Unit2   cell cycle
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Unit2 cell cycle

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Unit2 cell cycle Presentation Transcript

  • 1. UNIT 2: THE CELL CYCLE
  • 2. Cells need to build proteins (especially enzymes) to carry out the living processes of growth, reproduction, tissue repair, etc The genetic information the cell needs to make proteins is stored in the DNA. DNA is located in the nucleus of eukaryotic cells.
  • 3. We have aprox. 1.8m of DNA in each of our cells!! how can it fit inside the nucleus (6µm)? This is geometrically equivalent to packing 40 km of extremely fine thread into a tennis ball!
  • 4. DNA is tightly packaged: - When the cell is not dividing (INTERPHASE), DNA is associated with proteins (histones) forming a thread-like mass called CHROMATIN. - Just before cell division starts, the cell duplicates the DNA and condenses it into more tightly packaged structures called CHROMOSOMES.
  • 5. CHROMOSOME CHROMATIN DNA
  • 6. Chromosomes are rod-shaped structures with a constriction called CENTROMERE. The centromere divides the chromosomes in two ARMS that may be of equal or different length. After the DNA duplication before cell division, chromosomes consist of two identical CHROMATIDS joined together by the centromere. The terminal part of each chromatid is called TELOMERE. TELOMERE SHORT ARM CENTROMERE LONG ARM SISTER CHROMATIDS
  • 7. The number, shape and size of the chromosomes is characteristic of each species. The number of chromosomes does not correlate with the complexity of the organism.
  • 8. A KARYOTYPE is a picture of the chromosomes of an organism, ordered in pairs. The members of each pair are called HOMOLOGOUS CHROMOSOMES. The human karyotype has 23 pairs of chromosomes. All diploid cells have this 23 pairs, that is 46 chromosomes, except gametes, which are haploid and have only one set of chromosomes, that is, 23.
  • 9. Humans have a chromosomic number of 2n=46, where n stands for the number of homologous chromosomes.
  • 10. The chromosomes (X and Y) which determine the sex of an individual are called SEX CHROMOSOMES. The rest of the chromosomes are called AUTOSOMES.
  • 11. The chromosomes (X and Y) which determine the sex of an individual are called SEX CHROMOSOMES. The rest of the chromosomes are called AUTOSOMES.
  • 12. GENES are segments of DNA with the necessary information to build a protein. Genes are located linearly in the chromosomes. All the cells of an individual contain the same genes, BUT not all the genes are active in all the cells. The genes in the two sister chromatids in a chromosome are identical (they come from the replication of the same segment of DNA). The group of genes of an organism is called GENOME. Humans have a genome of around 35000 genes.
  • 13. The Cell Cycle:
  • 14. Why do the cells need to divide? - Growth - Repair of damaged cells and tissues - Renovation of cells
  • 15. CELL CYCLE (= period of time that goes from one cell division to the next one)
  • 16. A) INTERPHASE: the cell is not dividing, but is getting ready to divide - increase in cell size, number of organelles, cytoplasm volume - new organelles are produced - cell prepares to divide - protein synthesis G0 (e.g.neurons) DNA duplicates (replicates)
  • 17. B) CELL DIVISION (PHASE M) MITOSIS: division of nucleus and distribution of chromosomes CYTOKINESIS: division of cytoplasm, distribution of organelles in the two daughter cells
  • 18. VARIATION OF DNA CONTENT DURING CELL CYCLE
  • 19. Remember: we can only see chromosomes in the cell when it is dividing! During INTERPHASE: DNA is associated to proteins called histones forming CHROMATIN During MITOSIS DNA condenses into CHROMOSOMES. This ensures and makes easier its equal distribution in the two daughter cells
  • 20. MITOSIS - Division of the nucleus and distribution of chromosomes in the two daughter cells. - Most animal cells divide (epithelial cells, blood cells, etc) except neurons and muscle cells. - In plants, only the meristem cells undergo mitosis. This meristems originate all the other plant tissues. - makes sure that one cell originates two IDENTICAL daughter cells (asexual reproduction) IMPORTANCE of MITOSIS - keeps the number of chromosomes constant - allows growth: from zygotes (one cell) to adult (billions of cells) - originates new cells for tissue repair
  • 21. - The movement of the chromosomes during mitosis is controlled by the centriols and the mitotic spindle, which belong to the cytoskeleton. The starting point is G2 of interphase: the DNA is replicated, the nuclear envelope is complete, and the cell has two centriols. - Mitosis can be divided in the following stages: - PROPHASE - METAPHASE - ANAPHASE - TELOPHASE
  • 22. 1) PROPHASE - Chromatin starts to condense and chromosomes can be seen. - Nucleolus disappears. - Centriols start to migrate to opposite sides of the cell, and start to form the microtubules of the mitotic spindle, which will help the cromosomes to move. - The nuclear envelope starts to disappear. 2) METAPHASE - Nuclear envelope has disappeared. - Chromosomes attach to the microtubules of the mitotic spindle and align in the centre of the cell (equatorial plate).
