Transcript Cell Division 2015

Cell Growth & Division
Ch. 10
• Cells divide to maintain a
workable ratio of volume
to surface area. The
original cell is called the
parent cell and the
offspring are called
daughter cells. The
offspring receive a portion
of the cytoplasm,
organelles, and hereditary
information from the
parent.
• Cell size
Limits to Cell Growth:
• 1. DNA overload - more demands are placed
on the cell’s DNA
2. Material Exchange - the rate at which
food & oxygen are used up & waste products
are produced become too much for the cell
3. Ratio of Surface Area to Volume - volume
increases more rapidly that surface area
4. Cell Division - 2 new daughter cells form
Diploid Cells
• Diploid cells are also known as 2n because
they have the full set of
chromosomes. Somatic cells are diploid. The
diploid number for humans is 46.
Haploid Cells
• Haploid cells are also known as n because
they have half of the cells found in a typical
somatic cell. The haploid number for humans
is 23. An example of a haploid cell in humans
would be sperm or eggs.
Diploid vs. Haploid Division
• Diploid cells undergo cell division known as
mitosis while haploid cells undergo cell
division known as meiosis. All cells that
undergo mitosis have daughter cells with the
same number of chromosomes as the parent
cell, while cells that undergo meiosis have half
the number of chromosomes as the parent
cell.
Homologous Chromosomes
• Every body cell contains 2
chromosomes bearing genes for
the same set of characteristics, &
the 2 members usually have the
same size & shape. The
members of a pair are called
homologous chromosomes –
one from the mother & one from
the father.
• A locus is a particular site on a
chromosome.
Chromosome drawing:
Human Karyotype
Autosomes –
1st 22 pairs of
chromosomes
Sex
chromosomes
– last pair –
determines sex
samples
Karyotype
• Nondisjunction - failure of chromosomes to
separate properly in cell division – responsible
for Klinefelter Syndrome (XXY) , Turner
Syndrome (XO), & Trisomy-21 (Down
Syndrome)
Cell Division
• Some cells divide once a day, others are not
as frequent. Brain cells and other highly
specialized cells (mature muscle cells) don’t
divide at all.
THE CELL CYCLE
• 1. Interphase - time in between divisions – cell
carries on its usual life activities- metabolism is
very high in this stage
• 3 Stages of Interphase: 1) G1 - growth phase –
the cell decides if it will divide again – cellular
differentiation occurs
2) S - DNA replication occurs – chromosomes go
from being single to being double (2 sister
chromatids)
3) G2 - another growth phase – organelles
reproduce – prepares for division.
The Cell Cycle
Cell Cycle
• 1) Interphase – 90% of cell’s life
• 2) Mitotic Stage – broken up into:
•
a) Mitosis – splits nucleus & contents
– consists of prophase, metaphase, anaphase,
telophase
•
b) Cytokinesis – splits cytoplasm
Prophase
• Prophase - EARLY – centrioles begin to
migrate to opposite sides of the nucleus,
chromatin may become denser, MIDDLE –
nuclear membrane starts to break down,
nucleolus begins to disasppear, chromosomes
appear, centrioles migrate to poles & produce
the aster complex that makes the spindle
arising from 2 centrosomes, LATE – no
nucleolus or nuclear membrane,
chromosomes are floating within the cell,
spindle apparatus has formed
Metaphase
• Metaphase chromosomes align
themselves along the
equator – they are
attached to the spindle
fibers at the
kinetochore.
Anaphase
• Anaphase - splitting of the centromeres ,
spindle fibers contract & chromatids are
divided, these become chromosomes & are
pulled to the poles, the chromosomes are Vshaped in middle anaphase & J-shaped in late
anaphase, this stage is over when
chromosomes reach the poles.
Telophase
• Telophase - this is the reverse of prophase,
MIDDLE – begins the formation of the nuclear
membrane, the nucleolus reappears, granular
chromatin reappears, LATE – there is a
complete nuclear membrane, the aster
complex disappears, centrioles disappear
CYTOKINESIS
• The result of cell division is
the production of 2 identical
daughter cells. In animals a
cleavage furrow occurs that
pinches in the cell. This
begins during anaphase. In
plants a cell plate forms in
the middle of the dividing
cell – this eventually forms
the cell wall after migrating
to the edges of the cell.
Examples
MEIOSIS
• Meiosis occurs in sex cells. It is composed of 2
stages: Meiosis I & Meiosis II. Meiosis consists of 2
nuclear divisions in which the chromosomes divide
only once. The result is that mature gametes have
only 1 member of each homologous chromosome
pair (haploid). When the gametes unite, a zygote is
formed.
MEIOSIS I
• 1. Prophase I -the 2 members of
each pair of homologous
chromosomes come into side-by-side
contact . Each chromosome has
already replicated to form 2
chromatids, each of which will
become a new chromosome. The 2
chromatids are joined at 1 point, the
synapsis, so that each will become 4
future chromosomes, & is thus called
a tetrad. In this stage, crossing over
occurs in which chromatids exchange
segments (increases genetic
variation).
