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Cell Reproduction
Reproduction of Prokaryotes
Prokaryotes reproduce through a process
called binary fission
The single, circular chromosome (which is
attached to the plasma membrane) replicates
The cell grows & elongates, separating the
two chromosomes
The plasma membrane and cell wall pinch
inward, eventually forming two daughter cells
Eukaryotic Chromosomes and
Cell Division
Almost all of a eukaryotic cell’s genes
are found in the cell’s nucleus, and are
grouped into multiple chromosomes
Chromosomes are only visible when a
cell is in the process of dividing
The rest of the time, the DNA + protein
threads exist as a diffuse mass called
chromatin
Before dividing, the eukaryotic cell
replicates its chromosomes
The two copies of the chromosome are
called chromatids, and they are
connected to each other at a region
called the centromere
The pair of chromatids is still only one
chromosome, and the chromatids are
not considered chromosomes until they
separate from one another
The Eukaryotic Cell Cycle
A eukaryotic cell spends most of its time in
interphase, the period of time between cell
divisions
During interphase, metabolic activity is
high; the cell grows, produces cell parts,
and in the middle of interphase the cell’s
chromosomes duplicate
Interphase accounts for 90% of the total
time required for the cell cycle
Interphase
Interphase may be divided into three
subphases
The first subphase is called G1 (the G is
for gap); the cell is producing proteins and
organelles
The second subphase is the S phase,
which is when DNA synthesis occurs
The third subphase is called G2, which is a
time of metabolic activity; proteins needed
for cell division are produced
Mitotic Phase (M Phase)
Consists of two processes: mitosis and
cytokinesis
During mitosis, the chromosomes divide
and are distributed into two daughter
nuclei
During cytokinesis, the cytoplasm is
divided into two
These two processes result in the
production of two genetically identical
daughter cells
Phases of Mitosis - Prophase
Chromatin fibers condense
Nucleoli disappear
Each chromosome has two chromatids
joined at the centromere
The spindle begins to form; microtubules
grow out of the centrosomes, which
contain centrioles
The nuclear envelope fragments
Microtubules grow until they can reach the
chromosomes
Each chromatid has a special region at the
centromere, called the kinetichore, where
the spindle will attach
Spindles that don’t attach to kinetichores
extend toward the opposite side of the cell
The chromosomes begin moving toward
the center of the cell
Phases of Mitosis - Metaphase
The spindle is fully formed
All of the centromeres of the
chromosomes are lined up at the
metaphase plate (the middle of the
cell)
The kinetichores of the chromatids
face opposite sides of the cell
Phases of Mitosis - Anaphase
The centromeres of the chromosome
come apart and the sister chromatids
separate (they are now considered
daughter chromosomes)
Motor proteins on the kinetichores walk
the chromosomes toward opposite sides
of the cell
Kinetichore microtubules shorten and nonkinetichore microtubules lengthen (the cell
elongates)
Phases of Mitosis - Telophase
Basically the reverse of prophase
Cell elongation that began in anaphase
continues
Daughter nuclei appear-nuclear envelope
appears
Chromatin fibers uncoil
Nucleoli reappear
Spindle disappears
Cytokinesis
Occurs along with telophase
In animal cells, cytokinesis involves a
cleavage furrow, which pinches the
cell in two
In plants, cytokinesis involves a cell
plate, which grows outward and
accumulates cell wall material
The Cell Cycle & Cancer
Cancer is a disease of the cell cycle
Systems that control the cell cycle do
not work in cancer cells
As a result, cancer cells divide
excessively and can invade other
body tissues
The excessive growth can result in an
abnormal mass called a tumor
Tumors
Benign tumor-abnormal mass of normal
cells; can cause problems if they grow in
an organ; remain in their original site; can
usually be completely removed by surgery
Malignant tumor-cancerous; capable of
spreading to other body areas; displaces
normal tissue; can form new tumors by
breaking apart and spreading through the
circulatory system (called metastasis)
Types of Cancers
Carcinomas-originate in the external
or internal coverings of the body
(skin)
Sarcomas-originate in tissues that
support the body (bone and muscle)
Leukemias and Lymphomas-cancers
of blood forming tissues (bone
marrow, spleen, and lymph nodes)
Treatments for Cancer
Radiation- parts of the body with
cancerous tumors are exposed to high
energy radiation, which disrupts cell
division (damage to ovaries or testes may
cause sterility)
Chemotherapy-administration of drugs that
disrupt cell division; some interfere with
the formation or functioning of the spindle
Meiosis
Essential for the formation of gametes
(ova and sperm) that will be used for
sexual reproduction
Occurs in two consecutive cell
divisions
Reduces the chromosome number by
½ (for humans, this means creating
gametes that have 23 chromosomes)
Human Chromosomes
Occur in pairs (there are 23 pairs) in
somatic cells (typical body cells); these are
called diploid cells (di=2)
Occur unpaired (a single set of 23) in the
gametes; these are called haploid cells
The paired chromosomes in somatic cells
are called homologous chromosomes,
because they have genes for the same
traits
Human Chromosomes
22 of the chromosome pairs are called
autosomes
The 23rd pair consists of the sex
chromosomes
Human females have a pair of sex
chromosomes called X chromosomes (XX)
Human males have one X chromosome
and one Y chromosome (XY)
Inheritance of Human Chromosomes
One chromosome from each pair is
inherited from our mothers, and the other
chromosome in the pair is inherited from
our fathers
At the time of fertilization, the two haploid
gametes (sperm and ovum) unite to form a
diploid cell called the zygote
Fertilization results in the formation of a
diploid cell, thus restoring the normal
diploid number of human chromosomes
Meiosis Produces Gametes
Meiosis occurs in the reproductive organs