Download The Cell, Chapter 2

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Part II
The Cell – Cell Division, Chapter 2
Outline of class notes
Cellular Division Overview
• Types of Cell Division
• Chromosomal Number
• The Cell Cycle
– Mitoses
• Cancer Cells
In Vitro Fertilization
• Infertility
– Affects one in ten American couples
• In vitro fertilization (IVF) can help infertile couples.
– A sperm and an egg are joined in a petri dish.
– The embryo is implanted into the mother’s uterus at about the 8 cell stage.
– IVF is one of many reproductive technologies
Cell Division
• Cell Division
– Is the process whereby cells reproduce
themselves
– Functions:
• Replace diseased or damaged cells
• Growth of the organism
• Production of a new organism
– Two types of cell division are:
• Somatic cell division
• Reproductive cell division
Somatic Cell Division
• In somatic cell division:
– Cell undergoes a nuclear division called mitosis
and a cytoplasmic division called cytokinesis.
– Results in two daughter cells genetically identical exactly the same number and type of
chromosomes as the parent cell.
Reproductive Cell Division
• In reproductive cell division:
– Cells undergo a process of meiosis that produces gametes (eggs or sperm).
– The egg and sperm contain half the number of
chromosomes as the parent cell.
– Occurs in the ovaries and testes.
– Function: The creation of offspring by the fertilization of an
egg by a sperm
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Background on Chromosomes
• You inherited 46 chromosomes, 23 from each parent.
– Each somatic cell of the human body, except for sex cells, contains 46 chromosomes
(diploid number).
• These chromosomes are arranged in pairs, thus you have 23 pairs to give a total
of 46 chromosomes.
• Of the 23 pairs, one pair are the sex chromosomes which consist of two X
chromosomes if the person is female or an X and a Y chromosome if the
person is male
– The remaining 22 pairs of chromosomes are called autosomes
– Sex cells (sperm and egg) contain 23 individual chromosomes (haploid number), or half
that of somatic cells.
• The union of a sperm and egg during fertilization restores the chromosome
number to 46 (23 pairs).
A human karyotype has 22 pairs of autosomes and 1 pair of sex chromosomes (23 total pairs)
Cell Cycle
• The cell cycle is an sequence of events in the life of a dividing cell, it lasts from the beginning
of one division to the beginning of the next.
– Cell Cycle consists of two distinct phases:
1. Interphase
2. Mitotic (M) Phase
Interphase
• Interphase is the time in which the cell performs its normal functions, and if appropriate,
prepares for cell division. Consists of four stages:
– Go Phase: (G = gap) The normal functional state of a cell not dividing.
– G1 Phase: Cell prepares for replication. Cell produces all the enzymes required for
DNA replication and duplicates most of its organelles.
– S Phase: (Synthesis) Chromosomes are replicated and become double stranded. The
replicated chromosomes are attached together at the centromere and are known as
sister chromatids. Each is an exact copy of the other and is called a chromatid.
– G2 Phase: Devoted to last minute protein synthesis as cell prepares to divide.
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Cell Division in Somatic cells
• Cell division includes two overlapping processes – mitoses and cytokinesis.
– Mitoses: Division of the nucleus (chromosomes)
• The chromosomes within the nucleus divide and are evenly distributed, forming
two daughter nuclei
– Cytokinesis: Division of the cytoplasm
• Results in two genetically identical daughter cells
• Note: Mitoses is a continuum, but we can divide it into 4 stages: prophase, metaphase,
anaphase, and telophase
Mitoses: 4 stages
1. Prophase
• Chromatin
condenses to form
visible (with a
microscope)
chromosomes.
• Centrioles move toward opposite ends of the cell
• Centrioles produce spindle fibers (microtubules), which extend and attach to the
centromeres of each chromosome.
– Spindle fibers attach to the kinetochore of the centromere.
• Movement of chromosomes depends on the spindle fibers.
• Nucleolus and the nuclear membrane disappear
2. Metaphase
• Chromosomes (each consisting of sister chromatids) line up on the
metaphase (equatorial) plate which is at the center of the cell.
3. Anaphase
• Sister chromatids separate at the centromere forming two sets of
identical chromosomes.
– Note: At time of separation, each chromatid is now referred
to as a chromosome.
• The daughter chromosomes move toward the opposite ends of the cell.
– Spindle fibers from the centrioles assist in this process.
