Transcript Cell Division Mitosis PowerPoint

Cell Division
Cell Cycle & Mitosis
Why do cells divide?
DNA Overload – the larger the cell grows, the
greater the demands are placed on the cell’s
DNA. At some point there is not sufficient DNA
for the cell’s proper functioning.
Surface Area to Volume Ratio (SA:V) gets too
low: As the cell increases in size, the ratio of
the surface area to its volume decreases. If this
ratio gets too low, the cell cannot efficiently
move materials in & out of the cytoplasm.
THE CELL CYCLE
Cell cycle is a series of cellular events
during which the cell grows, prepares for
division & then divides.
• INTERPHASE – Made up of the G1, S & G2 phases
(Most of the cell life is spent here.)
• G1 (Gap 1)- period of cell growth
• S (Synthesis) -period of DNA replication
• G2 (Gap 2) – period of preparation for cell division.
• Cell Division is made up of Mitosis &
cytokinesis
Mitosis = division of the nucleus
Cytokinesis = division of the cytoplasm
Cyclins – proteins that regulate the cell cycle
ALSO: Some cells can stop dividing by
exiting the cell cycle and entering a
Go phase. There is no preparation for
cell division of DNA replication.
Central Nervous System cells, like
those in the spinal cord are examples
of cells that are in the Go phase.
Mitosis Phases &
Cytokinesis
Mitosis refers to cell division in somatic cells
(non sex-cells)
PMAT
Prophase-Metaphase-AnaphaseTelophase
(Interphase: “in-between” period of growth
between cell divisions.)
Interphase
• DNA –exists in the form of chromatin
• Centrioles – a grouping of microtubules that
organize the spindle (Not present in plant cells)
• During the G2, the chromatin strands change to
chromatid pairs (completed in the prophase)
EARLY PROPHASE:
(“pro” means before)
Chromatin condenses and coils into
Chromosomes (consisting of chromatid
pairs). This starts at the end of interphase
and is completed in prophase.
Nuclear membrane breaks down and
Nucleolus disappears.
Centrioles begin to move to opposite
poles of the cell. They eventually reach
the ends of the cell.
Astral rays appear
Chromosome Structure
• When chromosomes form from chromatin, the
DNA strands wrap themselves around histone
proteins, which help maintain the structure.
• The DNA wrapped around histones forms a
nucleosome.
• At the end of G2, (in the beginning of
“prophase” in mitosis) the DNA coils up
(known as “super coiling”) to become
chromosomes.
• Coiling of the DNA into chromosomes
allows for efficient cell division.
Human Chromosomes
• Chromosomes of similar size, shape and containing
similar genes are called homologous
chromosomes.
• Humans have a total of 23 pairs of chromosomes in
their body cells. (One of each pair comes from the father, and
is called “paternal,” and the other one comes from the mother and is
called “maternal”.)
• There are 22 pairs of non-sex chromosomes called
autosomes, and 1 pair of sex chromosomes.
Diploid v. Haploid number
• Diploid = 2N chromosome number = 2 sets of
homologous chromosomes. (The diploid
chromosome number for human cells is 46.)
• Haploid = 1N chromosome number = 1 set of
homologous chromosomes. (The Haploid
chromosome number for humans sex cells is 23.)
Haploid – “Half ”
Late Prophase:
Formation of the spindle, composed of fanlike
microtubules network that helps to separate the
chromosomes.
Two types of spindle
fibers: polar fibers
(pole to pole)
kinetochore
fibers (attach to
centromeres)
Chromatid pairs move toward the middle of the
cell. (equatorial plane)
Metaphase:
(“meta” means middle)
• chromatid pairs align themselves in the
middle of the cell.
• chromatid pairs are held in place by the
kinetochore fibers.
Early Anaphase:
(“ana”means away)
• centromeres split
• Spindle fibers are
disassembled at
the centrioles as the spindle fiber shorten.
• chromatid pairs are pulled apart as individual chromatid
(now called chromosomes) move to opposite poles.
• (Some spindle fibers push against others which help to
elongate the cell.)
Late Anaphase
• chromosomes are now at opposite poles
Telophase:
• chromosomes begin to uncoil into chromatin
• cytokinesis (cell splitting) begins as the
cytoplasm divides.
• The nuclear membrane and nucleolus begin to
reappear.
Cytokinesis: Plant and animal cell
cytokinesis is different.
• Animal Cell -cleavage furrow A
constricting actin ring forms around the
cell (divides outside –in).
Plant Cell: Cytokinesis
• cell plate forms (divides inside-out)
It is formed from Golgi vesicles
• At the end of cytokinesis there are now
two daughter cells in interphase.
• They are both diploid (2N) as was the
original cell. They are clones of one
another.