Download 2/23/10 Cell division is the process by which cells reproduce

2/23/10
 Cell division is the process by which cells reproduce themselves
 Is the process of a cell called a mother cell dividing into two daughter cells
 Reasons for Cell Division
 Cell too big (cell becomes inefficient and so they need to divide)
 Growth
 Repair and replacement of damaged parts
 Nerve cells do not reproduce
 Reproduction of species
 Asexual reproduction occurs in one-celled organisms
 Binary fission is a form of cell division used by prokaryotic cells
 Mitosis is the most common form of eukaryote division
 G1 means 1st Gap (Growth);
o cell doing its “everyday job”
o Cell grows
o Receives signal to divide
 S = DNA Synthesis
o Copies chromosomes
 G2 prepares for mitosis
 Mitosis: nuclear division (karyokinesis)
 Interphase: between phases; stage between division of cell and nucleus
o Cell spends most of its time in this phase
o Grows
o DNA is copied
o Prepares for cell division
o Chromosomes are not visible
 Prophase
o Chromosomes appear from chromatin
o Nuclear envelope and nucleolus disappear
o Centrioles move toward opposite poles
 Metaphase
o Shortest phase
 Anaphase
o Sister chromatids split apart at centromere
o Move toward opposite poles
 Telophase
o Chromosomes begin to untangle and form chromatin
o Nuclear envelope reforms, nucleolus reappears
o Spindles break apart
o Cell begins to squeeze in the middle
 Cytokinesis
o Complete division of the cytoplasm forming two new cells
o Differs in plant and animal cells
 Cytokinesis in plants
o Cell plate forms
o Vesicles line up at equator
 Derived from Golgi
3/1/10





3/2/10










3/4/10





3/5/10












3/22/10

Gap 1: Cells increase in size, produce RNA and synthesize proteins. An important cell cycle
control mechanism activated during this period (G1 Checkpoint) ensures that everything is ready
for DNA synthesis
Gap 0: There are times when a cell will leave the cycle and quit dividing. This may be a
temporary resting period i.e. liver cell, or more permanent, i.e. a cell that has reached an end stage
of development and will no longer divide (e.g. nerve cells in the brain).
Gap 2: The cell will continue to grow and produce new proteins required for cell division. At the
end of this gap is another control checkpoint (G2 checkpoint) to determine if the cell can now
proceed to enter M (mitosis) and divide
Synthesis phase: DNA replication takes place
Interphase = G1 + G2 + S
DNA must be copied or replicated before cell division
Each new cell will then have an identical copy of the DNA: semi-conservative replication
Half strand from parent strand, half new strand
Meiosis
Has two phases: meiosis I, meiosis II
Begins with two sets of chromosomes, just like mitosis
Meiosis I: called reduction- division
Instead of two daughter cells, there are four
Produces four haploid gametes rather than two diploid cells (they’re not identical)
Zygote: fertilized egg (combined egg and sperm)
Is the fundamental basis of sexual reproduction
Replicated copies are called sister chromatids
Held together at centromere
Replicated chromosome is X shape
Meiosis reduces genetic content
In late prophase, crossing-over happens (chromosomes condense)
Tetrad: four chromatids; happen in prophase I for crossing-over
Where chromosomes overlap: chiasmata
Chromosomes twist around each other and end up with each other’s genetic material
Oogenesis: 3 polar bodies that die and 1 egg
Polar bodies die because of uneven sharing of cytoplasm
Immature egg called oocyte
Oogonium: egg ready to divide
Spermatogenesis: produce 4 sperm
Spermetagonium
Mitosis: number of divisions 1; number of daughter cells 2; genetically identical? Yes;
chromosome # same as parent; where? Somatic cells; When? Throughout life’ Role: growth and
repair
Meiosis: number of divisions 2; number of daughter cells; 4: genetically identical: no;
chromosome number: half of parent: where: germ cells: when? At sexual maturity; role: sexual
reproduction