1. THE CELL AND CELL DIVISION

Dr. Rajendra A. Lavate

(M. Sc., M.Phil., Ph.D.)

Asst. Prof. in Botany

Raje Ramrao Mahavidhyalaya, Jath

Email: bryoraj@gmail.com

Mobile: 7588627653/9623420161

4.1 The Cell Theory

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• The cell theory, a foundation of modern biology, states that cells are the fundamental units of life

Measuring Cells

• One micrometer (μm) is one-thousandth of a millimeter

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Animalcules and Beasties

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• Van Leeuwenhoek was the first to describe small organisms seen through a microscope, which he called animalcules and beasties

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• Hooke was the first to sketch and name cells

Development of the Microscope

Cell Division

• Cell division is the process where a parent cell divides into two daughter cells.
• There are two types of cell division:
• Mitosis occurs in somatic cells.
• Meiosis occurs in the sex organs and produces sex cells (gametes).

The Centrosome

• All eukaryotic cells contain a centrosome,�also called the microtubular organizing center. It has a central role in cell division.
• Within an centrosome of animal and algal cells,�there is a pair of centrioles.
• During cell division, the centrosome divides and the centrioles replicate, producing two centrosomes, each with its own pair of centrioles.
• The two centrosomes move to opposite ends�of the nucleus.
• Each centrosome produces microtubules. These form the spindle responsible for separating the replicated chromosomes into two daughter cells.
• Plant cells have centrosomes, with a similar role to those in animal cells, but they lack centrioles.

Introduction to Mitosis

• During mitosis, an existing parent cell divides into two new daughter cells (right).
• The cells are genetically identical.
• There is no change in chromosomal number.
• Cells are diploid, containing two sets�of chromosomes.
• In humans the diploid number is 46
• Mitosis is associated with the growth �and repair of somatic cells in the body.

Mitosis and the Cell Cycle

• Mitosis is just one phase of the cell cycle.
• There are three main phases in the �cell cycle:
• Interphase (itself comprising three stages)
• Mitosis (nuclear division)
• Cytokinesis (division of the cytoplasm)

Interphase

• Interphase accounts for 90% �of the cell cycle.
• It is the longest phase of the cell cycle.
• Interphase consists of three stages:
• First gap phase (G₁)�The cell grows and develops
• Synthesis (S)�The cell duplicates its genetic�material (chromosomes).
• Second gap phase (G₂)
• The nucleus is well defined
• The chromosomes condense into chromatids in preparation for division
• The centrosome is replicated

M

G₁

G₂

S

C

The cell cycle

Nuclear membrane

Mitosis

• The mitotic cycle is broken down into six phases.
• The example below is for a plant cell.

Mitosis: Early Prophase

• Prophase is the first stage of mitosis. In early prophase:
• the nuclear membrane disintegrates
• the nucleolus disappears
• the chromatin condenses into visible chromosomes.

Replicated centrosomes

Mitosis: Prophase

• In late prophase:
• the chromosomes continue to coil�and appear as double chromatids.
• the chromatids are each joined by a centromere.
• the centrosomes move to opposite poles (ends) of the cell. As they do so, they form the mitotic spindle between the poles.
• the kinetochores mature and attach to the spindle.

A newt lung cell in late prophase (stained with fluorescent dyes). The mitotic spindles appear green and the nucleus appears blue.

Mitosis: Metaphase

• During metaphase the chromosomes become aligned at the equator of the cell.
• Kinetochores attach the chromosomes to the spindle and align them along �the metaphase plate at the equator of �the cell.
• The metaphase plate is an imaginary �plane equidistant between the two poles.
• Kinetochores are disc like structures to which the spindle fibers attach.
• The spindle fibers are made up of microtubules and associated proteins, �joined at the ends (the spindle poles).
• Some spindle fibers extend to the equator but do not attach to chromosomes.

Mitosis: Early Anaphase

• In anaphase, the chromosomes are pulled to opposite poles of the cell.
• the centromeres divide, freeing the two sister chromatids from each other.
• Each chromatid is now considered to be a chromosome.
• The spindle fibers begin moving the once-joined sisters to opposite poles of the cell.

Chromosomes

Spindle

Anaphase is the shortest mitotic phase

Mitosis: Late Anaphase

• By late anaphase, the chromosomes have moved to opposite poles.
• The kinetochore microtubules shorten as the chromosomes approach the poles.
• At the same time, non-kinetochore microtubules elongate the cell�(i.e. move the poles apart).
• By the end of anaphase, the two poles of the cell have equivalent, and complete, collections of chromosomes.

Mitotic spindle

Chromosomes

Centrosome

Mitosis: Telophase

• Telophase is characterized by the formation of two new nuclei.
• The non-kinetochore microtubules continue to elongate the cell.
• The daughter nuclei begin to form at the two poles of the cell where the chromosomes have gathered.
• The nucleoli reappear and the chromatin becomes less tightly coiled (less condenses).

In plant cells, the cell plate forms where the new cell wall will form.

Cytokinesis

• The division of the cytoplasm is termed cytokinesis.
• Cytokinesis is usually well underway by the end of telophase, so that the appearance of two new daughter cells follows shortly after the end of mitosis.
• In plant cells, the cell plate forms where the new cell wall will form.
• In animal cells, a cleavage furrow pinches the cell in two.
• The two daughter cells are now separate cells in their own right.

