slide 3 Flashcards
cell division
The ability of organisms to produce more of their own
kind distinguishes living things from nonliving matter.
The continuity of life is based on the reproduction of
cells, or cell division
In unicellular organisms,
division of one cell
reproduces the entire organism.
Multicellular organisms depend on cell division for
- Development from a fertilized cell
- Growth
- Repair
cell cycle
the
life of a cell from formation to its own division.
Cell division
is a controlled process resulting in
genetically identical daughter cells
Cells duplicate their genetic material
before
they divide, ensuring that each daughter cell
receives an exact copy of the genetic code
genome
All the DNA in a cell constitutes the cell’s genome
A genome can consist of
a single DNA molecule
(common in prokaryotic cells) or a number of DNA
molecules (common in eukaryotic cells)
chromosomes.
DNA molecules in a cell are packaged into
chromosomes.
chromatin
The combined DNA and protein complex is called chromatin
Every eukaryotic species has a characteristic number of
chromosomes. Chromosomes come in pairs with one of
each pair inherited from each parent
Humans have 23 pairs
= 46 chromosomes
sister chromatids.
During cell replication each chromosome is duplicated. It is
connected to its copy. These connected copies of the
chromosome are called sister chromatids
The centromere
is the narrow “waist” of the
duplicated chromosome, where the two
chromatids are most closely attached
The two sister chromatids separate and move into
two nuclei before the cell divides into two
Once separate, the chromatids are chromosomes
Eukaryotes have two types of cells
Somatic cells
and gametes
Somatic cells
– typical cells of the body, contain the
full genome, the full set of chromosomes
Gametes –
reproductive cells including sperm and
eggs, contain half the chromosomes in somatic cells
meiosis.
Gametes (sperm and eggs) are produced by a
variation of cell division called meiosis
Meiosis yields
nonidentical daughter cells that have
only one set of chromosomes, half as many as the
parent cell.
The cell cycle of somatic cells consists of
- interphase
- mitotic (M) phase
Interphase
(cell growth and copying of
chromosomes in preparation for cell division)
Mitotic (M) phase
(mitosis and cytokinesis)
interphase (about 90% of the cell cycle) can be
divided into subphases
– G1 phase (“first gap”)
– S phase (“synthesis”)
– G2 phase (“second gap”)
The cell grows during all three phases, but
chromosomes are duplicated only during the S phase.
interphase (about 90% of the cell cycle) can be
divided into subphases
– G1 phase (“first gap”)
– S phase (“synthesis”)
– G2 phase (“second gap”)
The cell grows during all three phases, but
chromosomes are duplicated only during the S phase.
Mitosis
the division of duplicated chromosomes
Mitosis is conventionally separated into five phases
– Prophase – Prometaphase – Metaphase – Anaphase – Telophase ***Cytokinesis overlaps the latter stages of mitosis.
G2 of interphase
Chromosomes have been
duplicated but aren’t condensed. Two centrosomes
have formed that will organize the mitotic spindle.
The mitotic spindle is
a structure made of microtubules that controls chromosome movement during mitosis.
An aster
(a radial array of
short microtubules) extends
from each centrosome in
the spindle
Prophase:
Chromatin is condensed into discrete
duplicated chromosomes that appear as pairs of
identical sister chromatids joined at their centromeres.
Prometaphase:
Nuclear envelope fragments. Each
of the two sister chromatids of each duplicated
chromosome has a kinetichore protein to which the
microtubules of the spindle attach.
Metaphase
The chromosomes are aligned at the
metaphase plate, a plane equal distance from the
two centrosomes.
Anaphase
Sister chromatids of each pair separate
and move towards opposite centrosomes. The cell
elongates as the microtubules not attached to
kinetichores lengthen
Telephase
Two daughter nuclei form in the cell. The
chromosomes become less condensed. Spindle
microtubules are depolymerized.
cleavage
In animal cells, cytokinesis occurs by a process
known as cleavage, forming a cleavage furrow
In plant cells, a cell plate
forms during cytokinesis
binary fission.
Prokaryotes (bacteria and archaea) reproduce by
a type of cell division called binary fission
In binary fission, the chromosome replicates
(beginning at an origin of replication),
and the
two daughter chromosomes actively move apart.
The plasma membrane pinches inward, dividing
the cell into two
Regulation of the Cell Cycle
The frequency of cell division varies with the type of cell. These differences result from regulation at the molecular level. Cancer cells manage to escape the usual controls on the cell cycle.
cell cycle control system
The sequential events of the cell cycle are directed
by a distinct cell cycle control system, which is
similar to a clock.
The cell cycle control system is regulated by both
internal and external controls.
The clock has specific checkpoints where the cell
cycle stops until a go-ahead signal is received
Regulation of the Cell Cycle
For many cells, the G1 checkpoint seems to be
the most important.
If a cell receives a go-ahead signal at the G1
checkpoint, it will usually complete the S, G2
,
and M phases and divide
G0 phase.
If the cell does not receive the go-ahead signal, it
will exit the cycle, switching into a nondividing
state called the G0 phase
An example of an internal signal is that kinetochores
not attached to spindle microtubules send a
molecular signal that delays anaphase
External signals include growth factors, proteins
released by cells that stimulate other cells to divide
Cancer cells do not respond normally to the body’s
control mechanisms
Cancer cells may not need growth factors to grow
and divide, rather they .
– may make their own growth factor
– may convey a growth factor’s signal without
the presence of the growth factor
– may have abnormal cell cycle control
