Biology Chapter on Cell Division and Cancer

Learning Objectives

• Understand the functions of cellular division and the organization of genetic material in chromatin/chromosomes.
• Define key chromosomal/genetic material terminology.
• Understand the major phases and processes occurring in the cell cycle during mitosis.
• Explain how the mitotic spindle contributes to chromosomal movement in all phases of mitosis.
• Understand the difference between animal and plant cell cytokinesis.
• Understand why cellular checkpoints are important and what happens when they malfunction.
• Explain how genetic material is passed through generations.
• Understand chromosomal numbers in humans, haploid vs diploid cells (and examples in the body), human life cycle.
• Describe the differences between asexual and sexual reproduction.
• Explain the major outcomes of meiosis, major phases of meiosis I and meiosis II, how genetic diversity occurs.
• Compare and contrast mitosis and meiosis.
• Cellular division is crucial for organism growth and survival
• The continuity of life is based on cell division.

Cellular Division

• The ability of organisms to produce more of their own kind.
• Mitosis and meiosis are types of cell division.
• Mitosis results in two identical daughter cells.
• Meiosis results in four non-identical daughter cells.

Functions of Cellular Division

• In unicellular organisms: Division of one cell reproduces the entire organism.
• In multicellular eukaryotes: Dependent on cell division for development from a fertilized egg, growth, and repair.

Introducing the Cell Cycle

• Cell division is an integral part of the cell cycle.
• Cell cycle: The life of a cell from formation to its own division.
• Most cell division results in two daughter cells with identical genetic information.
• The exception is meiosis -- a special type of division that can produce sperm and egg cells (gametes).
• Meiosis produces non-identical daughter cells.

Cellular Organization of the Genetic Material

• Genome: All the DNA in a cell
• A genome can consist of a single DNA molecule (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells).
• DNA molecules in a cell are packaged into chromosomes.
• Eukaryotic chromosomes consist of chromatin.
• Chromatin: A complex of DNA and protein (histones) that condenses during cell division.
• Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus.

Distribution of Chromosomes During Eukaryotic Cell Division

• In preparation for cell division, DNA is replicated, and the chromosomes condense.
• Each duplicated chromosome has two sister chromatids.
• Sister chromatids: Joined copies of the original chromosome, attached along their lengths by cohesins.
• Cohesins: Protein complexes that attach sister chromatids.
• The centromere: the narrow waist of a duplicated chromosome, where two chromatids are most closely attached.

The Two Major Phases of the Cell Cycle

• Interphase (G1, S, and G2): Cell growth and copying of chromosomes in preparation for cell division (about 90% of the cell cycle).
• G1 phase (First gap): Cell growth, increases in size, gets ready for S phase.
• S phase (Synthesis): DNA replication occurs.
• G2 phase (Second gap): Cell continues to grow, gets ready for mitosis.
• Mitotic phase (Mitosis and Cytokinesis): Mitosis: the division of the genetic material in the nucleus, and Cytokinesis: the division of the cytoplasm.

Mitosis Consists of 5 Stages

• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase and Cytokinesis

G2 of Interphase

• A nuclear envelope encloses the nucleus.
• Two centrosomes have formed, regions that organize microtubules.
• Chromosomes duplicated in S phase cannot be seen.

Prophase

• Chromatin fibers tightly coil into discrete chromosomes, becoming visible.
• Duplicated chromosomes appear as two sister chromatids joined at centromeres.
• Mitotic spindle begins to form.
• Centrosomes and the microtubules extending from centrosomes.
• Centrioles move away from each other

Prometaphase

• Nuclear envelope fragments, chromosomes are even more condensed.
• Microtubules from each centrosome invade nuclear area.
• Kinetochore: protein structure at the centromere on each sister chromatid.
• Some microtubules become kinetochore microtubules, jerking chromosomes back and forth.
• Any non kinetochore microtubules elongate the cell

Metaphase

• Centrosomes now at opposite poles of the cell.
• Chromosomes have arrived at the metaphase plate, an imaginary plane at the middle of the cell.
• Centromeres lie at metaphase plate.
• Kinetochore of each sister chromatid attach to kinetochore microtubules.

Anaphase

• Shortest phase of mitosis.
• Cohesins between chromatids are cleaved, allowing each pair to separate (becoming an independent chromosome).
• Two new daughter chromosomes moving towards opposite poles.
• Centromeres lead, because microtubules attached to kinetochore (at centromere).

Telophase and Cytokinesis

• Two daughter nuclei form.
• Nuclear envelope reappears.
• Chromosomes start to decondense.
• Mitosis is now complete, creating two genetically identical nuclei.
• Cytokinesis: Division of the cytoplasm.
• Involves formation of a cleavage furrow, pinching cell in two (animal cells).
• Cell plate forms during cytokinesis in plant cells.

Animal Cell Mitosis (time-lapse) and Plant Cell Cytokinesis (in onion root cells)

• Both animal and plant cells undergo mitosis and cytokinesis, but the mechanisms for cytokinesis differ.

