9 Announcements

• Chapter 8 Online Quiz Deadline: Tonight

• Highest exam grade thus far: 144/150 (96%)

• Have a wonderful Fall Break!

9Chromosomes, the Cell Cycle,

and Cell Division

9 Chromosomes, the Cell Cycle, and Cell Division

• Systems of Cell Reproduction

• Interphase and the Control of Cell Division

• Eukaryotic Chromosomes

• Mitosis: Distributing Exact Copies of Genetic Information

• Cytokinesis: The Division of the Cytoplasm

• Reproduction: Asexual and Sexual

• Meiosis: A Pair of Nuclear Divisions

• Meiotic Errors

• Cell Death

9 Henrietta Lacks & HeLa Cells

Henrietta Lacks (August 18, 1920 – October 4, 1951) was the involuntary donor of cells from her cancerous tumor, which were cultured by George Otto Gey to create an immortal cell line for medical research. This is now known as the HeLa cell line. Henrietta LacksPhoto and Text:

http://en.wikipedia.org/wiki/Henrietta_Lacks

“Over the past half-century, tens of thousands of research articles have been published using information obtained from Henrietta’s cells.” -Purves et al. (textbook)

9 HeLa Cells

“Cells descended from those Hopkins researchers took from Lacks' tumor before her death have been used to

seek cures for polio, cancer, and AIDS.” www.citypaper.com/news/story.asp?id=3426

9 Systems of Cell Reproduction• All living cells (even HeLa cells) are mortal, and

therefore cell reproduction is universal among living organisms.

• Unicellular organisms use cell division primarily to reproduce (b), whereas in multicellular organisms, such as ourselves, cell division also plays important roles in growth (a) and in the repair of tissues (c).

9 Systems of Cell Reproduction

In order for any cell to divide, four events must occur:

A signal to reproduce must be received.

Replication of DNA and vital cell components must occur.

DNA must be distributed to the new cells.

The cell membrane or cell wall must separate the two new cells.

9 Systems of Cell Reproduction

• Prokaryotes divide by fission.

Most prokaryotes have one circular chromosome.

As DNA replicates, each of the two resulting DNA molecules attaches to the plasma membrane.

As the cell grows, new plasma membrane is added between the attachment points, and the DNA molecules are moved apart.

Cytokinesis separates the one cell into two, each with a complete chromosome.

9 The Bacterial Chromosome

A bit about the bacterial chromosome

A single, circular chromosome

If in a circle, the circumference would be approximately 1.6 mm.

E. coli is ~ 4 um long. It’s interesting to know that

mitochondria and chloroplasts in eukaryotes also have circular chromosomes.

http://www.colorado.edu/MCDB/MCDB1111/intrographics/bacterial%20chromosome.jpg

- Purves et al.

Figure 9.2 Prokaryotic Cell Division

Binary Fission

9 Bacterial Reproduction

• Escherichia coli and Bacillus anthracis are prokaryotes. E. coli has the potential to divide every 20 minutes. Studies using Bacillus indicate that external factors (e.g., food supply) control the initiation of cell division. -Purves et al.

http://www.astrographics.com/GalleryPrints/Display/GP2144.jpg http://biochem.ultraevil.com/bio/Images/bioloskoorozje/anthrax/BacillusAnthrax.jpg

9 Systems of Cell Reproduction

• Eukaryotic cells divide by mitosis or meiosis.

• Eukaryotes usually have many chromosomes (e.g., humans have a diploid number of 46).

• Eukaryotes have a nucleus, which must replicate and, with few exceptions, divide during cell division.

9 Systems of Cell Reproduction

• The reproduction of eukaryotic cells is typically characterized by three steps:

The replication of the DNA within the nucleus

The packaging and segregation of the replicated DNA into two new nuclei (nuclear division)

The division of the cytoplasm (cytokinesis)

http://www.medicalprogress.org/benefits/breastcan/needs.cfm

9 Interphase and the Control of Cell Division

• The cell cycle has two phases: mitosis and interphase.

