Chapter 12: The Cell Cycle (How do cells divide?)

Chapter 12: The Cell Cycle (How do cells divide?) • cell division in prokaryotes • chromosomes • eukaryotic cell cycle • mitosis • cytokinesis

Prokaryotes binary fission generation time as short as 20 minutes

Define: • chromosome • chromatin • gene • genome • karyotype • Describe the human genome and karyotype in terms of: • number of basepairs • number of genes • number of chromosomes

Eukaryotic DNA in chromosomes • chromatin: long DNA molecule with associated proteins • chromosomes: densely packaged chromatin • during cell division • protects the DNA • sets up DNA distribution

each chromosome contains hundreds to thousands of genes • functional units of heredity • typically instructions for a protein or RNA • genome – organism’s complete DNA sequence • humans apparently have ~25,000 genes in the now-sequenced human genome

chromosomes • each species has a characteristic number of chromosomes • number varies between species • does not reflect complexity • humans: 46 • some ferns: 1000+ • chromosomes for an individual is the karyotype

chromosomes • function: chromosomes carry the genetic information of a cell • from one cell generation to the next • from one organism to its offspring

Define: • chromosome • chromatin • gene • genome • karyotype • Describe the human genome and karyotype in terms of: • number of basepairs • number of genes • number of chromosomes

Draw a circle diagram of the eukaryotic cell cycle. Label all phases. • Discuss what goes on in each of the phases on the diagram. Note where checkpoints exist. Also, discuss G0, and discuss cell cycle regulation in general terms.

Eukaryotic Cell Cycle • when cells reach a certain size, growth either stops or the cell must divide • cell division is generally a highly regulated process (not all will divide!) • the generation time for eukaryotic cells varies widely, but is usually 8-20 hours

Eukaryotic Cell Cycle

Eukaryotic Cell Cycle cyclins and cyclin-dependent protein kinases (Cdks) cytokinins; growth factors; suppressors; cancer cells

Draw a circle diagram of the eukaryotic cell cycle. Label all phases. • Discuss what goes on in each of the phases on the diagram. Note where checkpoints exist. Also, discuss G0, and discuss cell cycle regulation in general terms.

Describe what “PMAT” means. • With a partner, do the “chromosome dance” for mitosis. Make sure that you distinguish between chromosomes and chromatids, and note at each stage the number of sister chromatids per chromosome. • Discuss what happens in each stage of mitosis.

Mitosis 4 stages: prophase metaphase anaphase telophase (PMAT) *

Mitosis: prophase • chromatin condenses to form chromosomes • each chromosome (duplicated during S phase) forms a pair of sister chromatids • sister chromatids are joined at a centromere by protein tethers • centromeres contain a kinetochore where microtubules will bind • each sister chromatid has its own kinetochore • sister chromatids become attached by their kinetochores to microtubules from opposite poles

Mitosis: prophase • a system of microtubules, called the mitotic spindle, organizes between the two poles (opposite ends) of the cell • each pole has a microtubule organizing center (MTOC) • in animals and some other eukaryotes, centrioles are found in the MTOC

Mitosis: prophase • by the end of prophase: • the nuclear membrane has disappeared (actually divided into many small vesicles) • nucleoli have disintegrated • the sister chromatids are attached by their kinetochores to microtubules from opposite poles

Mitosis: prophase • some call the later part of prophase prometaphase, usually defined to include vesicularization of the nuclear membrane and attachment of kinetochores to microtubules • in some eukaryotes the nuclear membrane never vesicularizes

Mitosis: metaphase • chromosomes line up along the midplane of the cell (the metaphase plate) • chromosomes are highly condensed • the mitotic spindle, now complete, has two types of microtubules • kinetochore microtubules extend from a pole to a kinetochore • polar microtubules extend from a pole to the midplane area, often overlapping with polar microtubules from the other pole • the mitosis checkpoint appears to be here; progress past metaphase is typically prevented until the kinetochores are all attached to microtubules

Mitosis: anaphase • sister chromatids separate and are moved toward opposite poles • the protein tethers at the centromere between the chromatids are broken • each former sister chromatid can now be called a chromosome

Mitosis: anaphase • model for the mechanism that moves chromosomes to the poles • motor proteins move the chromosomes towards the poles along the kinetochores microtubules • kinetochore microtubules shorten behind the moving chromosomes • polar microtubules lengthen the entire spindle • motor proteins on the polar microtubules slide them past each other, pushing them apart (the microtubules may grow a bit, too) • this pushes the MTOCs away from each other, and thus has the effect of pushing kinetochore microtubules from opposite poles away from each other *

Mitosis: anaphase • overall, this process assures that each daughter cell will receive one of the duplicate sets of genetic material carried by the chromosomes

Mitosis: telophase • prophase is essentially reversed • the mitotic spindle is disintegrated • the chromosomes decondense • nuclear membranes reform around the genetic material to form two nuclei • each has an identical copy of the genetic information • nucleoli reappear, and interphase cellular functions resume *

Describe what “PMAT” means.

With a partner, do the “chromosome dance” for mitosis. Make sure that you distinguish between chromosomes and chromatids, and note at each stage the number of sister chromatids per chromosome. • Discuss what happens in each stage of mitosis.

Describe cytokinesis in both plant cells and animal cells, noting the differences. • Describe what is meant by the term “polar division” and why this process was (and still is) important in your development.

cytokinesis • divides the cell into two daughter cells • cytokinesis usually begins in telophase and ends shortly thereafter

cytokinesis • in animals, a cleavage furrow develops • usually close to where the metaphase plate was • a microfilament (actin) ring contracts due to interactions with myosin molecules, forming a deepening furrow • eventually, the ring closes enough for spontaneous separation of the plasma membrane, resulting in two separate cells *

cytokinesis • in plants, a cell plate develops • usually close to where the metaphase plate was • vesicles that originate from the Golgi line up in the equatorial region • the vesicles fuse and add more vesicles • grow outward until reaching the plasma membrane and thus separating the cells • the vesicles contain materials for making the primary cell wall and a middle lamella *

cytokinesis • cytoplasm (and with it most organelles) is usually distributed randomly but roughly equally between daughter cells • sometimes cell division is a highly regulated polar division that purposefully distributes some materialsunequally

Describe cytokinesis in both plant cells and animal cells, noting the differences. • Describe what is meant by the term “polar division” and why this process was (and still is) important in your development.

In the following slide, look for examples of interphase, prophase, metaphase, anaphase, and telophase/cytokinesis.

Chapter 12: The Cell Cycle (How do cells divide?)