Mitosis PDF

## CELL, DEVELOPMANT ## DIVERSITY AND CONSERVATION ### structure of chromosome: 1. Thread of DNA that only become visible during cell division, as the chromosomes condense becoming much shorter and denser, where each chromosome can be seen to be made up of 2 chromatids. 2. Where each chromatid con...

## CELL, DEVELOPMANT ## DIVERSITY AND CONSERVATION ### structure of chromosome: 1. Thread of DNA that only become visible during cell division, as the chromosomes condense becoming much shorter and denser, where each chromosome can be seen to be made up of 2 chromatids. 2. Where each chromatid contains one identical copy of DNA molecule which is composed of genes. 3. The 2 chromatids are held together by centromere, which can be at any length of the chromosome, but the position is characteristic for a particular chromosome. 4. In each chromatid, the DNA is wrapped around globular protein molecule called histones, thus holding the DNA molecule in position and prevent it from becoming tangled up helping the movement of chromosomes during cell division and easy replication and transcription. $DNA+histone=chromatin)$ Where the length of DNA molecule coils around 8 molecules of histones forming nucleosome. 5. The number of chromosomes is a characteristic of the species. 6. At both ends of each chromatid are regions of special DNA called telomeres act as a cap preventing loss of DNA during DNA replication. ### The difference between chromosome and chromatin: | Chromatid | Chromosome | |---|---| | Basically they are the same | Basically they are the same | | Unfolded and un condensed DNA. | Folded and condensed DNA. | | Found through out the interphase | Found only during cell division. | | Homologous chromosomes | Homologous chromosomes | One pair of chromosomes having their centromeres placed at the same position, length and carry the same genes placed at same loci (position)(but possibly different alleles), one from the father and the other from the mother. **Gene:** is a length of DNA containing a specific sequence of bases placed in a specific locus(position) that codes for a specific sequence of amino acids to form specific protein, giving a particular characteristic/ phenotype/ feature Each gene has a start and an end. ## THE CELL CYCLE The sequences of events taking place between one cell division and the next. It has three phases: 1. interphase, 2. nuclear division (mitosis) 3. cell division(cytokinesis). ### 1. interphase: The non dividing stage that takes up most of the cycle (80%) cell division. Cells increase in mass & size and carry out normal cellular activities and replicate their DNA ready for division, It is the stage between cell division. In this phase chromosomes are not visible as they are very thin and very long (uncondensed chromatin). **It consists of three phases** * **G1: the first growth phase:** - The cells takes in material, grows and develop where much protein synthesis and transport occur. - Organelles are copied and the amount of cytoplasm increase. - In this phase the cell becomes committed to dividing or not dividing. - In actively dividing cells, G1 is very short (hours or days), in other cells it can be months or even years. * **S phase: synthesis of DNA phase** - The cell enters this phase if its to undergo further division. - When the DNA replicates (semi conservative replication) - This produces two identical copies of DNA to form the chromatids. - Notice construction of kinetochores starts during this phase * **G2 phase: second growth phase** - cell continues to grow and new DNA is checked and any errors are usually repaired. - Enlargement of cytoplasm. ### Cell division ### Mitosis (20%) #### 1. Prophase (longest phase) * The chromosome/chromatin coil & condense and become Visible as two sister chromatids held by centromere. * The centrosome replicates just before prophase. * The two centrosomes (centrioles) begin to move to opposite ends of nucleus forming of spindle. * The spindles begin to form between centrosomes (microtubule assemble). * The nuclear membrane breaks up into small vesicles then disappear and nucleolus disappear. #### Metaphase * The nuclear membrane has broken down and the centrioles have moved to opposite poles of the cell. In moving apart, the centrioles have formed between them a set of microtubules that is known as the spindle. The chromatids appear to compete for position on the metaphase plate (equator) of the spindle during metaphase. They eventually line up along this position. * The centromere of each chromatid is associated with a separate microtubule of the spindle (see fig C). Plant cells also form a spindle but they do not have a centriole. Scientists still do not completely understand exactly how spindle formation takes place. ### Anaphase * Anaphase (shortest): - The centromeres will divide/break/split separating the 2 sister chromatids. - Spindle fibres which will shorten