1.6 Cell division

Question 1

what is interphase?

Answer 1

longest stage of the eukaryotic cell cycle
cells grow and synthesise new organelles, proteins and DNA in preparation for mitosis

Question 2

what is mitosis?

Answer 2

form of cell division
produces two genetically identical copies of genes (diploid) daughter cells

Question 3

why is mitosis important?

Answer 3

important for the growth of tissue and repair and replacement of cells.

Question 4

name the stages of mitosis

Answer 4

prophase
metaphase
anaphase
telophase

Question 5

summarise the process of mitosis.

Answer 5

prophase = nuclear envelope breaks down, chromosomes condense, centrioles move to opposite poles of the cell.
metaphase = sister chromosomes line up at the equator, centromeres attach to the spindle fibres
anaphase = sister chromatids separated and pulled apart to the opposite poles
telophase = nuclear envelope reforms, spindle fibres break down, chromosomes uncoil

Question 6

what are sister chromatids?

Answer 6

a pair of identical chromatids formed by DNA replication, joined by a centromere.

Question 7

what is cytokinesis?

Answer 7

the division of the cytoplasm at the end of mitosis to produce two new daughter cells.

Question 8

compare mitosis in plant and animal cells.

Answer 8

Animal cells/Plant cells
occurs in most tissues/ occurs in meristematic cells only
involved centrioles/no centrioles involved
involved micro filaments/ no micro filaments involved
spindle fibres disappear prior to cytokinesis/ some spindle fibres remain during cytokinesis

Question 9

what may unrestricted mitosis divisions lead to?

Answer 9

cancerous growth

Question 10

what is meiosis?

Answer 10

form of cell division

produces 4 genetically different daughter cells (gametes) with a haploid number of chromosomes

involves two divisions

Question 11

what is the significance of meiosis in reproduction?

Answer 11

gametes must be haploid so that when they combine during fertilisation, the full number of chromosomes is present within the resulting zygote. It also creates genetic variation.

Question 12

what is meiosis one?

Answer 12

first stage of meiosis
homologous chromosomes separated to form two haploid cells

Question 13

describe what happens during meiosis one.

Answer 13

homologous chromosomes pair for form bivalents

crossing over occurs at chiasmata

cell divides into two; independent segregation of homologous chromosomes, each cell contains either a maternal or paternal copy.

Question 14

what are homologous chromosomes?

Answer 14

pair of chromosomes with genes at the same locus.
one maternal one paternal.
some alleles may be the same while others are different

Question 15

define crossing over.

Answer 15

process in meiosis 1

homologous chromosomes pair up, their chromatids wrap around one another and their alleles are exchanged at equivalent portions of chromatids

creates genetic variation

Question 16

what is independent segregation?

Answer 16

the random separation of homologous chromosomes in meiosis one that produces genetic variation

Question 17

what is meiosis two?

Answer 17

second stage of meiosis

sister chromatids separated to form four haploid gametes

Question 18

describe what happens during meiosis two

Answer 18

independent segregation of sister chromatids

each cell divides again, producing four haploid cells

Question 19

how does meiosis produce genetic variation?

Answer 19

crossing over during meiosis one

independent assortment of homologous chromosomes and sister chromatids

results in new combinations of alleles

Question 20

Where in plants can cells undergoing mitosis be found?

Answer 20

Meristem tissue at shoot and root tips.

Question 21

What is the mitotic index?

Answer 21

The ratio of cells undergoing mitosis to the total number of cells in a sample.

Question 22

Outline the procedure to prepare a root tip slide.

Answer 22

1. Warm 1M HCl to 60°C in a water bath.
2. Cut a root tip using a scalpel and add to the HCl. Leave for 5 minutes.
3. Remove from HCl and wash with cold distilled water.
4. Dry and place on a slide. Macerate with needle to spread out the cells.
5. Add a few drops of stain to make chromosomes visible.

Question 23

State the formula for the mitotic index.

Answer 23

Mitotic index =
Number of cells with visible chromosomes / Number of cells in sample

Question 24

State the hazards and precautions for reagents used in this procedure.

Answer 24

HCl - corrosive, avoid contact with skin, wear eye protection

Toluidine Blue O stain - irritant, avoid contact with skin, wear eye protection

Scalpel - cut away from fingers

Question 25

Why is the root tip placed in hot HCI?

Answer 25

HCl dissolves the middle lamellae in order to break up the cellulose cell wall. This allows the stain to permeate and the tip to be squashed more easily.

Question 26

Why are the cells flattened on the slide?

Answer 26

To spread out the cells and reduce the thickness of the layers so that cells can be observed more clearly.

Question 27

Which cells in an anther undergo meiosis?

Answer 27

Pollen sacs contain pollen mother sacs that divide by meiosis.

Question 28

State the stages of meiosis.

Answer 28

Prophase, Metaphase, Anaphase,
Telophase one and two

Question 29

How is the magnification of a drawing calculated?

Answer 29

Calibrate the eyepiece graticule using a stage micrometer.
Use the eyepiece graticule to measure the actual size of the cell.
Use a ruler to measure the size of the cell drawing.

Calculate magnification using magnification = size of image / size of object.

Question 30

What can be observed in a cell undergoing prophase one and two?

Answer 30

Individual chromosomes visible, nuclear envelope absent.

Question 31

What can be observed in a cell undergoing metaphase one?

Answer 31

Pairs of chromosomes line up along the the cell equator.

Question 32

What can be observed in a cell undergoing metaphase two?

Answer 32

Individual chromosomes line up along the equator of the cell.

Question 33

What can be observed in anaphase one?

Answer 33

Chromosomes (consisting of 2
chromatids each) separated to opposite poles of the cell.

Question 34

What can be observed in a cell undergoing anaphase two?

Answer 34

Chromosomes (consisting of 1 chromatid each) are separated to opposite poles of the cell.

Question 35

What is observed in a cell undergoing telophase one and two?

Answer 35

2 groups of chromosomes at opposites poles of the cell, individual chromosomes no longer visible.