  • 23. 3) ANAPHASE - The sister chromatides of each chromosome are pulled apart to opposite poles of the cell as the spindle microtubules shorten, and they become new chromosomes. 4) TELOPHASE - The new chromosomes reach the two poles of the cell and start to decondense to form chromatin again. - Mitotic spindle disintegrates. - Nuclear envelope starts to form around each new nucleus. - Cytokinesis starts.
  • 24. CYTOKINESIS - Division of cytoplasm and organelles in the two daughter cells. - Starts during the Telophase. In animal cells a furrow forms Due to the rigid cell wall, there is no thanks to a contractile ring that contractile ring in plant cells. Instead, a cell splits the cell in two. plate is formed in the middle of the cell by vesicles coming from the Golgi Apparatus. This finally separates the two cells and originates the new cell wall
  • 25. MEIOSIS - Organisms that reproduce sexually have two types of cells: ● Somatic cells, are diploid (=with 2n chromosomes) ● Reproductive cells or GAMETES are haploid (= with n chromosomes, so that when they fuse in fertilization the zygote will have again 2n - Gametes are produced in a special type of cell division called MEIOSIS, in which the number of chromosomes is reduced to half.
  • 26. - Meiosis is actually TWO consecutive cell divisions (I and II), similar to mitosis, with no DNA duplication between them. - The result of meiosis is 4 daughter cells that are haploid (n). - In addition to this, the daughter cells are not identical: they contain different combination of genes (genetic recombination) due to the exchange of fragments of homologous chromosomes (crossing-over) during the prophase of the first meiotic division.
  • 27. MEIOSIS I 1) PROPHASE I: Chromosomes (formed of 2 chromatids) condense and thicken. Each pair of homologous chromosomes pairs and forms a tetrad (4 chromatids). The chromatids of homologous chromosomes exchange fragments (crossing-over). This will result in genetic recombination.
  • 28. 2) METAPHASE I: Centriols are already in opposite poles of the cell. Each pair of homologous (tetrad) attaches the spindle fibers and aligns in the equatorial plate. 3) ANAPHASE I: When the microtubules of the spindle shorten, homologous chromosoms (with 2 chromatids each) separate and migrate to different poles of the cell.
  • 29. 4) TELOPHASE I: Chromosomes are now in different poles of the cell. Spindle disintegrates, a new nuclear envelope starts to form, and cytokinesis begins After the first meiotic division, the two daughter cells are now haploid (n). Each chromosome is made of two chromatids. The second meiotic division will start without replication of DNA.
  • 30. MEIOSIS II 1) PROPHASE II: Similar to a normal mitotic prophase. 2) METAPHASE II: Chromosomes align in the middle of the cell and attach to the microtubules of the spindle.
  • 31. 3) ANAPHASE II: Chromatids separate as the fibers shorten, and migrate to opposite poles of the cell. 4) TELOPHASE II: The spindle disappears, a new nuclear envelope forms, cytoplasm divides. The result of the process are 4 haploid cells.
  • 32. MEIOSIS: GAMETOGENESIS (Spermatogenesis and Oogenesis)
  • 33. SEXUAL REPRODUCTION AND VARIABILITY Biological Evolution needs VARIABILITY, that is, genetic diversity, to occur. Sexual reproduction creates this genetic variability through 3 mechanisms: - Random distribution of homologous chromosomes in meiosis I. - Genetic Recombination due to crossing-over in Prophase I. - Random fertilization of gametes
  • 34. SEXUAL REPRODUCTION versus ASEXUAL REPRODUCTION