BIO 101 BIO 101 BIO 101 BIO 101 –––– LaboratoryLaboratoryLaboratoryLaboratory August 2009August 2009August 2009August 2009

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EXERCISE V

Cell Growth and Division

2 General Processes of Cell Division: 1. Karyokinesis – organization of DNA strands into chromosomes & separation of chromosomes (nuclear

division)

2. Cytokinesis – division of the cell body (cytoplasm & its organelles)

Cell Cycle � events that occur from the completion of one division until the beginning of the next division constitute the

cell cycle

Interphase Stage – interval between divisions

� devoted to growth and metabolism

� the time for chromosomal replication

1. G1 (1st Gap) – initiation of new cycle

� Cells may become non-dividing or continue through G1

2. S Phase – replication of the DNA of each chromosome; DNA Synthesis

3. G2 (2nd

Gap) – volume of cell has doubled, DNA has been replicated, mitosis is initiated

MITOSIS

� a form of cell division resulting in the production of two cells, each with the same chromosome & genetic

complement as the parent cell

� somatic cell division

� occurs on cells during embryonic development, growth, repair of injured tissues & replacement of worn-out

tissues

� Specimen used:

� Allium cepa root tips (plant cell)

� Root tips → region of active cell division (meristematic)

� Most cells are in Interphase and Prophase stage

� Whitefish blastula (animal cell)

� Blastula → embryonic stage in the development of a fertilized egg

MEIOSIS

� the process that produces haploid gametes in diploid organisms

� the cell undergoes two consecutive divisions – Meiosis I & Meiosis II

� four daughter cells result from the divisions, in contrast to the two daughter cells produced from mitotic

division

� 2 divisions of meiosis are preceded by only one duplication of chromosomes → each of the 4 daughter cells

resulting from meiosis has only half as many chromosomes as the starting cells – a single haploid set of

chromosomes

� Specimen used: Grasshopper testis (animal cell)

BIO 101 BIO 101 BIO 101 BIO 101 –––– LaboratoryLaboratoryLaboratoryLaboratory August 2009August 2009August 2009August 2009

This only serves as your guide. You still have to read your manuals

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MITOSIS

Allium cepa – root tips

Interphase

Interphase

� Cell is undergoing respiration and synthesis of DNA, RNA, and protein in

preparation for mitosis

Prophase

Prophase

� Chromosomes become distinguishable as long, thin tubules in the nucleus

� Nuclear membrane begins to break down

� Chromosomes are distributed in the cytoplasm as coiled mass

� Each chromosome is composed of 2 separate strands – sister chromatids

which are identical

� Sister chromatids are joined together at a region of attachment,

centromere

� Within this region is the kinetochore from which projects the spindle to

the 2 poles of the cell

Metaphase

Metaphase � Double stranded chromatids are arranged near the center of the cell at

right angles to the long axis of the spindle fibers forming the metaphase

plate

Anaphase

Anaphase � Chromatids are pulled by the microtubules to opposite poles of the cell

� This stage is recognized by the V-shaped chromosomes in 2 groups at

opposite sides of the cell

� Anaphase ends when separated chromatids, now called chromosomes

reached the opposite poles

Telophase

Telophase

� Cell plate starts as a fine line across the center of a cell

� When complete, the cell plate divides the original cell into 2 daughter

cells

� Nuclei begins to reorganize; chromosomes uncoil & become longer &

thinner, nuclear membrane re-forms, and nucleoli begins to reappear

� Karyokinesis is completed at the end of telophase and cytokinesis begins

BIO 101 BIO 101 BIO 101 BIO 101 –––– LaboratoryLaboratoryLaboratoryLaboratory August 2009August 2009August 2009August 2009

This only serves as your guide. You still have to read your manuals

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MITOSIS

Whitefish blastula

Interphase

Interphase � with distinct nucleus bounded by a nuclear membrane

� with identifiable nucleolus

� presence of centrosome (or cell center) containing the centrioles

Prophase

Prophase – stage of preparation

� 2 pairs of centioles start to move apart to opposite poles

� Microtubules – radiate from each pair of centrioles like spokes on a wheel

forming a configuration called Aster

� disappearance of the nuclear membrane

� appearance of spindle fibers (microtubules + spindle)

� short chromosomes become visibly doubled → each chromosome consists

of 2 sister chromatids

Metaphase

Metaphase – stage of separation

� chromosomes move & align themselves along the equatorial plane

forming the metaphase plate

Anaphase

Anaphase – stage of migration � pair of chromatids are pulled apart by microtubules of the spindle and

become daughter chromosomes

� then they are pulled apart toward opposite poles of the cell

Telophase

Telophase – stage of nuclear reconstruction & cytosome division

� spindle disappears, 2 daughter nuclei are organized

� nucleoli appear, nuclear membranes are formed

� cleavage furrow; cytokinesis takes place

� results into 2 daughter cells with the same genetic material as the parent

cell.

BIO 101 BIO 101 BIO 101 BIO 101 –––– LaboratoryLaboratoryLaboratoryLaboratory August 2009August 2009August 2009August 2009

This only serves as your guide. You still have to read your manuals

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MEIOSIS I

The resulting cell at the end of Meiosis I is reduced from the diploid to the haploid condition

1. Prophase I � Homologous chromosomes – each are composed of two chromatids

� Substages:

� Leptotene – chromosomes are long, thin, optically single threads with little coiling

� Zygotene – homologous chromosomes begin to pair (synapsis) and lie adjacent to one another to form

bivalents � Pachytene – exchange of genetic components called crossing over is manifested by “X” arrangements

(chiasmata) of portions of homologous chromosomes; portions of each of the paired chromosomes

separates into 2 sister chromatids (structure at this stage is then called a tetrad).

� Diplotene – chromosomes clump on the center of the cell

� Diakinesis – chromosomes pairs as well separated through the cell. Nucleolus generally disappears and

nuclear membrane begins to break down

2. Metaphase I � Homologous chromosomes (called tetrads) line up side by side at the metaphase plate

� Spindle fibers join the centromeres to the poles

� Nuclear membrane is completely gone

3. Anaphase I

� Homologous chromosomes in each pair separate from each other and move to opposite poles of the cell

4. Telophase I - Chromosomes are regrouped at the poles as in mitosis.

BIO 101 BIO 101 BIO 101 BIO 101 –––– LaboratoryLaboratoryLaboratoryLaboratory August 2009August 2009August 2009August 2009

This only serves as your guide. You still have to read your manuals

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MEIOSIS II

Equational division resulting to four haploid cells

1. Prophase II

� Similar to mitosis, it involves the contraction and coiling of chromosomes

� Synapsis and crossing over does not occur anymore but the remnants of the crossing over from prophase I is

carried over.

� Chromosomes are already reduced from diploid to haploid

2. Metaphase II

� Chromosomes line up at the metaphase plate; spindle fibers join the chromosomes to the opposite poles

3. Anaphase II

� Sister kinetochores move separately to the poles pulling with them the chromatids to which they are

attached (sister chromatids move to opposite poles).

4. Telophase II

� Interphase nuclei are reconstituted and cell walls are laid down to give 4 cells known as a tetrad.