Powerpoint chromosomes, mitosis


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  • The illustration represents a cell in the process of dividing. The coloured structures are chromosomes. Full details are given in slides 20-27
  • Stains can be chosen for particular parts of the cell; the cytoplasm, the nucleus, the cell wall or structures within the cytoplasm. Staining usually kills the cell.
  • The living content of the nucleus is called nucleoplasm to distinguish it from cytoplasm.
  • * The 8 chromosomes have already replicated so some of them appear ‘double’.
  • The centromere appears as a constriction in the chromosome but in subsequent drawings it is represented by a circle to make its position clear.
  • Although the shortening of the spindle fibres and the corresponding movement of the chromosomes are clearly associated, it should not be assumed that the fibres are physically tugging the chromosomes apart.
  • Although No.1 doesn’t look much like the drawing, it represents the stage when the chromosomes are clearly visible but not yet organised on the spindle. Possible web sites www.microscopy-uk.org.uk/mag/artaug99/mitosis.html Photomicrographs of cell division in broad bean root tip www.johnkyrk.com/mitosis.html Simple animation of mitosis www.biologyinmotion.com/cell_division Interactive ‘drag and drop’ test on mitosis and meiosis www.tcb.cl/1535/article-56384.html Time-lapse video of mitosis in newt lung cell Possible web sites www.microscopy-uk.or.uk/mag/artaug99/mitosis.html
  • Powerpoint chromosomes, mitosis

    1. 1. 1
    2. 2. In a living cell seen underthe microscope, it is oftendifficult to see the nucleusWhen certain stains are applied, thenucleus takes up the stain morestrongly than the cytoplasm does,making the nucleus easily visibleStaining 2
    3. 3. By means of some special techniques ofillumination, the nucleus and other structuresmay be seen in the living cell.But in this presentation and in most textbooks, thenucleus is shown dark, as if it had been stainedIt is the differences in chemical compositionbetween nucleus and cytoplasm that make one takeup a particular stain more strongly than the other3
    4. 4. INHERITANCE• Is the transmission of genetic informationfrom generation to generation.
    5. 5. Terms you should know• CHROMOSOME: thread of DNA, made up of a string ofgenes.• GENE: a length of DNA that is the unit of heredity andcodes for a specific protein. A gene may be copied andpassed on to the next generation.• ALLELE: any of two or more alternative forms of a gene.• HAPLOID NUCLEUS: a nucleus containing a single setof unpaired chromosomes (e.g. sperm and egg)• DIPLOID NUCLEUS: a nucleus containing two sets ofchromosomes (e.g. in body cells)
    6. 6. In a cell which is not about to divide, thestructures in the nucleus are not distinctnuclearmembranecytoplasmCell 1 4
    7. 7. Just before cell division, thread-likestructures appear in the nucleusCell 2 5
    8. 8. These structures are called chromosomesThey get shorter and thicker and take upstains very stronglychromosomesCell 36
    9. 9. The shortening and thickening continues.Now the chromosomes are seen to bein pairsTo continue the sequence for cell division click hereCell 47
    10. 10. It may not be obvious from this illustration, but thechromosomes are always in pairs. They are distinguishedby their size.Chromosome pairs8
    11. 11. 9
    12. 12. The chromosomes are always in pairs because oneof them is derived from the male parent and theother from the female parentDifferent species have different numbers andshapes of chromosomesMembers of the same species have identicalsets of chromosomes10
    13. 13. kangaroo (6 pairs) a sedge (21 pairs) hawkweed (4 pairs*)chicken (18 pairs) fruit fly (4 pairs) human (23 pairs)Chromosome numbers 11
    14. 14. Although chromosomes can be seen distinctlyonly at the time of cell division, they are presentand active all the timeThe chromosomes carry the DNADNA controls all the chemical reactions in the cellDNA also determines the species of organismand its individual characteristics (See thepresentation on ‘DNA’ for more detail)12
    15. 15. Chromosomes consist of DNA molecules supportedby a ‘scaffold’ of proteins. The diagram illustrates suchan arrangement but it is really more complex than thisChromosomes and DNAchromosomeDNAproteindouble helix13
    16. 16. The DNA in the chromosomes carries the genesConsequently, the genes are spaced out alongthe chromosomeThe genes consist of distinct stretches of the DNABy means of their DNA content, the genes controlthe activities of the cell, the type of cell it becomes,the species of the organism and the individualcharacteristics of that organism14
    17. 17. genes for eye colourgenes for hair coloursingle genechromosomeChromosomes and genesgenes for tallnessThe diagram illustrates the relationship between chromosomesand genes but it does not represent an actual chromosome.The genes for these characteristics are not necessarily on thesame chromosome and the number of genes shown is arbitrary15
    18. 18. ABCDEFGHIabcdefghIBecause the chromosomes are inpairs, the genes they carry are alsoin pairsThe individual genes of a pair, controlthe same characteristic, e.g. B and bcould control eye colour; G and g couldcontrol hair colourEach member of a pair of genes comesfrom either the male or the femaleparent just as the chromosomes doChromosomes and genes 16
    19. 19. Just before cell division, it can be seen that the chromosomeshave replicated; that is each chromosome has made a copy ofitself (including its DNA). These copies and the originals arenow called chromatidschromatidsChromatids 17
    20. 20. The chromatidstend to separate butare held together by aspecial region calledthe centromerechromatidsCentromere 18
    21. 21. CELL DIVISION:MITOSIS• Is a nuclear division giving rise togenetically identical cells in which thechromosome number is maintained by theexact duplication of chromosomes.
