Chapter 12: the Cell Cycle (How Do Cells Divide?)

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

 cell division in prokaryotes

 chromosomes

 eukaryotic cell cycle

 mitosis

 cytokinesis

• 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

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.

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

 cell division in prokaryotes

 chromosomes

 eukaryotic cell cycle

 mitosis

 cytokinesis

. • 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.

• Discuss what happens in each stage of mitosis.

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

 cell division in prokaryotes

 chromosomes

 eukaryotic cell cycle

 mitosis

 cytokinesis

• 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 materials unequally

• 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.

. *