Pullin the chromatids apart at the centromeres (causing centromere to split) towards the opposite poles. - The sister chromatids move towards opposite poles(ends) of the cell. - As soon as they separate, the chromatids become single chromosomes of the two daughter cells. ### Telophase * The chromosomes reach the poles of the spindle. * The spindle breaks down / disassembles. * A new nuclear envelop/membrane reforms around each set of chromosomes (from ER). * New nucleoli and centrioles reforms. * The cytoplasm constricts and cytokinesis begins. ### What is a chromatid * One strand containing One of two DNA molecules that make up a chromosome (i.e each chromatid contains one DNA molecule), which is joined to the other chromatid with a centromere. These 2 DNA molecules are the result of DNA replication taking place during the S phase. ### Important notice #### • Role of spindle fibres during mitosis 1. Spindle fibres (microtubules), attach to centromere at kinetochore. 2. Attach to the centrioles (which act as MTOC) 3. Arranging the chromosomes at the equator. 4. Fibres shorten (microtubules disassemble). 5. Move /pull the sister chromatids to opposite poles. 6. So equal number of chromosomes in each daughter cell/nucleus... #### - role of centromeres 1. Hold the 2 sister chromatids together. 2. allow the spindle fibres to attach to them during mitosis when they arrange themselves at the equator of the cell. #### - Remember role of spindle fibre in each stage: 1. Prophase: to attach to chromosomes. 2. Metaphase: to align chromosomes. 3. Anaphase: to separate chromatids at centromere. #### • cytokinesis: * It is initiated during telophase (but not considered as part of mitosis). * Over all it involves division of cytoplasm and the formation of 2 genetically daughter cells with the same number of chromosomes(2n). ##### • In animal cells this involves the breakdown of remains of spindle fibres and constriction( formation of furrows) of the cytoplasm from the edges of the cell, where a ring of contractile fibres tightens around the centre of the cell, which continue to contract until two cells have separated. ### • In case of plant cells A. There are no centrioles( centrosomes) in plant cell And in plant cells a new cellulose cell wall builds up from the inside of the cell outwards. B. In plant, a plate will be formed between the two daughter cells. Some spindle fibres remain and guide Golgi vesicles containing wall materials and enzymes to equator of the cell. The vesicles fuse with the cell surface membrane, then these vesicles enlarge and fuse together forming cell plate Perforated by plasmodesmata. Then formation of cell membrane. ### Remember **Advantages of using light microscope rather than electron microscope for study of cell cycle** 1. Living cells can be viewed. 2. Can watch the cell cycle happen in real 3. All chromosomes can be seen 4. Can see all stages of mitosis or cell cycle. 5. Don't need to take section to see 6. Dyes/stain can be used. ### Function of cell Cycle 1. Control growth of organism/replace dead cells/repair damaged tissues. 2. Increase number of cells/production of new cells. 3. Asexual reproduction/production of clones/ genetically identical cells. 4. Synthesis of new cytoplasm /new organelles/DNA replication. **fig H** Once the cell leaves interphase and enters mitosis, the chromosomes can be seen. You can use this when counting cells to work out the mitotic index. ### (Assessment questions) | Phase | Number of cells | Cells in each phase (%) | |---|---|---| | prophase | 25 | 10 | | metaphase | 10 | 4 | | anaphase | 5 | 2 | | telophase | 10 | 4 | | interphase | 200 | | **(ii) Calculate the percentage of cells that are in interphase. (1 mark)** * 200/250 x 100 = 80% (1). **(b) Assume that the mean total duration of the cell cycle is 18 hours for these cells. Calculate the actual time, in minutes, spent in anaphase. Show your working. (2 marks)** * 18x60=1080 (minutes) OR 2/100x18=0.36 (1) x1080-22 OR 0.36x60=22 (minutes) (1) Allow 21.6. **6. Describe how cells from the root of a plant could be prepared in order to observe mitosis. (5 marks)** * Reference to use of the root tip /area with meristem (1) * Add acid/named acid /acetic alcohol (1) * Add appropriate named stain, e.g. acetic orcein or toluidine blue (1) Warm /use a water bath at 55 °c (to intensify staining) (1) * Break apart tip with a (mounted) needle /equivalent (1) * Mount on a slide and (gently) squash under a coverslip (1) **7. Some students are planning to take measurements of root tip cells and compare the size of cells in different stages of mitosis. They would like to make sure that their results are valid. accurate and reliable. (a) Explain what is meant by the term 'valid results'. (3marks)** * Valid results measure what they are supposed to (or equivalent wording).(1) * The measurements that have been made are affected by a single independent variable /only by