Transcript Cell Growth and Reproduction

Cell Growth and Reproduction
Why Cells Must Divide
• In multi-celled organisms (like humans) cells
specialize for specific functions thus the original
cells must divide to produce different kinds of
cells
• Cells can only take in nutrients and excrete
waste products over the surface of the
membrane that surrounds them. The surface to
volume ratio decreases with the square of the
volume (unless special accommodations are
made)
Cell Reproduction
• Cell division is the process by which new
cells are produced from one cell
• Results in two cells that are identical to the
original, parent cell
• All organisms grow and change; worn=out
tissues are repaired or are replaced by
newly produced cells
Chromosomes
• Just before cell division, several short,
stringy structures suddenly appear in the
nucleus
• Once cell division is complete these
structures seem to vanish
• These structures, which contain DNA and
becomes darkly colored when stained, are
called chromosomes
Chromosomes
• Chromosomes are the carriers of the
genetic material that is copied and passed
from generation to generation of cells
Sister chromatids
Centromere
Eukaryotic Chromosomes
• For most of a cell’s lifetime, chromosomes
exist as chromatin
• Chromatin are long strands of DNA
wrapped around proteins called histones
• Before cell division, chromatin must be
reorganized into chromosomes
Cell Cycle
•
The cell cycle is the sequence of growth
and division of a cell
• Two general periods:
1. Period of growth
2. Period of division
Interphase
• The majority of a cell’s life is spent in the
growth period known as interphase
• During interphase, a cell grows in size and
carries on metabolism
• Chromosomes are duplicated in
preparation for the period of division
Mitosis
• Following interphase, a cell enters its
period of nuclear division called mitosis
• Mitosis is the process by which two
daughter cells are formed, each containing
a complete set of chromosomes
• Following mitosis, the cytoplasm divides,
separating the two cells
Cell Cycle
Phases of Mitosis
1.
2.
3.
4.
Prophase
Metaphase
Anaphase
Telophase
Prophase
• During prophase, 1st and longest phase,
the long, stringy chromatin coils up into
visible chromosomes
• Each duplicated chromosome is made up
of two halves, sister chromatid, they are
connected by a centromere
• As prophase continues, the nucleus
begins to disappear as the nuclear
envelope and the nucleolus disintegrate
Prophase
• By late prophase, these structures are
completely absent (nuclear envelope and
nucleolus)
• In animal cells, two important pairs of structures,
the centrioles, begin to migrate to opposite ends
of the cell
• The second structure that forms is the spindle,
which is a football-shaped, cagelike structure
• In plant cells the spindle forms without the
centrioles
Mother cell
Stages Of Mitosis
Interphase
Disappearing
Prophase
nuclear
membrane
Mitotic
spindle
Metaphase
Anaphase
Telophase
Metaphase
• During metaphase, the doubled
chromosomes become attached to the
spindle fibers by their centromeres
• The chromosomes are pulled by the
spindle fibers and begin to line up on the
midline, or equator, of the spindle
• Each sister chomatid is attached to it’s
own spindle fiber
Mother cell
Stages Of Mitosis
Interphase
Disappearing
Prophase
nuclear
membrane
Mitotic
spindle
Metaphase
Anaphase
Telophase
Anaphase
• The separation of sister chromatids marks
the beginning of anaphase
• During anaphase, the centromeres split
apart and chromatid pairs from each
chromosome separates from each other
• The chromatids are pulled apart by the
shortening of the spindle fibers
Mother cell
Stages Of Mitosis
Interphase
Disappearing
Prophase
nuclear
membrane
Mitotic
spindle
Metaphase
Anaphase
Telophase
Telophase
• Final phase of mitosis, begins as the
chromatids reach the opposite poles of the
cell
• Many of the changes that occurred during
prophase are reversed as the new cells
prepare for their own independent
existence
Telophase
• The chromosomes, which had been tightly
coiled since the end of prophase, now unwind so
they can begin to direct the metabolic activities
of the new cell
• The spindle begins to break down, the nucleolus
reappears, and an new nuclear envelope forms
around each set of chromosomes
• A new double membrane (plasma membrane0
begins to form between the two new nuclei
Mother cell
Stages Of Mitosis
Interphase
Disappearing
Prophase
nuclear
membrane
Mitotic
spindle
Metaphase
Anaphase
Telophase
Cytokinesis
• Following telophase, the cell’s cytoplasm
divides in a process called cytokinesis
• Cytokinesis differs between plants and
animals
• In animals, the plasma membrane pinches
in along the equator
• In plants, they form a cell plate that is laid
across the equator, and a cell wall will
form along this plate
Results of Mitosis
• Mitosis is a process that guarantees
genetic continuity
• The new daughter cells produced will carry
out the same function as the parent or
original cell