q132/random_k4/random_3.md

LAB 9 – EUKARYOTIC CELL DIVISION: MITOSIS AND MEIOSIS

Objectives

Identify plant and animal cells in each stage of mitosis.

1.
2.  Model each stage of mitosis and meiosis.
3.  Assess the generation of genetic diversity due to the independent assortment of

chromosomes.

INTRODUCTION

BINARY FISSION: Prokaryotic cells (bacteria) reproduce asexually by binary fission.  Bacterial cells have a
single  circular  chromosome,  which  is  not  enclosed  by  a  nuclear  envelope.    During  binary  fission  the
bacterial chromosome is duplicated, the cell elongates, and the two chromosomes migrate to opposite
ends of the cell.  Each daughter cell receives one chromosome and is identical to the parent cell.  Binary
fission is a relatively fast and simple process.

MITOSIS:    The  increased  complexity  of  eukaryotic  cells  causes  several  logistical  problems  during  cell
division.    Eukaryotes  are  diploid,  which  means  they  have  two  sets  of  chromosomes;  one  set  of
chromosomes is inherited from each  parent.  Eukaryotic DNA is enclosed by a nuclear envelope.  The
proper sorting and distribution of multiple chromosomes during cell division is a complex process that
requires  the  temporary  dissolution  of  the  nuclear  envelope.    Eukaryotic  organisms  carry  out  mitosis
throughout their entire life to grow and to replace old or damaged cells.  Some eukaryotic organisms use
mitosis  to  reproduce  asexually.    The  daughter  cells  produced  by  mitosis  are  diploid  and  genetically
identical to each other and the parent cells that produced them.

CELL CYCLE:  INTERPHASE & MITOSIS

Cells  only  spend  a  small  part  of  their  life  dividing.  The  time  between  consecutive  mitotic  divisions  is
referred to as interphase.  Eukaryotic cells spend most of their  time in interphase.  During interphase
the  cell’s  genetic  material  is  in  the  form  of  chromatin  (uncoiled  DNA),  nucleoli  are  present,  and  the
nuclear  envelope  is  clearly  visible.    Shortly  before  mitosis,  the  cell  duplicates  its  DNA  during  the  S
(synthesis) phase of interphase.

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Mitosis can be divided into distinct phases as shown below:

I. Prophase:  Nuclear envelope and nucleoli disappear.  Chromatin condenses into chromosomes, which
are made up of two identical sister chromatids joined by a centromere.  In animal cells, centrioles start
migrating  to  opposite  ends  of  the  cell  (centrioles  are  not  present  in  plant  cells).  The  mitotic  spindle
forms and begins to move chromosomes towards the center of the cell.

NOTE:  Prometaphase is a transitional phase between prophase and metaphase.

II. Metaphase:  Brief stage in which chromosomes line up in the equatorial plane of the cell.  In animal
cells, one pair of centrioles are visible at both ends of the cell.  The mitotic spindle is fully formed.
III. Anaphase:  Sister chromatids begin to separate, becoming individual chromosomes, which begin to
migrate to opposite ends of the cell.

IV. Telophase:  A full set of chromosomes reaches each pole of the cell.  The mitotic spindle begins to
disappear.  The nucleus and nucleoli begin to reappear.  Chromosomes begin to unravel into chromatin.

Cytokinesis or cytoplasmic division usually occurs at the end of telophase.  In plant cells cytokinesis is
accomplished by the formation of a cell plate.   Animal cells separate by forming a cleavage furrow.  In
some cells (e.g., muscle cells and certain embryonic cells) cytokinesis does not occur or is delayed until
multiple  nuclear  divisions  have  occurred.    So  although  cytokinesis  is  associated  with  telophase  of
mitosis, it is separate from mitosis and may not occur at all.  Mitosis is therefore the process of nuclear
division, distinct from cytokinesis or cytoplasmic division.

EXERCISE 1 – Observing mitosis under the microscope

Examine prepared slides of both plant cells (onion/allium root tip) and animal cells (whitefish blastula) under the
microscope at 400X.  Even though the cells in these tissues are rapidly dividing, most of the cells you see will be in
interphase (between cell divisions).  Using your microscope, scan the slides to find a cell in interphase and each
one  of  the  four  stages  of  mitosis.    Draw  a  schematic  representation  of  your  observations  for  both  plant  and
animal cells at each stage in the spaces provided on your worksheet and be sure to indicate and clearly label the
important features or events of each stage.

