CLOZE
Chromosomes, the Cell Cycle, and Cell Division
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 ________ Information
• Cytokinesis: The Division of the Cytoplasm
• Reproduction: Asexual and Sexual
• Meiosis: A Pair of Nuclear Divisions
• Meiotic Errors
• Cell Death
Systems of Cell Reproduction
• Since all living cells are mortal, cell ________ or r________ is universal among living organisms.
• This may consist of simple replacement, differentiation, or specialization.
Systems of Cell Reproduction
• Unicellular organisms use cell division primarily to r________ whereas in multicellular organisms cell division also plays important roles in g________ and in the r________ of tissues.
Systems of Cell Reproduction
• Four events occur before and during cell division:
§ A signal to reproduce must be received.
§ ________ 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.
Systems of Cell Reproduction
• Prokaryotes divide by fission.
§ Most prokaryotes have one ________ 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.

Systems of Cell Reproduction
• Eukaryotic cells divide by mitosis or meiosis.
• Eukaryotes usually have ________ chromosomes.
• Eukaryotes have a nucleus, which must replicate and, with few exceptions, ________ during cell division.
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 ________ of the replicated DNA into two new nuclei (nuclear division)
§ The division of the cytoplasm (cytokinesis)
Systems of Cell Reproduction
• Meiosis is specialized cell division used for ________ reproduction. The genetic information in the chromosomes is shuffled, and the cells, called gametes, typically get ________ of the original DNA complement.
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 nerve and muscle cells, lose the capacity to divide altogether and stay in interphase indefinitely, while other cells divide regularly or occasionally.
Interphase and the Control of Cell Division
• Interphase consists of three subphases:
§ G1 is Gap 1, the period just after mitosis and before the beginning of DNA synthesis.
§ Next is S phase (________ ) which is the time when the cell's DNA is replicated.
§ G2 is the time after S and prior to mitosis.


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 ________ 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.

Interphase and the Control of Cell Division
• A protein called RB, or retinoblastoma protein, is the key to progressing past the restriction point.
• Cyclins D and E activate Cdk 4 and 2, which in turn inactivate RB by phosphorylating it.
• When RB becomes inactivated, the cell can progress past G1 into S phase.
Interphase and the Control of Cell Division
• Cyclin-Cdk complexes act as ________ 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.
Interphase and the Control of Cell Division
• Some cells which no longer go through the cell cycle may respond to growth factors provided by other cells.
• Examples include platelet-derived growth factor, interleukins, and erythropoietin.
• Growth factors act by binding to target cells, and triggering events within the target cell that initiate the cell cycle.
• Cancer cells cycle ________ because they either make their own growth factors or no longer require them to start cycling.
Eukaryotic Chromosomes
• The basic unit of the eukaryotic chromosome is a gigantic, linear, double-stranded molecule of DNA ________ 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.

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 ________ 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.

Mitosis: Distributing Exact Copies of
Genetic Information
• 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 ________ determines the cell's plane of division.
• Centrosomes are regions where microtubules form.
Mitosis: Distributing Exact Copies of
Genetic Information
• 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.



Mitosis: Distributing Exact Copies of
Genetic Information
• During prometaphase, the nuclear l________ disintegrates and the nuclear envelope breaks into small vesicles, permitting the fibers of the spindle to "invade" the nuclear region.
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.

Mitosis: Distributing Exact Copies of
Genetic Information
• Anaphase begins when the centromeres separate.
§ Molecular motors at the ________ 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.
Mitosis: Distributing Exact Copies of
Genetic Information
• Telophase begins when the chromosomes finish moving.
§ Nuclear envelopes and ________ coalesce and re-form.
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.
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 ________ which is the beginning of the new cell wall.


Reproduction: Asexual and Sexual
• Mitosis by repeated cell cycles can give rise to vast numbers of genetically ________ cells.
• Meiosis results in just ________ progeny, which usually do not further duplicate. The cells can be genetically different.

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 ________ or ________ effects.
Reproduction: Asexual and Sexual
• Sexual reproduction involves meiosis.
• 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.
Reproduction: Asexual and Sexual
• In multicellular organisms, somatic cells each contain two sets of chromosomes.
• In each recognizable pair of chromosomes, one comes from each of the two parents.
• The members of the pair are called ________ chromosomes and have corresponding but generally not ________ genetic information.
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.
Reproduction: Asexual and Sexual
• Haplontic organisms have a predominant life cycle in a 1n (haploid) state.
• Some organisms have an ________ of generations that includes both a 1n vegetative life stage and a 2n vegetative life stage.
• In diplontic organisms, which include animals, the organism is usually diploid.





Reproduction: Asexual and Sexual
• Cells in metaphase can be killed and prepared in a way that spreads the chromosomes around a region on a glass slide.
• A photograph of the slide can be taken, and images of each chromosome can be organized based on size, number, and shape. This spread is called a ________
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.
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.

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 ________ 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.

Meiosis: A Pair of Nuclear Divisions
• The homologous chromosomes separate in anaphase I.
• The individual chromosomes are pulled to the poles, with one homolog of a pair going to one pole and the other homolog going to the opposite pole.


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 ________ 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.



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.
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.

Meiotic Errors
• One reason for aneuploidy may be a lack of cohesins.
• Failure of chromosome 21 to separate in humans results in trisomy 21—________ syndrome.
• Translocation, a process in which part of a chromosome attaches to another, can also cause abnormality.

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.
Cell Death
• Cells die in one of two ways: ________ and apoptosis.
• Necrosis occurs when cells either are damaged by poisons or are starved of essential nutrients. These cells swell and burst.
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.

Video 9.1 Division of bacteria, Salmonella enteritidis
Video 9.2 Formation of mitotic spindles
Video 9.3 Mitosis in a newt lung epithelial cell
Video 9.4 Mitosis in a plant cell
Video 9.5 Human melanoma cells dividing in culture
Video 9.6 Cytokinesis in the euglenoid Phacus
Video 9.7 Cytokinesis in a green alga, Micrasterias
Video 9.8 Meiosis in a cranefly spermatocyte
Video 9.9 Apoptosis
Animation 9.1 Mitosis
Animation 9.2 Meiosis