Topic 2.3 Cell division

Question 1

G1 (part of interphase)

Answer 1

Gap 1
(Longest stage)
-DNA replicates
-All about growth (cell size doubles)
-New organelles are produced
-New proteins are synthesised ready for cell division

Question 2

S phase (part of interphase)

Answer 2

-Synthesis stage
-Chromosomes are replicated by semi-conservative replication
-They become chromatids held by the centromere

Question 3

G2 (part of interphase)

Answer 3

Gap 2
-Organelles and proteins required for mitosis are synthesised/divided
eg. microtubules spindle fibres

Question 4

Interphase

Answer 4

-DNA/chromosomes are copied by semi-conservative replication
-New organelles are produced eg. mitochondria
-New proteins synthesised
-ATP generated
(Made up of G1, S phase, G2)

Question 5

Prophase

Answer 5

-Nuclear membrane breaks down
-Chromasomes condense and become visible (X-shaped sister chromatids joined at centromere)
-Centrioles move towards the poles (opposite ends of cell) and develop spindles

Question 6

Metaphase

Answer 6

Sister chromatids line up at cell equator and attach to the spindle fibres.

Question 7

Anaphase

Answer 7

-Spindle fibres contract
-Centromere divides
-Sister chromatids separate, chromosomes move apart (V-shaped, facing each other).

Question 8

Production of eukaryotic chromosomes

Answer 8

-DNA is linear
-However it condenses and associates with proteins called histomes (nucleosomes: when wrapped around 8)
-This is now described as chromatin
-This is then coiled (protein and DNA together)
-This is then supercoiled to form a chromosome

Question 9

Telophase

Answer 9

-Spindle fibre break down
-Nuclear membrane forms around the chromosomes
-Chromosomes become less dense (decondense) and invisible

Question 10

Cytokinesis in animals

Answer 10

-Ring of contractile protein forms around the cell (contractile fibres)
-Contracts and causes indentation of the cell called ‘cleavage furrow’
-The ‘cleavage furrow’ is formed by filaments which the splits the cell into two

Question 11

Cytokinesis in plants

Answer 11

-A cell plate forms within the plant cell
-The cell plate extends outwards splitting the parent plant cell
-Plant cell produces new inflexible/rigid cellulose cell walls

Question 12

Producing spores

Answer 12

-Sporulation involves mitosis and the production of the asexual pores that are capable of growing into new individuals
-These spores can usually survive adverse conditions and are also easily spread over great distances
-This form of asexual reproduction is most common in fungi and plants such as mosses and ferns.

Question 13

Regeneration

Answer 13

Regeneration is a very dramatic form of asexual reproduction, occurring when organisms replace parts of the body that have been lost eg. lizard.

Question 14

Fragmentation

Answer 14

Fragmentation is a more dramatic process than regeneration when an organism can reproduce themselves asexually from fragments of their original body, eg. starfish.
(The organism splits into pieces and each piece develops into a new individual).

Question 15

Budding

Answer 15

-In reproductive budding there is an outgrowth from the parent organism that produces a smaller but identical individual, produced by mitotic cell division (not the production of buds containing flowers and leaves).
-The ‘bud’ eventually becomes detached from the parent and has an independent existence.
-Budding is relatively rare in the animal kingdom

Question 16

Asexual reproduction

Answer 16

-Single individual is the sole parent
-Single parent passes all its genes to its offspring
-Results in a clone
-Rarely genetic differences may occur because of a mutation

Question 17

Sexual reproduction

Answer 17

-Two parents give rise to offspring
-Each parent passes half its genes on
-Offspring have a uniques combination of genes inherited from both parents
-Results in greater genetic variation
-Offspring vary genetically from their siblings and parents

Question 18

Advantages of asexual reproduction

Answer 18

-Large amounts
-Very fast
-Desired characteristics
-If conditions remain, all have a selected advantage

Question 19

Advantages of sexual reproduction

Answer 19

Conditions change; variation will allow some to have a selected advantage.

Question 20

Vegetative propagation

Answer 20

In vegetative propagation a plant forms a structure that develops into a fully differentiated new plant which is identical to the parent, and eventually becomes independent.
The new plant may be propagated from the stem, leaf, bud or root of the parent. It involves only mitotic cell division.

Question 21

Why is vegetative propagation advantageous?

Answer 21

It is advantageous as its an easy way of increasing plant numbers cheaply. They will also have the same characteristics as their parents so they will be the same colour or have the potential to produce fruit just as good.

Question 22

How to prepare a temporary root top squash?

Answer 22

1) Place root in hydrochloric acid to halt cell division and hydrolyse middle lamella
2) Stain root tip with a dye that binds to chromosomes (Aceto-orcein)
3) Macerate (soften) tissue in water using mounted needle
4) Use mounted needle at 45° to press down coverslip and obtain a single layer of cells. (Avoid trapping air bubbles).

Question 23

Procedure for root tip squash experiment

Answer 23

1) Prepare a temporary mount of root tissue
2) Focus an optical microscope on the slide. Count total number of cells in the field of view and number of cells in a stage of mitosis
3) Calculate mitotic index (proportion of cells undergoing mitosis).

Question 24

2 dyes that bind to chromosomes

Answer 24

-Toluidine blue (blue)
-Acetic orcein (purple-red)

Question 25

Why is root tip used when calculating root tip index?

Answer 25

Meristematic cells at root tip are actively undergoing mitosis.
Cells further from root tip are elongating rather than dividing.

Question 26

The process taking place inside a cell during specialisation

Answer 26

-Genes would be activated/deactivated
-Active genes transcribed/mRNA produced
-Translation (of mRNA) to produce proteins
-Proteins modify cell/ structure of cell altered perminantely