MITOSIS / MEIOSISMITOSIS / MEIOSIS

Lab 7

OBJETIVESOBJETIVES• Name the stages of the cell cycle and

describe their characteristics.

• Name the phases of mitosis and meiosis and describe their characteristics.

• Identify the phases of mitosis and meiosis from diagrams, pictures or micrographs.

• Compare the processes and end products of mitotic and meiotic cell division.

• Describe the significance of mitotic and meiotic cell divisions.

Be able to define the following terms in writing:

a- chromosome l- spindle

b- chromatid m- equatorial plane

c- centromere n- cytokinesis

d- chromatin o- furrow

e- nucleolus p- cell plate

f- centriole q- daughter cell

g- poles

h- diploid

i- crossing-over

j- haploid

k- tetrad

Cell Cycle: period between two sequential divisions

• Interphase– Growth (G1),

– Synthesis (S),

– Growth (G2)

• Mitotic phase– Mitosis and

cytokinesis

Figure 3.30

MitosisMitosisGrowth in our bodies, and in those of other many-celled organisms, is basically a process of increasing the number of cells. This involves two processes; the distribution of copies of the genetic information from the parent cell to the two daughter cells and the cytoplasmic division. In this way, a body grows or wounds are repaired. Each of the new cells will include all the genetic information possessed by all of the other living cells of the body.

• Mitosis is an orderly series of events that flow without interruption. We have artificially divided this smooth flow into stages: prophase, metaphase, anaphase, and telophase. It is a convenience to be able to classify parts of the process in this way, but it can be misleading if we forget that there is really no pause or interruption in the events.

• When we look at a plant or animal cell that has been killed and stained while in the process if mitosis, we are looking at the stopped action.

Early and Late Prophase

• Asters are seen as chromatin condenses into chromosomes

• Nucleoli disappear

• Centriole pairs separate and the mitotic spindle is formed

Early prophase

Early mitotic spindle

Pair of centrioles

Centromere

Aster

Chromosome, consisting of two sister chromatids

Late prophase

Fragments of nuclear envelope

Polar microtubules

Kinetochore

Kinetochore microtubule

Spindle pole

Metaphase• Chromosomes cluster

at the middle of the cell with their centromeres aligned at the exact center, or equator, of the cell

• This arrangement of chromosomes along a plane midway between the poles is called the metaphase plate

Metaphase

Metaphase plate

Spindle

Anaphase

• Centromeres of the chromosomes split

• Motor proteins in kinetochores pull chromosomes toward poles

Daughter chromosomes

Anaphase

Telophase and Cytokinesis• New sets of

chromosomes extend into chromatin

• New nuclear membrane is formed from the rough ER

• Nucleoli reappear

• Generally cytokinesis completes cell division

Telophase and cytokinesis

Nucleolus forming

Contractile ring at cleavage furrow

Nuclear envelope forming

Cytokinesis

• Cleavage furrow formed in late anaphase by contractile ring

• Cytoplasm is pinched into two parts after mitosis ends

MeiosisMeiosis If a species is to retain the original

number of chromosomes and produce offspring with chromosomes from both male and female parent, then the parents must have a mechanism to produce cells with half the number of chromosomes. The production of these cell with the haploid number (one-half of the parental number of chromosomes) results from the special cell division called Meiosis.

Meiosis consists of two cell divisions.

Meiosis

• Many organisms reproduce sexually. Sexual reproduction means the formation of new individual by a combination of two sex cells (gametes). Gametes are product of meiosis and they are haploid

• Meiosis is the type of the cell division that reduces the is the type of the cell division that reduces the number of chromosomes in the daughter cells by number of chromosomes in the daughter cells by halvehalve..

• Meiosis is the process that includes two consecutive cell divisions. First (Meiosis I) or reduction. Second division (Meiosis II) or equation division.

• During the first division of meiosis the chromosomes line up at the equator of the cell in pairs. Each pair separates and one member of the pair moves to one pole; the other member of the pair moves to the other pole. When the two nuclei reorganize and cytokinesis occurs, the resultant two cells have one-half as many chromosomes as the testis cell had. Reduction from the diploid number of chromosomes to the haploid number of chromosomes has been accomplished.

Meiosis I or Reduction division• Prophase I:

Chromosomes descondensed even farther. Homologous chromosomes pair and form very close contacts. As a result of synapses, the exchange of DNA (crossing over) between sister chromatids may occur.

Pairs of homologous chromosomes are called bivalents (2 chromosomes and 4 chromatides).

Meiosis I (cont)

• Prometaphase I: The nuclear m. dissapears.One kinetochore

forms per chromosomes some rather than one per chromatid, and the chromosomes attached to spindle fibers begin to move.

• Metaphase I: • Bivalents, each composed of two

chromosomes (four chromatids) align at the metaphase plate. (50/50 chance for the daughter cells to get either mother or father’s homologues)

Meiosis I (cont)

• Anaphase I: Homologous chromosomes separate and move to the opposite poles. There is haploid set of chromosomes at each pole but each chromosome has two chromatids. Each daughter receive the reduce number of chromosomes. For this reason, the first division of meiosis is called the reduction division . In this stage occurs a misbalance between # chrom. and DNA , to restore the balance , the second meiosis is needed

Meiosis I (cont)

• Telophase I: Nuclear envelopes may or may not form around the chromosomes. Chromosomes stay condensed

• Interkinesis: This stage is analagous of cytokinesis of mitosis. Two complete daughter cell form. Chromosomes stay condensed

• Each of the two cells produced as a result of meiosis I go through a second division (Meiosis II).

• This second division results in the formation of four cells. During this division, chromosomes line up on the equators of the cells. The centromeres divide and one of the chromatids moves to one pole; the other chromatid moves to the other pole.

Meiosis II or Equation division

• Prophase II:

Centrosomes begin to move to oppossite poles of the cell and microtubules cross the cell to form the mitotic spindle.

Meiosis II (cont)

• Prometaphase:• Microtubules attach at the kinetochores, and the

chromosomes begin moving

• Metaphase II

Spindle fibers align the chromosomes along the equator of the cell in the metaphase plate. All chromosomes are laying in one plane, their sister chromatids leaning toward the opposite poles. This organization helps to ensure that each new nucleus will receive one copy of each pre-S-phase chromosomes

Meiosis II (cont)

• Anaphase II: The paired sister chromatids separate at the centromeres and move to opposite poles of the cell. Now each chromosome has only one DNA molecule.

(# chromosomes = # DNA molecules)

For this reason, the second meiosis division is called equation division.

• Telophase II: Chromatids arrive to the opposite poles of the cell, and new nucleus reconstructed around them

Cytokinesis• In animal cells, cytokinesis occur when

cell membrane contracts pinching the cell into two daughter cells, each with one nucleus.

• In plant cells, the rigid cell wall requires the partitioning membrane to be synthesized between the two daughter cells.

• One diploid cell is entered meiosis. Four haploid daughter cells emerged as the result of the type of cell division.

• The four cells that result from these meiotic divisions may mature into sperm cells. Each cell has the haploid number of chromosomes and each chromosome is composed of a single chromatid. These four cells are not necessarily alike in the genetic information that they contain. Each cell contains similar packages of information, but the information in each package could be quite different.