = (the number of cells in mitosis / the total number of cells)

Question 8

How long does each stage of mitosis take?

Answer 8

• Varies and depends on cell type and environmental conditions
• Count total number of cells and number of cells undergoing certain mitosis stage
• Calculate proportion of time cells spend in stage by dividing number of cells in certain stage of mitosis by total number of cells
• Multiply this by time to complete cell cycle

Question 9

Changes in Cell Mass During Mitosis

Answer 9

• The mass of the cell grows continually during the cell cycle as it makes DNA, proteins and organelles
• Mass increases during interphase - increased size, more organelles and more DNA
• Mass increases during mitosis - two cells form, more cytoplasm
• Mass halves during cytokinesis - cells splits into two

Question 10

Changes in DNA Mass During Interphase & Mitosis

Answer 10

• DNA is only synthesised in S part of interphase so mass of DNA increases for a short part of the cell cycle
• G1 phase - no increased mass, DNA just becomes visible
• S phase - DNA replicates
• G2 - no change, cell prepares to divide
• Cytokinesis - mass of DNA halves

Question 11

What is a chromosome?

Answer 11

• A chromosome is a thread-like structure made up of one long DNA molecule

Question 12

What is a chromatid?

Answer 12

• A chromatid is one ‘arm’ of a double stranded chromosome