Molecular Cell Biology. Prof. D. Karunagaran. Department of Biotechnology. Indian Institute of Technology Madras

Transcription

1 Molecular Cell Biology Prof. D. Karunagaran Department of Biotechnology Indian Institute of Technology Madras Module 6 Cell Division, Cell Cycle, Cell Growth and Differentiation, Programmed Cell Death Lecture 1 Cell Division

2 Cell division Cell division results in genetically identical daughter cells Cells duplicate their genetic material, DNA The process is carried out in such a way that each daughter cell receives an exact copy DNA from the parent cell Cell Division helps in the continuity of life especially in unicellular organisms since it is the basis for their reproduction In multicellular organisms cell division helps in the development of the fertilized cell, growth and repair mechanisms and it is also an integral part of the cell cycle Cell division in bacteria, plants and animals Prokaryotes (bacteria) reproduce by a type of cell division called binary fission In animal cells cytokinesis (division of cytoplasm) occurs by a process known as cleavage, forming a cleavage furrow In plant cells, during cytokinesis a cell plate forms. Prokaryotic Cell Cycle Prokaryotic chromosome is circular loop in shape The changes take place through binary fission The chromosome first attaches to one point on the plasma membrane Then the chromosome is replicated from the origin of replication and the replicated chromosome is attached to plasma membrane at a different point cell elongates new plasma membrane is added between chromosomes, pushing them towards opposite ends of cell plasma membrane grows inward at middle of cell parent cell is now divided into two identical daughter cells.

3 Eukaryotic cells divide through cell cycle The cell cycle consists of The mitotic phase Interphase Interphase can be divided into sub phases G 1 phase S phase G 2 phase Thus the mitotic phase alternates with interphase in the cell cycle

4 The mitotic phase is made up of mitosis and cytokinesis Mitosis, the division of the nucleus Cytokinesis, the division of the cytoplasm In meiosis sex/germ cells are produced after a reduction in chromosome number Mitosis Mitosis consists of four distinct phase Prophase Metaphase Anaphase Telophase The mitotic spindle is an apparatus of microtubules that controls chromosome movement during mitosis The spindle arises from the centrosomes and includes spindle microtubules and asters Some spindle microtubules attach to the kinetochores of chromosomes and move the chromosomes Non kinetochore microtubules from opposite poles overlap and push against each other, elongating the cell Microtubules and centrosomes When the cell enters S phase DNA is replicated. Similarly in the cytoplasm, the centrosome ( central body ), an organelle that lies near the nucleus, doubles, forming a pair of centrosomes. Generally each centrosome has a pair of centrioles, each one is a hollow tube lined with nine microtubules. The two tubes are at right angles to each other.

5 Significance of Mitosis Genetic stability Maintain same number of chromosomes as a parental cell. Daughter cells are genetically identical to the parental cell with no variation in genetic information Growth Growth in multicellular organisms Cell replacement Replacement of damaged cells and tissues Regeneration Some animal are able to regenerate whole parts of the body Asexual reproduction Production of new individuals of a species by one parent organism

6 Mitosis in a plant cell Prophase: The chromatin is condensing. The nucleolus is beginning to disappear. The mitotic spindle is starting to form. Metaphase: The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate. Anaphase: The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of cell as their kinetochore microtubules shorten. Telophase: Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell. CYTOKINESIS IN PLANT CELL

7 Vesicles-containing cell wall material (cellulose) derived from golgi apparatus move to the middle of the cell vesicles line up at equator and fuse, producing a structure called the cell plate Cell plate enlarges and fuses with the parent cell plasma membrane Formation of 2 new cells. cell plate becomes new cell wall between the two cells CYTOKINESIS IN ANIMAL CELL In animal cells, a ring of actin fibers (microfilaments are composed of actin) forms around the cell equator and contacts, pinching the cell in half.

