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INTRODUCTION

A special type of nuclear division that

separates one copy of each homologous

chromosome into each new gamete is theprocess known as meiosis. Mitosis conserves

the original ploidy level (for example, one

diploid 2n cell producing two diploid 2n cells,

one haploid n cell producing two haploid n cells,

etc.) while meiosis reduces the number of sets

of chromosomes by half. This is the reason why

when gametic recombination (fertilization)

happens the ploidy of the parents will be re-

established. A specific type of meiosis is

spermatogenesis, a process that forms sperms

(Pierce, 2005).

The spermatogonium that is contained

in the male testes having diploid cells matures

to become sperm. To turn each one of the

diploid spermatogonium into 4 haploid sperm

cells is the basic function of spermatogenesis. In

this experiment, the process of

spermatogenesis will be studies using prepares

slide of the testes of a frog.

METHODOLOGY

RESULTS AND DISCUSSION

Meiosis is a process that reduces the

amount of genetic material by one-half. It

produces gametes or spores with only one

haploid set of chromosomes. In meiosis,

homologous chromosomes pair together; they

synapse. Each synapsed structure, called a

bivalent, gives rise to a unit, the tetrad,

consisting of four chromatids. The presence of

four chromatids demonstrates that both

chromosomes have duplicated. In order to

achieve haploidy, two divisions are necessary

(Klug et. al, 2000).

In meiosis I, a reductional division,

components of each tetrad separate, yields two

dyads. Each dyad is composed of two sister

chromatids joined at a common centromere.

The initial stage, prophase I, is subdivided into

five substages: leptonema, zygonema,

pachynema, diplonema, and diakinesis. During

the leptonene stage, the interphase chromatin

material begins to condense and the

chromosomes become visible. The

chromosomes continue to shorten and thicken

during the zygotene stage. At the completion of

zygonema, the paired homologues take the

form of bivalents. During the pachytene stage,

homologous pairs of chromosomes undergo

synapsis. Each homologue is a double structure,

showing the DNA replication of each

chromosome. Each bivalent contains four

member chromatids. During the diplotene

stage, within each tetrad, each pair of sister

MEIOSIS

Bernadas, Mary Anne, Cabico, Farhanna, Calisin, Cherie Ann,

Salgado, Cris Adams

University of the Philippines - Baguio, College of Science, Department

of Biology,

Gov. Pack Road, Baguio City, 2600

July 2, 2013

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chromatids separate but one or more areas

remain contact where chromatids are

intertwined called chiasma or chiasmata. This is

where crossing over, a genetic exchange

process, occurs between synapsed homologues.

The final stage of prophase I is diakinesis where

the chiasmata remains and moves toward the

ends of the tetrad. This process is called

terminalization. During this final period of

prophase I, centromeres of each tetrad attaches

to the spindle fibers.

The next stage is Metaphase I where random

arrangement of paternal and maternal

chromosomes at the metaphase plate occurs.At Anaphase I, one dyad is pulled toward each

pole of the dividing cell. The chromosomes are

separated from one another. At telophase I, a

nuclear membrane is formed around each dyad.

During meiosis II, an equational division, each

dyad splits into two monads of one

chromosome each thus producing four haploid

cells. During prophase II, each dyad is

composed of one pair of sister chromatids

attached by a common centromere. During

metaphase II, the centromeres are positioned

on the equatorial plate. At anaphase II, sister

chromatids of each dyad are pulled to opposite

poles. The last stage, telophase II, reveals one

member of each pair of homologous

chromosomes present at each pole. Each

chromosome is called a monad. Each meiotic

event results to four haploid gametes (Klug et.

al, 2000).

One of the gametogenesis in animal

species observed in the laboratory is

spermatogenesis. Spermatogenesis takes place

in the testes, the male reproductive organs. The

process begins with the expanded growth of an

undifferentiated diploid germ cell called a

spermatogonium. This cell enlarges to become

a primary spermatocyte, which undergoes the

first meiotic division. The products of thisdivision, called secondary spermatocytes,

contain a haploid number of dyads. The

secondary spermatocytes then undergo the

second meiotic division, and each of these cells

produces two haploid spermatids. Spermatids

go through a series of developmental changes,

Figure 1. Five stages of Prophase I.

Figure 2. Process of Meiosis I.

Figure 3. Process of Meiosis II.

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spermiogenesis, and become highly specialized,

motile spermatozoa or sperm. All sperm cells

produced during spermatogenesis receive equal

amounts of genetic material and cytoplasm

(Klug et. al, 2000).

CONCLUSION

ANSWERS TO QUESTIONS

REFERENCES

Pierce, B. (2005). Genetics: A Conceptual

Approach. New York City: W. H.

Freeman and Company.

Klug, W., et. al. 2000. Concepts of Genetics,

Sixth Edition. Prentice Hall, Inc.

Figure 4. Stages in Spermatogenesis.