Cell Reproduction
Cells are limited in the size the can grow
Recall the surface area:volume agar lab
As cells get bigger, there is less opportunity for exchange
at the membrane
DNA overload would also eventually occur in huge cells,
the limited DNA in the nucleus would be insufficient to
carry out all cell functions
Cells overcome their limits by division called Mitosis
Chromosomes:
Chromosomes carry the genetic material. They are made
of DNA and protein. All cells of an organism have the
same number of chromosomes, and ultimately the same
DNA information. Humans have 23 chromosomes, fruit
flies have 8, etc.
Before cell division chromosomes are copied.
Chromosomes are made of sister chromotids connected
by a centromere.
Centromeres are usually near the middle of a chromosome
Telomeres are at the ends of each chromosome
The cell cycle:
Interphase – the period of time between active divisions
of cells. Cells spend the vast majority of their time in
interphase. It consists of 3 separate phases
G1 – Gap phase 1, cells in this phase do most of
their growing. New proteins and organelles are
formed
S phase – Chromosomes replicate in preparation of
replication
G2 – a short phase where the cell gets ready for
mitosis
The next events take place rather rapidly. In order, they
are Prophase, Metaphase, Anaphase, Telophase, and
cytokinesis.
Mitosis
Prophase
Chromosomes become visible
Centrioles separate and go to opposite sides of the
nucleus
Spindle fibers connect to centromeres of
chromosomes
Nuclear envelope (membrane) breaks down
Metaphase
Chromosomes line up at center of the cell
Anaphase
Centromeres joining sister chromatids separate
Chromosomes move to end of the spindles
Telophase
Chromosomes uncoil and tangle
Nuclear envelopes reform around each set of
chromosomes
Spindles break apart
Cytokinesis
The actual separation of cytoplasm and connection
of cell membrane down the middle
Cell division is complete and the cell enters the G1 phase
of interphase again
Cell Cycle Regulation
Timing of mitosis is regulated by cyclins. Cyclins are
proteins that initiate spindle formation.
There are internal and external regulators of mitosis.
Internal regulators are proteins produced inside the cell.
External regulators are proteins that respond to
circumstances surrounding the cell. These are especially
important in healing and embryonic growth.
Cancer
Cancer is the result of uncontrolled cell growth. Cancer
cells do not respond to the signals regulating growth in
most cells. As a result large masses of such cells form
called tumors. Some cells can break loose and spread
throughout the system.
Meiosis
Chromosome number:
Somatic cells = nonreproductive cells
Gametes = sex cells (sperm/egg)
Somatic cells have 2 copies of each chromosome which
we refer to as 2N or diploid
Gametes have 1 copy of each chromosome which we
refer to as 1N or haploid
In humans, 2N = 46, 1N = 23
If there are more than 2 copies, we refer to that as
polyploidy
A lot of domestic strains of plants are the result of
polyploidy
Meiosis – turn to page 276 in book and follow along
Vocab: homologous the 2 copies of each chromosome are
called homologous
Meiosis I
Interphase 1
Cells duplicate chromosomes
Prohase 1
Each chromosome pairs with its homologous
chromosome to form a tetrad. A tetrad is simply the
four chromatids of the 2 chromosomes.
Metaphase 1
Spindle fibers attach to the chromosomes –
chromosomes line up at the middle
Anaphase 1
Chromosomes are pulled to the poles
Telophase 1/cytokinesis
New nuclear membrane forms, the cell splits
Meiosis II
Prophase II
Chromosomes do not replicate
Centromeres move to poles
Metaphase II
Chromosomes line up in the middle
Anaphase II
Sister chromotids move toward opposite poles
Telophase II/cytokenesis
4 daughter cells are produced
Each daughter cell is haploid