4. Telophase
• A nuclear membrane forms around each set of chromosomes
• Chromosomes uncoil becoming chromatin
• Nucleoli reappear
• Cell prepares to divide
• Cleavage furrow begins to form
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Cytokinesis
• Cytokinesis (cleavage) is the division of the cytoplasm.
– Typically begins during telophase.
• The cell membrane constricts along the plane of the metaphase (center) plate forming a
cleavage furrow.
Nemonic Phrase
• Cytoplasm divides resulting in two daughter cells each with DNA
I’ve
Interphase
that is identical to the DNA of the parent cell
Probably Prophase
Made
Metaphase
Mnemonic to Remember Mitosis
All
Anaphase
This
Telophase
Correct
Cytokinesis
Control of the Cell Cycle
• Normal animal cells have a cell cycle control system.
– Special proteins within the cell send “stop” and “go-ahead” signals at certain key points
during the cell cycle.
– Example: Nerve and muscle cells are in a permanent nondividing state (Go).
• Other cells like our skin cells, are constantly receiving signals to divide.
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Cancer Cells: Growing Out of Control
• Cancer is a group of diseases characterized by uncontrolled cell proliferation - is a disease of
the cell cycle.
– Kills one out of every 5 people in the U.S.
• Cancer cells do not respond normally to the cell cycle control system.
– They divide excessively and can invade other tissues of the body.
Benign vs. Malignant Tumor
• Cancer cells can form tumors (neoplasms), which are abnormal growths (masses) of body
cells. Two types of Tumors:
1. Benign tumors: Tumor cells remain at the original site and are often harmless (Ex:
wart).
• Removed surgically if interferes with normal function
2. Malignant tumors: Tumor cells spread into neighboring tissue by way of the circulatory
and lymphatic vessels and may form new tumors.
• Metastasis is the spread of cancer cells.
• An individual with a malignant tumor is said to have cancer.
How Cancers are Named
• Cancers are named according to where they originate and are grouped into 4 categories:
1. Carcinomas: Originate in the external or internal coverings of the body (skin, intestine
etc), lungs, breasts, pancreas, glands.
• Most commonly diagnosed cancers
2. Sarcomas: Originate in support tissues/structures, such as in bone, fat, and muscle
3. Leukemias: Originate in the bone marrow
4. Lymphomas: Originate in the lymph nodes
Genetic Basis of Cancer
• Proto-oncogenes are genes that code
for growth factors that regulate growth
and development - stimulate cell
division.
– When functioning normally they
keep the rate of cell division at the
normal level.
• Oncogenes are mutated forms of the
proto-oncogenes and can cause cancer
to develop.
– Oncogenes transform a normal
cell into a cancerous cell.
• Ex: Causes excessive production of growth factors
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Tumor-suppressor Genes
• Tumor-suppressor genes produce proteins that help prevent
uncontrolled cell growth.
– Mutation of a Tumor-suppressor gene called p53 on
chromosome 17 is the most common genetic change leading to
a wide variety of tumors, including breast and colon cancers.
– Normal p53 protein:
• Arrests cells in the G1 phase, which prevents cell
division.
• Assist in repair of damaged DNA and induces apoptosis
in cell where DNA repair was not successful.
• Nicknamed “the guardian angle of the genome”
Oncogenic Viruses
• Certain viruses (oncogenic viruses)
carry genes called oncogenes that
can cause cancer.
– Viruses can insert their DNA
containing oncogenes into
the host chromosomes and
stimulate abnormal
proliferation of cells – cancer
cells.
Human Papillomavirus (HPV)
• HPV causes most cervical cancer in woman
– Symptoms and virulence depend on the strain of
virus
– Virus produces a protein that causes proteasomes
to destroy the p53 protein.
– Absence of this suppressor protein causes cells to
proliferate without control.
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Cancer Treatment
• Cancer treatment can involve:
– Surgery: Removes the tumor or cancer.
• Is usually the first step
– Radiation therapy: High energy radiation damages DNA and disrupts cell division.
• Cancer cells divide more often than other cells.
• Can cause nausea and hair loss
– Chemotherapy: Uses drugs that disrupt cell division.
• Usual site is to disrupt spindle fiber formation or function
Cancer Prevention and Survival
• Cancer prevention includes changes in lifestyle:
– Not smoking
– Exercising adequately
– Avoiding exposure to the sun
– Eating a high-fiber, low-fat diet
– Visiting the doctor regularly
– Performing regular self- and doctor mediated examinations
• Breast
• Prostate
• Cervical
• Testicular
• Colon