CHAPTER 3 The Chromosomal Basis of Heredity 3 1 Each species has a characteristic set of chromosomes Carl Nageli first discovered that in dividing cells the nucleus divided first Chromosomes were discovered in 1880 3 2 The daughter cells of mitosis have identical chromosomes The ends or poles of the spindle where the microtubules converge mark the locations of the centrosomes which are the microtubule organizing centers where tubulin polymerization starts Three types of microtubules in spindle anchor centrosomes to cell membrane arch between centrosomes and attach to the chromosomes 3 3 Meiosis results in gametes that differ genetically The first meiotic division reduces the chromosome number by half Prophase I Leptotene chromosomes become visible thread like dense granules called chromomeres appear along their length Zygotene pairing or synapsis of homologous chromosomes mediated by synaptonemal complex which holds the chromosomes together each pair of synapsed homologous chromosomes is a bivalent Pachytene chromosomes continue to shorten and thicken tetrad of four chromatids crossing over takes place Diplotene synaptonemal complex breaks down synapsed chromosomes begin to separate but are held together at intervals by cross connections resulting from crossing over known as chiasma breakage and rejoining between nonsister chromatid a chiasma results from physical exchange between chromatids of homologous chromosomes Diakinesis homologous chromosomes repel each other maximum condensation formation of spindle and nuclear envelope breaks down Genes on different chromosomes undergo independent assortment because nonhomologous chromosomes align at random on the metaphase plate in meiosis I Chromosome separation at anaphase is the cellular basis of the segregation of alleles The second meiotic division is equational Chromosome replication never takes place between the two divisions Each chromatid is considered a separate chromosome after anaphase II 3 4 Eukaryotic chromosomes are highly coiled complexes of DNA and protein Chromosome sized DNA molecules can be separated by electrophoresis The nucleosome is the basic structural unit of chromatin The DNA of chromosomes is associated with protein molecule in a stable ordered aggregate called chromatin The major class of chromosomal proteins are the histone proteins largely responsible for the structure of chromatin five types H1 H2A H2B H3 and H4 small mostly lysine and Arginine Both these amino acids are positively charged allowing the histones to bond to the negatively charged phosphate groups in the sugar phosphate backbone of DNA The beadlike units in chromatin are called nucleosomes Additional DNA called linker DNA links adjacent core particles The amino ends of the histone proteins which constitute about 25 of the total length are known as histone tails Acetylated histones tend to bind DNA more loosely and usually render chromatin more accessible to transcription whereas methylated histones can either promote or impede transcription depending on the particular histone residue that is modified Chromatin fibers form discrete chromosome territories in the nucleus The metaphase chromosome is a hierarchy of coiled coils Chromosome condensation is an ordered energy consuming process orchestrated by a protein complex called condensin that works to actively coil the chromatin The scaffold is composed of a number of nonhistone chromosomal proteins Heterochromatin is rich in satellite DNA and low in gene content Heterochromatin DNA that is compact and heavily stained in interphase rest is euchromatin Heterochromatin is located near the centromeres and at the telomeres contains satellite DNA which are highly repeated short base sequences The number of genes located in heterochromatin is small relative to the number in euchromatin 3 5 The centromeres and telomere are essential parts of chromosomes The centromeres is essential for chromosome segregation The telomere is essential for the stability of the chromosome tips The mechanism of restoring the ends of a DNA molecule in a chromosome relies on an enzyme called telomerase which adds tandem repeats of a simple sequence to the 3 end of DNA A guide RNA contains sequences complementary to the telomere repeat and serves as a template for telomere synthesis and elongation 3 6 Genes are located in chromosomes Special chromosomes determine sex in many organisms Hemophilia is a classic example of human x linked inheritance Affected persons lack a blood clotting protein called factor VIII that is needed for normal clotting For any rare trait due to an x linked recessive allele affected individuals are exclusively male Affected males who reproduce have normal sons Experimental proof of the chromosome theory came from nondisjunction Nondisjunction is the failure of the two X chromosomes to separate during meiosis Embryos that lack an X chromosome die early in development Chromosome Theory of Heredity genes are contained in the chromosomes 3 7 Genetic data analysis makes use of probability and statistics Progeny of crosses are predicted by the binomial probability formula The addition rule of probability deals with possible outcomes of a genetic cross that are mutually exclusive USE NOTES Chi square tests goodness of fit of observed to expected numbers If the observed numbers deviate too much from the expected numbers reject the hypothesis Reject the hypothesis is p 0 05 or lower
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