Biology class 9thClass Matric Part 1 Notes

UNIT 5 Matric Part 1 Class 9TH BIOLOGY

Matric Part 1 Class 9TH BIOLOGY Unit 5: CELL CYCLE Short And Simple Questions And Answers

These 9th-class biology notes are prepared according to the syllabus of all Punjab Boards. Other boards other than Punjab do not follow class 9 biology notes. These Punjab boards are Gujranwala Board, Lahore Board, Faisalabad Board, Multan Board, Rawalpindi Board, Bahawalpur Board Sargodha Board, DG Khan Board, Sahiwal Board.

Question And Answer

Q.1. What is the cell cycle and what are its main phases?

Ans: Cell cycle: It is the series of events from the time a cell is produced until it
completes mitosis and produces new cells.
Phases of cell cycle: Cell cycle has two major phases.
(a) Interphase: It lasts for at least 90% of the total time required for the cell cycle.
(b) Mitotic phase (M phase): It is a short period of cell cycle.

Q.2. Define Interphase? Write a note on different phases of Interphase?

Ans: Interphase: In interphase metabolic activity of cell is very high and cell performs many functions in it.
Phases of interphase: It is divided into three phases.
(1) G1 (first gap) phase
(3) G2 (second gap) phaseG1 phase: It is first phase of cell cycle. During G1 phase:
(a) Cell increases its supply of proteins.
(b) Number of organelles increases
(c) Cell size increases.
(d) Many enzymes are synthesized that are required in next phase (S phase).S phase: In this phase, cell duplicates its chromosomes. As a result, cach chromosome has two sister chromatids. CG2 phase: In this phase cell prepares proteins. These proteins are important in mitosis for the production of spindle fibres.

Q.3. What is Go phase?

Ans: G0 phase: It is a phase during which cell stops division temporarily or permanently. It is a state of
quiescence. Cell enters in GO phase from G1 and stop dividing,
a) Neurons (nerve cells) remain in Go for indefinite period.
b) Some cells of liver and kidney remain in GQ semi-permanently (for short time)
c) Epithelial cells do not enter Go and continue to divide.

Q.4. What is the difference between somatic cells and germ line cells?

Ans: Somatic cells: These cells make the body of organisms. Mitosis occurs in these cells. Germ line cells: These cells form gametes. Meiosis occurs in these cells.

Q.5 What is mitosis? Describe its various phases?

Ans: Mitosis (M phase): It is a type of cell division in which cell divides into two daughter cells, each cell
has same number of chromosomes as in parent cell.
Mitosiş occurs only in eukaryotic cells.
Mitosiş occurs only in somatic cells
Phases of mitosis: There are two major phases of mitosis.
A. Karyokinesis: The division of nucleus is called karyokinesis.
B. Cytokinesis: The division of cytoplasm is called cytokinesis.
Phases of karyokinesis: It is further divided into four phases.
(1) Prophase
(3) Anaphase
(2) Metaphase
(4) Telophase
1) Prophase: Following changes occurs during prophase.
a) Chromatin condenses into chromosomes.
Chromatin: A loose thread-like genetic material in nucleus is called chromatin. b) Each chromosome contains two sister chromatids which are attached together at centromere.
c) A complex protein structure is present at centromere of each chromosome which is called
kinetochore. It is the point where spindle fibers attach.
d) A pair of centrioles is called a centrosome. Each centriole duplicates and forms two daughter centrosomes, which move towards opposite poles of cell.
e) Centrosomes join with tubulin proteins and form microtubules. These microtubules are called spindle fibres. Complete set of spindle fibres is known as mitotic spindle.
f) Nucleolus disappears (degrade).
g) Nuclear envelope disappears.
h) Spindle fibres invade in the centre of cell.
2) Metaphase: During metaphase:
a) Kinetochore fibres from opposite poles attach with kinetochores of chromosomes.
b) Chromosomes arrange themselves at equator of cell and form a metaphaseplate.
c) Non-kinetochore fibres from opposite poles alsotattach with each other.
3) Anaphase: During anaphase:
a) Kinetochore fibres pull toward their original poles as a result sister chromatids of a chromosome are separated from eachother.
b) At the end of anaphase, identical copies of chromosomes into two groups have been separated.
4) Telophase: Telophase is reverse of prophase. During Telophase: a) Nuclear envelope is formed around each set of chromosomes.
b) Spindle fibres disappear.
c) Chromosomes are changed into chromatin.
d) Nucleolus also reappears.
Cytokinesis: Cytokinesis is the division of cytoplasm.
Cytokinesis in animal cells: In animal cells cytokinesis occurs by a process known as cleavage. A cleavage furrow is developed (at equator). The furrow finally divides the parent cell into two daughter
Cytokinesis in plant cells: In plant cells cytokines phragmoplast. through cell plate or
a) The vesicles originate from Golgi apparatus are fused in the middle of cell to form a membrane-bounded dise called all plate or phragmoplast.
b) This plate grows outward and more vesicles fuse with it. c) Membranes of cell plate fuse with plasma membrane and finally join with
parental cell wall. d) As a result two daughter cells lite formed.
e) Each daughter cell has its its own plasma membrane and cell wall.

