1: Mitosis

Question 1

In eukaryotes, where is nearly all of the DNA of a cell stored?

Answer 1

in the nucleus

Question 2

How many metres of DNA does a human nucleus contain? How long is the human nucleus in diameter? How can all the DNA fit in the small nucleus?

Answer 2

• 2m
• 5um
• DNA molecule is so narrow - its width is is 2nm (0.002um)

Question 3

Is a DNA molecule visible with a light microscope?

Answer 3

no

Question 4

What do DNA molecules have attached to them in eukaryotes? What structure does this form?

Answer 4

have proteins attached to them, forming structures called chromosomes

Question 5

What happens to the chromosomes during mitosis? What is this called? How does ‘this’ occur?

Answer 5

• chromosomes become shorter and fatter
• condensation
• through complex process called ‘supercoiling’

Question 6

When is it possible to see chromosomes through a light microscope?

Answer 6

when they are condensed during the early stages of mitosis

Question 7

What sort of structure do chromosomes have when it is possible to see them with a light microscope?

Answer 7

a double structure

Question 8

During the early stages of mitosis, what are the two parts of the chromosome called? What is the name of the point that holds the two parts of the chromosome together?

Answer 8

• sister chromatids

- the centromere

Question 9

What does the term ‘sister’ indicate in ‘sister chromatid’? How are they produced?

Answer 9

• that the two chromatids contain an identical DNA molecule

- produced by DNA replication before the start of mitosis

Question 10

What happens to the centromere during mitosis? What is the result of this?

Answer 10

• centromere divides

- the sister chromatids separate

Question 11

What are the sister chromatids referred to as after the centromere divides and they separate?

Answer 11

as chromosomes, rather than chromatids

Question 12

State all the stages of mitosis.

Answer 12

1. early prophase
2. late prophase
3. metaphase
4. anaphase
5. early telophase
6. late telophase

Question 13

What happens in mitosis: early prophase

Answer 13

• spindle microtubules are growing

- chromosomes are becoming shorter and fatter by supercoiling

Question 14

What happens in mitosis: late prophase

Answer 14

• spindle microtubules extend from each pole to the equator
• each chromosome consists of two identical chromatids formed by DNA replication in interphase and held together by a centromere

Question 15

What happens in mitosis: metaphase

Answer 15

• spindle microtubules from both poles are attached to each centromere, on opposite sides
• nuclear membrane has broken down and chromosomes have moved to equator

Question 16

What happens in mitosis: anaphase

Answer 16

• spindle microtubules pull the genetically identical chromosomes to opposite poles
• centromeres have divided and the chromatids have become chromosomes

Question 17

What happens in mitosis: early telophase

Answer 17

• spindle microtubules break down

- all chromosomes have reached the poles and nuclear membranes form around them

Question 18

What happens in mitosis: late telophase

Answer 18

• chromosomes uncoil and are no longer individually visible

- cell divides (cytokinesis) to form two cells with genetically identical nuclei

Question 19

Draw and label a cell undergoing mitosis (only include relevant parts of cell). (p14)

Answer 19

-

Question 20

What is the mitotic index?

Answer 20

the ratio between the number of cells in mitosis in a tissue and the total number of observed cells

Question 21

State the formula for mitotic index.

Answer 21

= number of cells in mitosis/ total number of cells

Question 22

How is the mitotic index used?

Answer 22

by doctors to predict how rapidly a tumour will grow and therefore what treatment is needed

Question 23

What does a high mitotic index indicate?

Answer 23

a fast-growing tumour

Question 24

What is cytokinesis?

Answer 24

the division of the cytoplasm to form two cells

Question 25

When does cytokinesis occur?

Answer 25

after mitosis and is different in plant and animal cells

Question 26

How does cytokinesis occur in plant cells?

Answer 26

a new cell wall is formed across the equator of the cell, with plasma membrane on both sides. This divides the cell in two.

Question 27

How does cytokinesis occur in animal cells?

Answer 27

the plasma membrane at the equator is pulled inwards until it meets in the centre of the cell, dividing it in two