Metaphase I
• 2. Metaphase I - the chromosomes align on
the equator of the cell
Anaphase I
• 3. Anaphase I - unlike mitosis, the
centromeres holding the chromatids together
do NOT divide – each chromatid is pulled
toward each pole (1st Reductive Division
occurs)
Telophase I
• 4. Telophase I - the daughter cells contain 1 of
each of the homologous chromosomes - total
chromosome number is now 23 in humans.
Meiosis I
Meiosis II
MEIOSIS II
• The second meiotic division more closely resembles
the events in mitosis. The chromosomes are split at
the beginning of meiosis II by division of the
centromeres, & single-stranded chromosomes move
toward each pole. Each chromatid now exists in a
separate cell. The essential difference is that you
start with haploid cells. 4 cells are now formed. In
the male, all 4 cells are functional sperm, but in the
female, only 1 is functional & the others become
polar bodies.
Oogenesis
Spermatogenesis
Comparing Mitosis & Meiosis
• Similarities: Both are types of cell division;
Chromosomes duplicate only once
• Differences: Mitosis used for somatic cells,
meiosis used for sex cells; Mitosis yields 2
diploid genetically identical cells, meiosis
yields 4 haploid genetically different cells;
Mitosis has 1 division of the nucleus, meiosis
has 2
Cell Differentiation
• All cells contain the
same DNA. In cell
differentiation, some
of the DNA is repressed
and some is
expressed. This allows
cells to have different
functions.
Cell Differentiation & Regulation
• Stem cells – Unspecialized cells that can
differentiate into any type of body cell – 2
types are embryonic & adult
• Apoptosis – programmed cellular death (all
cells have a pre-determined life span)
Cell cycle regulators
• 1. Internal regulators - proteins respond to
events inside the cell to determine when
mitosis occurs – cyclins bind to cyclindependent kinases that start stages in the cell
cycle
2. External regulators - proteins respond to
events outside the cell – direct cell to speed
up or slow down the cell cycle – cells grow
until they touch each other
Types of Cell Regulation
• Density-dependent inhibition – cells stop
dividing when they touch each other – based
on cell surface proteins (will close wounds)
• Anchorage dependence – cells must be in
contact with a solid surface (matrix of tissue)
to divide
Cancerous Division
• Tumor – abnormally growing mass of cells
• Benign – stays enclosed in a “wall” & remains
at the original site
• Malignant – spreads into neighboring tissues
& causes angiogenesis (lures new blood
vessel formation)
• Metastasis – spread of cancer through the
body via the circulatory/lymphatic system
Types of cancers
• Carcinomas – in coverings of skin/lining of
intestines
• Sarcomas – in bone, muscle – in areas that
support the body
• Leukemias /lymphomas – in blood-forming
tissues (marrow, spleen, lymph nodes)
• Most chemotherapy agents work to freeze or
prevent spindle formation.
Cancer Development
• Hypertrophy - cells increase in size
Hyperplasia - cells increase in number
Dysplasia - cells change their shape (atypical
cells)
Neoplasia - producing new cells
Apoptosis - is programmed cellular death
Asexual vs. sexual reproduction
• Asexual reproduction only takes one parent.
Asexual reproduction is the primary form of
reproduction for unicellular organisms.
Asexual reproduction is relatively rare among
multicellular organisms.
• While asexual reproduction may have short
term benefits when rapid population growth is
important or in stable environments, sexual
reproduction offers a net advantage by
allowing more diversity, allowing adaptation
to changing environments.
Types of asexual reproduction:
• 1. Budding - the formation of a new
organism by the protrusion of part of
another organism. This is very common in
plants, bacteria, and fungi, but may be found
in animal organisms, such as the hydra, as
well.
• 2. Binary fission - in single-celled organisms
by which one cell divides into two cells of the
same size, used by most bacteria. This
process results in the reproduction of a living
cell by division into two equal or near-equal
parts.
• 3. Vegetative propagation - Plants are
produced using material from a single parent
and as such there is no exchange of genetic
material, therefore vegetative propagation
methods almost always produce plants that
are identical to the parent. Vegetative
reproduction uses vegetative plants parts or
roots, stems and leaves.
• 4. Spore formation – alternation of
generations allows spore formation with
meiosis (not true asexual method), some fungi
& algae, however, can form spores without
meiosis
• 5. Fragmentation – new organism grows from
a fragment of the parent (regeneration)
• 6. Parthenogenesis - form of reproduction
found in females where growth and
development of an embryo or seed occurs
without fertilization by males (plants, bees,
aphids, wasps, some reptiles, some fish,
some amphibians).
Sexual reproduction
• 1. Conjugation - transmission of DNA without
fusion of gametes (some bacteria & protists)
• 2. Fertilization - sperm & egg (gametes) join
to form embryo