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MEIOSIS

During sexual reproduction in eukaryotes, a haploid sperm cell fuses with a haploid egg cell to produce
a  diploid zygote  or  fertilized  egg.  In  most  species,  it  is  very  important  that  the  offspring  produced  by
fertilization  have  the  same  number  of  chromosomes  as  the  parents.    Even  a  single  extra  or  missing
chromosome can be lethal or extremely deleterious to an individual (e.g., Down’s syndrome in humans).
Meiosis is a special type of cell division that produces haploid gametes (sperm cells or ova).  Meiosis only
occurs in an individual’s gonads, during their reproductive years.

Meiosis involves two cell divisions and ultimately produces four haploid gametes.  The haploid gametes
produced by meiosis are different from each other as well as from the parent cells due to the crossing
over  of  genetic  material  between  homologous  chromosomes  and  the  random  distribution  of
homologous chromosomes.

Meiosis is different in males and females:

Spermatogenesis

•

In males four functional sperm cells are produced by meiosis.

Oogenesis

•  Due to unequal distribution of cytoplasm in during meiosis, one large functional egg

(ovum) and three small polar bodies are produced.

STAGES OF MEIOSIS

MEIOSIS I

This is a reductive division in which one diploid (2N) cell produces two haploid (1N) cells.

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Prophase I:  Similar to prophase of mitosis with one important difference:  Crossing Over in which pairs
of  homologous  chromosomes  synapse  together  to  form  tetrads  and  exchange  genetic  information
(DNA). Crossing over creates new, recombinant chromosomes.  Homologous chromosomes contain the
same arrangement of genes and are of the same size.  Although they are very similar (hence the term
“homologous”), they differ slightly in DNA sequence since one comes from an individual’s mother, and
the other comes from an individual’s father.

Metaphase I:  Brief stage in which tetrads line up in the equatorial plane of the cell.

Anaphase I:  Homologous chromosomes separate and migrate to opposite ends of cell.  Sister
chromatids DO NOT separate.

Telophase I/Cytokinesis:   A full set of chromosomes reaches each pole of the cell.  The cells produced
contain half of the original number of chromosomes and are considered haploid.  Each chromosome
consists of two sister chromatids attached at the centromere.

Interphase may be very brief or absent between meiosis I and meiosis II.

MEIOSIS II

This division is very similar to mitosis – sister chromatids are distributed into different cells.

Prophase II:  DNA condenses into chromosomes.  No crossing over occurs.

Metaphase II:  Individual duplicated chromosomes line up in the equatorial plane of the cell.

Anaphase II: Sister chromatids separate and begin to migrate to opposite poles of the cell.

Telophase II/Cytokinesis:  A full set of chromosomes reaches each pole of the cell.  Four

     genetically unique gametes are produced.

Exercise 2A – Modeling the stages of meiosis

1.  Use the chromosome bead models to construct a single pair of homologous chromosomes, each with two sister
      chromatids.  Use red for the maternal chromosome and yellow for paternal chromosome.  Use the rubber
      tubing with magnets for centromeres and attach 10 beads to each end of the centromere.

2.  Model each stage of meiosis with your single pair of homologous chromosomes, diagramming each stage on
      your worksheet using different colors to indicate maternal and paternal chromosomes.  Also, be sure to include
      one crossover event during Prophase I.

INDEPENDENT ASSORTMENT OF CHROMOSOMES

For  each  homologous  pair  of  chromosomes,  maternal  and  paternal  chromosomes  are  randomly
distributed  into  daughter  cells  during  meiosis  I.    The  chromosomes  of  each  homologous  pair  are
distributed  independently of  the other  homologous pairs, a phenomenon we refer to as  independent
assortment.    Independent  assortment  can  create  a  staggering  number  of  possible  chromosome
combinations in gametes depending on the haploid chromosome number (n).