BioFlix Animation: Mitosis

• Digital animation.
• Demonstrates stages of mitosis

The Mitotic Spindle

• A structure made of microtubules that controls chromosome movement during mitosis.
• In animal cells, assembly of spindle microtubules begins the centrosome (Microtubule-organizing center [MTOC])
• The centrosome replicates during interphase and two centrosomes migrate to opposite ends of the cell during prophase and prometaphase
• During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes, and begin to move the chromosomes.
• Kinetochores are protein complexes associated with centromeres
• At metaphase, the chromosomes are all lined up at the metaphase plate.
• In anaphase, cohesins are cleaved by the enzyme separase.
• Sister chromatids separate and move along the kinetochore microtubules toward opposite sides of the cell.
• The microtubules shorten by depolymerizing at their kinetochore ends.
• Non-kinetochore microtubules from opposite poles overlap and push against each other, elongating the cell.
• Cytokinesis begins during anaphase or telophase, and the spindle eventually disassembles

Comparing cytokinesis in animal and plant cells

• In animal cells, cytokinesis involves a cleavage furrow.
• A contractile ring of microfilaments forms during cytokinesis in animal cells.
• In plant cells, a cell plate forms during cytokinesis.
• Vesicles contain cell wall material derived from the Golgi apparatus.

How do cells know when to divide?

• Cell Cycle is tightly regulated.
• Some cells divide frequently (intestinal cells) and others rarely divide (neuronal cells).
• Cancerous cells escape the usual controls and proliferate.

Molecular Clocks

• The cell cycle is driven by specific chemical signals present in the cytoplasm.
• The cell cycle is directed by a distinct cell cycle control system, which is similar to a clock.
• The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received.

Cellular Checkpoints

• Changes in regulatory protein concentrations drive the cell cycle.
• Three important checkpoints are in the G1, G2, and M phases
• If the cell doesn’t receive the go-ahead signal, it will exit the cycle and switch to a non-dividing state (G0 phase).

Cancer Cells

• Lose their cell cycle checkpoints due to internal and/or external factors.
• Become immortal and continuously divide provided they have enough nutrients
• HeLa cells: Harvested from a woman named Henrietta Lacks (1951)

Chapter 13: Meiosis

• A special type of cell division that can produce sperm and egg cells (gametes).
• Produces non-identical daughter cells.
• Heredity is the transmission of traits from one generation to the next.
• Variation is demonstrated by the differences in appearance that offspring show from parents and siblings.
• Genetics is the scientific study of heredity and variation.

Inheritance of Genetic Material

• Offspring inherit genetic material from parents.
• Genes are the units of heredity and are made of segments of DNA.
• Locus (plural, loci): A gene's specific position along a chromosome.
• Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs).
• Offspring acquire genes from parents by inheriting chromosome
• Most DNA packaged into chromosomes.
• Human somatic cells have 23 pairs of chromosomes (total of 46).
• Karyotype: An ordered display of the pairs of chromosomes from a cell.

Comparison of Asexual and Sexual Reproduction

• In asexual reproduction, a single individual passes all of its genes to its offspring.
• A clone is a group of genetically identical individuals from the same parent.
• In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents.

Cellular Organization of the Genetic Material

• Somatic cells: Non-reproductive cells, with two sets of chromosomes (diploid).
• Gametes: Reproductive cells (sperm and eggs), with half as many chromosomes as somatic cells (haploid). In an unfertilized egg, the sex chromosome is X, in a sperm either X or Y.

Sets of Chromosomes in Human Cell

• Human somatic cells have 23 pairs of chromosomes.
• The sex chromosomes are called X and Y.
• Human females have a homologous pair of X chromosomes (XX).
• Human males have one X and one Y chromosome.
• The remaining 22 pairs of chromosomes are called autosomes.
• Aneuploidy: Abnormal number of chromosomes.

Sets of Chromosomes in a Cell

• The homologous chromosomes, or homologs, are the two chromosomes in each pair.
• Chromosomes in a homologous pair are the same length and shape, and carry similar genes.
• Each pair of homologous chromosomes includes one chromosome from each parent.
• Nonsister chromatids are chromatids in a homologous pair, one from each parent.

Behavior of Chromosome Sets in the Human Life Cycle

• A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism.
• Fertilization: The union of gametes.
• The fertilized egg is called a zygote, which has one set of chromosomes from each parent.
• The zygote produces somatic cells by mitosis, and develops into an adult.

Animal Life Cycles

• Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism.
• Only diploid cells can undergo meiosis.
• The halving and doubling of chromosomes contributes to genetic variation in offspring.

Meiosis

• Meiosis reduces the number of chromosome sets from diploid (2n) to haploid (n).
• Meiosis is preceded by replication of chromosomes
• Meiosis takes place in two consecutive cell divisions: meiosis I and meiosis II.
• Meiosis I involves separation of homologous chromosomes.
• Meiosis II involves separation of sister chromatids.
• The two cell divisions result in four haploid daughter cells, rather than the two daughter cells in mitosis.
• Each daughter cell has only half as many chromosomes as the parent cell.
• Crossover allows for genetic recombination.

Comparing Mitosis and Meiosis

• Mitosis:
• Conserves the number of chromosome sets.
• Produces cells that are genetically identical to the parent cell.
• Meiosis:
• Reduces the number of chromosomes sets from two (diploid) to one (haploid).
• Produces cells that differ genetically from each other and from the parent cell.

Events Unique to Meiosis I

• Synapsis: The pairing of homologous chromosomes followed by crossing over.
• Crossing over: Exchange of genetic material between homologous chromosomes.
• Homologous pairs at the metaphase plate
• Separation of homologs during anaphase I

Meiosis I Separates Homologues

• Two consecutive cell divisions = four haploid daughter cells
• Synapsis and crossing over
• Homologous pairs at metaphase plate
• Separation of homologs

Meiosis II Separates Sister Chromatids

• Very similar to mitosis.
• Sister chromatids separate during anaphase II.
• Four haploid daughter cells result

Results of Meiosis

• Chromosomes duplicate before meiosis.
• Pairs of homologs and crossover at chiasma allows for genetic recombination.
• The chromatids are sorted into four haploid daughter cells.