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

CytokinesisCleavage

furrow Contracting ring ofmicrofilaments

Daughtercells

Cleavagefurrow

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.

Independent orientation at metaphase I

Independent orientation of chromosomes in meiosis and random fertilization lead to

varied offspring