Cell Division, part 1 Cell Division, part 1

I.I. Prokaryotes – Binary FissionProkaryotes – Binary Fission

II.II. Eukaryotes – MitosisEukaryotes – MitosisA.A. ProphaseProphase

B.B. MetaphaseMetaphase

C.C. AnaphaseAnaphase

D.D. TelophaseTelophase

Cellular Division OverviewCellular Division Overview

Cell division requiresCell division requires Duplication, organization and sorting of Duplication, organization and sorting of

chromosomeschromosomes

Single-celled organisms: Single-celled organisms: asexual reproductionasexual reproductionMulti-celled organisms: Multi-celled organisms: growth, replacementgrowth, replacement

I. Prokaryotes – Binary FissionI. Prokaryotes – Binary Fission

Binary Fission: Prokaryotes, Yeast & Amoeba

II. Eukaryotes: II. Eukaryotes: MitosisMitosis

1.1. Karyokinesis – Karyokinesis – nuclear divisionnuclear division

2.2. Cytokinesis – Cytokinesis – cytoplasmic divisioncytoplasmic division

InterphaseInterphase

Comprises G1, S, G2 phases of cell cycleComprises G1, S, G2 phases of cell cycle

Key event = Key event = replication of DNA during Sreplication of DNA during S, , each chromosome will become two sister each chromosome will become two sister chromatidschromatids

Sister chromatids are Sister chromatids are notnot visiblevisible during during this phase, & the nucleus remains in tact.this phase, & the nucleus remains in tact.

mitosismitosis1)1) interphaseinterphase

2)2) Prophase, Prophase, prometaphaseprometaphase

3)3) MetaphaseMetaphase

4)4) AnaphaseAnaphase

5)5) TelophaseTelophase

6)6) interphaseinterphase

Mitosis at the DNA level:

MITOSIS

A. Prophase, prometaphaseA. Prophase, prometaphaseFirst part of M phaseFirst part of M phase

Chromatin condenses into visible Chromatin condenses into visible chromosomes, chromosomes, sister chromatidssister chromatids are joined at are joined at the centromere the centromere (genetically identical to one (genetically identical to one another)another)

Nucleoli disappear, and nuclear membrane Nucleoli disappear, and nuclear membrane begins to break downbegins to break down

Centrosomes appear; Spindle fibers formCentrosomes appear; Spindle fibers form

whitefish blastula cells

Once the spindle apparatus is in place it attaches to the kinetochore on the centromere.

Homologues move toward the “equator”

Prometaphase – chromosomes begin to move

B. MetaphaseB. Metaphase

Each chromosome lines up on Each chromosome lines up on the equatorial plane the equatorial plane ((metaphase platemetaphase plate), led by the ), led by the centromerecentromere via the spindle via the spindle apparatusapparatus

C. AnaphaseC. Anaphase

Sister chromatids of each chromosome Sister chromatids of each chromosome separate from each other and migrate separate from each other and migrate to opposite ends of the cellto opposite ends of the cell

Each centromeric region is split in two, Each centromeric region is split in two, once this occurs each chromatid is once this occurs each chromatid is referred to as a referred to as a daughter chromosomedaughter chromosome

Movement of each chromosome made Movement of each chromosome made possible by the spindle apparatuspossible by the spindle apparatus

D. TelophaseD. Telophase

Final stage of mitosisFinal stage of mitosis Cytokinesis occursCytokinesis occurs – cytoplasm is partitioned to each half, – cytoplasm is partitioned to each half,

cleavage furrow forms in animal cells; cell plate forms in plant cleavage furrow forms in animal cells; cell plate forms in plant cellscells

Chromatids Chromatids de-condensede-condense into chromatin into chromatin Nuclear membrane reforms around each nucleus; cell divides Nuclear membrane reforms around each nucleus; cell divides

into two into two genetically identicalgenetically identical daughter cells daughter cells

Cleavage furrow forms in animal cells,Cell plate in plants…

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.biology.arizona.edu/cell_bio/tutorials/cell_cycle/main.html

http://www.pbs.org/wgbh/nova/miracle/divide.html

2n = 4

Know the number of chromosomes/chromatids during each phase

2n = 12, 24, 46… etc.

