9th Class Biology Chapter 5 Notes
Important Notes of Chapter No.5: Cell Cycle 9th Class or 9th class biology chapter 5 notes written by Honorable Sir Adnan Haider Suib. These notes are very helpful in the preparation of 9th class biology chapter 5 notes of the 9th class Biology Urdu Medium and these are according to the paper patterns of all Punjab boards.
Summary and Contents:
Topics which are discussed in the notes are given below:
- Very Important Multiple Choice Questions (MCQs) of Chapter No.5 Cell Cycle Urdu Medium.
- Cell cycle is the series of events from the time a cell is produced until it completes mitosis and
produces new cells. Cell cycle consists of two major phases i.e. interphase and mitotic phase (M
phase). Mitotic phase is a relatively short period of cell cycle. It alternates with the much longer
interphase, where cell prepares itself for division.
Interphase is the time when a cell’s metabolic activity is very high, as it performs its various functions.
It is divided into three phases, G1 (first gap), S (synthesis), and G2 (second gap).
- G1 phase: After its production, a cell starts its cell cycle in G1 phase. During this phase, cell increases
its supply of proteins, increases the number of its organelles (such as mitochondria, ribosomes),
and grows in size. This phase is also marked by the synthesis of various enzymes that are required
in next phase i.e. S phase for the duplication of chromosomes. Typically,interphase lasts for at least 90% of the total time required for the cell cycle.
- S phase: In this phase, cell duplicates its chromosomes. As a result, each chromosome consists of
two sister chromatids.
- G2 phase: In the G2 phase, cell prepares proteins that are essential for mitosis, mainly for the
production of spindle fibres.
- After the G2 phase of interphase, cell enters the division phase i.e. M phase. It is characterized by
mitosis, in which cell divides into the two daughter cells.
- Cells that have temporarily or permanently stopped dividing are said to have entered a state of
quiescence, called G0 phase. Inhibition of protein synthesis during G2 phase prevents cell from undergoing mitosis.
- G0 phase: In multicellular eukaryotes, cells enter G0 phase from G1 and stop dividing. Some cells
remain in G0 for indefinite period e.g. neurons. Some cells enter G0 phase semipermanently e.g.
some cells of liver and kidney. Many cells do not enter G0 and continue to divide throughout an
organism’s life, e.g. epithelial cells. The events of cell cycle are ordered and directional i.e each event occurs in a sequential
fashion and it is impossible to “reverse” the cycle.
- Mitosis: In 1880s, a German biologist Walther Flemming observed that in a dividing cell, nucleus passes
through a series of changes which he called mitosis. Mitosis is the type of cell division in which a
cell divides into two daughter cells, each with the same number of chromosomes as were present
in parent cell.
- Mitosis occurs only in eukaryotic cells. In multicellular organisms, the somatic cells undergo mitosis.
Prokaryotic cells undergo a process similar to mitosis called binary fission. They do not undergo
proper mitosis. Why?
- Phases Of Mitosis: The process of mitosis is complex and highly regulated. There are two major phases i.e. the division
of nucleus known as karyokinesis; and the division of cytoplasm known as cytokinesis.
- A. Karyokinesis: The division of nucleus is further divided into four phases i.e. prophase, metaphase,
anaphase and telophase.
- i. Prophase: Normally, the genetic material in nucleus is in a loose thread-like form called chromatin. At the
onset of prophase, chromatin condenses into highly ordered structures called chromosomes.
Since the genetic material has already been duplicated earlier in S phase, each chromosome is
made of two sister chromatids, bound together at the same centromere. Each chromosome also
has kinetochore at centromere. Kinetochore is a complex protein structure that is the point where
spindle fibers attach.