• A typical eukaryotic cell will spend most of its life in interphase, the period between divisions of the cytoplasm.

• Some cells, such as human RBCs, nerve and muscle cells, lose their ability to divide altogether and stay in interphase indefinitely, while other cells divide regularly or occasionally.

http://www.astrographics.com/GalleryPrints/Display/GP2120.jpg

Figure 9.3 The Eukaryotic Cell Cycle

Figure 9.4 Cyclin-Dependent Kinases and Cyclins Trigger Transisions in the Cell Cycle

9 Interphase and the Control of Cell Division

• Transitions from G1 to S and G2 to M depend on activation of a protein called cyclin-dependent kinase, or Cdk.

• A kinase catalyzes phosphorylation of a protein.

• Cdk is activated by binding to a second type of protein called cyclin.

• Several different cyclins exist, which, when bound to Cdk, phosphorylate different target proteins.

• Cyclin-Cdk complexes act as checkpoints. When functioning properly, they allow or prevent the passage to the next cell cycle stage, depending on the extra- and intracellular conditions.

• In cancer cells, these cyclin-Cdk controls are often disrupted.

9 Eukaryotic Chromosomes

• The basic unit of the eukaryotic chromosome is a gigantic, linear, double-stranded molecule of DNA complexed with many proteins to form a dense material called chromatin.

• After the DNA of a chromosome replicates during S phase, each chromosome consists of two joined chromatids.

9 Eukaryotic Chromosomes

• Interphase chromosomes are wrapped around proteins called histones.

• These wraps of DNA and histone proteins are called nucleosomes and resemble beads on a string.

• The core of a nucleosome contains eight histone molecules, two each from four of the histone classes.

• One molecule from the remaining histone class, histone H1, clamps the DNA to the core.

• During mitosis and meiosis, the chromatin becomes even more coiled and condensed.

Figure 9.6 DNA Packs into a Mitotic Chromosome

9 Mitosis: Distributing Exact Copies of Genetic Information

• Centrosomes are regions where microtubules form. Microtubules move the chromosomes.

• When the cell enters S phase and DNA is replicated, the centrosome replicates to form two centrosomes.

• During G2-to-M transition, the two centrosomes separate from each other and move to opposite ends of the nuclear envelope.

• The orientation of the centrosomes determines the cell’s plane of division.

9 Mitosis – Layered Figure

• Layered Figure: Mitosis [Figure 09.08]

9 Mitosis: Distributing Exact Copies of Genetic Information

Some notes that may be helpful:

• Prophase marks the beginning of mitosis.

Chromosomes compact and coil, becoming more dense and visible.

Polar microtubules form between the two centrosomes and make up the developing spindle.

The mitotic spindle serves as a “railroad track” along which chromosomes will move later in mitosis.

Late in prophase, the kinetochores develop in the region around the centromere and are the sites where microtubules attach to the chromatids.

Figure 9.8 Mitosis (Part 1)

9 Mitosis: Distributing Exact Copies of Genetic Information

• During prometaphase, the nuclear lamina disintegrates and the nuclear envelope breaks into small vesicles, permitting the fibers of the spindle to “invade” the nuclear region.

9 Mitosis: Distributing Exact Copies of Genetic Information

• During metaphase, the kinetochores arrive at the equatorial plate.

Chromosomes are fully condensed and have distinguishable shapes.

Cohesins break down.

DNA topoisomerase II unravels the interconnected DNA molecules at the centromere, and all the chromatids separate simultaneously.

Figure 9.8 Mitosis (Part 2)

9 Mitosis: Distributing Exact Copies of Genetic Information

• Anaphase begins when the centromeres separate.

Molecular motors at the kinetochores move the chromosomes toward the poles, accounting for about 75 percent of the motion.

About 25 percent of the motion comes from shortening of the microtubules at the poles.

Additional distance is gained by the separating of the mitotic centers.

