Transcript Document 7626945

MITOTIC CELL DIVISION
JANUARY 7, 2011
LAB JANUARY 13, 2011
• In eukaryotic cells the genetic material is
organized into a complex structure composed of
DNA and proteins and localized in a specialized
compartment, the nucleus.
• This structure was called chromatin (from the
Greek "khroma" meaning coloured and "soma"
meaning body).
• Close to two meters of DNA in each cell must be
assembled into a small nucleus of some mm in
diameter.
Packaging of DNA into Nucleus
• Despite this enormous degree of compaction,
DNA must be rapidly accessible to permit its
interaction with protein machineries that regulate
the functions of chromatin:
• replication,
• repair and
• recombination.
• The organization of chromatin structure
influences all functions of the genome.
• The fundamental unit of chromatin, termed
the nucleosome, is composed of DNA
and histone proteins.
• This structure provides the first level of
compaction of DNA into the nucleus.
Within an interphase nucleus chromatin is
organized into functional territories.
Nucleosome
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Chromatin has been divided into:
euchromatin and
heterochromatin.
Heterochromatin was defined as a structure that
does not alter in its condensation throughout the
cell cycle whereas euchromatin is decondensed
during interphase.
• Heterochromatin is localized principally on the
periphery of the nucleus and euchromatin in the
interior of the nucleoplasm.
Chromatin and Chromosomes
• Packed inside the nucleus of every human
cell is nearly 6 feet of DNA, which is
subdivided into 46 individual molecules,
one for each chromosome and each about
1.5 inches long.
• Collecting all this material into a
microscopic cell nucleus is an
extraordinary feat of packaging.
• For DNA to function when necessary, it can't be
haphazardly crammed into the nucleus or simply
wound up like a ball of string.
• During interphase, DNA is combined with
proteins and organized into a precise, compact
structure, a dense string-like fiber called
chromatin, which condenses even further into
chromosomes during cell division.
• Each DNA strand wraps around groups of small
protein molecules called histones, forming a
series of bead-like structures,
called nucleosomes, connected by the DNA
strand.
• Under the microscope, uncondensed chromatin
has a "beads on a string" appearance. The
string of nucleosomes, already compacted by a
factor of six, is then coiled into an even denser
structure known as a solenoid that compacts
the DNA by a factor of 40.
• The solenoid structure then coils to form a
hollow tube.
• This complex compression and structuring
of DNA serves several functions.
• The overall negative charge of the DNA is
neutralized by the positive charge of the
histone molecules, the DNA takes up
much less space, and inactive DNA can be
folded into inaccessible locations until it is
needed.
• There are two basic types of
chromatin. Euchromatin is the genetically
active type of chromatin involved in
transcribing RNA to produce proteins used
in cell function and growth. The
predominant type of chromatin found in
cells during interphase, euchromatin is
more diffuse than the other kind of
chromatin, which is
termed heterochromatin.
• Heterochromatin tends to be most
concentrated along chromosomes at
certain regions of the structures, such as
the centromeres and telomeres.
• Genes typically located in euchromatin
can be experimentally silenced (not
expressed) by relocating them to a
heterochromatin position.
• Throughout the life of a cell, chromatin
fibers take on different forms inside the
nucleus.
• During interphase, when the cell is
carrying out its normal functions, the
chromatin is dispersed throughout the
nucleus in what appears to be a tangle of
fibers.
• This exposes the euchromatin and makes
it available for the transcription process.
• When the cell enters metaphase and prepares
to divide, the chromatin changes dramatically.
• First, all the chromatin strands make copies of
themselves through the process of DNA
replication.
• Then they are compressed to an even greater
degree as they undergo a 10,000-fold
compaction into specialized structures for
reproduction, the chromosomes.
• As the cell divides to become two cells,
the chromosomes separate, giving each
cell a complete copy of the genetic
information contained in the chromatin.
• The number of chromosomes within the
nuclei of an organism's cells is a speciesspecific trait.
• Human diploid cells (those that are not gametes)
characteristically exhibit 46 chromosomes, but
this number can be as low as 2, as is the case
for some ants and roundworms, or more than a
thousand, as exemplified by the Indian fern
(Ophioglossum reticulatum), which has 1,260
chromosomes.
• The number of chromosomes a species has
does not correlate to the complexity of the
organism.
Human male genome
Ophioglossum reticulatum
genome
Chromosomes, Chromatin and
DNA
DNAProteinsPhenotype
• We have seen therefore that the genes and
transcription units along the DNA molecule are
responsible for the proteins made in the body
(protein synthesis).
• These proteins are then expressed which is
essentially the phenotypic representation of the
gene.
• These exprressions may be external (e.g. hair,
skin or eye colour) or internal (production of
hormones and enzymes).
IMPORTANCE OF DNA
REPLICATION
• Chromosomes are the most structures
during cell division.
• Chromosomes are composed of
deoxyribonucleic acid (DNA) and protein
and are present in the nuclei of all cells.
• Chromosomes contain genetic information
in the form of genes.
• They contain DNA and are responsible for
transferring hereditary material through
generations.
• Before the nucleus divides an exact copy of the
DNA/chromosomes must be made.
• The two parts of the chromosomes are called
the chromatids.
• Each pair of chromatids contain identical
DNA/genes.
• Chromosomes of different species are
distinctly different in number and
appearance.
• For example every human cell has 46
chromosomes, while dog cells have 98
and mosquito cells have 6.
GENES
• All along the length of chromosomes are
found genes.
• Genes are the basic unit of heredity.
