Mitosis

AS Unit BIOL2: The Variety of Living Organisms During the cell cycle, genetic information is copied and passed to genetically identical daughter cells During the cell cycle, genetic information is copied and passed to genetically identical daughter cells During the cell cycle, genetic information is copied and passed to genetically identical daughter cells.

You should know: Cell cycle Mitosis and the cell cycle. DNA is replicated and this takes place during interphase. Candidates should be able to relate their understanding of the cell cycle to cancer and its treatment.

Mitosis During mitosis, the parent cell divides to produce two daughter cells, each containing an exact copy of the DNA of the parent cell. Mitosis increases the cell number in this way in growth and tissue repair. Candidates should be able to name and explain the events occurring during each stage of mitosis. They should be able to recognise the stages from drawings and photographs.

The Cell cycle The Cell cycle The process that all body cells in a Multicellular organism use to grow and divide. The Cell cycle Nuclear division is part of the cell cycle.

Phases of the cell cycle Cell cycle consists of a period of growth and DNA replication, called interphase and a period of cell division, called mitosis. Cell cycle starts when a new cell is produced by cell division and ends with the cell dividing into 2 identical daughter cells. M phase= Mitosis = cell cycle starts and ends here. G 1 phase = growth phase 1 = cell grows and new organelles and proteins are made. S phase = synthesis = DNA replication, ready for mitosis. G 2 phase = Gap phase 2 = cell growing and makes proteins needed for cell division.

Cell cycle

(a) Interphase Sometimes known as the resting stage. This stage takes the longest time. Cell grows to normal size after cell division. Carries out normal functions and synthesising substances (especially proteins). Has 3 phases: First growth (G 1 ) phase = proteins produced which will be used to synthesise the cells organelles. Cell grows twice the size. More organelles are produced, as are nucleotides, and histone proteins. Synthesis (S) phase = DNA is replicated. DNA is unraveled and replicated, to double its genetic content. It combines with the newly produced histones to double the amount of chromatin in the cell. Second growth (G 2 ) phase = organelles grow and divide and so are replicated. Energy stores are also increased, in the form of ATP. Specialist proteins called tubulins are synthesized. These are used to make the spindle fibres, which will separate the chromosomes. At some point gets signal that cell should divide.

Mitosis This is the process of nuclear division which happens in body cells so that the new cells produced by cell division have identical nuclei. MITOSIS is divided into 4 stages to help description but it is a continuous process with no sharp breaks between one stage and the next. Root tip cell of onions divide every 20 hours Epithelial cell in human gut divides every 10 hours.

Interphase

Interphase Chromosomes present as chromatin. Cell growing and carrying out normal functions. Prepares for cell division by making additional organelles (mitochondria, ribosomes etc) DNA is replicated so daughter cells have identical information to the parent cell to produce enzymes and other proteins. Process – semi-conservative replication.

Prophase

Prophase

Key events of prophase Chromatin condenses into thick, short chromosomes. Each chromosome consists of 2 identical chromatids held at a centromere. Fibres (microtubules) radiate out from centrioles to form a spindle. Nuclear envelope and nucleolus disappear

Metaphase Staff.jccc.net/pdecell/celldivision/mitosis1.html

Key events of metaphase The chromosomes line up at random on the equator of the spindle. The centromeres are attached to spindle fibres.

Anaphase

Key events of anaphase The centromere splits and the chromatids separate. The microtubules move the chromosomes towards opposite poles of the spindle.

Telophase

Key events of telophase The chromosomes have arrived at the spindle poles. A nuclear membrane forms around them. A nucleolus forms. The spindle breaks down. The chromosomes gradually uncoil to take on their interphase character.

Cytokinesis Cell organelles are evenly distributed. Cell membrane constricts in the middle of the cell so that it becomes cut into two. Plant cells lay down a cell wall.

Significance of mitosis Produces new cells for growth and repair. Is used in asexual reproduction. The cells produced are genetically identical to the original cell.

The centromere is made up of: A sequence of DNA bases not transcribed but is required for the segregation of the chromatids. A protein based structure called a kinetochore to which spindle fibres attach.

spindle There are 2 types of spindle fibres: Polar spindle fibres which extend from 1 pole of the cell to the other. Spindle fibres that extend from the pole of the cell and attach to the centromere of a chromatid.

Spindle

Interphase Botit.botany.wisc.edu/ …

prophase

metaphase

Anaphase

Telophase

Mitosis in plant and animal cells.

Useful websites Bomi.ou.edu/…/mitosis/mitosis.html t05.html t05.html

animation

Questions 7. In the mitotic cell cycle of a human cell how many chromatids are present as the cell enters mitosis? 8. How many DNA molecules are present? 9. How many chromatids are present in the nucleus of each daughter cell just after mitosis and cell division? 10. How many chromatids are present in the nucleus of a cell after replication of DNA? 11. Draw a simple diagram of a cell which contains only one pair of homologous chromosomes (i) at metaphase of mitosis, (ii) at anaphase of mitosis.

Answers (46 chromosomes. Each 1 has 2 chromatids) Each chromatid contains one

Growth - Nuclei of 2 daughter cells have same number of identical chromosomes as parent nucleus. Allows unicellular zygote to grow into multicellular organism. Animal - growth over entire body Plants growth confines to growing points of plants, like the meristem. DifferentiationDifferentiation – cells change or differentiate to become specialised cells. These different cells divide by mitosis to give tissues of specialised cells, adapted to a particular function. Repair - Mitosis followed by cell division allows for repair of tissues and replacement of cells. Cell replacement in human skin is rapid and in lining of gut. Some animals can regenerate whole parts of body, like starfish arms. Mitosis is basis of asexual reproduction, (the production of new individuals of a species by 1 parent organism.

Meiosis To pass DNA onto the next generation is different. In sexual reproduction 2 specialised sex cells or gametes fuse to form a single cell = the zygote. Sex cells must have half the normal amount of DNA, so when they fuse, normal amount is restored. The cells that give rise to sex cells divide by meiosis.

Meiosis ii. Meiosis = Must be a point when number of chromosomes is halved if sexual reproduction occurs in life cycle. Gametes contain one set only of chromosomes. Gametes are always haploid (n) as a result of meiosis. Meiosis produces gametes that are different-they vary

Variation is achieved by meiosis because: Haploid gametes when they fuse at fertilisation combine the genetic material from parents. Pairs of homologous chromosomes arrange themselves randomly and so are re-sorted in the daughter cells. Known as independent assortment. During meiosis, chromatids break at points called chiasmata separates linked alleles and creates new genetic combinations.