Cell Division Mitosis and Meiosis

Transcription

1 Cell Division Mitosis and Meiosis students will describe the processes of mitosis and meiosis o define and explain the significance of chromosome number in somatic and sex cells o explain the events of the cell cycle o compare the formation of identical and fraternal twins Chromosome Structure and Vocab Haploid (n) gametes (eggs and sperm) 23 chromosomes in humans Diploid (2n) Somatic cells (non gametes) 46 chromosomes in humans

2 The Cell Cycle Growth and division of somatic cell How multicellular organisms grow in size Two phases: Interphase and Division Phase Division Phase Cell Cycle Interphase Longest phase in the cell cycle Cell grows in size Doubles cytoplasmic organelles Duplicates chromosomes Three parts: G1, S, G2 o G1: cell growth o S: synthesis of DNA (chromosome duplication o G2: cell growth and preparation for division Interphase Division Phase Mitosis and Cytokinesis Cell splits into two identical cells

3 Mitosis 4 stages: Prophase, Metaphase, Anaphase, Telophase Prophase o Chromosomes condense o Centrioles form and move to opposite ends of cell o Spindle fibers form and attach to centromeres o Nuclear membrane starts to dissolve Metaphase o Chromosomes (sister chromatids) move to center of cell (equatorial plate) o Nuclear membrane completely dissolved o Chromosomes most visible at this stage o Chromatids can become intertwined Anaphase o Centromeres divide o Sister chromatids (now separate chromosomes) move to opposite ends of cell o Should be identical chromosomes at each end Telophase o Chromosomes lengthen (expand) o Spindle fibers dissolve o Nuclear membrane reforms around each group of chromosomes Cytokinesis Cytoplasm divides End with two identical cells o Both still 2n

4 Mitosis and Aging (pg 562 and 570 in text) Limited # of divisions Biological clock may regulate # of cell divisions allowed Study on heart cells Specialization affects mitosis rate o More specialized cells divide fewer times Telomere (cap at end of chromosome) controls # of divisions o In normal cells: telomeres shorten every time the cell divides until it can t shorten anymore without affecting the chromosome. Cell is at max age and dies Telomere Black bands represent genes o In abnormal cells: cell keeps dividing and chromosome shortens = lose genetic information = abnormal cells o In cancer cells: telomeres don t shorten with each division, so cell never stops dividing This is due to the presence of enzyme telomerase in cancer cells that prevents shortening of telomeres.

5 Cloning Forming identical offspring from a single cell or tissue in the parent organ Clone and parent have identical DNA Happens naturally in nature (pg 565) o Budding o Runners o Identical twins Allows for production of strains of plants with predictable characteristics Animal Cloning (1960s +) Frog cloning (pg 567) o Blastocyst nucleus inserted into enucleated (no nucleus) egg cell o Egg cell divides mitotically to develop into normal embryo and then frog o New frog is genetically identical to original embryo o Nucleus from older cells (gastrulation stage) did not work as well cells already too specialized to undergo mitosis Mammal cloning o Same process as frogs o Only seemed to work with nucleus from blastocyst cells o Then Dolly happened o Took nucleus from adult cells (udder cells) of Finn Dorsett and put in enucleated egg cel of Poll Dorsett o New embryo (with transplanted nucleus) put into surrogate sheep s womb (Scottish Blackface) o Lamb was born that was genetically identical to first adult (Finn Dorsett) Review questions Pg 571: #1 4

6 students will describe the processes of mitosis and meiosis o describe the process of meiosis and the necessity for the reduction of chromosome number o compare the processes of mitosis and meiosis o describe the processes of crossing over and nondisjunction Meiosis cell division to form sex cells (aka gametes, sperm, eggs) happens only in testes and ovaries gametes are haploid (n) goes through 2 divisions similar to Mitosis starts with G1, S, G2 Meiosis I homologous pairs separate Prophase I o Nuclear membrane dissolves o Centrioles form and move to opposite ends o Spindle fibers form and attach to centromeres o Synapsis occurs: homologous pairs come together and intertwine o (tetrad = 4 chromatids) o Crossing over may occur (pieces of chromatids break off and exchange) = variability i offspring Synapsis Metaphase I o Tetrads line up on equatorial plate ** For simplicity, I will assume no crossing over happened for the next stages** Anaphase I o Homologous chromosomes separate to opposite ends (segregation) o Chromosome number has now been reduced to n **Which ones (red or blue) go left or right is completely random for each pair. ** Telophase I o Nuclear membrane forms around each set o These two new cells are not identical (unlike in mitosis) because one has one set of the homologous pair and the other has another set. Cytokinesis o Cell membrane separates and you are left with two new haploid cells o Then both start Meiosis II without going through G1, S, G2

7 Meiosis II Sister chromatids separate (like in mitosis) Prophase II o Nuclear membrane dissolves o Centrioles form and move to opposite ends o Spindle fibers form and attach to centromeres Metaphase II o Sister chromatids line up on equatorial plate Anaphase II o Sister chromatids separate to opposite ends o Chromosomal number is still haploid Telophase II o Nuclear membrane forms around each set o These two new cells should be identical if there was no crossing over previously. Cytokinesis o Cell membrane separates and you are left with four new haploid gametes

8 students will describe the processes of mitosis and meiosis o describe the diversity of reproductive strategies by comparing the alternation of generations in a range of organisms Key Terms Sporophyte: structure that releases spores Diploid Spores: grow into gametophyte Gametophyte: structure that makes gametes Haploid Know the vocabulary and which ones are diploid or haploid. Don't have to know specific plants.

9

10

11

12 Abnormal Meiosis Nondisjunction o 2 homologous chromosomes fail to separate during meiosis or mitosis o 1 daughter cell has too many, 1 has too few chromosomes o o o chromosomes Too much or too little, don t function properly More consequences in meiosis = fetus that s missing or has too many chromosomes Polyploidy o More than 2n (nondisjunction of all chromosomes) o 3n triploidy o 4n tetraploidy Failure of 2n zygote to divide after replicating (4n embryo) o Common and chemically encouraged in plants such as wheat, oats, tobacco, and potat Humans and nondisjunction o Split at meiosis II produces gametes of 22 and 24 chromosomes (instead of normal 23 23) o Therefore after fertilization with a normal gamete, you get embryo of either 45 or 47 1 chromosome where there should be 2 = monosomy 3 chromosomes where there should be 2 = trisomy As individual develops, every cell will either be 1 short or have 1 too many Disorders Down Syndrome Turner Syndrome Klinefelter syndrome Trisomic female