The Mitotic Cell Cycle PDF

# THE MITOTIC CELL CYCLE ## 1. Replication and division of nuclei and cells ### 1.1 Chromosome structure - Chromosome is a thread-like structure of DNA, carrying genetic information in the form of genes (gene is a sequence of nucleotides that forms part of a DNA molecule) - Chromosomes are made of...

# THE MITOTIC CELL CYCLE ## 1. Replication and division of nuclei and cells ### 1.1 Chromosome structure - Chromosome is a thread-like structure of DNA, carrying genetic information in the form of genes (gene is a sequence of nucleotides that forms part of a DNA molecule) - Chromosomes are made of one very long, condensed DNA molecule associated with proteins (in eukaryotic cells) - The main proteins present are the large positively charged globular proteins called histones, their role is to organise and condense the DNA tightly so that it fits into the nucleus - The other proteins are enzymes used in copying and repairing the DNA - The tightly coiled combination of DNA and proteins is called chromatin - this is what chromatids, and therefore chromosomes, are made of - They contain two genetically identical sister chromatids joined at a point called centromere ### Role of telomeres - Telomeres are present at the ends of chromosome - made of non-coding DNA which is made of sequence of nucleotide bases that are repeated - The main function of telomeres is to ensure that the ends of the DNA molecules are included in DNA replication during mitosis (the copying enzyme responsible for DNA replication is unable to run right to the very end of the DNA molecule) - To avoid the risk of losing genes most cells have an enzyme called telomerase that adds additional bases at each end (the telomeres) - In each subsequent cell division, a little more genetic information would be lost - length of telomeres determines the life span of cells ## 1.2 The cell cycle - The cell cycle is the regulated sequence of events that occurs between one cell division and the next - The length of the cell cycle is affected by environmental conditions, the cell type and the organism - The movement from one phase to another is triggered by chemical signals called cyclins ### Interphase - Interphase consists of three phases: - G1 phase - S phase - G2 phase - During the G₁ phase the cell synthesis proteins including enzymes for growth - there will be many mRNA presents for protein synthesis - During the S phase the DNA is replicated (resulting in each chromosome consisting of two identical sister chromatids) - During the G2 phase the cell continues to grow, mitochondria divides and there is further increase in the quantity of other cell organelles. By the end of G2 phase there is a large quantity of centrioles to form spindle during mitosis. ## Nuclear division (mitosis) - Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent nucleus - Cell growth stops during the mitosis phase - **prophase:** - condensation of chromatin - visible as two sister chromatids - spindle fibre form - nuclear membrane disintegrates - nucleolus disappears - **metaphase:** - centrosomes reach opposite poles. - microtubules/spindle fibres attach to centromere/kinetechore - chromosomes align at the equator - **anaphase:** - centromere of each chromosome divides - sister chromatids split at the centromere - chromatids pulled to the opposite poles - spindle fibres shorten - **telophase:** - chromosomes decondense - sister chromatids at poles - nucleolus reforms - nuclear membrane reassembles - spindle fibre disassembles - **cytokinesis:** - cytoplasm divided into two organelles are equally shared between two daughter cells - in plants cell wall forms ### Importance of mitosis - growth of multicellular organisms - enables unicellular zygotes to divide by mitosis and grow into multicellular organism - replacement of damaged cells, and repair tissues - cells can be repaired by forming genetically identical cells by mitosis followed by cell division - asexual reproduction - it is the production of new individuals of a species by a single parent and offspring are identical to the parent - this is a result of cell division through mitosis ## 1.3 Observing mitosis - Growth in plants occurs in specific regions called meristems - The root tip meristem can be used to study mitosis - In the root tip meristem, there is a zone of cell division that contains cells undergoing mitosis ### Method - Garlic or onion root tips are most commonly used (the bulbs can be encouraged to grow roots by suspending them over water for a week or two) - Remove the tips of the roots (about 1cm) and place in a suitable stain (eg. warm, acidified acetic orcein, which stains chromosomes a deep purple) - The stained root tip is gently squashed on a glass slide using a blunt instrument (eg. the handle of a mounting needle) - Cells undergoing mitosis (similar to those in the images below) can be seen and drawn ### Analysis - **Prophase:** - **Metaphase:** - **Anaphase:** ## 1.4 Role of stem cells - A stem cell is a cell that can divide (by mitosis) an unlimited number of times - Each new cell (produced when a stem cell divides) has the potential to remain a stem cell or to develop into a specialised cell such as a blood cell or a muscle cell (by a process known as differentiation) - this ability of stem cell is known as potency ### There are three types of potency: - **Totipotency** - totipotent are stem cells that can differentiate into any cell type found in an embryo, as well as extra-embryonic cells (the cells that make up the placenta). - **Pluripotency** - pluripotent are embryonic stem cells that can differentiate into any cell type found in an embryo but are not able to differentiate into extra-embryonic cells (the cells that make up the placenta) - **Multipotency** - multipotent are adult stem cells that have lost some of the potency associated with embryonic stem cells and are no longer pluripotent ### Multipotent adult stem cells - Having differentiated and specialised to fulfil particular roles, most adult cells gradually lose the ability to divide until, eventually, they are no longer able to divide - However, small numbers of stem cells - adult stem cells remain to produce new cells for the essential processes of growth, cell replacement and tissue repair - Although these adult stem cells can divide (by mitosis) an unlimited number of times, they are only able to produce a limited range of cell types - they are multipotent - For example, the stem cells found in bone marrow are multipotent adult stem cells - they can only differentiate into blood cells (red blood cells, monocytes, neutrophils and lymphocytes) - In adults, stem cells can be found throughout the body (eg. in the bone marrow, skin, gut, heart and brain) ## 1.5 Formation of tumour - Cancers start when changes occur in the genes that control cell division. A change in any gene is known as a mutation – causes uncontrolled cell division (mitosis) that results in cancer - Most mutations either result in early cell death or result in the cell being destroyed by the body's immune system - As most cells can be easily replaced, these events usually have no harmful effect on the body - The mutations that result in the generation of cancerous cells do not result in early cell death or in the cell being destroyed by the body's immune system - allowing mutation to pass onto to all the cells - Cancerous cells divide repeatedly and uncontrollably, forming a tumour (an irregular mass of cells) - Carcinogens are any agents that may cause cancer (eg. UV light, tar in tobacco smoke and X-rays). If the agent causes cancer. - In cancer cells checkpoints are not controlled during cell cycle. Have a short interphase - not full growth - Some tumours (such as warts) do not spread from their original site - these are known as benign tumours and do not cause cancer - Some tumours spread through the body, invading and destroying other tissues - these are known as malignant tumours and cause cancer - Malignant tumours interfere with the normal functioning of the organ / tissue in which they have started to grow (eg. they may block the intestines, lungs or blood vessels) - Malignant tumour cells can break off the tumour and travel through the blood and / or lymphatic system to form secondary growths in other parts of the body - The spreading of cancers in this way is known as metastasis - difficult to detect, locate and remove secondary cancers