Cell Division

Question 1

What is mitosis?

Answer 1

A type of nuclear division that results in the production of two daughter nuclei that are genetically identical to the parent cells This means that the daughter nuclei have the same number of chromosomes as the parent nucleus

Question 2

What are the three stages in cell cycle

Answer 2

Interphase, mitosis, and cell division ( cytokinesis )

Question 3

What happens during interphase?

Answer 3

The cells absorb nutrients, synthesize new organelles, build up protoplasm and replicates DNA
Before the cell enters the nuclear division phase, the chromatin threads replicate, the centrioles replicate and divide

Question 4

What happens during DNA replication?

Answer 4

The chromatin threads replicates to produce two identical chromatin threads that coil and shorten to become chromosomes. Chromosomes are only visible during cell division as short, thick, x-shaped structures. Each chromosome contains two identical DNA molecules known as sister chromatids. The sister chromatids are joined at a point known as the centromere

Question 5

What are the 4 stages of mitosis?

Answer 5

Prophase
Metaphase
Anaphase
A\telophase

Question 6

What happens during prophase?

Answer 6

Chromatin threads condense, coil and shorten to become chromosomes that each consists of 2 sister chromatids attached at the centromere.
They now become visible under a microscope as x-shaped structures

In an animal cell, asters form around the centrioles.
The 2 pairs of centrioles move apart to opposite ends/poles of the cells.
The nucleolus and nuclear envelop disappears and a spindle forms with spindle fibres extending from one pole of the cell to the other

Question 7

What happens during metaphase?

Answer 7

Chromosomes line up along the equatorial plane of the spindle.
The centromere of each chromosome is attached on both sides to a spindle fibre

Question 8

What happens during anaphase?

Answer 8

Each centromere divides.
The spindle fibres shorten and pull the chromatids apart to opposite poles of the cell
Once the chromatids are separated, they are known as daughter chromasomes

Question 9

What happens during telophase?

Answer 9

Spindle fibres break down
A nuclear envelope forms around the chromosomes at each pole of the cell
A nucleolus reforms in each nucleus and the chromosomes uncoil and lengthen to become thin chromatin threads

Question 10

What happens during cytokinesis?

Answer 10

The cytoplasm divides.
In animal cells, cleavage / furrows appear in the cytoplasm between the two nuclei
The furrows deepen and 2 identical daughter cells are finally produced

Question 11

What is the difference between mitosis in plant and animal cells?

Answer 11

Animal: centrioles are present
Plant: centrioles are absent

Animal: cleavage of cytoplasm occurs during cytokinesis
Plant: cleavage of cytoplasm does not occur during cytokinesis

Animal: cell plate does not form between 2 daughter nuclei and instead cleavage of cytoplasm occurs
Plant: cell plate is formed between 2 daughter nuclei, dividing the cell into 2. The cell plate is formed by the fusion of a small fluid-filled vesicles produced by the Golgi apparatus

Question 12

Why is mitosis important?

Answer 12

It enables the growth of an organism

Mitosis is needed for the repair of worn-out parts of the body

Mitosis allows asexual reproduction to occur

Question 13

What are gametes?

Answer 13

Reproductive cells that contains half the number of chromosomes as the normal body cells

Question 14

What is meiosis?

Answer 14

A form of nuclear division that produced daughter nuclei containing half the number of chromosomes as the parent nucleus

Question 15

How do gametes differ from normal body cells?

Answer 15

Gametes have half the number of chromosomes as compared to normal body cells

Question 16

What happens during prophase 1?

Answer 16

Chromatin threads condense,coil and shorten to become chromosomes

Homologous chromosomes pair along their whole length. This is called a synapsis.

The homologous chromosomes cross over; the point where they cross over is called a chiasma

The strength of their coiling may cause them to break and exchange parts in a process called crossing over
Crossing over results in new combinations of alleles across the chromosomes

The chromosomes shorten further

Asters form around the centrioles which moves apart to opposite poles of the cells

The nuclear envelope and nucleolus disappear

Spindle fibres form

Question 17

What happens during metaphase 1?

Answer 17

The pairs of homologous chromosomes arrange themselves along the equatorial plane of the spindle
Each chromosome is attached to a spindle fibre

Question 18

What happens during anaphase 1?

Answer 18

The homologous chromosomes separate and
are pulled to opposite poles of the cell
As the spindle fibres shorten

Question 19

What happens during telophase 1?

Answer 19

A nuclear envelope forms around the chromosomes at each pole

Question 20

What happens during cytokinesis 1?

Answer 20

The cytoplasm cleaves into 2, producing 2 daughter cells
Each with the haploid number of chromosomes
The centrioles divide

Question 21

What happens during prophase 2?

Answer 21

The 2 pairs of centrioles move to opposite poles of the cell
The nuclear envelope disappears
Spindle fibres appear

Question 22

What happens during metaphase 2?

Answer 22

Chromosomes arrange themselves singly along the equatorial plane of the spindle

Question 23

What happens during anaphase 2?

Answer 23

The centromeres divide
Sister chromatids separate to become daughter chromosomes
Which are pulled towards opposite ends of the cell

Question 24

What happens during telophase 2?

Answer 24

Spindle fibres disappear
Nuclear envelope forms around 2 daughter chromosomes at each pole
A nucleolus reforms

Question 25

What happens during cytokinesis 2 ?

Answer 25

Cleavage of cytoplasm results in 4 daughter cells being produced, each with half the number of chromosomes as the parent cell

Question 26

Why is meiosis important?

Answer 26

Meiosis produces haploid gametes
Haploid gametes fuse with another haploid gamete to form a diploid number gamete to maintain the normal diploid number of chromosomes in the species

Meiosis results in variations in the gametes produced
Variations occur due to crossing over and independent assortment of chromosomes

Question 27

What is independent assortment?

Answer 27

The process whereby one chromosome from each pair can combine with either chromosome of the pair resulting in 4 different gametes being produced from 2 chromosomes.
Since fertilisation is random , such variations in the gametes produce variations in the offspring

Question 28

Why is variation important?

Answer 28

Variations increase the chances of survival of the species during changes in the environment. When environmental conditions change, nature will ‘select’ those organisms with favourable characteristics to survive, while others die. Those that survive will pass on their favourable genes to their offspring, ensuring the continuity of the species

Question 29

What are the differences in meiosis and mitosis?

Answer 29

Mitosis: daughter cells contain the same number of chromosomes as the parent cell
Meiosis: daughter cells contain half the number of chromosomes as the parent cell

Mitosis: pairing of homologous chromosomes does not occur
Meiosis: pairing of homologous chromosomes may occur

Mitosis:daughter cells are genetically identical to the parent cell
Meiosis: daughter cells are not genetically identical to the parent cell

Mitosis: 2 daughter cells are produced from 1 parent cell
Meiosis: 4 daughter cells are produced from 1 parent cell

Mitosis:involves 1 nuclear division
Meiosis: involves 2 nuclear divisions

Mitosis:occurs in normal body cells during growth/repair of body parts
Meiosis:occurs only in the gonads during gamete formation

Question 30

In which stages in meiosis results in increased genetic variation?

Answer 30

Crossing over of homologous chromosomes to produce new chromosomes of alleles along the chromosome in prophase 1

Independent assortment of chromosomes at metaphase 1

Random fertilisation of gametes