Q.6.Write a note on significance of mitosis?

Ans: Significance of mitosis:
Maintenance of chromos e of chromosomes: In mitosis each daughter cell has chromosomes that are alike in composition and equal equal int number to the chromosomes of parent cell.
Processes where mitosis occurs:
1) Development and growth: Mitosis increases the number of cells and this is the basis of development and growth. The formation of multicellular body from a single cell (zygote) is called
Cell replacement: When the cells of skin and digestive tract are broken down then new cells are formed by mitosis. Similarly, red blood cells have short life span of about 4 months. The new red
blood cells are formed by mitosis.
3) Regeneration: Some organisms can regenerate their body parts by mitosis. For example sea star
(star fish) regenerates its lost arm by mitosis.
4) Asexual reproduction: In asexual reproduction genetically similar offspring is formed. Mitosis is
involved in asexual reproduction. Some examples are given below.
a) Budding in hydra: Hydra reproduces asexually by budding. A mass of cells called bud is formed on the surface of hydra by mitosis. Mitosis continues and bud grows into a new individual.
b) Vegetative propagation: Asexual reproduction in plants takes place by mitosis and is called vegetative propagation.

Q.7. What errors can take place in mitosis?

Ans: Errors in mitosis: Errors in control of mitosis may cause cancer. (OR) The uncontrolled mitosis is
called cancer. Different genes control the timing and number of mitosis. Tumors: Mutation in genes causes growth of abnormal cells which are called tumors.
Types of tumors: There are two types of tumors.
a) Benign tumor: It is a tumor which remains in its original position.
b) Malignant (cancerous) tumor: It is a tumor which invades other tissues. Its cells are called cancer cells.
Metastasis: The spreading of cancer cells in other parts of body is called metastasis.

Q.8. Define diploid and haploid cell?

Ans: Diploid (2n) cell: A cell in which chromosomes are present in pairs.
Haploid (n) cell: A cell having half number of chromosomes. (Chromosomes are not in pair form)

Q.9. Define meiosis? Explain different phases of meiosis in detail?

Ans: Meiosis: It is a type of cell division in which one diploid eukaryotic cell divides into four haploid daughter cells.
➤ The word meiosis comes from Greek word “meioun” meaning “to make smaller”. ➤In 1876 first time, a German biologist Oscar Hertwig discovered and described meiosis,
Phases of meiosis:
Interphase: Interphase of meiosis is divided into the same three phases as in mitosis i.e. GI, S phase, and G2. Interphase is followed by meiosis I and meiosis II,
1) Meiosis I: Meiosis I occurs in two main steps i.e. karyokinesis and cytokinesis.
Karvokinesis: Karyokinesis of Meiosis I is subdivided into prophase I, metaphase I, anaphase I, and
telophase I.
A. Prophase I: It is longest phase in meigsis and following changes occurs in it.
a) Chromatin condenses into chromosomes.
b) Homologous chromosomes form pairs and this process is called synapsis.
c) Each pair of homologous chromosomes is called bivalent or tetrad (due to four chromatids).
d) Non-sister chromatids of homologous chromosomes join each other at some points. These points are called chiasmata,
e) Non-sister chromatids exchange their segments and this process is called crossing over. It
results in recombination of genetic information.
f) Nucleolus and the nuclear envelope disappear.
g) Centrioles after duplication move toward opposite poles and form spindle fibres.
h) Kinetochore spindle fibres attach with the kinetochores of chromosomes.
B. Metaphase I: During metaphase I:
a) Pairs of homologous chromosomes arrange themselves at equator and form a metaphase plate,
C. Anaphase I; During anaphase I:
a) Kinetochore fibres pull toward their original poles as a result chromosomes of each pair are
separated from eachother.
b) Two haploid sets of chrome somes are formed at each pole. Each chromosome contains two
sister chromatids.
D. Telophase I: During telophase
a) Spindle fibres are disappeared.
b) Nuclear envelope is formed around each haploid set.
c) Nucleolus also reappears.
d) Chromosomes change into chromatin.
Cytokinesis: Two haploid daughter cells are formed after cytokinesis through cleavage or cell plate. Interkinesis or Interphase II: It is a period of rest in which both haploid daughter cells enter after
meiosis I.
It is different from the interphase of mitosis and meiosis I.
There is no S-phase in it. So there is no duplication of chromosomes during this stage.
2) MEIOSIS II: It is similar to mitosis.
It is subdivided into prophase II, metaphase II, anaphase II, and telophase II.
A. Prophase II:
a) It takes less time than prophase I.
b) Chromatin condenses into chromosomes.
c) Nucleolus and the nuclear envelope disappear.
d) Centrioles after duplication move toward opposite poles and form spindle fibres.
B. Metaphase II:
a) Kinetochore fibres from opposite poles attach with kinetochores of chromosomes.
b) Chromosomes arrange themselves at equator of cell and form a metaphase plate.
C. Anaphase II:
1) Kinetochore fibres pull toward their original poles as a result sister chromatids of a chromosome are separated from eachother. Sister chromatids are now called sister chromosomes.
D. Telophase II:
a) Spindle fibres are disappeared,
b) Nuclear envelope reappears.
c) Nucleolus also reappears.
d) Chromosomes change into chromatin.

