Lecture 2

Topic: Time organization of cell. Cell cycle. Mitosis and its periods. Structural organization of chromosomes. Components of chromosome and dynamics in cell cycle. Morphofunctional characteristics and classification of chromosomes. Karyotype of man. Reproduction. Meiosis. Parthenogenesis.

Plan of lecture:

1. Cell cycle, periods.

2. Mitosis

3. Eukaryotic chromosomes, location, morphology, physical structure, ultrastructure.

4. Classification of chromosomes, types. Karyotype of man. Idiogram. Polytene chromosomes. (Denwer’s and Paris classification)

5. Reproduction: Sexual and asexual. Reproduction as universal property of living.

6. Meiosis. Spermatogenesis and oogenesis. Gametogenesis. Parthenogenesis.

1. Cell cycle is well – ordered sequence of events between the time a cell divided to form two daughter cells and the time those daughter cells divide. Includes a doubling of a cell’s cytoplasm and reproduction of cellular organelles, precise duplication of DNA, mitosis and cytokinesis.

Cell cycle duration varies with the type of cell. Some cells divide each hour, others take more 24 hours. The cell cycle alternates between M phase, or dividing phase, and interphase, the nondividing phase.

M phase, the shortest part of the cell cycle and the phase during which the cell divides, includes:

1. Mitosis – division of the nucleus

2. Cytokinesis – division of the cytoplasm.

Interphase, nondividing phase which is about 90% of the cell and includes most of a cell’s growth and metabolic activities.

It is a period of intense biochemical activity during which the cell growth and copies its chromosomes in preparation for cell division. Consist of three periods;

1- G₂ - period (Gap two - 2n 2chr. 4С)–cell growths

2- Mitosis (prophase, metaphase, anaphase, telophase)

3- G₁ – period (Gap one - 2n 1chr. 2С),

4 – S-period - DNA synthesis ( 2n 2 chr. 4С)

Gap one

● Enzymes and substrates are made ready;

● Cell growth occurs;

● Mitochondria, chloroplasts, lysosomes, endoplasmic reticulum, Golgi complex, Vacuoles and vesicles are produced;

● Structural and functional proteins are formed.

● Nucleus produces r-RNA, m-RNA, t-RNA;

● Ribosome’s are synthesized.

● Metabolic rate of the cell becomes very high.

Duration of the period from same hours to some months.

S- Synthesis

• DNA synthesis takes place (replication of DNA take place);

• Protein molecules called histones are synthesized that cover each strand of DNA;

• Each chromosome is in the form of two chromatids. Duration of S-period – 6-10 hours.

Gap two

● Spindle and aster proteins are formed. ( There is no DNA synthesis during Gap one and Gap two)

● Centriole replicates

● Mitotic spindle begins to form

● Energy store increases.

● Increased metabolic rate of cell.

Duration Gap two stage 3-4 hours.

2. Mitosis is indirect division of cell. It occurs in somatic cells. There is equal division of genes and DNA daughter cells which are identical to the mother cells.

Nuclear division occurs in four main stages – Prophase, metaphase, anaphase and telophase. It is followed by an equal division of cytoplasm or cytokinesis and organelles into each daughter cells. It helps in growth, replacement and repair of cells.

Significance of mitosis

1. Identical gene composition

   • The cells contain the same number of chromosomes as the parental cell.

   • The cells carry the same hereditary information as in the parental cells.

   • There is no variation in genetic information

   • It gives a genetic stability within a population.

2. Growth. The increase in number of cells brings about body growth and scope for cell differentiation.

3. Cell replacement. The replacement of lost cell like wound healing, skin cells, blood cells, cells of digestive tract, occur to varying degrees.

4. Regeneration and sexual reproduction.

In some animals regeneration of the lost part leads to a mode of multiplication e.g. Planaria, sponges.

In plants asexual reproduction by which the number of species increases.

3. Hofmeister discovered nuclear filaments in the nuclei of the pollen mother cells of Tradscantia in 1848. these nuclear filaments were called chromosomes by Waldeyer in 1888. The term “chromosome” means a colored body (G. chrome - color; soma – body). It points to the fact that the chromosomes easily take up biological stains.

Chromosomes are unique cell organelles capable of replication (production of exact copies). They store and transmit coded information about biological activities.

The eukaryotic chromosomes occur in the nucleus and certain other organelles, an are respectively called nuclear and extranuclear chromosomes (these are present in the mitochondria and plastids).

Nuclear chromosomes are long, double-stranded DNA molecules of linear forms and with a protein coat. Extranuclear chromosomes are short, double-stranded DNA molecules f circular form and without a protein coat like the prokaryotic chromosomes. These may be called organelle prochromosomes. They have genetic information for the synthesis of only some proteins for the organelles containing them. Mitochondria and plastids transmit their chromosomes to daughter organelles.

Morphology. In the interphase stage of the cell cycle, the eukaryotic chromosomes are extended into long, thin chromosomes fibres, which often lie criss-cross to form the chromatin reticulum. They replicate in the S-phase and become double, each now consist of two chromatids that remain held together at one point called centromere.

Physical Structure at metaphase. A metaphase chromosome consists of two identical components, the chromatids, which lie side-by-side along their length and are held together at one point the centromere. At this region each chromatid has a plate-like kinetochore where spindle microtubules join the chromosome during the cell division. The centromere appears as a constriction in the chromosome. The part of the chromatids on the two sides of the centromere a known as arms. The arms may be equal (isobrachial) or unequal (heterobrachial), depending upon the position of the centromere.

The end of the chromosomes are called telomeres. Primary construction , a chromosome may have an additional one termed secondary constructions. Part of the chromosome beyond the secondary constrictions is called satellite. The chromosome bearing a satellite is known as sat chromosome.