Mitosis and Meiosis PDF

2.4 Mitosis and Meiosis Another type of Nuclear Division: Meiosis  Meiosis – nuclear division where a single cell divides TWICE to produce FOUR cells containing HALF the amount of original DNA of the cell.  These resulting FOUR cells are our sex cells (gametes): sperm in males, eggs in females...

2.4 Mitosis and Meiosis Another type of Nuclear Division: Meiosis  Meiosis – nuclear division where a single cell divides TWICE to produce FOUR cells containing HALF the amount of original DNA of the cell.  These resulting FOUR cells are our sex cells (gametes): sperm in males, eggs in females. How does this compare with mitosis?  Mitosis is a type of nuclear division where a single cell divides ONCE to produce TWO daughter cells containing the SAME amount of original DNA  Mitosis vs Meiosis x x No me sa tt lica he 2 rep t c Ex a x 2 x x x x x 1/ 1/ 1/ 1/ 2x 2x 2x 2x MITOSIS – to produce MEIOSIS – to produce clones gametes Some important terms – diploid and haploid  Our cells have 2n DNA (n from father, n from mother), so our cells are termed “diploid”  In mitosis, the purpose is to “make more” of the same type of cells (one 2n parent cell → TWO 2n daughter cells)  In meiosis, the purpose is to produce sex gametes i.e. sperm cells and egg cells (one 2n parent cell → FOUR daughter cells with half of 2n i.e. n)  Later, 1 sperm (n, haploid) will fertilize 1 egg (n, haploid) → produce 1 diploid (2n) fertilized egg (zygote)  Mitosis vs Meiosis 2 n 2 n No me sa tt lica he rep t c 2x2 Ex a n 2x2 n 2 2 n n 2 2 n n n n n n MITOSIS – to produce MEIOSIS – to produce clones gametes Sexual Reproduction Sexual reproduction is a mode of reproduction involving fusion of female gamete (egg) and male gamete (sperm), which forms zygote that potentially develops into genetically distinct offspring  2 steps to achieve sexual reproduction: 1. Meiosis + cytokinesis: a process to produce haploid cells (gametes) from a diploid cell (e.g., 2n → n) 2. Fertilization (sperm meets oocyte): a process where 2 haploid gametes unite to form a diploid cell (called zygote) (e.g., n + n → 2n) Meiosis What does MEIOSIS involve?  2 rounds of Nuclear Division: o 1st meiotic division (Meiosis I) o End meiotic division (Meiosis II) – resembles mitosis Meiosis What does MEIOSIS involve?  2 rounds of Nuclear Division: Image credit: Ali Zifan, for o 1st meiotic division Wikimedia (Meiosis I) o End meiotic division (Meiosis II) – resembles mitosis Stages of Meiosis I: PROPHASE I Nuclear envelope breaks up Spindle fibres form Centrioles appear Homologous pairs of chromosomes are tangled together by a process “Crossing Over” o Exchange of genes between homologous chromosomes so that some paternal genes are mixed with some maternal characteristics o 1st strategy for generating genetic variation Image credit: Ali Zifan, for Crossing over gives rise to genetic variation Identical sister chromatids Crossing over occurs These are the non-sister chromatids where crossing over is going to take place 1 2 3 4 Stages of Meiosis I: METAPHASE I Spindle fibres attach to centromeres of the chromosomes Homologous chromosomes “meet in the middle” at the equator of the cell – “Random Assortment” o Paternal and maternal chromosomes align themselves on equator in a random way that is up to chance. o Not ALL the paternal DNA will go to one side and the maternal DNA to the other side o 2nd strategy for generating genetic variation Image credit: Ali Zifan, for Stages of Meiosis I: ANAPHASE I Spindle fibres contract Each homologous chromosome gets pulled Apart to be separated to opposite poles of cell Sister chromatids are still joined at their centromeres o Question: Are the sister chromatids still identical to each other? The answer is here Image credit: Ali Zifan, for Stages of Meiosis I: TELOPHASE I & Cytokinesis Each homologous chromosome reaches the pole of cell DNA decondenses to form chromatin Spindle fibres disappear Nuclear membranes form Cytokinesis follows after Telophase I TWO new haploid daughter cells formed Image credit: Ali Zifan, for What does MEIOSIS involve?  2 rounds of Nuclear Division: o 1st meiotic division (Meiosis I) o End meiotic division (Meiosis Image credit: Ali Zifan, for II) – resembles mitosis Wikimedia  Meiosis II  TWO daughter cells generated from Meiosis I will undergo a 2nd round of nuclear division  Between Meiosis I and Meiosis II, DNA is NOT replicated  Meiosis II is very similar to mitosis Stages of Meiosis II: PROPHASE II Chromatin condense to form chromosomes Nuclear envelope breaks up Spindle fibres form Centrioles appear and move to opposite poles Sounds exactly like Prophase of Mitosis, what about remaining 3 stages? Image credit: Ali Zifan, for Stages of Meiosis II: METAPHASE II Sister chromatids are arranged at the equator of cell Spindle fibres attach to the centromere “Meet in the middle” Image credit: Ali Zifan, for Stages of Meiosis II: ANAPHASE II Centromere break apart Spindle fibres contract Sister chromatids are pulled Apart to the opposite poles of cell Each chromatids is now called a daughter chromosome Image credit: Ali Zifan, for Stages of Meiosis II: TELOPHASE II Each daughter chromosome reaches the poles Spindle fibres break down New nuclear envelope forms around each group of chromosomes (now chromatin) Nucleolus reappears in each daughter nucleus Cytokinesis follows after Telophase II FOUR non-identical HAPLOID daughter cells formed Image credit: Ali Zifan, for Why is MEIOSIS important?  To produce sperm cells and egg cells with half the number of chromosomes i.e. haploid (n) Image credit: Ali Zifan, for  Allows genetic variations in the Wikimedia gametes (which affects the genetic variation in the zygote/offspring) due to:  CROSSING OVER between non- sister chromatids of homologous chromosomes in Meiosis I (Prophase I)  RANDOM ASSORTMENT of homologous chromosomes at equator/metaphase plate during Meiosis I (Metaphase I)  19 Spermatogenesis Brook et al., Biology, 2011, 2nd ed  20 Oogenesis (formation of oocyte/egg cell) Brook et al., Biology, 2011, 2nd ed Summary – meiosis:  Each parent cell undergoes meiosis to give rise to FOUR genetically different daughter cells – takes place in reproductive organ  Parent cell: 2n (diploid)  daughter cells: n (haploid)  Involves TWO nuclear divisions:  Meiosis I  Crossing over  Random assortment  Meiosis II  Mitosis vs Meiosis MITOSIS MEIOSIS Products TWO genetically FOUR genetically non- identical daughter cells identical daughter cells Daughter cells (diploid) Daughter cells (haploid) contain same number contain half the number of chromosomes as the of chromosomes as the parent cell parent cell When and Occurs in normal body Occurs in the Where cells – growth and reproductive organs repair during sperm and egg formation Nuclear and One round Two rounds Cell Division Genetic No Yes – because crossing Variation over and random assortment Summary: Checklist CAN YOU:  Describe cell cycle in eukaryotes?  List the stages of mitosis?  Describe the events that occur during mitosis?  Describe the events that occur during meiosis?  Compare mitosis and meiosis?