Notes de cours
CIE A level biology Notes - unit 05 - the mitotic cell cycle
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Full notes covering the checkpoints of topic 5 - the mitotic cell cycle.
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5.1 - Replication and division of nuclei and cells:Karyotype - the number and appearance of chromosomes in the nucleus of a eukaryotic cell.
• The number of chromosomes is characteristic of the species.
◦E.g. in human cells there are 46 chromosomes (or 23 pairs of chromosomes).
• When chromosomes are found in pairs in the cell they are diploid.
• When chromosomes are unpaired in the cell they are haploid.
Structure of chromosomes:
• Chromosomes are made of two identical structure called chromatids
(threadlike strands containing a double helix of DNA) - this is because during
the period between nuclear divisions (interphase - no chromosomes visible and
one or more nucleoli visible), each DNA molecule in a nucleus makes an
identical copy of itself.
• Each chromatid contains one of these DNA copies and the chromatids are held
together be a centromere (the positioning of the centromere is characteristic of
di erent chromosomes).
• When cells divide, one chromatid goes into one daughter cell and the other
goes into another daughter cell, making the cells genetically identical.
• Histones are alkaline proteins found in the cell nuclei that package and order
the DNA into structural units called nucleosomes (a cylindrical shape made up
of 8 histone molecules wrapped in DNA).
• Telomeres - a region at each end of a chromatid which protects the end of the
chromosome from deterioration or from fusion of neighbouring chromosomes, by preventing the loss of genes from the ends
of the chromosomes.
◦As you get older they get shorter and when they are shorter they are linked with premature ageing and many diseases.
Mitosis - cell division of somatic cells to produce genetically identical daughter cells (clones):
• Growth of multicellular organisms
• Replacement of damaged/dead cells and repair of tissues (cell replacement in human skin is rapid)
• Asexual reproduction - production of new individuals by one parent
• immune responses - cloning of lymphocytes
The mitotic cell cycle:
• During interphase, the cell grows to its normal size and carries out many of its
normal functions (e.g. synthesis of substances).
• Interphase consists of 3 sections: G1, S and G2.
• During G1, cells synthesises the RNA, enzymes and other proteins needed for
growth.
• During S, DNA in the nucleus is duplicated so that each chromosome consists
of 2 sister chromatids.
• During G2, the cell continues to grow and the new DNA is checked for any
errors and any identi ed errors are repaired.
• Nuclear division (mitosis) then occurs, splitting the contents of the cell - after
this cytokinesis then happens (cell division).
Stem cells - special human cells that are able to develop into many di erent cell types which are used in cell replacement and
tissue repair by mitosis.
• Each new cell can develop into a new stem cell or a specialised stem cell such as a blood cell or muscle cell.
• The extent of the power of a stem cell to produce di erent cell types is referred to as its potency.
◦Totipotent - stem cells that can turn into anything
◦Pluripotent - stem cells that can turn into any adult cell
◦Multipotent - stem cells that are programmed to become a certain type of cell
• Stem cell therapy is the introduction of new adult stem cells into damaged tissues to treat disease or injury.
Cancer - the formation of a tumour due to repeated and uncontrolled cell division.
◦Tumours can be benign or malignant (invade other tissues around the body and destroy them)
◦When a cell breaks o and spreads around the body (like a malignant tumour), by blood circulation, this is called
metastasis.
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