Cell Division, part 2 - MeiosisCell Division, part 2 - Meiosis

I.I. Sexual reproduction requires gametesSexual reproduction requires gametesII.II. Meisos v. mitosisMeisos v. mitosisIII.III. Prophase IProphase I

A.A. Letoneme stageLetoneme stageB.B. Zygoneme stageZygoneme stageC.C. Pachyneme stagePachyneme stageD.D. Diploneme stageDiploneme stageE.E. DiakinesisDiakinesis

IV.IV. Metaphase, Anaphase, Telophase IMetaphase, Anaphase, Telophase IV.V. The second meiotic divisionThe second meiotic divisionVI.VI. Gamete development in animalsGamete development in animals

A.A. SpermatogenesisSpermatogenesisB.B. OogenesisOogenesis

I. Sexual reproduction requires I. Sexual reproduction requires gametesgametes

Meiosis = cell division that halves the Meiosis = cell division that halves the genetic content for genetic content for sexual reproductionsexual reproductionMeiosis is critical to the successful sexual reproduction of all diploid organisms-

Mechanism by which 2n is reduced to nLeads to the formation of gametes Basis for the production of extensive genetic variation among members of a population

Gametes:Gametes:

Haploid Gametes – isogamous vs. Haploid Gametes – isogamous vs. heterogamousheterogamous

In many species, haploid (n) gametes In many species, haploid (n) gametes are descended from germ cells that are are descended from germ cells that are originally diploid (2n) (via meiosis)originally diploid (2n) (via meiosis) hapliod – they contain ½ the genetic hapliod – they contain ½ the genetic

contentcontent

Gametes then combine in fertilization to Gametes then combine in fertilization to reconstitute the diploid complement reconstitute the diploid complement found in parental cellsfound in parental cells

II. Meiosis, different from mitosis:II. Meiosis, different from mitosis:1)1) TwoTwo successive nuclear divisions, MI & MII, which successive nuclear divisions, MI & MII, which

produces haploid gametes that differ geneticallyproduces haploid gametes that differ genetically

2) Homologous chromosomes pair up (synapse), forming tetrads

3) Non-sister chromatids exchange homologous sections of DNA (crossing over)

III. Prophase IIII. Prophase I

A.A. Leptotene stageLeptotene stage

Chromatin begins to condense, Chromatin begins to condense, chromosomes become visiblechromosomes become visible

ChromomeresChromomeres develop along each develop along each chromosomechromosome

Localized condensations Localized condensations

Homology searchHomology search underway – essential to underway – essential to the initial pairing of homologsthe initial pairing of homologs

B. B. Zygotene stageZygotene stage

Continued chromosomal condensationContinued chromosomal condensation

Homologous chromosomes undergo initial Homologous chromosomes undergo initial alignment, wherein "pairing sites" on the alignment, wherein "pairing sites" on the chromosomes are matched chromosomes are matched

Synaptonemal complexSynaptonemal complex begins to form begins to form between the homologsbetween the homologs

Upon completion of this stage, paired Upon completion of this stage, paired homologs are referred to as homologs are referred to as bivalentsbivalents

C. C. Pachytene stagePachytene stageFurther development of synaptonemal complex Further development of synaptonemal complex occurs between the two members of each occurs between the two members of each bivalent…leading to bivalent…leading to SynapsisSynapsis

ChromomeresChromomeres align in the bivalents, producing a align in the bivalents, producing a distinctive pattern for each pair (completes the distinctive pattern for each pair (completes the ““homology searchhomology search”)”)

Physical exchange between non-sister Physical exchange between non-sister chromatids occurs (but not visible yet)chromatids occurs (but not visible yet)

An identical pattern of twinned loops occurs on both pairs of sister chromatids

D. D. Diplotene stageDiplotene stage

Tetrads highly visible, each consisting Tetrads highly visible, each consisting of two pairs of sister chromatidsof two pairs of sister chromatids

Within each teterad, each pair of sister Within each teterad, each pair of sister chromatids begins to separatechromatids begins to separate

One or more areas remain in contact One or more areas remain in contact where chromatids are intertwined = where chromatids are intertwined = Chiasma Chiasma (crossing over visible)(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: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. E. DiakinesisDiakinesis

