Chapter 16 - Cell Division

Question 1

Define growth.

Answer 1

Growth is a characteristic of all living things. It is a permanent increase in size. It is accompanied by cell division and differentiation to form tissues and organs.

Question 2

What are 2 differences in growth for plants and animals?

Answer 2

1) growth of plants is restricted to growing points such as tips of shoots and roots, while in animals, growth occurs throughout body
2) growth occurs throughout the life of a plant while for animals, growth stops after a certain age, when the animal reaches maturity

Question 3

What are 6 differences between mitosis and meiosis?

Answer 3

1) mitosis is involved in growth and development, while meiosis is involved in reproduction
2) mitosis produces 2 genetically identical cells, while meiosis produces 4 varied cells (gametes)
3) in mitosis, daughter cells contains same number of chromosomes as the parent cell. In meiosis, daughter cells contain half the number of chromosomes as the parent cell
4) in mitosis, pairing of homologous chromosomes does not occur. In meiosis, homologous chromosomes pair at prophase I
5) there is no crossing over in mitosis, but crossing over may occur in meiosis
6) mitosis involves only 1 nuclear division while meiosis involves 2 nuclear divisions.

Question 4

Define mitosis.

Answer 4

Mitosis is a form of nuclear division that produces 2 daughter nuclei which have the same number of chromosomes as the parent nucleus, and is genetically identical to the parent.

Question 5

How many chromosomes does humans have?

Answer 5

Every cell has 23 PAIRS of chromosomes.

Question 6

Why is it important to produced genetically identical daughter cells during mitosis?

Answer 6

DNA replication copies all the information stored within the chromosomes. This ensures that daughter cells produced by mitosis contain all sections of DNA needed for subsequent cell division and differentiation.

Question 7

What are the stages in a cell cycle during mitosis? (6)

Answer 7

1) interphase
2) mitosis: prophase
3) mitosis: metaphase
4) mitosis: anaphase
5) mitosis: telophase
6) cytokinesis

Question 8

What happens during interphase? (5)

Answer 8

1) absorb nutrients
2) build up protoplasm
3) synthesis new organelles
4) chromosomes appears as long thin threads called chromatin. Just before the cell enters mitosis, the chromatin threads replicate, producing 2 identical chromatin threads joined at a point called centromere.
5) centrioles replicate and divide

Question 9

Why must DNA replication and mitosis be precisely controlled?

Answer 9

It is to ensure all daughter cells are genetically stable. If an error occurs during replication, the new DNA strand formed would be different from original, which is known as gene mutation, and passes on to the daughter cell. Apart from that, some form of gene mutation may lead to uncontrolled division of cells(cancer: might be fatal)

Question 10

What happens during early prophase of mitosis? (4)

Answer 10

1) The sister chromatids coil and shorten to form chromosomes, attached at the centromere.
2) in animal cells, asters form around centrioles.
3) The 2 pairs of centrioles move apart to opposite poles of the cell
4) nucleolus disappears

Question 11

What happens during late prophase of mitosis? (2)

Answer 11

1) nuclear envelope disappears

2) a spindle forms with spindle fibres extending from one pole of the cell to the other

Question 12

What happens during metaphase in mitosis?(2)

Answer 12

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

Question 13

What happens during anaphase in mitosis?(3)

Answer 13

1) each centromere splits
2) the spindle fibres shorten and pull the sister chromatids apart to opposite poles of the cell.
3) once the sister chromatids are separated, they are called daughter chromosomes

Question 14

What happens during telophase in mitosis? (4)

Answer 14

1) spindle fibres disintegrate
2) a nuclear envelope forms around the chromosomes at each pole of the cell
3) a nucleolus reforms in each nucleus
4) chromosomes uncoil and lengthen to become thin chromatin threads

Question 15

What happens during cytokinesis in mitosis? (2)

Answer 15

1) division of cytoplasm
2) in animal cells, cleavage/furrows appear in the cytoplasm between two nuclei. Furrows deepen and split the cell into two identical daughter cells.

Question 16

What are 2 differences between mitosis in plants and animal cells?

Answer 16

1) in plants, centrioles are absent while centrioles are present in animals
2) in plants, a cell plate forms between 2 daughter nuclei by the fusion of fluid filled vesicles made by the Golgi Apparatus, dividing the cell into 2. In animals, cleavage furrows appear during cytokinesis and divides the cell into 2.

Question 17

Why is mitosis important?(3)

Answer 17

1) new cells are required for the growth of an organism
2) it repairs worn-out parts of tissue, heals wounds, replaces dead cells
3) asexual reproduction in plants such as the development of shoots and roots in storage organs like rhizomes and bulbs, allowing them to grow into new identical daughter plants.

