Chapter 11: Cell Division

Mitosis

(prophase, metaphase, anaphase, telophase) -sorting process that ensures that each daughter cell will obtain the correct number and types of chromosomes -mitotic spindle apparatus is responsible for organizing and sorting the chromosomes during mitosis -beings after cell progresses through G1, S, G2

Mitotic Cell Divison

- a cell divides to produce 2 new cells genetically identical to the original -original called mother, new cells called daughter -involves mitosis and cytokinesis -can be for asexual reproduction or for production and maintenance of multicellularity

Cell Division Prep

-DNA replicated -sister chromatids (2 identical copies with associated proteins) -tightly associates at centromere -serves as attachment site for kinetochore used in sorting chromosomes

Meiosis vs. Mitosis

-Mitosis produces two diploid daughter cels that are genetically identical (6 chromosomes in 3 homologous pairs) -Meiosis produces four haploid daughter cells (each daughter has random mix of 3 chromosomes)

Mitotic Spindle

-animal cell mitotic spindle -single centrosome duplicates at the beginning of M phase -each defines a pole -animal cells have centrioles (not found in many other eukaryotes)

Prophase

-chromosomes have already replicated to produce 12 chromatids, joined as 6 pairs of sister chromatids -nuclear membrane dissociates into small vesicles -chromatids condense into highly compacted structures that are readily visible by light microscopy -sister chromatids condense and spindle starts to form; nuclear membrane begins to dissociate into vesicles

Telophase

-chromosomes have reached their respective poles and decondense -nuclear membranes now re-form to produce two separate nuclei (cleavage furrow)

G1 Phase (prior to Meiosis)

-chromosomes replicate and then condense

DNA Compaction

-compaction is not uniform -chromosomes compact further when the cell is preparing to divide -most transcriptional activities cease during cell division

Anaphase

-connections b/n pairs of sister chromatids are broken -each chromatid is an individual chromosome and is linked to only one of the two poles by one or more kinetochore microtubules -kin mic. shorten, pulling chromosomes toward pole to which they are attached -two poles move farther away from each other as overlapping polar microtubules lengthen and push against each other

Human Chromosomes

-eukaryotic chromosomes occur in sets (humans have 23 pairs) -diploid 2n -haploid 1n -gametes have 1 member of each pair

Synapsis

-happens in meiosis -also called crossing over -homologous pairs of sis chromatids associate with each other, lying side by side to form bivalent

Heterochromatin

-highly compacted and transcriptionally inactive -some localized regions in nondividing cells

Cell Divison

-interphase, prophase, metaphase, anaphase, telophase, cytokinesis

Euchromatin

-less condensed and capable of gene transcription -most chromosomal regions in nondividing cells -all euchromatin converted to heterochromatin

Haploid-Dominant Species

-many fungi and some protists -multicellular organism is haploid -haploid cells unite to form diploid zygote, then proceeds immediately through meiosis to make 4 haploid spores

Metaphase

-mitotic spindle is completely deformed -centrosomes move apart and demarcate the 2 poles -spindle fibers interact w/ sister chromatids -nuclear membrane has completely dissociated into vesicles and spindle is fully formed; sister chrom. attach to spindle via kinetochore microtubules -pairs of sister chromatids are aligned along plane halfway b/n poles -organized into single row along metaphase plate

Diploid-Dominant Species

-most animal species -haploid gametes are a specialized type of cell

Meiosis II

-no S phase b/n meiosis I and meiosis II -sorting events of meiosis II are similar to those of mitosis -sister chromatids are separated during anaphase II, unlike anaphase I

Crossing Over

-physical exchange b/n chromosome pieces -may increase genetic variation of species -chiasma = arms of the chromosomes tend to separate but remain adhered at crossover site -number of crossovers carefully controlled by cells 1. homologous chromosomes condense 2. synapsis begins 3. bivalents form 4. crossing over occurs

Life Cycle

-sequence of events that produces another generation of organisms -for sexually reproducing organisms, involves an alternation b/n haploid cells or organisms and diploid cells or organisms

Meiosis

-sexual reproduction requires a fertilization event in which two haploid gametes unite to create a diploid cell called a zygote -meiosis is the process by which haploid cells are produced from a cell that was originally diploid -begins after cell progressed through G1, S, and G2 phases of cell cycle *homologous pairs form bivalent or tetrad

Cytokinesis

-two nuclei are segregated into separate daughter cells -process of cytokinesis is different in plants & animals

Homologous Chromosomes

-when a species is diploid, members of a pair of chromosomes are called homologues -one of each of these pairs comes from the mother and one from the father -slight differences provide variation in gene function -sex chromosomes differ in size and genetic composition -have alleles for same genes at specific loci

Binary Fission

1. circular bacterial DNA molecule is attached by proteins to the inner membrane 2. DNA replication begins at specific location and proceeds bidirectionally around the circle 3. newly synthesized DNA molecule is also attached to the inner membrane near the attachment site of the initial molecule 4. as replication proceeds, cell elongates symmetrically around the midpoint separating the DNA attachment sites 5. cell division begins with the synthesis of new membrane and wall material at the midpoint 6. continued synthesis completes the constriction and separates the daughter cells

General Cell Cycle

1. prior to division, mother cell has 6 chromosomes 2. chromosome replication produces 6 pairs of sister chromatids 3. replication is completed, cell prepares to divide 4. replicated chromosomes condense in prep for mitosis 5. chromosomes separate during mitosis and 2 cells are formed during cytokinesis

Kinetochores

attached to centromere (contains microtubules)

Cytokinesis in Plants

cell plate forms a cell wall b/n two daughter cells

Meiosis II: Metaphase II

chromosomes align in center of cell

Cytokinesis in Animals

cleavage furrow constricts like a drawstring to separate the cells

Meiosis I: Telophase I

daughter cells are ready to move into prophase II: -sister chromatids have reached their respective poles and decondense and nuclear membranes reform -cytokinesis -original diploid cell had its chromosomes in homologous pairs while the two cells produced at the end of meiosis I are haploid - but they do not have pairs of homologous chromosomes

Meiosis I: Prophase I

homologous chromosomes separate Prophase I: 1. chromosomes first become visible as thin threads (DNA rep is already complete) 2. homologous chromosomes continue to condense and undergo synapsis (gene-for-gene pairing) 3. when synapsis is complete, each pair of homologous chromosomes forms a bivalent (each consists of 2 sister chromatids) 4. the chromosomes continue to shorten and thicken and the chiasmata b/n non-sister chromatids become apparent 5. nuclear envelope begins to break down

Meiosis I: Anaphase I

homologous chromosomes separate, but sister chromatids do not: -connections b/n bivalents break, but not connections b/n sis chromatids -each joined pair of chromatids migrates to one pole, and the homologous air moves to the opposite pole

Meiosis I: Metaphase I

homologous pairs line up in center of cell with bivalents oriented randomly with respect to each other: -spindle apparatus complete, the chromatids are attached to kinetochore microtubules -bivalents organized along metaphase plate as double row

Meiosis II: Telophase II

nuclear envelope re-forms

Interphase

phase of cell cycle during which the chromosomes are decondensed and found in the nucleus -chromosomes have already replicated during interphase

Sister Chromatids

sister chromatids of duplicated chromosomes have same alleles for each gene

Meiosis II: Anaphase II

sister chromatids separate

Meiosis II: Prophase II

the nuclear envelope breaks down and the chromosomes condense