Q.10. Write a note on significance of meiosis?

Ans: Significance of meiosis:
In 1890, German biologist August Weismann described the significance of meiosis.
He pointed out that meiosis was necessary to
1) Maintain the number of chromosomes in the next generation
2) Produce variations in next generation.
1) Maintenance of the chromosome number: Meiosis is essential for sexual reproduction.
Examples are given below.
a) Humans:
In humans, diploid gamete-mother cells produce haploid gametes after meiosis.
Male and female gametes unite to form diploid zygote.
Zygote develops into a new human after repeated mitosis.
b) Alternation of generation in plants:
Plants life cycle shows alternation of generations. Diploid sporophyte generation alternates
with the haploid gametophyte generation it is called alternation of generations.
Diploid sporophyte generation produces haploid spores after meiosis. These spores grow into haploid gametophyte generations.
Gametophyte generation produces haploid gametes through mitosis.
Gametes combine to produce diploid zygote.
Zygote becomes diploid sporophyte after repeated mitosis.
c) Fungi and protozoans:
They are mostly haploid.
They produce haploid gametes through mitosis.
Zygote produces haploid cells after meiosis.
These cells after mitosis form haploid organism.
2) Genetic variations: Due to process of crossing over in meiosis genetic variations are produced
in daughter cells. The gametes fuse and form zygote which has different genetic makeup from
both parents. Due to these variations organisms adapt the changes of environment.

Q.11. Describe some errors in meiosis?

Ans: Errors in meiosis (OR) Non-disjunction: When the separation of chromosomes in anaphase I or sister chromatids in anaphase II of meiosis is not normal it is called non-disjunction. Non-disjunction produces gametes having more or less number of chromosomes than normal value.
When such abnormal gamete fuses with a normal gamete, then number of chromosomes becomes abnormal in next generation. For example, 47 or 45 chromosomes in humans.
Disjunction: When the separation of chromosomes in anaphase I or sister chromatids in anaphase 11 of meiosis is normal it is called disjunction.

Q.12. Write a note on apoptosis?

Ans: Apoptosis: It is a programmed cell death.
Events in apoptosis: Following biochemical events occur in apoptosis. a) Cell shrinks and becomes rounded due to breakdown of cytoskeleton.
b) Chromatin condenses and nuclear envelope breaks.
c) Nucleus spreads in the form of many chromatin bodies.
d) Cell membrane makes irregular buds called blebs,
e) Blebs break off from cell and are called apoptotic bodies.
f) These bodies are phagocytosed by other cells.
Significance of apoptosis:
a) Apoptosis occurs when a cell is damaged or present in stress conditions.
b) It removes the damaged cell so the bell cannot get further nutrients.
c) It prevents from infections (by removing of damaged cell).
d) It is important during development. For example, during formation of fingers, cells between them undergo apoptosis and the digit separate.

