Cell Division Cycle Is essential for Growth Mother and daughter cells are identical Maintenance Cell turnover Components of the nucleus and cytoplasm divide during cell cycle Cytokinesis is the division of the cytoplasm
Cell Cycle Control of cell division is complex & precise Steps occur in sequence Must know when to proceed/wait
Normal Growth Conditions Protein content increases RNA synthesis continues, except during cell division (M phase) Overall growth is a steady process
Interphase Decision making time for cell Should cell replicate?? Mitosis?? Can lead to cancer Often looks like nothing is happening Makes up 90% of the time Has 3 subphases G1 – separates the end of mitosis from the next round of DNA replication S phase – DNA replication G2 phase – gap between S phase and Mitosis Cells can remain in G1 for weeks or even years
G0 Phase There are times when a cell will leave the cycle and quit dividing. This may be a temporary resting period Can be permanent ie a cell that has reached an end stage of development and will no longer divide (e.g. neuron).
G1 phase Cells increase in size Produce RNA and synthesize protein. An important cell cycle control mechanism activated during this period (G1 Checkpoint) ensures that everything is ready for DNA synthesis.
S Phase To produce two similar daughter cells the complete DNA instructions in the cell must be duplicated. DNA replication occurs during this S (synthesis) phase.
G2 Phase Cells continue to grow and produce new proteins. At the end of this gap is another control checkpoint (G2 Checkpoint) to determine if the cell can now proceed to enter M (mitosis) and divide.
Mitosis Mitosis = process of nuclear division Occurs in somatic cells (not germ-line) http://www.rebeccastmartin.com/mnemonicdevices/phasescelldevelopment.htm
Mitosis M-Phase is divided into 6 stages Based on observations using the light microscope on living cells and a mixture of light and electron microscopy Interphase Prophase Prometaphase Metaphase Anaphase Telophase cytokinesis (non-nuclear)
Mitosis - Prophase Prophase (beginning of mitosis) Transition from G2 to M is not clear Chromosomes condense (shorted and thicken) Nuclear envelope disintegrates Chromosomes are composed of 2 identical (sister) chromatids Centromeres have divided but remain together Spindle fibres begin to grow from the centrosome
Mitosis - Metaphase Chromosomes move to the equator of the cell (metaphase plate) Spindle fibres become attached to the sister chromatids Spindle apparatus is complete
Mitosis - Anaphase Sister chromatids begin to separate and form chromosomes Chromatids move towards opposite poles (1 um per minute Can be delayed until all chromosomes are on the metaphase plate
Mitosis - Telophase Sister chromatids arrive at opposite poles Reverses the steps of prophase to return to interphase Chromosomes uncoil & protein synthesis begins Nuclear envelope reforms Cytokinesis takes place Cell entered G1 phase
Meiosis Grant McKenzie http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect16.htm
What is meiosis? Gamete formation Sperm/pollen and egg From diploid (2 copies) to haploid (1 copy) Resorted NOT doubled 4 daughter cells contain a complete set of chromosomes Promotes genetic diversity http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.figgrp.3679
Genetic Diversity Meiosis gives rise to genetic diversity Crossing over Occurs between homologous chromosomes not sister chromatids Chance combination of gametes during fertilisation Genetically diverse sperm and eggs can unite
Meiosis Involves two cell Divisions Meiosis I – a reduction from 2n to 1n Meiosis II resembles mitosis (1n to 1n)
Meiosis – Prohase I Similar to prophase of mitosis chromosome duplication & spindle formation Chromosomes condense and are attached to the nuclear membrane Paring of homologous chromosomes Homologous chromosomes join along their length Chromosomes fuse together and are referred to as bivalents
Overview of DNA Synthesis In each cycle of cell division, the cell first makes a copy of all of the DNA in each of the chromosomes http://student.ccbcmd.edu/~gkaiser/biotutorials/dna/fg13.html
Meiosis – Prohase I Crossingover occurs throughout prophase I
Crossing Over Homologous chromosomes Chiasma Recombinant chromatids Adjecent homologous chromosomes break and swap genetic material Greatly increases the genetic variation Tetrads form chiasmata, X shapped configuration that marks crossing over http://www.unis.org/UNIScienceNet/IBHbio2_knowledge.html
Meiosis - Metaphase I Nuclear membrane breakdown Development and attachment of the machinery to pull the tetrads (chromosomes) apart (kinetochore & microtubules) Tetrads move to the equatorial plate There is no splitting of the sister chromatids like mitosis
Meiosis – Anaphase I Cohesin breaks down between the centromeres homologous chromosomes are pulled apart Called a reductional division For every tetrad there is now 1 chromosome in the form of a chromatid pair called a dyad Located at each pole Each daughter cell is haploid
Meiosis - Telophase I & Prophase II Telophase I does not occur in all species Chromosomes enter an interphase configuration as cytokinesis takes place No chromosome duplication occurs Nuclear envelope reforms Cell enters Prophase II
Meiosis II – Prophase II Meiosis II is basically a mitotic division Chromatids from each chromosome are pulled apart to opposite poles For each original cell entering Meiosis I, 4 haploid cells emerge at telophase II Meiosis II is an equational division Reduces the amount of genetic material per cell by half, does not reduce the chromosome number by 2
Cell Division - Cytokinesis Contractile ring forms Does not always occur after mitosis Start by a furrowing of the membrane Organelles need to be produced need not be distributed equally between daughter cells http://www.sparknotes.com/biology/cellreproduction/mitosis/section3.rhtml