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Chapter 11
Stages of cell cycle
Cell growth, nuclear division and cytoplasmic division
Nuclear division: Mitosis and Meiosis
1.
Which cell theory is related to cell division?
All cells come from pre-existing cells. All cells are produced by pre-existing cells by cell division.
2.
A cell produces another cell by cell division. What is the name of the original cell and the cell produced?
Parent cell divides into daughter cells.
3.
Cell division consists of two steps. What are they?
Nuclear division and cytoplasmic division
4.
What chemical is found in a chromosome? What is contained in a DNA?
Protein and DNA are found in a chromosome. Genetic information is found in a DNA.
5.
What are chromatin, chromosome and chromatid?
Chromosomes become thinner and longer and are not visible in a non-dividing nucleus. When a cell starts to divide,
chromosomes become visible. They become thicker and shorter. When a cell starts to divide, chromosomes start to replicate
first. Therefore, when we see a cell is dividing, we can see replicated chromosomes which consist of two chromatids joining
together at a point called centromere.
6.
What is homologous chromosome?
Homologous chromosomes are a pair of chromosomes. One of them comes from male parent and one of them comes from
female parent. (Homologous chromosomes come from two different parents.) They are of similar size and shape. The
positions of their centromeres are the same. They control the same characteristics but they carry different genetic materials.
7.
What is diploid cell and haploid cell?
All our body cells are diploid cells. There are homologous chromosomes in the diploid cells. There are two sets of
chromosomes. We call these cells 2N cells. In human cells, there are 23 pairs of homologous chromosomes. (There are 46
chromosomes.) All our body cells come from the fertilized egg which is diploid by mitosis.
In our testes and ovaries, some diploid cells carry out division to form haploid cells. Haploid cells are cells without
homologous chromosomes. There is only one set of chromosomes. We call these cells N cells.
8.
How many chromosomes are there in human cells?
All our body cells come from the 2N zygote. Muscle cell, skin cell, nerve cell, blood cells all contain 2 sets of chromosomes.
In male, there are 22 pairs of homologous chromosomes and one X and one Y chromosome. In female, there are 22 pairs of
homologous chromosomes and two X chromosomes.
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9.
Is there any relationship between the number and size of chromosome and the amount of genes?
NO. There is no relationship between the amount of genes in an organism and the number of chromosomes in the organism.
There is no relationship between the amount of genes stored and the size of chromosomes
10. What is the use of chromosome?
It contains DNA. It contains genetic information. It controls the activity of the cells. It controls characteristics of organism.
11. What kinds of cell can carry out cell division?
Single-celled organisms can carry out mitosis to produce daughter cells. This is a kind of asexual reproduction.
Zygote can carry out mitosis to produce more cells to grow into an embryo.
All our body germ cells can carry out mitosis to increase number of cells. (Germ cells divide mitotically to form new cells.
Then, the cells become specialised by cell differentiation. After cell differentiation, the cell cannot carry out cell division any
more. Therefore, only germ cells in each organ can carry out mitosis. Germ cells in nervous system do not carry out mitosis.)
Only the germ cells in testes and ovaries can carry out meiosis to form gametes.
Sperm, pollen and ovum cannot carry out meiosis. Only 2N cells can carry out meiosis.
12. What are the main stages of cell cycle?
Cell cycle consists of cell growth stage, nuclear division and cytoplasmic division.
13. What happens during cell growth?
It is the longest stage. It is called interphase. New organelles are formed. DNA, chromosomes are duplicated. The
chromosomes exist as two chromatids joining at a point of centromere (THE CHROMOSOMES ARE NOT VISIBLE.)
14. What happens during mitosis?
Mitosis is a type of nuclear division. Mitotic cell division = mitosis + cytoplasmic division
Prophase of mitosis: chromosomes are visible; nuclear membrane breaks
Metaphase of mitosis: chromosomes line up at the middle of the cell
Anaphase of mitosis: chromatids separate and move to the opposite poles of the cells
Telophase of mitosis: chromosomes start to become longer, thinner, to become chromatin again; nuclear membrane forms
15. How do animal cells and plant cells carry out cytokinesis (cytoplasmic division)?
In animal cell, the cell membrane constricts inwards to form two daughter cells.
In plant cell, cell plates are formed between the two daughter cells. Finally, the plates form the cell walls.
16. What is the significance of mitosis?
Mitosis can produce 2 daughter cells. The number of chromosomes of the daughter cells is the same as the parent cells.
Mitosis is important in producing new cells for growth, repair and replacement of tissue. It is also used in asexual
reproduction in organisms such as bacteria, amoeba, yeast and spore formation.
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17. Give examples of mitosis for growth, repair and replacement of tissue.
A boy grows in height. The number of body cells increases. This is growth.
