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Chapter 12

Ch. 12 chromosomal basis of behavior.docx

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Karen Williams

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Ch. 12: chromosomal basis of inheritance Qs 12.1- 12.13 Chromosomes and cellular reproduction Eukaryotic Chromosomes -many eukaryotes have two copies of each type of chromosome in their nuclei, so their chromosome complement is said to be diploid, or 2N. Diploid eukaryotes are produced by the fusion of two haploid gametes (mature reproductive cells that are specialized for sexual fusion), one from the female parent and one from the male parent. -the fusion produces a diploid zygote, which then undergoes embryological development -each gamete has only one set of chromosomes and is said to be haploid (N) -the complete compendium of genetic information in a haploid chromosome set is the genome -two examples of diploid organisms are humans, with 46 chromosomes (23 pairs), and Drosophila melanogaster, with 8 chromosomes (4 pairs). -Yeast are an example of haploid. 16 chromosomes -In diploid organisms, the members of a chromosome pair that contain the same genes and that pair during meiosis are called homologous chromosomes; each member of a pair is called a homolog, and one homolog is inherited from each parents -chromosomes that contain different genes and that do not pair during meiosis are called nonhomologous chromosomes -in animals and in some plants, male and female cells are distinct with respect to their complement of sex chromosomes (the chromosomes that are represented differently in the two sexes in many eukaryotic organisms. -one sex has a matched pair of sex chromosomes, and the other sex has an unmatched pair of sex chromosomes or a single sex chromosome. For example, human females have two X chromosomes (XX), whereas human males have one X and one Y (XY) -autosomes: chromosomes other than sex chromosomes -each chromosome has constriction along its length called a centromere, which is important for the behavior of the chromosomes during cell division -the location of the centromere is in one of four places 1.metacentric chromosome: has the centromere at about the center, so the chromosome appears to have two approximately equal arms 2. Submetacentric chromosomes: have one arm longer than the other 3. acrocentric chromosomes: have one are with a stalk and often with a bulb (called a satellite) on it (i.e. almost all the way at the top (or bottom) of the arm occurs the centromere) 4. telocentric: have only one arm, because the centromere is at the very end -chromosomes also vary in size. BUT chromosome length and centromere position are constant for each chromosome and help in identifying individual chromosomes -a complete set of all metaphase chromosomes in a cell is called a cells karyotype. -it is customary in karyotypes to arrange chromosomes in order according to their sizes and positions of their centromere. -conventionally, the longest pair of homologous chromosomes is designated 1, the next largest 2, and so on (except 21 is shorter than 22) -chromosomes with similar morphologies may be arranged under the letter designations A through G -based on size and morphology alone, the different chromosomes are hard to distinguish unambiguously when they are stained evenly, Fortunately, a number of procedures stain certain regions (bands) of the chromosomes more intensely than other regions. Banding patterns are specific to each chromosome, enabling us to clearly distinguish each chromosome. -one of these techniques is called G banding -chromosomes are teated with mild heat or proteolytoic enzymes (enzymes that digest proteins) to digest the chromosomal proteins partially and then are stained with Giesma stain to produce dark bands called G bands - each chromosome has two arms separated by the centromere -the smaller arm is designated p, and the larger arm is designated q -numbered regions and numbered subregions are then assigned from the centromere outward -region 1 is closes to the centromere -location 17q21, meaning that is on the long arm of chromosome 17 in region 21. Subregions are indicated by decimal numerals after the region number. For instance, the cystic fibrosis gene spans subregions 7q31.2-q31.3 -that is, it spans both subregions 2 and 3 of region 31 of the long arm of chromosome 7. -a more modern technique: the DNA probes are labeled with fluorescent molecules that have various wavelengths of fluorescence emissions. The method, known as chromosome painting, has several variations that enable users to paint each chromosome either with several colors or with one distinct color -used rarely Mitosis -in both unicellular and multicellular eukaryotes, cellular reproduction is cyclical procces of growth, mitosis (nuclear division or karyokinesis), and (usually but not always) cell division (cytokinesis). The cycle of growth, mitosis, and cell division is called the cell cycle -in proliferating somatic cells, the cell cycle consists of two phases: the mitotic (or division) phase (M) and an interphase between divisions. -interphase consists of three stages: G 1gap 1), S, and G (2ap 2). During G (t1e presynthesis stage), the cell prepares for DNA and chromosome replication, which take place in the S stage. In G (the 2 postsynthesis stage), the cell prepares for cell division (M stage) -SUMMARY: chromosome replication takes place in interphase and then mitosis occurs, resulting in the distribution of a complete chromosome set to each of two progeny nuclei -during interphase, the individual chromosomes are elongated and difficult to see under the light microscope. The DNA of each chromosome is replicated in the S phase, giving two exact copies, called sister chromatids, that are held together by the replicated but unseperated centromeres -chromatid: one of the two distinct longitudinal subunits of all replicated chromosomes that becomes visible between early prophase and metaphase of mitosis -when the centromeres separate, the sister chromatids become known as daughter chromosomes -replicate = sister chromatids, separate = daughter chromosomes -mitosis occurs in both haploid and diploid cells. Commonly divided in 5 stages - in the 2 stage of the cell cycle, just prior to the start of M, each chromosome consists of two sister chromatids, and the centrioles have duplicated to produce two pairs 1. Prophase: the chromatids condense. By late prophase, each chromosome, which was duplicated during the preceding S phase of interphase, can be seen to consist of two sister chromatids. While condensation is occurring, the nucleolus shrinks and eventually disappears -many mitotic events depend on the mitotic spindle, a structure consisting of fibers composed of microtubules made of special proteins called tubulins. The mitotic spindle assembles outside the nucleus during prophase. In most animal cells, the centrioles are the focal points for spindle assembly (on higher plant cells:usually lack centrioles but they have mitotic spindle) -centrioles are arranged in pairs and before the S phase, the cells centriole pair is duplicated. Then, during mitosis, each new centriole pair becomes the focus of a ra50dial array of microtubules called the aster. Early in prophase, the two asters are next to one another and close to the nuclear envelope; bylate prophase, the two aster have moved far apart along the outside of the nucleus and are spanned by the microtubule spindle fibers 2. Prometaphase: the nuclear envelope breaks at the end of prophase, denoting the beginning of prometaphase. The developing spindle now enters the former nuclear area. A specialized multiprotein complex called a kinetochore binds to each centromere. The Kinetochores are the sites for the attachment of the chromosomes to spindle microtubules known as kinetochore microtubules. Kinetochore microtubules from one pole bind to a sister chromatid while an equivalent number of kinetochore microtubules from the other pole attach to the kinetochore of the other chromatid. Nonkinetochore microtubules: spindle microtubules that do not bind to kinetochores, but also originate from each spindle pole and overlap in the middle of the spindle 3. Metaphase: the kinetochore microtubules orient the chromosomes so that their centromeres become aligned at the metaphase plate. Chromosome condensation occurs and (occurs again at the end of anaphase: this alte condensation serves to minimize the potential problem of chromosome arms extendin
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