Cell Cycle, Mitosis and Meiosis
Covered in these slides, in the Concepts & Connections book- Chapter 8 to page 143In Raven and Johnson Book- Chapter 10 pages192-204
And in your packet on The Cell Cycle
The cell cycle is an ordered sequence of events that extends
– from the time a cell is first formed from a dividing parent cell
– until its own division.
The cell cycle multiplies cells
© 2012 Pearson Education, Inc.
During cytokinesis, the cytoplasm is divided into separate cells.
The process of cytokinesis differs in animal and plant cells.
Cell division is a continuum of dynamic changes
The cells within an organism’s body divide and develop at different rates.
Cell division is influenced externally by
– the presence of essential nutrients,
– growth factors, proteins that stimulate division, there are over 50 different growth factors which work for one or more cell type
– density-dependent inhibition, in which crowded cells stop dividing,
– anchorage dependence, the need for cells to be in contact with a solid surface to divide.
Anchorage, cell density, and chemical growthfactors affect cell division
The cell cycle control system is a cycling set of molecules in the cell that
– triggers and
– coordinates key events in the cell cycle.
Checkpoints in the cell cycle can
– stop an event or
– signal an event to proceed.
Growth factors signal the cell cycle control system
There are three major checkpoints in the cell cycle.
Growth factors signal the cell cycle control system
G1- commitment to divide, growth factors present?, Size of cell ok?,
G2- check for proper DNA replication
M- all chromosomes attached to spindle fibers
Cell Cycle progresses by action of Cdks
Cyclinsproteins produced by the cell during cell division
Cyclin-dependent kinases (Cdk)cyclin is required to activate these enzymesactivates cell proteins by phosphorylating them
(proteins needed for S phase)needed to go through G1 checkpoint
MPFMaturation-promoting factor (mitosis promoting factor)aka Mitosis- promoting factor is a cyclin-Cdk complexphosphorylates proteins needed for mitosisneeded to go through G2 checkpoint
Rate of Cell Division
• Differs from one cell type to the next– Examples:
• red bone marrow cells divide every 12 hours to replace RBCs that wear out
• Cells at tip of root divide about every 19 hours.• Neurons (nerve cells) normally never divide again once brain is fully
formed in utero
• Control of Division, lost = CANCER – Cancer is different depending on the tissue affected– Common theme is lack of control over cell division– Abnormal, uncontrolled cell division– Mutation in genes (including p53) that target and control
abnormal cells.– Abnormal cells impede functioning of normal cells
p53 gene ( tumor suppressor gene)
• Key role in G1 checkpoint• P53 protein monitors DNA• Found absent or damaged in most cancer cells
Cancer is failure of cell cycle control
• Tumor suppressor genes- prevents the development of mutated cells, prevents cancer/tumors
• Oncogenes- cancer causing genes
• Proto-oncogenes- normal genes that become mutated
Meiosis
• Production/formation of __________
• Basis of sexual reproduction• Only germ cells undergo meiosis
Haploid gametes (n 23)
Egg cell
Sperm cell
Fertilization
n
n
Meiosis
Ovary Testis
Diploidzygote(2n 46)
2n
MitosisKey
Haploid stage (n)Diploid stage (2n)
Multicellular diploidadults (2n 46)
A pair ofhomologouschromosomesin a diploidparent cell
A pair ofduplicatedhomologouschromosomes
Sisterchromatids
1 2 3
INTERPHASE MEIOSIS I MEIOSIS II
How meiosis halves chromosome number…
Centrosomes(with centriolepairs) Centrioles
Sites of crossing over
Spindle
Tetrad
Nuclearenvelope
Chromatin Sisterchromatids Fragments
of thenuclearenvelope
Centromere(with akinetochore)
Spindle microtubulesattached to a kinetochore
Metaphaseplate Homologous
chromosomesseparate
Sister chromatidsremain attached
Chromosomes duplicate Prophase I Metaphase I Anaphase IINTERPHASE:
MEIOSIS I: Homologous chromosomes separate
Prophase II Metaphase II Anaphase II
MEIOSIS II: Sister chromatids separate
Sister chromatidsseparate
Haploid daughtercells forming
Telophase IIand Cytokinesis
Meiosis Leads to Genetic Diversity
Three ways genetic diversity is increased by meiosis:
1. 2 parents contribute ½ of the genetic material to offspring
2. Crossing-over in Prophase I
3. Chromosome Alignment in Metaphase I
Meiosis produces cells that are NOT identical, unique gametes
Tetrad(pair of homologouschromosomes in synapsis)
Breakage of homologous chromatids
Joining of homologous chromatids
Chiasma
Separation of homologouschromosomes at anaphase I
Separation of chromatids atanaphase II andcompletion of meiosis
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosomeGametes of four genetic types
1
2
3
4
C
c e
E
C
c e
E
c e
C E
C e
e
C E
c
c E
C E
C e
c E
ec
Crossing Overincreases genetic diversity by producing “new” chromosomes.