9 Mitosis: Distributing Exact Copies of Genetic Information

• Telophase begins when the chromosomes finish moving.

Nuclear envelopes and nucleoli coalesce and re-form.

9 Cytokinesis: The Division of the Cytoplasm

• Animal cells divide by a furrowing (a “pinching in” or constriction) of the plasma membrane.

• Microfilaments of actin and the motor protein filament myosin first form a ring beneath the plasma membrane.

• Actin and myosin contract to produce the constriction.

9 Cytokinesis: The Division of the Cytoplasm

• Plants have cell walls and the cytoplasm divides differently.

• After the spindle breaks down, vesicles from the Golgi apparatus appear in the equatorial region.

• The vesicles fuse to form a new plasma membrane, and the contents of the vesicles combine to form the cell plate, which is the beginning of the new cell wall.

Figure 9.10 Cytokinesis Differs in Animal and Plant Cells

9 Until next time…

Have an enjoyable Fall Break!

9 Announcements

• Welcome back!• Just 60% of the semester and 70% of your

grade remaining.• Graded exams will be distributed at the end of

class.

9 Post-Break Big Picture

• Recall the role of cell division: Growth Reproduction Repair

9 Prokaryotes vs. Eukaryotes: Cell Division

Review: Cell Division in Prokaryotes & Eukaryotes• Prokaryotes

Binary Fission• Eukaryotes

Mitosis: Growth, Repair, and Asexual Reproduction Meiosis: Sexual Reproduction (Formation of Gametes)

http://www.astrographics.com/GalleryPrints/Display/GP2061.jpghttp://biochem.ultraevil.com/bio/Images/bioloskoorozje/anthrax/BacillusAnthrax.jpg

9 Mitosis – Layered Figure

Review of Mitosis• Layered Figure: Mitosis [Figure 09.08]

9 Reproduction: Asexual and Sexual

• Asexual reproduction involves the generation of a new individual that is essentially genetically identical to the parent. It involves a cell or cells that were generated by mitosis.

• Variation of cells is likely due to mutations or environmental effects.

Fungal asexual spores – products of mitosis

9 Reproduction: Asexual and Sexual

• Sexual reproduction involves meiosis. Meiosis is specialized cell division used for sexual reproduction. The genetic information in the chromosomes is shuffled, and the cells, called gametes, typically get one-half of the original DNA complement.

• Two parents each contribute a set of chromosomes in a sex cell or gamete.

• Gametes fuse to produce a single cell, the zygote, or fertilized egg. This creates variety among the offspring beyond that attributed to mutations or the environment.

9 Reproduction: Asexual and Sexual

• In multicellular organisms, somatic cells each contain two sets of chromosomes. In contrast, germ cells (e.g., gametes) contain only one set of chromosomes. [“germ” = to beget]

• In each recognizable pair of chromosomes, one comes from each of the two parents.

• The members of the pair are called homologous chromosomes and have corresponding but generally not identical genetic information.

• Haploid cells contain just one homolog of each pair. The number of chromosomes in a single set is denoted by n.

• When haploid gametes fuse in fertilization, they create the zygote, which is 2n, or diploid.

9 Reproduction: Asexual and Sexual

• Haploid cells contain just one homolog of each pair. The number of chromosomes in a single set is denoted by n.

• When haploid gametes fuse in fertilization, they create the zygote, which is 2n, or diploid.

http://www.astrographics.com/GalleryPrints/Display/GP2061.jpg

Figure 9.13 Human Cells Have 46 Chromosomes

9 Meiosis: A Pair of Nuclear Divisions

• Meiosis consists of two nuclear divisions that reduce the number of chromosomes to the haploid number. The DNA is replicated only once.

• The functions of meiosis are:

To reduce the chromosome number from diploid to haploid.

To ensure each gamete gets a complete set of chromosomes.

To promote genetic diversity among products.

9 Meiosis: A Pair of Nuclear Divisions

• Meiosis I is preceded by an interphase in which DNA is replicated.