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The number of possible gametes generated by independent assortment alone (i.e., without considering
the effects of crossing over) is 2n, where n is the haploid number of chromosomes in a cell.  For example,
in  Homo  sapiens  (humans)  n  =  23  thus  a  human  cell  can  produce  over  8  million  different  gametes  by
independent  assortment  (223  =  8.4  million).    The  effect  of  crossing  over  further  increases  genetic
variation  in  gametes  generated  by  an  individual  such  that  the  number  of  possibilities  is  essentially
infinite.

Exercise 2B – Modeling independent assortment

Use the chromosome bead models to model meiosis with one, two, or three pairs of homologous chromosomes.
By  doing  this  exercise  you  should  be  able  to  determine  all  the  different  combinations of  chromosomes  that  are
possible in gametes due to the independent assortment of homologous pairs.  Diagram all possible arrangements
in the circles provided using red for maternal chromosomes and yellow for paternal chromosomes.

NOTE: You will disregard crossing over between homologous chromosomes for this exercise.

Before  you  leave,  please  make  sure  your  table is  clean,  organized,  and  contains  all  supplies
listed below so that the next lab will be ready to begin.  Thank you!

Lab Supply List

Lens paper

•
•  Chromosome bead kit

Also, please attend to the following before you leave:

•  Return all prepared slides to the correct trays.
•  Put the compound microscopes away as indicated below, and make sure there is no slide on

turn off power

the stage:
o
o  secure the power cord to the back of the microscope
o  set to the lowest magnification (4X objective)
o  cover and place “arm out” in the cupboard at your table

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LABORATORY 9 – WORKSHEET

Name ________________________

Section _______________________

Ex. 1 – Observing mitosis under the microscope

Diagram a single cell in each of the indicated stages of mitosis both Onion Root Tip (plant) and
Whitefish Blastula (animal):

ONION ROOT TIP (Allium)

WHITEFISH BLASTULA

Interphase

Interphase

Prophase

Prophase

Metaphase

Metaphase

Anaphase

Anaphase

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ONION ROOT TIP (Allium)

WHITEFISH BLASTULA

Telophase

Telophase

Ex. 2A – Modeling the stages of meiosis

Diagram each stage of meiosis as it unfolds for a cell with 1 pair of homologous chromosomes.  Be sure to use
different colors for maternal and paternal chromosomes and to have one crossover event.

   MEIOSIS I

Prophase I

Important Events

Tetrad formation
and crossing over

Metaphase I

        Alignment of tetrads

Anaphase I

Telophase I &
Cytokinesis

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Separation of
homologous
chromosomes

 2 haploid cells
     produced

MEIOSIS II

Prophase II

           Metaphase II

            Anaphase II

Telophase II &
Cytokinesis

       Alignment of
        individual
   chromosomes

         Chromatids
            separate

      Four gametes
                                     Produced

Ex. 2B – Modeling independent assortment

1.  A cell with 1 pair of homologous chromosomes (n = 1)

Assemble two duplicated chromosomes, one red and one yellow, with 6 beads on each end of the centromere.  By
modeling meiosis, determine all possible combinations of chromosomes in gametes when n = 1, assuming there is
NO crossing over, and diagram each unique combination in the squares below using different colors for maternal
and paternal chromosomes.

21  =   ________ (possible chromosome combinations when n = 1)

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2.  A cell with 2 pairs of homologous chromosomes (n = 2)

Assemble another homologous pair of duplicated chromosomes with 4 beads on each side of the centromere.  Use
this homologous pair with the longer homologous pair from the previous exercise to model meiosis when n = 2 and
determine  all  possible  combinations  of  chromosomes  in  gametes,  assuming  there  is  no  crossing  over.    Diagram
each unique combination in the squares below using different colors for maternal and paternal chromosomes.

22  =   ________ (possible chromosome combinations when n = 2)

3.  A cell with 3 pairs of homologous chromosomes (n = 3)

Assemble another homologous pair of duplicated chromosomes with 2 beads on each side of the centromere.  Use
this homologous pair along with the homologous pairs from the previous exercise to model meiosis when n = 3 and
determine  all  possible  combinations  of  chromosomes  in  gametes,  assuming  there  is  no  crossing  over.    Diagram
each unique combination in the squares below using different colors for maternal and paternal chromosomes.

23  =   ________ (possible chromosome combinations when n = 3)

Question:  How many gametes would be produced by independent assortment alone, in a cell

      with 8 pairs of homologous chromosomes? __________________

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