* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Chromosomes and Cell Division
Survey
Document related concepts
Site-specific recombinase technology wikipedia , lookup
Extrachromosomal DNA wikipedia , lookup
History of genetic engineering wikipedia , lookup
Designer baby wikipedia , lookup
Microevolution wikipedia , lookup
Y chromosome wikipedia , lookup
Polycomb Group Proteins and Cancer wikipedia , lookup
Vectors in gene therapy wikipedia , lookup
X-inactivation wikipedia , lookup
Transcript
Chromosomes and Cell Division Chromosomes • Visible only during cell division. • Most condensed form of DNA. –DNA double helix tightly coiled around proteins. Chromatin = loosely coiled form of DNA An interphase cell has no visible chromosomes. Although not visible, every chromosome in a nondividing cell is made up of only 1 chromatid. Only if a cell is preparing to divide, does a chromosome become made up of 2 chromatids. Prior to DNA replication After DNA replication Human Chromosomes 46 Chromosomes 23 pairs Diploid Cells • Diploid = 2n – n = # of types (sizes) of chromosomes • n = 23 for humans, so the diploid # for humans is 46 (2 x 23) – Diploid cells have pairs of chromosomes • Homologous chromosomes: chromosomes of the same shape and size, carry genes for the same traits. – Each species has a unique diploid number. Diploid numbers of some commonly studied organisms Homo sapiens (human) Mus musculus (house mouse) Drosophila melanogaster (fruit fly) Caenorhabditis elegans (microscopic roundworm) 46 40 8 12 Saccharomyces cerevisiae (budding yeast) Canis familiaris (domestic dog) Gallus gallus (chicken) 32 78 78 Zea mays (corn or maize) Muntiacus reevesi (the Chinese muntjac, a deer) Myrmecia pilosula (an ant) 20 23 2 Cambarus clarkii (a crayfish) 200 Equisetum arvense (field horsetail, a plant) 216 Homologous Chromosomes e.g.: eye color From “Dad” From “Mom” Fertilization produces diploid cells • Example: Humans – The mother’s egg cell has 23 chromosomes – The father’s sperm cell has 23 chromosomes – When these cells fuse, the fertilized egg has 46 chromosomes (23 pairs); it is diploid. • Mitosis produces trillions of body cells that are all diploid. – Each of the 23 pairs of chromosomes is called a “homologous pair”. Haploid cells • Gametes (reproductive cells) – Egg cells (ova) and sperm cells – Produced in the reproductive organs, ovaries or testes • Haploid = 1n – n = 23 for humans so human haploid cells contain 23 chromosomes. – Haploid cells have one of each chromosome; they do NOT have pairs of chromosomes. Diploid vs Haploid Cells 2 of each type of chromosome is present in the cell. Only 1 of each type of chromosome is present in the cell. Note: the chromosomes in both pictures have 1 chromatid each. The # of chromatids does NOT relate to diploid/haploid. Cells must be haploid to maintain the chromosome # of a species during reproduction • If reproductive cells were diploid, then after fertilization a human zygote (fertilized egg) would have 96 chromosomes. • To have the 46 chromosomes of a typical human cell, each reproductive cell should only have 23 chromosomes. Meiosis produces Haploid cells • Involves 2 cell divisions: Meiosis I and Meiosis II. – Each cell division consists of PMAT. – Results in 4 cells. For cell division, what needs to occur within a cell? • 2 copies of every DNA molecule – DNA replication during the S phase of interphase • DNA packaged into easily moveable units – Prophase: DNA “condenses” (becomes more tightly coiled around proteins) • Dissolving of the nuclear membrane to allow for distribution of genetic material (prophase) • Structures to attach to, and physically move, the chromosomes – Spindle fibers (microtubules) – Made by centrosomes • An organized way of distributing the genetic material equally between two cells. – Metaphase and Anaphase • Separation of the material into distinct cells. – Telophase: formation of new nuclear membranes, cytokinesis (division of cytoplasm) Meiosis I • Prior to the beginning of cell division, DNA is copied (replicated). • Prophase I: – Similar to mitosis: dissolving of nuclear membrane, condensing of chromosomes, formation of spindle fibers. What is unique is: – Synapsis: pairing of homologous chromosomes (formation of tetrads). – Crossing-over: chromatids of homologous chromosomes exchange genetic material. • Results in genetic recombination: a new, unique combination of genes. Prophase I • Letter T = tetrads – Each red box surrounds a tetrad. • Letter C = centrosomes – Produce spindle fibers. • Crossing-over is occurring in the lower tetrad. Crossing-over Metaphase I • Lining up of tetrads (homologous pairs) along the “equator” of the cell. – The arrangement of the chromosomes is random. – This allows for “independent assortment”. • The random distribution of genes from different chromosomes to different gametes. • Genetic recombination: new, unique combination of genes will be present in the daughter cells. Metaphase I • Chromosomes arranging like in “A” is just as likely as chromosomes arranging like in “B”. • Each time cells in the testes/ovaries go through meiosis, unique cells are produced. How many arrangements are possible? • For our simulation? • For humans? How many arrangements are possible? • For our simulation: – 4 possibilities – 2n = 2 3 = 8 • For humans: – 2n = 223 = 8,388,608 – Wow! – And this doesn’t even include recombination resulting from crossing-over! Anaphase I • Separation of homologous chromosomes to opposite sides of the cell. – Sister chromatids do NOT separate. • The direction they are pulled depends on their alignment during metaphase I. • Chromosomes are “independently assorted”. Anaphase I Telophase I: • Chromosomes are segregated at opposite poles, one group on each side of the cell. Cytokinesis begins, forming two daughter cells. Each chromosome consists of two chromatids. Telophase I: • Are the new cells diploid or haploid? • HAPLOID – Only one of each chromosome Meiosis II • After PMAT I the daughter cells are haploid. So why round two of PMAT? – The chromosomes are still in a “duplicated” state, two DNA molecules per chromosome. – The normal state of genetic material in a cell is one DNA molecule per chromosome. Prophase II Nucleus dissolves and spindle fibers form again Metaphase II Chromosomes line up “single-file” along the midline of the cell. Anaphase II Sister chromatids of each chromosome detach from each other and move to opposite sides of the cell. Telophase II A nuclear membrane forms around each set of chromosomes and cytokinesis begins, forming 4 daughter cells. After Telophase II and Cytokinesis • A new nuclear membrane should be shown. • Cells are in Interphase. • Review: Separation of chromosomes during meiosis.