Cell Division, part 1 Prokaryotes – Binary Fission Eukaryotes – Mitosis Prophase Metaphase Anaphase Telophase

Cellular Division Overview Cell division requires Duplication, organization and sorting of chromosomes Single-celled organisms: asexual reproduction Multi-celled organisms: growth, replacement

I. Prokaryotes – Binary Fission

Binary Fission: Prokaryotes, Yeast & Amoeba

II. Eukaryotes: Mitosis Karyokinesis – nuclear division Cytokinesis – cytoplasmic division

Interphase Comprises G1, S, G2 phases of cell cycle Key event = replication of DNA during S, each chromosome will become two sister chromatids Sister chromatids are not visible during this phase, & the nucleus remains in tact.

mitosis interphase Prophase, prometaphase Metaphase Anaphase Telophase

Mitosis at the DNA level:

MITOSIS

A. Prophase, prometaphase whitefish blastula cells A. Prophase, prometaphase First part of M phase Chromatin condenses into visible chromosomes, sister chromatids are joined at the centromere (genetically identical to one another) Nucleoli disappear, and nuclear membrane begins to break down Centrosomes appear; Spindle fibers form

Prometaphase – chromosomes begin to move Once the spindle apparatus is in place it attaches to the kinetochore on the centromere. Homologues move toward the “equator”

B. Metaphase Each chromosome lines up on the equatorial plane (metaphase plate), led by the centromere via the spindle apparatus

C. Anaphase Sister chromatids of each chromosome separate from each other and migrate to opposite ends of the cell Each centromeric region is split in two, once this occurs each chromatid is referred to as a daughter chromosome Movement of each chromosome made possible by the spindle apparatus

D. Telophase Cleavage furrow forms in animal cells, Cell plate in plants… Final stage of mitosis Cytokinesis occurs – cytoplasm is partitioned to each half, cleavage furrow forms in animal cells; cell plate forms in plant cells Chromatids de-condense into chromatin Nuclear membrane reforms around each nucleus; cell divides into two genetically identical daughter cells

SIGNIFICANCE: results in 2 new identical daughter cells, the exact distribution of the DNA (via chromosomes) to the daughter cells ensures the stability of cells and the inheritance of traits from one generation to the next. http://www.pbs.org/wgbh/nova/miracle/divide.html http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/main.html

2n = 4 Know the number of chromosomes/chromatids during each phase 2n = 12, 24, 46… etc.

Cell Division, part 2 - Meiosis Sexual reproduction requires gametes Meisos v. mitosis Prophase I Letoneme stage Zygoneme stage Pachyneme stage Diploneme stage Diakinesis Metaphase, Anaphase, Telophase I The second meiotic division Gamete development in animals Spermatogenesis Oogenesis

I. Sexual reproduction requires gametes Meiosis = cell division that halves the genetic content for sexual reproduction Meiosis is critical to the successful sexual reproduction of all diploid organisms- Mechanism by which 2n is reduced to n Leads to the formation of gametes Basis for the production of extensive genetic variation among members of a population

Gametes: Haploid Gametes – isogamous vs. heterogamous In many species, haploid (n) gametes are descended from germ cells that are originally diploid (2n) (via meiosis) hapliod – they contain ½ the genetic content Gametes then combine in fertilization to reconstitute the diploid complement found in parental cells

II. Meiosis, different from mitosis: Two successive nuclear divisions, MI & MII, which produces haploid gametes that differ genetically Homologous chromosomes pair up (synapse), forming tetrads Non-sister chromatids exchange homologous sections of DNA (crossing over)

III. Prophase I Leptotene stage Chromatin begins to condense, chromosomes become visible Chromomeres develop along each chromosome Localized condensations Homology search underway – essential to the initial pairing of homologs

B. Zygotene stage Continued chromosomal condensation Homologous chromosomes undergo initial alignment, wherein "pairing sites" on the chromosomes are matched Synaptonemal complex begins to form between the homologs Upon completion of this stage, paired homologs are referred to as bivalents

