CELL DIVISION Mitosis
Why do cells divide? • Growth • Repair • Replace • Reproduction
Mitosis • Interphase • Growth • Cell activities (make proteins, transport nutrients,eliminate waste) • Nucleus is visible • Genetic material is called chromatin (long thin strands) • Replication of DNA • Sister chromatids joined by centromere
Mitosis • Prophase • Chromatin coils into chromosomes • Nuclear membrane disappears • Centrioles migrate to poles • Spindle fibers form
Mitosis • Metaphase • Chromosomes (composed of sister chromatids) align at the equator, that are attached to spindle fibres
Mitosis • Anaphase • Centromeres divide and sister chromatids (chromosomes) separate and move to opposite poles
Mitosis • Telophase • New nuclear membranes • Spindle disappears • Chromosomes unwind
Cytokinesis • Cytoplasm is divided in half • cleavage furrow (animal) • cell plate (plant)
Animation and Real Footage • http://www.youtube.com/watch?v=VlN7K1-9QB0 • http://www.youtube.com/watch?v=rgLJrvoX_qo&feature=related • http://www.youtube.com/watch?v=Q6ucKWIIFmg
CELL DIVISION Meiosis
The need for meiosis (do not copy) • During mitosis, the complete set of chromosomes is duplicated. Every new cell produced by mitosis receives a full set of genes that control its development. • Sexual reproduction, introduces a new problem. Suppose that an egg nucleus contains one full set of chromosomes. Let the symbol N represent the number of chromosomes in this set. Then the sperm nucleus will bring another set, another N. • EGG + Sperm ------------------ ZYGOTE • N + N ---------------- 2N • When the egg and sperm unite, the zygote will contain 2N – a double set of chromosomes. Suppose the process of mitosis were to maintain this double set of chromosome in every cell of the embryo and eventually the adult. At the next fertilization, if the number of chromosomes doubled again then • 2N + 2N ----------------4N And again • 4N + 4N-----------------8N • You see that eventually there would be too many chromosomes for the space in the nucleus.
Meiosis • Is a special kind of nuclear duplication of sex cells • The double set of chromosomes (2N or diploid number) is reduced to a single set. The number of chromosomes in the single set is called the HAPLOID NUMBER (N). • Both the egg and sperm contain the haploid number. In humans the diploid number is 46 the haploid number is 23 (46/2= 23). Egg and sperm cells contain 23 chromosomes.
Two Types of Cell Division • Handout
ACTIVITY Cut & Paste Meiosis Things you need to know before you begin • Meiosis resembles mitosis, both processes begin with chromosomes duplicating, thickening and shortening • There is a difference in the way the duplicated chromosomes line up in the middle of the cell. The chromosomes line up in matching pairs. Some chromosomes may break and exchange parts with their partners • A second cell division immediately follows the first. Result 4 Haploid cells which are formed from the one original cell Label Centromere, nuclear membrane, sister chromatids, tetrad, Crossing Over, cleavage furrow, spindle fibres, Prophase I & II, Metaphase I & II, Anaphase I & II, Telophase I & II, Cytokinesis, 4 haploid cells
Sexual Reproduction • Genetic diversity is created through 3 process: • Fertilization • Crossing over • Random Assortment The family of Sissy Spacek and Jack Fisk exhibits genetic variation. Sexual reproduction produces variation in the gametes and unique combinations of genes in the offspring.
Fertilization • Given the number of different individuals in the population, the number of different combinations of parents is vast. • Any of a number of different gametes can fuse to make a zygote and each one would be unique.
Crossing Over • Prophase I of meiosis • Homologous chromosomes form tetrads • During synapsis pieces of the chromosomes are exchanged • Maternal and paternal genes are carried on the same chromosome now
Random Assortment • There are numerous patterns of alignment during Metaphase I • Maternal and paternal homologues segregate randomly
Meiosis • Reproduction (plants, fungi, animals) • Produces gametes (haploid) • Fertilization forms a zygote (diploid) • DNA replicated once, cell divides twice • http://www.youtube.com/watch?v=AgEqWPyO8z0&feature=related
Meiosis • FEMALES • Begins during 3-5th month of fetal development • Continues from puberty to menopause • One large egg cell and 3 polar bodies
Meiosis • MALES • Begins before birth • From puberty until death or damage • Temperature and age dependent
Determining Sex/Gender • All eggs and sperm carry 23 chromosomes (human) • Autosomes are #1-22 • Sex chromosome is #23 • Eggs contain X • Sperm may contain either X or Y
CELL DIVISION Atypical or Abnormal Cell Division
Mitosis Gone Awry • Mutations in the genes that control cell division trigger an abnormal cell cycle. • Accumulation of these mutations due to environmental stimuli leads to tumour growth. • Carcinogens, radiation and viruses can all cause mutations. http://www.youtube.com/watch?NR=1&v=Hm03rCUODqg Breast cancer cells dividing (3 days time lapse) Mutations in somatic cells aren’t passed to offspring
Damage to Chromosome Structure • Large scale mutations that damage chromosome structure can occur when something unusual happens during crossing over. Translocation – the transfer of a chromosome segment to a non-homologous chromosome. Deletion - occurs when a chromosome segment breaks off, resulting in the loss of some genes Duplication – occurs when part of a chromosome is repeated Inversion – occurs when a piece of a chromosome is rotated so that the order of genes in the segment is reversed.
Atypical Meiosis • NONDISJUNCTION – the uneven separation of homologous chromosomes during Anaphase.
Mutations in gametes • Mutations in meiosis are inherited by offspring though no disorder was evident in the parent • By age 42, 90% of a woman’s eggs are chromosomally abnormal. Thus, at age eighteen, 1:2,000 live births results in Down Syndrome. By age 42 that rises to 1:25
Trisomy 21 • Down Syndrome – there is an extra chromosome in either the egg or sperm making 47 in the zygote
Genetic Screening • Amniocentesis – withdraw fluid and cells from the amniotic sac surrounding the fetus to look at chromosomal abnormalities http://video.about.com/pregnancy/Amniocentesis.htm
Trisomy 8,9,13,16,18 • http://www.youtube.com/watch?v=-QJF0Cs4f9s • March is Trisomy Awareness month. I photographed this series of portraits at the SOFT (Support for Families with Trisomy 18, 13 and Related Disorders) conference in Roanoke, Virginia during July 2009. I am trying to raise awareness that while only 10% of these kids survive their first year the ones that do live a rich life. Expectant parents are often told that Trisomy is incompatible with life and I am trying to share their unique beauty through these digital images. These children and young adults do not pose for photographs but can be captured being themselves, living in the moment. Most are non-verbal so I see it as an honor to help tell their stories visually. • http://www.youtube.com/watch?v=-u2nbfgpP3U
Activity • Quick Lab pg. 122 (construct and Analyze Karyotypes • Homework pg. 120 #1~3