Question

1. Starting with a diploid cell that has 8 chromosomes, draw out/compare and contrast mitosis with meiosis. Label and describe what happens during each stage as well as the maternal and paternal chromosomes. Also label the reductional and equational division for both processes when appropriate.

Answer 1

Mitosis

Mitosis is an elaborate process that occur in 2 stages.

1) Karyokinesis - Karyokinesis is the division of nucleus. It is a continuous process that occur in 4 stages :

a) Prophase - In prophase, there is condensation of chromatin material, each chromosome appears double and consists of two sister chromatids joined by a centromere. The centrioles in the animal cell begin to move towards the opposite poles of the cell with initiation of assembly of the mitotic spindles.

b) Metaphase - The disintegration of the nuclear envelope marks the beginning of metaphase. The chromosomes during this stage are the thickest and shortest hence, it is best to study their morphology. Mitotic spindle formation is complete and chromosomes start aligning themselves at the equator with the spindle fibres movement which is attached to the kinetochores present on the centromere of each chromosomes. All the chromosomes gst align at the equator which is called the metaphasic plate.

c) Anaphase - The centromere that holds up the two chromatids together splits and the seperated sister chromatids are reffered as chromosomes. The spindle fibres attaached to the kinetochores now shorten and daughter chromosomes begin to move towards the opposite poles. During migration, the centromere of the daughter chromosome move towards the pole while the arms of chromosomes trail it. Hence, it is the best stage to study shape of chromosomes (V, L, J, I).

d) Telophase - The chromosomes upon reaching the opposite poles, mitotic spindle disappears. The chromosomes gradually uncoil and become thin, slender, long and lose their identity. They become indistinguishable mass and collect at the poles. Nucleolus, endoplasmic reticulum, golgi bodies and nuclear envelop reappear in the daughter cells.

2) Cytokinesis - Cytokinesis is the division of cytoplasm. This phase marks the end of cell division. In animal cells, cytokinesis is achieved by the formation of a furrow. The furrow in the plasma membrane of the cell deepens gradually. It moves centripetally and ultimately joins in the centre dividing the cell cytoplasm into two. In plant cells, the cytokinesis is achieved by the formation of a cell plate. Phragmoplast is formed by golgi complex and grows centrifugally to form the cell plate which ultimately joins the cell plate, dividing the cytoplasm into two.

Meiosis

Meiosis is the double division in which a diploid cell form four haploid cells, each having half the number of chromosomes. It is studied in two divisions Meiosis-| and Meiosis-||.

1) Meiosis-| - It is the reduction division in which the number of chromosomes is reduced to half. It is divides into four stages :

a) Prophase | - Prophase | is more elaborate, prolonged, complex than prophase of mitosis which is further sub-divided into 5 stages :

1) Leptotene - The chromatin material condenses to form distinct chromosomes which gradually become visible under the light microscope.

2) Zygotene - A diploid cell contains two sets of chromosomes. The two chromosomes which are similar in size, form and structure are called homologous chromosomes. One of which is paternal chromosome and other is maternal chromosome. During zygotene, these homologous chromosomes start pairing together. These chromosomes come to lie side by side in pairs and this pairing is known as synapsis. And the complex formed by this pairing is called bivalent.

3) Pachytene - The two chromatids of same chromosomes are called sister chromatids while of two different homologous chromosome is called non-sister chromatids. During pachytene, crossing over occurs between the non-sister chromatids of the homologous chromosomes. The exchange of genetic material between the non-sister chromatids of the homologous chromosomes is known as crossing over. The site where crossing occurs form a recombination nodule. An enzyme called recombinase is involved in this process.

4) Diplotene - During this stage, the crossing over is complete and the two homologous chromosomes begin to separate from each other. Therefore, the homologous chromosomes separate except in the region of crossing over. The point of attachment between the homologous chromosomes is called chiasmata.

5) Diakinesis - During this stage, there is terminalisation of chiasmata which occurs by slipping-off or sliding from the tip of the chromosomes. Spindle fibres assembles to prepare homologous chromosomes for separation.

b) Metaphase | - The bivalent chromosomes align themselves on the equatorial plate. The centromeres of the two chromosomes are arranged in two rows at the equator. So double metaphasic plate is formed. This arrangement is random and spindle fibres from opposite poles attach to the chromosomes.

c) Anaphase | - The two homologous chromosomes separate from each other and start moving towards the opposite poles. In this phase division of centromere does not occur. Since only one chromosomes out of a pair reaches the pole, the number of chromosomes become half in this phase.

d) Telophase | - The chromosomes reach the pole and spindle fibre disappear. The nucleolus and nuclear membrane reappear. The chromosomes uncoil and elongate but remain straight in this phase. They do not reach the extreme extended stage.Cytokinesis - It generally follows the first nuclear division, so two daughter cells are formed.

Interkinesis - It is a metabolic stage between telphase | and prophase ||. It is a gap which exists between meiosis | and meiosis ||. There is protein synthesis.

2) Meiosis || - It is similar to mitosis but not exactly same as meiosis-|| occur in haplois cellsCytokinesis - The cytoplasm divides by furrow formation in animal cell and by cell plate formation in plant cell which result in four separate haploid cells from a single diploid cell.The above diagrams has 4 chromosome number but for 8 chromosomes also the diagram will be samw just with more chromosomes. The above details will give you the knowlwdge to compare both the processes.