• During prophase I, synapsis occurs: The two homologs are joined together by a complex of proteins.

• This forms a tetrad, or bivalent, which consists of two homologous chromosomes with two sister chromatids.

http://www1.moe.edu.sg/edsoftware/ir/files/bio-meiosis/glossary/tetrad.jpg

Figure 9.14 Meiosis (Part 1)

9 Meiosis: A Pair of Nuclear Divisions

• At a later point, the chromosomes appear to repel each other except at the centromere and at points of attachments, called chiasmata, which appear x-shaped.

• These chiasmata reflect the exchange of genetic material between homologous chromosomes, a phenomenon called crossing-over.

• This crossing-over increases genetic variation by reshuffling the genes on the homologs.

Figure 9.16 Crossing Over Forms Genetically Diverse Chromosomes

9 Meiosis: A Pair of Nuclear Divisions

• Meiosis leads to genetic diversity.

• Synapsis and crossing-over during prophase I mix genetic material of the maternal with that of the paternal homologous chromosomes.

• Which member of a homologous pair segregates or goes to which daughter cell at anaphase I is a matter of chance. This phenomenon is called independent assortment. http://faculty.clintoncc.suny.edu/faculty/Michael.

Gregory/files/Bio%20101/Bio%20101%20Lectures/meiosis/meiosis_spindle.gif

9 Meiosis – Layered Figure

• Layered Figure: Meiosis [Figure 09.14]

9 Mitosis & Meiosis – Layered Figure

• Layered Figure: Mitosis & Meiosis: A Comparison [Figure 09.17]

9 Announcements

• Exam 1 Grades: (high 96%, low 26%)• Keys to exams are posted outside the MacLab

(SC132). Three versions are posted; compare your 23rd question with those posted with the keys, this will identify your particular version.

• Keys will be posted until Exam 2, but then permanently removed. Before the keys are removed, record the correct answers so you can learn from your mistakes.

• The grade on your answer sheet is without the curve.

• Grades (w/ dynamic curve) are posted on the web.

Figure 9.14 Meiosis (Part 2)

Figure 9.14 Meiosis (Part 3)

9 Meiosis: A Pair of Nuclear Divisions

• The second meiotic division separates the chromatids.

• Meiosis II is similar to mitosis but one difference is that DNA does not replicate before meiosis II.

• The number of chromosomes in the resulting cells is therefore half that found in diploid mitotic cells.

• In meiosis II, sister chromatids are not identical and there is no crossing-over.

Figure 9.14 Meiosis (Part 4)

Figure 9.14 Meiosis (Part 5)

Figure 9.14 Meiosis (Part 6)

9 Meiotic Errors

• Nondisjunction occurs when homologous chromosomes fail to separate during anaphase I, or sister chromatids fail to separate during anaphase II.

• The result is a condition called aneuploidy.

Figure 9.18 Nondisjunction Leads to Aneuploidy

9 Meiotic Errors

• One reason for aneuploidy may be a lack of cohesins.

• Failure of chromosome 21 to separate in humans results in trisomy 21—Down syndrome.

• Translocation, a process in which part of a chromosome attaches to another, can also cause abnormality.

9 Meiotic Errors

• Polyploids have extra whole sets of chromosomes, and this abnormality in itself does not prevent mitosis.

• Triploids are 3n; tetraploids are 4n.

• Although mitosis usually is unimpaired, meiosis is problematic, especially for odd numbers of sets, as in triploidy.

9 Cell Death

• Cells die in one of two ways: necrosis and apoptosis.

• Necrosis occurs when cells either are damaged by poisons or are starved of essential nutrients. These cells swell and burst.

9 Cell Death

• Genetically programmed cell death is called apoptosis:

The cell may no longer be needed, e.g., cells of the weblike tissue between the fingers of a developing human fetus.

Cells that are old or damaged may need to be replaced.

The cell death cycle is controlled by signals.

The cell becomes isolated, chops up its own chromatin, and gets ingested by surrounding living cells.