In a wound, there are some damaged cells. The new cells are produced to repair the wound.
Old red blood cells are killed. Bone marrow in bone produces new red blood cells to replace the old ones.
18. How can we see mitosis?
We get a root tip. (Region of cell division of the root tip) We use stain to stain the chromosomes. We examine the root tips
under the microscope. The number of cells in different phases is different.
Relative time required for each stage = (number of cells in the stage / total number of cell) x 720 minutes
19. What is the change in chromosome number in mitosis?
Chromosome number of parent cell is 2N (2 sets of chromosomes). During interphase, prophase, metaphase, anaphase and
telophase, it is also 2N. (2 sets of duplicated chromosome) Chromosome number in daughter cell is also 2N.
20. What is the change in the amount of DNA (genetic materials) in mitosis?
Amount of DNA of parent cell is ‘x’. During interphase, prophase, metaphase, anaphase and telophase, the amount is ‘2x’
because the DNA is replicated. In daughter cells, amount of DNA is ‘x’ (separation of chromatids in anaphase & cytokinesis)
21. What happens during meiosis?
Meiosis consists of 2 nuclear divisions. The second meiotic division resembles mitosis.
Prophase I: chromosomes are visible; nuclear membrane breaks; homologous chromosomes pair up
Metaphase I: homologous chromosomes pairs line up at the middle of the cell
Anaphase I: homologous chromosomes separate and move to the opposite poles of the cells
Telophase I: nuclear membrane forms
(In some cases, telophase I is absent.)
Prophase II: chromosomes are visible; nuclear membrane breaks
Metaphase II: chromosomes line up at the middle of the cell
Anaphase II: chromatids separate and move to the opposite poles of the cells
Telophase II: chromosomes start to become longer, thinner, to become chromatin again; nuclear membrane forms
22. What is the change in chromosome number in meiosis?
Chromosome number of parent cell is 2N (2 sets of chromosomes). During interphase I, prophase I, metaphase I, anaphase I
and telophase I, it is also 2N. (2 sets of duplicated chromosome) After cytokinesis, the number of chromosome in the 2
daughter cells is N. During prophase II, metaphase II, anaphase II and telophase II, its chromosome number is also N. After
cytokinesis, the number of chromosome in the 4 daughter cells is N.
23. What is the change in the amount of DNA (genetic materials) in meiosis?
Amount of DNA of parent cell is ‘x’. During interphase, prophase I, metaphase I, anaphase I, the amount is ‘2x’ because the
DNA is replicated. In telophase I, the amount is x (separation of homologous chromosomes in anaphase I and cytokinesis) In
prophase II, metaphase II, anaphase II and telophase II, the amount is still ‘x’. Amount of DNA in daughter cell is ‘x/2’
(separation of chromatids in anaphase II and cytokinesis)
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24. Mule is infertile. Why?
Mules have 63 chromosomes, a mixture of the horse's 64 and the donkey's 62. The different structure and number prevents
the chromosomes from pairing up. It cannot carry out meiosis to produce gametes.
25. What is the significance of meiosis?
It produces 4 haploid daughter cells. These cells are gametes for sexual reproduction. During fertilization of gametes, the
haploid cells fuse together to form a diploid zygote. The diploid number of chromosome can be restored.
The four daughter cells have different genetic combinations. It is because the homologous chromosomes randomly distribute
and randomly separate in the middle of the cell. This is called independent assortment. Meiosis provides genetic variation too.
The genetic variations increase the chance for a species to survive when the environment changes.
26. What cells can carry out mitosis and meiosis?
All body cells can carry out mitosis. Only gamete-producing cells can carry out meiosis. Sperm-producing cell,
ovum-producing cell can carry out meiosis. Sperm and ovum are haploid gametes. They cannot carry out mitosis and
meiosis.
27. What is a cancer?
Cancer cells are abnormal cells. The DNA of the cell is damaged. The cells continue to divide uncontrollably to form a
tumour. If the tumour cells produce toxin, the toxin may kill other cells. The tumour cells may spread to other parts of body
to cause cancer in other parts of body.
28. In a graph showing the change in DNA content, the DNA content suddenly doubles or halves. What do they mean?
DNA content suddenly increases because of fertilization. Two haploid cells fuse to form a diploid cell. DNA content
suddenly halves because of cell division. The cell forms two daughter cells with half amount of DNA.
29. What are differences between mitosis and meiosis?
Mitosis
Meiosis
Number of division
one
two
Number of daughter
2
4
Chromosomes in daughter cells
2N
N
Genetic materials
same as parent cell
different from parent cell
daughter cells are identical
daughter cells are genetically different
Pairing up of homologous chromosome
no
yes (in Prophase I)
Occurrence
all body cells
gamete-producing cells
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