C. Pachytene stage Further development of synaptonemal complex occurs between the two members of each bivalent…leading to Synapsis Chromomeres align in the bivalents, producing a distinctive pattern for each pair (completes the “homology search”) Physical exchange between non-sister chromatids occurs (but not visible yet) An identical pattern of twinned loops occurs on both pairs of sister chromatids

D. Diplotene stage Tetrads highly visible, each consisting of two pairs of sister chromatids Within each teterad, each pair of sister chromatids begins to separate One or more areas remain in contact where chromatids are intertwined = Chiasma (crossing over visible)

Crossing over at the DNA level – Holliday Structure

Significance of CROSSING OVER: Crossing over results in recombinant chromosomes that now have paternal alleles at some loci and maternal alleles at other loci.

Additional Crossing over info: The exchange is reciprocal, such that each chromosome gets the same region of the chromosome segment from the other parent that it donated to the other chromosome. Crossovers are frequent and there is usually at least one on each chromosome and 3-4 on the larger chromosomes.

E. Diakinesis Final stage of prophase I Chromosomes pull farther apart, but nonsister chromatids remain loosely associated via the chaismata Terminalization occurs = Chiasmata move toward the ends of the tetrad Nucleolus and nuclear envelope break down & the two centromeres of each tetrad attach to spindle fibers

IV. Metaphase, Anaphase & Telophase I Metaphase I – tetrads move to the metaphase plate, alignment is random Anaphase I – ½ of each tetrad (dyad) is pulled to one or the other pole at random, and the the other ½ moves to the opposite pole [disjunction] Telophase I – nuclear membrane forms around the dyads, cleavage furrow forms, nucleus enters short interphase

tetrad dyad

V. Second meitotic division Prophase II Metaphase II Anaphase II Telophase II monad

nondisjunction

Review questions A cell containing 64 chromatids at the start of mitosis would, at its completion, produce cells containing how many chromosomes? What if the same cell undergoes meiosis – how many chromatids would there be in the daughter cells after MI, and how many chromosomes after MII? When ½ of the gametes produced are trisomic, what has occurred during meiosis? (be specific) Which of the following is FALSE in comparing prophase I of meiosis and prophase of mitosis? The chromosomes condense Homologs pair up side by side The nuclear envelope disassembles A spindle apparatus forms from centrosomes Each chromosome has two chromatids 32 32, 16 First division nondisjunction

The significance of meiosis is that it produces genetic variation by reducing the genetic material by ½ that is recombined, to novel produce gametes for fertilization Meiosis produces new Combinations of genes in 3 ways: Random assortment of maternal and paternal chromosomes, and the alleles of genes they contain each monad is a combination of maternal & paternal genetic information Recombination due to crossing over and exchange of chromosome parts between non-sister chromatids Random fertilization

Random assortment

The number of possible combinations of maternal and paternal homologues is 2n, where n = the haploid number of chromosomes. In this diagram, the haploid number is 3, and 8 (23) different combinations are produced.

VI. Gamete development in animals Spermatogenesis takes place in testes Germ cell = spermatogonium, enlarges to become a primary spermatocyte Primary spermatocyte undergoes MI, producing 2 secondary spermatocytes – producing haploid spermatids Spermatids undergo modifications – becoming spermatozoa

B. Oogenesis Occurs in ovary Germ cell = oogonium, enlarges to form primary oocyte Primary oocyte undergoes MI, producing 1 large secondary oocyte & 1 small polar body Unequal division of cytoplasm results in polar body Secondary oocyte undergoes MII, producing 1 large haploid ootid & 1 small polar body Ootid differentiates into mature ovum

2n = 8, what phase of mitosis, MI or MII is this cell in? This cell is in metaphase II, if it were in mitosis there would be 8 pairs of sister chromatids!

2n = 4, what phase of cell division is each cell in? Metaphase of mitosis Interphase Metaphase I Metaphase II