Final stage of prophase IFinal stage of prophase IChromosomes pull farther apart, but Chromosomes pull farther apart, but nonsister chromatids remain loosely nonsister chromatids remain loosely associated via the chaismataassociated via the chaismataTerminalizationTerminalization occurs = Chiasmata move occurs = Chiasmata move toward the ends of the tetradtoward the ends of the tetradNucleolus and nuclear envelope break Nucleolus and nuclear envelope break down & the two centromeres of each tetrad down & the two centromeres of each tetrad attach to spindle fibersattach to spindle fibers

IV. Metaphase, Anaphase & Telophase IIV. Metaphase, Anaphase & Telophase I

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

dyad

tetrad

V. Second V. Second meitotic meitotic divisiondivision

Prophase IIProphase II

Metaphase IIMetaphase II

Anaphase IIAnaphase II

Telophase IITelophase II

monad

nondisjunctionnondisjunction

Review questionsReview questions

1.1. A cell containing 64 chromatids at the A cell containing 64 chromatids at the startstart ofof mitosismitosis would, at its completion, produce cells containing how would, at its completion, produce cells containing how many chromosomes?many chromosomes?

2.2. What if the same cell undergoes What if the same cell undergoes meiosismeiosis – how many – how many chromatids would there be in the daughter cells chromatids would there be in the daughter cells afterafter MI, MI, and how many chromosomes after MII?and how many chromosomes after MII?

3.3. When ½ of the gametes produced are When ½ of the gametes produced are trisomictrisomic, what has , what has occurred during meiosis? (be specific)occurred during meiosis? (be specific)

4.4. Which of the following is Which of the following is FALSEFALSE in comparing prophase in comparing prophase I of meiosis and prophase of I of meiosis and prophase of mitosismitosis??a.a. The chromosomes condense The chromosomes condense b.b. Homologs pair up side by sideHomologs pair up side by sidec.c. The nuclear envelope disassemblesThe nuclear envelope disassemblesd.d. A spindle apparatus forms from centrosomesA spindle apparatus forms from centrosomese.e. Each chromosome has two chromatidsEach chromosome has two chromatids

32

32, 16

First division nondisjunction

The significance of meiosis is that it produces The significance of meiosis is that it produces genetic variation by reducing the genetic material genetic variation by reducing the genetic material

by ½ that is recombined, to novel produce by ½ that is recombined, to novel produce gametes for fertilizationgametes for fertilization

1.1. Random assortmentRandom assortment of maternal and of maternal and paternal chromosomes, and the alleles of paternal chromosomes, and the alleles of genes they contain genes they contain

each monad is a combination of maternal & each monad is a combination of maternal & paternal genetic informationpaternal genetic information

2.2. RecombinationRecombination due to crossing over and due to crossing over and exchange of chromosome parts between exchange of chromosome parts between non-sister chromatidsnon-sister chromatids

3.3. Random fertilizationRandom fertilization

Meiosis produces new Combinations of genes in 3 ways:Meiosis produces new Combinations of genes in 3 ways:

Random assortmentRandom 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.

A.A. Spermatogenesis Spermatogenesis takes place in testestakes place in testesGerm cell = Germ cell = spermatogoniumspermatogonium, enlarges , enlarges to become a to become a primary spermatocyteprimary spermatocytePrimary spermatocyte undergoes MI, Primary spermatocyte undergoes MI, producing 2 producing 2 secondary spermatocytessecondary spermatocytes – – producing producing haploid spermatidshaploid spermatidsSpermatids undergo modifications – Spermatids undergo modifications – becoming spermatozoabecoming spermatozoa

VI. Gamete development in animalsVI. Gamete development in animals

B. OogenesisB. Oogenesis

Occurs in ovaryOccurs in ovary

Germ cell = oogonium, enlarges to form primary Germ cell = oogonium, enlarges to form primary oocyteoocyte

Primary oocyte undergoes MI, producing 1 large Primary oocyte undergoes MI, producing 1 large secondary oocyte & 1 small polar bodysecondary oocyte & 1 small polar body Unequal division of cytoplasm results in polar bodyUnequal division of cytoplasm results in polar body

Secondary oocyte undergoes MII, producing 1 Secondary oocyte undergoes MII, producing 1 large haploid ootid & 1 small polar bodylarge haploid ootid & 1 small polar body

Ootid differentiates into mature ovumOotid 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?

Metaphaseof mitosis

Metaphase II Interphase Metaphase I