Nucleus

Cell wall

Mitosis: Review

Mitosis in the Root Tip

• Mitosis in plant cells occurs only in regions of meristematic tissue.
• The meristematic tissue is located at the tip of every stem and every root.
• In contrast, mitosis can occur throughout the body of a growing animal.

Root cap

Root tip growing in this direction

Apoptosis

• Apoptosis or programmed cell death (PCD) has many crucial roles in the body, including:
• maintenance of adult cell numbers. In humans, 50-70 billion cells undergo apoptosis each day to make way for new cells.
• defense against damaged or dangerous cells, such as:
• virus-infected cells
• cells with DNA damage
• the transformation and “sculpting” of embryonic tissue during its development:
• formation of fingers and toes in a fetus
• sloughing of the endometrium during menstruation
• formation of proper connections (synapses) between neurons in the brain

A normal leukocyte (top) and one undergoing apoptosis (bottom). Note the bulbous appearance of the membrane. This is called blebbing.

The Process of Apoptosis

• As series of morphological changes occur when a cell undergoes apoptosis.

Control of Apoptosis

• Apoptosis is a complicated and tightly controlled process, distinct from cell necrosis (uncontrolled cell death), when the cell contents are spilled.
• Regulation occurs through a combination of:
• positive signals, required for cell survival.
• negative signals, triggering cell death.
• When these are unbalanced one of two things may occur:
• The rate of apoptosis becomes too high, e.g. HIV infected helper T-cells induce apoptosis in neighboring T-cells, limiting the immune response to the virus.
• The rate of apoptosis becomes too low, �e.g. a low rate of lymphocyte apoptosis �is associated with an overactive �immune system.

Incomplete differentiation of the toes (syndactyly) as a result of lack of apoptosis.

Wikipedia Commons http://commons.wikimedia.org/wiki/Image:Celldeath.jpg

Apoptosis in mouse liver showing the apoptotic cells (stained orange).

Laboratory of Experimental Pathology, Division of Intramural Research, NIEHS (NIH)

Controls of Apoptosis

• Negative signals for inducing apoptosis are those that trigger the cellular changes leading to cell death. They include:
• Intrinsic inducer signals generated from within the cell in response to stress, e.g. DNA damage, starvation, or reactive oxygen.
• Extrinsic inducer signals or death activators, which promote apoptosis, e.g. certain �cytokines (signaling proteins and peptides) �such as lymphotoxin.
• Positive signals prevent apoptosis and allow a cell to function normally. They include:
• interleukin-2 and bcl-2 protein
• certain cytokines and growth factors
• inhibitors of apoptosis proteins

Apoptosis and Cancer

• Cell proliferation and death are controlled by two gene families:
• Proto-oncogenes, which promote cell growth.
• Tumor suppressor genes, which inhibit cell growth.
• Mutations to (or bypassing of) these genes gives rise to uncontrolled cell division and results in the formation of immortal cancer cells.

Apoptosis and Cancer

• Cancer cells can divide rapidly and spread because they are able to prevent apoptosis.
• Human papilloma virus (HPV), which is linked to cervical cancer, is able to inactivate an apoptosis promoter and continue to spread.
• Other cancer cells express high levels of proteins, such as bcl-2, which suppress apoptosis and allow the cell to divide rapidly �and form tumors.

Cancer in a bisected kidney. Most of the kidney has been replaced by gray and yellow tumor tissue. Only a small amount of healthy kidney tissue still present

CDC

Features of a Cancer Cell

• Cancer cells do not differentiate into a specialist cell type.
• A cancer cell is parasitic, taking nutrients from surrounding cells by forming large numbers of blood vessels to supply it.
• A cancer cell undergoes uncontrolled division, which is not inhibited by contact with surrounding cells.
• Cancer cells can be motile, enabling them to spread (metastasize) around the body.

Inducing Apoptosis�in Cancer Cells

• Understanding how apoptosis is controlled can help researchers find ways to treat cancer (e.g. by inducing cancer cells to undergo apoptosis).
• Several apoptosis-inducing drugs are being developed.
• Some suppress the production of anti-apopotic proteins such as bcl-2. If levels of bcl-2 decrease sufficiently, apoptosis will occur.
• Other drugs are designed to be used with existing cancer treatments such �as chemotherapy.

Apoptosis and Limb Development

• Apoptosis is important for the normal development of animal embryos.
• Apoptosis removes unnecessary tissue and sculpts the embryo.
• A good example is the formation of the fingers and toes in the human fetus.
• 41 days after fertilization (top right), the digits of the hands and feet are webbed, making them look like small paddles.
• The webbing is superfluous, and is removed by apoptosis. By 56 days after fertilization, the webbing has completely disappeared and each of the digits can be individually seen (right).

Significance of Apoptosis

• It has been studied in some animals. It is also observed in some plant organs during senescence and during growth of embryo.
• Similarly, bacteria growing in culture medium exhibit appotosis in some cells.
• Thus apoptosis occurs in all living organisms.
• Apoptosis /The programmed cell death is used to kll the cells for 3 different reasons
• For proper development of the system & to maintain internal stability required to compensate the changes in surrounding environment.
• For defence/as a protective mechanism against stress in culture medium &
• For aging/senescence process.

The cells which are killed by apoptosis are:

1. Having no function/are no more necessary for any physiological function.
2. Produced in excess than the requirement of the organ.
3. Not healthy & show improper development.
4. Harmful & need to be eliminated from the system.