• They control the structure and functioning
of the cells by controlling the proteins they
make, mainly enzymes.
• Genes therefore control the characteristics of
organisms.
• All cells of one organism contain an identical
combination of genes that make the organism
unique (e.g. hair, skin and eye colour).
• Within any cell however, only some genes may
be active e.g. in a muscle cell, muscle cell genes
are active and eye colour genes are of course
inactive.
HUMAN KARYOTYPE
THE DIVIDING CELL
WHY AND WHEN CELLS DIVIDE
• When studying about the processes
involved in cell divisions, here are some
important facts to remember to make it a
little easier:
• 1). The parts of the cells that we are
concerned with are the nucleus and
centrioles.
• 2). The nucleus of a cell contains
chromosomes which have genes on them.
These genes are responsible for giving the
cell instructions about what protein to
make and it is these proteins that give our
hair colour, eye colour etcetera.
• 3). When a cell divides, it is therefore
very important that the two new cells
(daughter cells) each to get exactly
the same number and kinds of
chromosomes as the original (parent)
cell.
MITOSIS
• Mitosis is the division of plant or animal
cells in order to enable an organism to
grow or repair damaged parts of itself.
• Mitosis is the cell division which occurs in
all body cells except for gamete (sex cell)
formation. Mitosis results in the formation
of two genetically identical cells, each
containing the same number of
chromosomes as the parent cell (the
diploid or 2n number).
• Mitosis ensures that the species number
of chromosomes is maintained.
• Mitosis ensures that each daughter cell
receives an identical combination of
genes.
• Mitosis is the method by which all cells of
a multi-cellular organism are formed from
the zygote, and therefore is essential for
growth.
• Mitosis is the method by which organisms
reproduce asexually forming offspring
identical to the parent.
MITOSIS MAKES IDENTICAL
OFFSPRING
CHROMOSOMES AND
MITOSIS
CENTRIOLOES AND MITOSIS
• These are organelles found in the
cytoplasm close to the nuclear envelope in
animal and simpler plant cells.
• Approximately 500nm long and 200 nm
wide.
• Composed of 9 groups of microtubules
arranged in triplets.
• Neighboring triplets are attached to each
other by fibrils.
• Microtubules are long hollow tubes 25nm
in diameter and made of the protein
tubulin.
• The centrioles lie in a poorly defined area
called the cemtrosome.
• This area actually makes the spindle
fibres.
• Spindle fibres shorten by the removal of
tubulin to pull chromatids apart.
• The addition of the chemical colchicine to
actively dividing cells inhibits spindle
formation and the chromatid pairs remain
in their metaphase positions.
• What do you think may be an advantage
to scientists of adding colchicine to
dividing cells?
ANSWER
• This technique enables the number and
structure of chromosomes to be examined
under the microscope.
THE CELL CYCLE
• The sequence of events which occurs
between one cell division and the next is
called the cell cycle.
• It has three main stages:
1 INTERPHASE
• This is a period of synthesis and growth.
• The cell produces many materials required
for its own growth and for carrying out all
its functions.
• DNA replication occurs during interphase.
2 MITOSIS
• This is the process of nuclear division.
3 CELL DIVISION
• This is the process of division of the
cytoplasm into two daughter cells
(cytokinesis).
INTERPHASE
MITOSIS
• The events occurring within the nucleus
during mitosis are usually observed in
cells which have been fixed and stained
(hence killed).
• This gives the appearance of a series of
snapshots even though mitosis is a
continuous process.
• There are four stages which occur during
mitosis:
1.PROPHASE
2.METAPHASE
3.ANAPHASE
4.TELOPHASE
PROPHASE
METAPHASE
ANAPHASE
TELOPHASE
LATE TELOPHASE
CYTOKINESIS
• This is the division of the cytoplasm.
• Normally follows telophase and leads into
the G1 phase of interphase.
• Cell organelles distribute to the two poles
along with the the chromosomes
(telophase).
• In animal cells, cell surface membrane
invaginates to create a furrow around the
outside surface of the cell.
• The cell surface membranes on the furrow
eventually meet up an completely
separate the two cells.
• In plant cells, the spindle fibres begin to
disappear during telophase everywhere
except for in the region of the equitorial
plane.
• Here they move outward and for a barrelshaped region known as the
phragmoplast.
• Microtubules, mitochondria, ribosomes,
endoplasmic reticulum and Golgi
apparatus are attached to this region.
• The Golgi apparatus releases small fluidfilled vesicles which fuse to form a cell
plate which grows across the equatorial
plane.
• The contents of the vesicles contribute to
the formation of the new middle lamella
and cell walls of the daughter cells.
• These are called primary cell walls which
are thickened by suberin, lignin and
cellulose to form secondary cell walls.
• Where vesicles of the cell plate fail to fuse
and the cytoplasm of neighboring daughter
cells remain intact form channels called
plasmodesmata.
MITOSIS IN ACTION!
MITOSIS IN ACTION!
MITOSIS IN PLANT AND ANIMAL
CELLS
•
1.
2.
3.
PLANT
No centriole present.
No aster forms.
Cell division involves
formation of a cell
plate.
4. Occurs mainly at
meristemmatic
regions.
• ANIMAL
1. Centrioles present.
2. Asters form.
3. Cell division involves
furrowing and
cytoplasm cleavage.
4. Occurs in most
tissues throughout
the body.
SIGNIFICANCE OF MITOSIS
1. Genetic stability
2. Growth
3. Cell replacement
4. Regeneration
5. Asexual reproduction
MITOSIS SUMMARY