- Describe how chromosome number
changes throughout the human life cycle
- Compare cytokinesis
in animals and plants
- Describe the roles of checkpoints,
cyclin, Cdk, and MPF
in the cell-cycle control system
- Describe the process of synapsis during prophase I and explain how genetic
recombination occurs
- List sources of genetic variation
Mitosis
and Meiosis.
Ch 12 &
13
Chromosome--The term was proposed for the
individual threads within a cell nucleus (gk.
chroma, colour; soma,
body). The self-replicating genetic structures of cells containing the cellular
DNA that bears in its nucleotide sequence the linear array of genes. In
prokaryotes, chromosomal DNA is circular, and the entire genome is carried on
one chromosome. Eukaryotic genomes consist of a number of chromosomes whose DNA
is associated with different kinds of proteins.
Genome-All the genetic material in the chromosomes
of a particular organism; its size is generally given as its total number of
base pairs
Mitosis-The most frequent
process of nuclear division (karyokinesis) in cells
that produces daughter cells that are genetically identical to each other and
to the parent cell. The mitosis is divided into four (or five) phases: prophase, prometaphase, metaphase, anaphase, telophase. Mitosis and interphase
make the cell cylcle.
Diploid A full set of
genetic material, consisting of paired chromosomes one chromosome from each
parental set. Most animal cells except the gamete have a diploid set of
chromosomes. The diploid human genome has 46 chromosomes
Polyploid Having three or more (Gk. polys,
many) sets of homologous chromosomes
Bacteria DNA-1 chromosome splits, attaches to
membrane and pulls apart.
What is MitosisTHE
PROCESS BY WHICH TWO NEW NUCLEII ARE FORMED, PRECEDING CELL DIVISION
1 chromosome (duplicated) splits into two
identical chromosomes
DNA Replicating and thus the chromosome
replicating from 1 strand into 2 identical strands…A goes wit T and C goes with
G
INTERPHASE is the
normal lifetime of a
cell, after being
“born” by division,
and before it divides
itself.
Prokaryotes-chromosomes move apart to opposite
ends of cell..maybe attached
to cell membrane. Dinoflagellates-nuclear
envelope remains intact, chromosomes attach to envelope, microtubules
pass through nucleus and pull it apart.
Diatoms the envelope remains intact but
microtubules form a spindle inside nucleus which separate
the chromosomes and the nucleus splits into 2.
Eukaryotes, spindle forms outside of nucleus and
envelope breaks down during mitosis, reforming afterwards.
Meiosis is a specialized type of cell division
that occurs in the formation of gametes such as egg and sperm. Although meiosis
appears much more complicated than mitosis, it is really just two divisions
in sequence, each one of which has strong similarities to mitosis.
MEIOSIS IS THE PROCESS
OF CUTTING BY HALF THE
NUMBER OF CHROMOSOMES
FOUND IN GAMETES.
homologous chromosomes
Meaning: chromosomes, that are similar in size and
genetic constitution; one member of each pair of homologous chromosomes comes
from the mother and the other from the father.
Every cell
has pairs
of chromo-
somes
(except
gametes).
1. What is meiosis ?
2. Explain why meiosis is necessary.
3. Name the only 2 kinds of human cells that do
NOT have a full 2N set.
4. What is meiosis in females called
? males ?
5. Where does meiosis in females occur ?
6. Where does meiosis in males occur
?
7. Give the mathematical formula for fertilization
using chromosome numbers.8. What would to chromosome numbers after a few generations
without meiosis ?
The Cell
Cycle Control
The timing
and rate of cell division differ between different organisms and also between
different cells of an organism.
Compare skin
cells with muscle or nerve cells.
What is
controlling the rate of cell division, how cells "know" that it is
time to divide? Why cancer cells do not stop dividing?
The cell
cycle is regulated at the molecular level
Accumulating
experimental evidences appearing since early seventies suggested that the cell
cycle is driven by specific chemical signals present in the cytoplasm.
Most of the
experiments were conducted with cell cultures.
Many types of
animal and plant cells can be removed from an organism and cultured in an
artificial environment.
Cell cycle is
controlled
Cultured
mammalian cells can be induced to fuse, forming a single cell with two nuclei.
The results
of fusing cells at two different phases of the cell cycle suggested that
particular chemicals control the progression of phases.
For example, when
a cell in M phase was fused with one in any other phase, the nucleus from the
latter cell immediately began mitosis.
If the second
cell was in Gl, the condensed chromosomes that
appeared had single chromatids.
Short summary
of interphase
·
Comprised by three (four) subphases: G1, (GO), S and
G2.
• Common feature of all (except GO) subphases: growth of the cell by producing proteins and _ cytoplasmic organelles.
·
G 1- "first gap"
·
S phase - chromosome duplication
G2 -
"second gap", DNA repair
Cell cycle control
A checkpoint in the cell is a critical control
point where stop and go signals can regulate the cycle
Animal cells have built-in "stop"
signals that halt the cell cycle at checkpoints until overridden by
"go" signals
The most important decision to make is: to
continue the cell division after the exit from M phase or not.
Cells that do not receive the "go"
signal at the G 1 checkpoint, switch into a nondividing
state called the GO phase.
A good example of these
cells are
liver cells.
They can be called back to the cell cycle by
growth factors released during injury.
Cyclins
and cyclin-dependent kinases
(Cdks)
Regulatory
molecules of the cycle transition are proteins of two main types: protein kinases and cyclins.
Protein
kinases are proteins that regulate the activity of
the others by phosphorylating them. "Go"
signal at the G 1 and G2 checkpoints is regulated by
particular protein kinases.
To be active,
such a kinase must be attached to a cyclin, a protein that gets its name from its cyclically
fluctuating concentration in the cell .
This kinases are called cyclin-dependent
kinases - Cdks
Cdk-cyclin
complex called MPF, acts at the G2 checkpoint to trigger mitosis.
•
The graph shows how MPF activity
fluctuates with the level of cyclin in the cell. The cyclin level rises throughout interphase
(G 1, S, and G2 phases), then falls abruptly during mitosis (M phase). The Cdk itself is present at a constant level.
(b)1. By the
G2 checkpoint (red bar), enough cyclin is available
to produce many molecules of MPF.
2. MPF
promotes mitosis by phosphorylating various proteins,
including other enzymes.
3. One effect
of MPF is the initiation of a sequence of events leading to the breakdown of
its own
cyclin.
4. The Cdk component of MPF is recycled
•