Question 18

What is reduction division?

Answer 18

During meiosis, it results in the production of four daughter nuclei, which contains half the number of chromosomes of the parent cells.

Question 19

What are gametes?

Answer 19

Gametes are reproductive cells that contain half the number of chromosomes (haploid) as normal body cells. (Diploid)

Question 20

Why are gametes produced for?

Answer 20

Sexual reproduction involves fusion of 2 gametes. The nucleus of the male gamete fuses with the nucleus of the female gamete to form a diploid zygote.

Question 21

Define meiosis.

Answer 21

Meiosis is a form of nuclear division that produces 4 daughter nuclei containing haploid number of chromosomes as the parent nucleus.

Question 22

What are the stages of meiosis? (3)

Answer 22

1) interphase
2) meiosis I - prophase, metaphase, anaphase, telophase, cytokinesis
3) meiosis II - prophase, metaphase, anaphase, telophase, cytokinesis

Question 23

What happens during interphase of meiosis?(2)

Answer 23

1) Chromatid threads replicate, producing 2 identical sister chromatids (not visible under microscope)
2) centrioles divide

Question 24

What happens during prophase I of meiosis?(6)

Answer 24

1) chromatin threads condense, coil and shorten into visible chromosomes
2) synapsis occurs: homologous chromosomes that each pair up, and each pair is known as a bivalent.
3) crossing over between chromatids of homologous chromosomes may occur at the chiasma. They cross and twist around each other, and the strength of coiling may cause them to break and exchange parts, producing a new combination of alleles along the chromosomes.
4) asters form around the centrioles which move apart to opposite poles of the cell
5) nuclear envelope and nucleolus disintegrates.
6) spindle fibres form

Question 25

What are homologous chromosomes?

Answer 25

They are 2 chromosomes of the same shape, gene and length, each inherited from 1 parent.

Question 26

What happens during metaphase I of meiosis? (2)

Answer 26

1) pairs of homologous chromosomes arrange themselves along the equatorial plane of the spindle.
2) two chromosomes of each pair face opposite poles of the cell, which each chrosomes attached to one spindle fibre.

Question 27

What happens during Anaphase I of meiosis?

Answer 27

1) homologous chromosomes separate

2) the homologous chromosomes are pulled to opposite poles of the cell.

Question 28

What happens during telophase I in meiosis? (3)

Answer 28

1) spindle fibres disintegrate
2) nuclear envelope reforms (for animal cells only)
3) most plant cells do not undergo telophase I and move straight to metaphase II

Question 29

What happens during cytokinesis I of meiosis?

Answer 29

1) centrioles replicate(复制) into 2

2) cytoplasm cleaves into 2, producing 2 haploid daughter cells.

Question 30

What happens during prophase II of meiosis?(3)

Answer 30

1) two pairs of centrioles move to opposite poles of the cell
2) nuclear envelope breaks down and disappears
3) spindle fibres appear

Question 31

What happens during metaphase II? (1)

Answer 31

Chromosomes line up singly along the equatorial plane of the spindle. This differs from Metaphase I: homologous chromosomes do not lie next to each other

Question 32

What happens during Anaphase II of meiosis?

Answer 32

1) centromeres divide and sister chromatids are separated, becoming daughter chromosomes
2) daughter chromosomes are pulled to opposite poles of the cell

Question 33

What happens during telophase II of meiosis? (2)

Answer 33

1) spindle fibres disintegrate and disappear

2) nuclear envelopes form around 2 daughter chromosomes at each pole. Nucleolus also reforms.

Question 34

What happens during cytokinesis II of meiosis?

Answer 34

Cleavage of cytoplasm results in four daughter cells being produced, each with a haploid number of chromosomes.

Question 35

Why is meiosis important in sexual reproduction? (3)

Answer 35

1) it produces haploid gametes, which fuses together during fertillisation to form diploid cells (zygote)
2) it gives rise to genetic variation
3) increases chances of survival of species during changes in the environment

Question 36

How does meiosis give rise to genetic variation?(3)

Answer 36

1) crossing over between homologous chromosomes at prophase I results in new combination of alleles
2) independent assortment of chromosomes: random arrangement of homologous pairs of chromosomes at the equator at metaphase I Ieads to random separation of homologous chromosomes at anaphase I, which produces different combinations of genetic material in gametes
3) since fertilisation is random, where any sperm fuses with any egg, such variations in gametes produce variations in the offspring.

Question 37

How does variation increase chances of survival of species during changes in the environment?

Answer 37

When environmental conditions change, nature selects those with the most favourable characteristics to survive, while the others die, so that they can pass on favorable genes to their offspring, ensuring their species live on, and prevent the whole species to be wiped out if they had the same genes.