    22. 22. The next sequence of slides shows the way thechromosomes are distributed during cell divisionFor clarity, only two pairs of chromosomes arerepresentedThe chromosomes coloured blue are derived fromthe male parent; the chromosomes shown in redare from the female parent19
    23. 23. Two pairs of chromosomes.Each chromosome hasreplicated to form chromatidsCell division 120
    24. 24. The nuclear membranedisappearsFibres appear in thecytoplasm and form aspindleThe chromosomes moveto the ‘equator’ of thespindleCell division 221
    25. 25. The spindle fibresshorten and appearto pull the chromatidsapart by their centromeresCell division 3 22
    26. 26. The chromatids are nowchromosomes.The chromosomesmigrate to opposite endsof the cell as the spindlefibres shortenCell division 423
    27. 27. The cell begins to divideCell division 5 24
    28. 28. The nuclear membraneforms againThe cell constrictsThe chromosomesbecome less distinctCell division 625
    29. 29. Two cells formedEach cell now has a full setof chromosomes identical tothe parent cellThe chromosomes revert totheir elongated thin shapeand eventually cannot beseenCell division 7 26
    30. 30. Plant cells divide bybuilding a new cell wallAnimal cells divide bya constriction of theircytoplasmPlant and animal cells 27
    31. 31. This process of cell division, which produces cellscontaining identical sets and numbers ofchromosomes, is calledMITOSISMitosis 28Mitosis ensures that every cell of an organismcarries an identical set of genesCan you see a problem with this?
    32. 32. For example, what can a gene for brown eyes doin a stomach cell?The problem is that if every cell carries the sameset of genes, how do cells become specialised inshape and function to do different jobs?The answer is that the genes which are not relevantto the cell’s function are not activated.We say the gene for brown eyes is not expressedin a stomach cell29
    33. 33. 1234567Mitosis 30
    34. 34. The next slide shows a photomicrograph of onion root cells.In a root tip, a great many cells are dividing bymitosis, leading to rapid growth.The preparation is made by softening the root tip tissue,squashing it on a microscope slide and staining thechromosomes and nuclei.The ‘squash’ technique spreads the cells out.See if you can associate the various stages of cell divisionwith the stages 1-6 in the previous slide.Bear in mind that the previous slide is a purelydiagrammatic representation.31
    35. 35. 244517Root tip squash32© McLeish & SnoadMacmillan
    36. 36. Mitosis is involved in growthand development
    37. 37. GROWTH• Permanent increase in size and dry mass byan increase in cell number or cell size orgrowth.
    38. 38. DEVELOPMENT• Increase in complexity.
    39. 39. Question 1Staining techniques show up the nucleus because(a) The nucleus is enclosed in a nuclearmembrane(b) Nucleus and cytoplasm have differentchemical properties(c) The cytoplasm is less concentrated thanthe nucleoplasm(d) The nucleus is in the middle of the cell33
    40. 40. Question 2When are chromosomes present in the nucleus ?(a) Only just before cell division(b) Only during cell division(c) Only after cell division(d) All the time34
    41. 41. Question 3Chromosomes are in pairs because(a) They have replicated(b) Each is derived from either the maleor female parent(c) They are joined at the centromere(d) They have to be shared at cell division35
    42. 42. Question 4How many chromosomes are there in a human cell?(a) 100+(b) 92(c) 46(d) 2336
    43. 43. Question 5Which of these statements are correct ?DNA controls(a) The species of the organism(b) The function of the cell(c) Features of the organism such as size(d) Chemical reactions in the cell37
    44. 44. Question 6Which of these statements are correct?A chromosome contains(a) Protein(b) Cellulose(c) DNA(d) Genes38
    45. 45. Question 7The process by which a chromosome makes acopy of itself is called(a) Reproduction(b) Recombination(c) Relocation(d) Replication39
    46. 46. 40What is the correct sequence of events in mitosis ?a b c d e f(c) c, b, a, e, d, f(a) c, d, a, b, e, f(b) b, c, a, d, e, f(d) c, b, d, a, e, fQuestion 8
    47. 47. AnswerCorrect
    48. 48. AnswerIncorrect