5 The Mitotic Cell Cycle PDF

5 The mitotic cell cycle Structure of a chromosome the total length of DNA of 46 chromosomes of an adult human is about 1.8m, this needs to be fit into a Chromosomes are thread-like structures made of...

5 The mitotic cell cycle Structure of a chromosome the total length of DNA of 46 chromosomes of an adult human is about 1.8m, this needs to be fit into a Chromosomes are thread-like structures made of DNA. nucleus of 6μm therefore, the DNA is wound around the outside of proteins called histones - histone proteins – basic proteins which associate with DNA to form nucleosomes these histone proteins (and therefore, nucleosomes) condense DNA to chromatin - chromatin – a complex of nucleic acids (DNA and RNA) and other proteins whose primary function is to compress DNA into a compact unit which will fit inside the nucleus Nucleosomes Chromatid – one half of two identical copies of a replicated chromosomes Centromere – the region of a chromosome to which the microtubules of the spindle attach, via the 11 nm wide by 6 nm long kinetochore, during cell division. made of 8 histone molecules Telomeres – protective sequence of nucleotides found at the ends of chromosomes Telomeres DNA (deoxyribonucleic acid) cap the ends of chromosomes and are needed for successful cell division DNA is the molecule of inheritance and is made up of a series of genes. permit continued replication prevent the loss of genes a gene is a section of DNA that codes for one protein DNA molecules are in sister chromatids so that their genes are identical when cells divide, one chromatid goes into one daughter cell and the other to the other daughter cell, making the daughter cells genetically identical size of DNA molecule – 2nm Image: https://www.tasciences.com/what-is-a-telomere.html telomeres are made of DNA with short base sequences that are repeated multiple times the copying enzyme, telomerase, stops a little short of the end of DNA more bases are added, which don’t have useful information, but allows telomerase to complete Image: https://mrleehamber119.wordpress.com/ copying the meaningful DNA 1 www.alevel-notes.weebly.com The cell cycle Functions of mitosis Processes of growth, development, and reproduction. growth cell replacement repair of tissues asexual reproduction immune response (cloning B and T-lymphocytes) The stages of mitosis 1) Prophase centrosomes migrate to Image: https://www.brainkart.com/article/Cell-Cycle_33013/ opposite poles of mitotic spindle Interphase (G1 + S + G2) chromatin starts to condense Longest phase Normal cell work/functions nuclear envelope and nucleolus begin to disappear The cell grows to its normal size centrosomes and migrate to opposite poles of the cell Time spent between cell divisions and form poles of mitotic spindle asters and spindle fibres form 1) G1 phase cells make RNA, enzymes and other proteins needed 2) Metaphase for growth centrosomes are at poles at the end of G1 phase, the cell becomes committed to of the cell and organise dividing or not the production of spindle microtubules 2) S phase chromosomes line up synthesis of DNA across equator of the spindle the DNA in the nucleus replicates so that each chromosome consists of 2 identical chromatids: DNA each chromosome splits at centromere replication chromosomes start to be pulled apart by microtubules 3) G2 phase the cell continues to grow 3) Anaphase new DNA is checked, and any errors are repaired chromatids move to opposite poles, centromeres preparations are made to start cell division e.g., a are first pulled by sharp increase in production of the protein tubulin is microtubules observed which is needed to make microtubules for mitotic spindle spindle fibres contract and chromosomes are pulled to the opposite ends M phase (mitosis) 4) Telophase Nuclear division that produces 2 genetically identical nuclear envelope and daughter cells. nucleolus reforming Maintains genetic stability nucleus divides Ensures cells retain function chromosomes are at poles of cell Maintains chromosome number Image: https://www.tekportal.net/telophase/ 2 www.alevel-notes.weebly.com Cytokinesis The division of cytoplasm. Cytokinesis in plant cells cell plate forms across equator of cell; cellulose, cell wall laid down cytoplasm is hence divided into 2 2) Centrosome an organelle found in animal cells that acts as the microtubule organising centre (MTOC) for construction of the spindle Image: https://courses.lumenlearning.com/ present at the poles of cell Cytokinesis in animal cells contractile ring forms, pushing the equator of cell inwards, forming a cleavage furrow this cleavage furrow deepens as active filaments in the ring contract eventually the cell splits into 2 Image: https://www.qsstudy.com/biology/centrosome-definition-function 3) Centrioles act as MTOCs assemble/form spindle fibres during prophase contraction of spindle fibres during anaphase Image: https://courses.lumenlearning.com/ separates sister chromatids Differences in plant and animal cell mitosis Stem cells plant cells don’t have any visible centriole or aster Stem cells are cells that can divide an unlimited number of times by mitosis. in plant cells, daughter cells get separated by cell plate, there is no cleave furrow formation Centromeres, centrosome, centrioles 1) Centromere needed for separation of chromatids during mitosis site of attachment of spindle microtubules each metaphase chromosome has 2 kinetochores at its centromere and microtubules - kinetochore – protein molecules which bind to DNA in centromere and microtubules - bundles of microtubules called spindle fibres extend from kinetochores to poles of spindle during mitosis Image: https://www.yourgenome.org/facts/what-is-a-stem-cell 3 www.alevel-notes.weebly.com  extent of the power of a stem cell to produce different cell types is called its potency stem cell therapy – introduction of new adult stem cells into damaged tissue to treat a disease or injury Cancer a result of uncontrolled mitosis; cancerous cells divide repeatedly and form a tumour carcinogen – agent that causes cancer e.g., radiations (UV, X-rays), tar, nuclear fallout Cellular changes that occur in development of cancer mutation occurs in gene responsible for cell division leading to formation of an oncogene from a proto- oncogene this causes uncontrolled cell division coordination of cell cycle is lost (cell does not receive/respond to signals that tell it to stop dividing) loss of function and lack of specialisation occurs a tumour is formed, an irregular mass of cells showing an abnormal change in shape 4 www.alevel-notes.weebly.com