Q.13. Write a note on necrosis?

Ans: Necrosis: It is accidental death of cells and tissues. It is less sequential than apoptosis.
Events in necrosis:
a) Lysosomes release special enzymes.
b) These enzymes break cellular parts.
c) These enzymes also break surrounding cells when they are released from outside the cell. d) Cells that die through necrosis may damage other cells.
Causes of necrosis: Following are the causes of necrosis:
(a) injury
(b) infection (c) cancer
(d) if a cell is given hypoxic (with less oxygen) environments
(e) Spider bites cause necrosis in some areas of body.
(f) Due to lack of proper care to a wound site.

Q.14. Contrast mitosis and meiosis, emphasizing the events that lead to different outcomes?

Ans: Meiosis II is similar to mitosis so meiosis I actual difference between these two cell division.
Homologous chromosomes form pairs.
Crossing over takes place.
Homologous chromosomes do not form pairs.
There is no crossing over.
Single chromosomes align to form metaphase plate.
Sister chromatids are pulled towards poles
Daughter cells contain diploid number of
chromosomes, each has single chromatid.
Pairs of homologous chromosomes align to form metaphase plate.
Whole chromosomes a Daughter cells contain ploid number of chromosomes, each ha
Q.15. The S-phase of interphase is important and a cell can never divi?

Ans: A cell can never divide without S-phase because a chromosomes chromatids.

Q.16. A nerve cell does not divide after its formation. In which phase?

Ans: It is present in GO phase. pulled towards poles. vo tout it? Justify. tes in it and forms two sister. le it is?
Q.17 How is cytokinesis different in plant cells as compared to animal cell?

Ans: Cytokinesis in animal cell occurs by cleavage. During cleavage cell membrane grows inward and divides the cells. In pants Golgi vesicles fuse to form phraginoplast. It forms new cell wall and then divides the cell.
Q.18. What type of cell division occurs when our wounds are healed?

Ans: Our wounds are healed by mitosis.

Q.19. T.H.Morgan is famous for what?

Ans: He observed the phenomenon of crossing over in fruit fly (Drosophila melanogaster).

Q.20. Plants do not make their gametes by meiosis. How is that?

Ans: In plants diploid sporophyte generation produces haploid spores by meiosis. These spores are changed into haploid gametophyte generation. This generation produces haploid gametes by mitosis.

Q.21. What happened when protein synthesis is inhibited during G2 phase?

Ans: Inhibition of protein synthesis during G2 phase prevents celi from undergoing mitosis.

Q.22. Is it possible to reverse a cell cycle?

Ans: The events of cell cycle are ordered and directional so it is impossible to reverse the Eycle.

Q.23. Walther Flemming is famous for what?

Ans: In 1880 German biologist Walther Flemming observed that in a dividing cell, nucleus passes through a series of changes which he called mitosis.

Q.24. Nucleus is visible only in interphase while chromosomes are only visible in cell division stage. Why?

Ans: Nuclear membrane breaks during cell division so there is rib proper nucleus. Nuclear material chromatin condenses and changes into chromosomes during prophase of cell division.

Q.25. Define binary fission? How is it different from mitosis?

Ans: Binary fission: It is a type of reproduction in prokaryotic cells in which a cell divides into two cells by simple division. Prokaryotic cell divides by binary fission because this type of tell has no proper nucleus. This cell has a single chromosome and no centromere. There is no spindles formation in binary fission so it is different from mitosis.
Q.26. How spindles fibres are formed in highly vacuofated plant cells?

Ans: Plant’s cells have no centrioles sb-spindle fibres are formed by the aggregation of tubulin proteins on the surface of nuclear envelope during prophase.

Q.27. How many cells in humans die daily due to apoptosis?

Ans: 50-70 billion cells

Q.28. Define mitotic appunatus?

Ans: It consists of spindle fibres, Centrioles and asters.

Q.29. What is the most important step of mitosis?

Ans: Afhaphase is most important in mitosis. Because sister chromatids of chromosomes are separated in it.

Q.30, Why tell division is necessary for continuation of Efe? (OR) What is the basic characteristic of life?

Ans: The most basit characteristic of life is reproduction.
Reproduction is a biological process of production of new structures and individuals similar to the existing ones.
Reproduction occurs at different levels of organization as in chromosomes, cells and individuals. Thus for continuation of life including all aspects of reproduction is based on the reproduction of cells.
Cellular reproduction is known as cell division

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