8 Major differences between mitosis in plant and animal cells Animal cells have centrioles and plant cells do not have centrioles In animal cells cytokinesis occurs by the constriction of microtubules cleavage furrow. In plant cells cytokinesis occurs by the growth of a cell plate through the fusion of vesicles Sites of mitosis In growing tissues mitosis occurs. Examples include the tissues from foetus, babies and growing children. In adults, most tissues do not proliferate However in some adult tissues mitosis occurs regularly Red bone marrow for production of blood cells (erythropoiesis) Lymphoid tissue - formation of lymphocytes (lymphooiesis) Testes for spermatogenesis (production of spermatozoa) Epidermis - replacement of superficial skin cells Hair follicles - hair growth Gastro-intestinal tract - renewal of epithelium Mitosis does not normally occur in: Neurons and Muscle cells Stages of Mitosis Prophase The chromosomes gradually condense and appear as strands that become thicker and shorter The nuclear envelope breaks up. Metaphase The chromosomes are condensed; A mitotic spindle is formed of microtubules; Microtubules attach to the centromeres on chromosomes and to the centrioles at opposite poles of the cell

9 Anaphase The chromatids separate and move to opposite poles Telophase The chromatids are at the opposite poles Nuclear envelope formation occurs Cytokinesis Division of the cytoplasm is mediated by actin filaments. Prophase Chromatin become more tightly coiled & condensed into chromosomes that consist of 2 sister chromatids. Mitotic spindle begins to form Centrosomes move away from each other to the opposite poles Nuclear envelope disintegrates Nucleolus disappears

10 Metaphase Metaphase is a short period in which the chromosomes are lined up on the equator of the cell, usually called metaphasic plate The sister chromatids arranged themselves at metaphase plate The centrosomes align at opposite ends and the spindle fibers are attached to the centromeres Anaphase

11 In anaphase, the centromeres divide. The sister chromatids separate and move away from each other Then the spindle fibers contract, and the chromosomes are pulled to opposite poles, towards the centrosomes Telophase and cytokinesis In telophase the cell actually divides. The chromosomes are at the poles of the spindle. The spindle disintegrates. The nuclear envelope re-forms around the two sets of chromosomes. The cytoplasm is divided into 2 separate cells

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13 Molecular changes during mitosis Evolution of mitosis Since prokaryotes preceded eukaryotes by billions of years it is likely that mitosis evolved from bacterial cell division Certain protists exhibit types of cell division that seem intermediate between binary fission and mitosis carried out by most eukaryotic cells Prokaryotes: In prokaryotes binary fission occurs. During this process, the origins of the daughter chromosomes move to opposite ends of the cell. The proteins may pull the daughter chromosomes to specific sites on the plasma membrane.

14 Din flagellates: They are unicellular protists. During their cell division the nuclear envelope remains intact and the chromosomes attach to the nuclear envelope. Microtubules pass through the nucleus inside cytoplasmic tunnels The nucleus divides in a fission process very similar to bacterial division. Evolution of mitosis Diatoms. They are unicellular protists. In diatoms the nuclear envelope remains intact during cell division. But the microtubules form a spindle within the nucleus. Microtubules separate the chromosomes, and the nucleus splits into two daughter nuclei. Eukaryotes. In eukaryotes, including plants and animals, the spindle forms outside the nucleus, and the nuclear envelope breaks down during mitosis. Microtubules separate the chromosomes, and the nuclear envelope is then formed again Meiosis Meiosis is the cell division process that occurs during the formation of sex or germ cells. The ovaries and testes produce haploid gametes by meiosis Unlike somatic cells Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes Meiosis is a form of nuclear division in which the chromosome number is halved from the diploid number (2n) to the haploid number (n). In Plants and some algae, the life cycle includes both diploid and haploid multicellular stages In most fungi and some protists Meiosis produces haploid cells that give rise to a haploid multicellular adult organism The haploid adult carries out mitosis, producing cells that will become gametes

15 Meiosis I and II Meiosis I Reduces the number of chromosomes from diploid to haploid Meiosis II Produces four haploid daughter cells

16 A Comparison of Mitosis and Meiosis Differences between mitosis and meiosis Meiosis II is a mitotic division that ultimately results in the formation of 4 haploid daughter cells The major difference is in Meiosis I prophase in which the homologous chromosomes overlap with each other and exchange (synapsis and crossover) This has implications in the genetic variation generated by meiosis At metaphase of meiosis I, paired homologous chromosomes (tetrads) are positioned on the metaphase plates and this is different from mitosis At anaphase of meiosis I, homologous pairs with attached sister chromatids move toward opposite poles of the cell In anaphase of meiosis II, the sister chromatids separate

17 Study Questions 1. Bring out the differences between mitosis and meiosis. 2. How do the plant cells divide? 3. Nuclear envelope remains intact during cell division in a) Animals b) Prokaryotes c) Flagellates d) Diatoms 4. Match the following Crossover Meiosis Diatoms Cytokinesis Protists Cytoplasm Haploid Synopsis 5. Prokaryotes reproduce by