14 - C THE NUCLEUS CHROMOSOMES AND EXPRESSION OF THEIR...

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C THE NUCLEUS, CHROMOSOMES, AND EXPRESSION OF THEIR GENES Genomic DNA in most of the animals of interest to the reader consists of approximately 3-4 billion base-pairs of nucleotides arranged as linear polymers, in a variable number of chromosomes. Somatic cells (i.e. non-gametes) have a pair of sex chromosomes along with characteristic numbers of paired autosomes where each chromosome within the pair is derived from each of the two parents. In mammals where the sire's gamete determines sex of the offspring, males possess an X and a Y chromosome, the Y being paternally- derived. Female offspring result from the sire contributing an X chromosome to the X present in the oocyte, resulting in an XX karyotype. Avian sex determination is the responsibility of the female parent and the non-autosomal chromosomes are called WZ in females and ZZ in the homogametic males. 2007 version – page 99
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Chapter 14. CHROMOSOMES HAVE DNA AND NUCLEOPROTEINS Each linear polymer of DNA within a chromosome has discrete domains. Those of special importance to applications made within this text will be briefly described. The telomere , mentioned in Chapter 9 as the "molecular counter" that normally determines how often a cell can replicate its DNA, is found at each end of the DNA and some minimal length is needed to permit DNA replication during S -phase. A centromere region serves as the site of adhesion of newly replicated DNA strands (during S -phase) then, along with proteins making up the kinetochore , it provides for the attachment of chromosomes to microtubules of the spindle apparatus during M -phase. Along the length of chromosomes are multiple origins of replication , which allow replication to occur simultaneously at multiple locations, vastly accelerating the overall process of S -phase. A mammalian genome of approximately 3 billion base-pairs, organized in, say, 30 pairs of chromosomes (see Table 14-1 for the number of pairs of autosomal chromosomes in a variety of species of likely interest), each of average size of 100 million base pairs, completes S -phase in just a few hours. Of the 3 billion base-pairs, only a very small portion (perhaps <5%) actually codes expressed genes. Using an estimate of approximately 30,000 genes and an "average-sized" protein of 30,000 Daltons means that barely 1% of the total nucleotides are needed to totally specify the amino acid sequences of the proteins. Table 14-1. Number of chromosomes in selected species. The following tabulation is the number of pairs of autosomal (i.e. non-sex determining) chromosomes. Animals of broad agricultural importance: Cattle 30 Goat 30 Sheep 27 Pig 19 Horse 32 Duck 39 Goose 40 Chicken 38 Turkey 39 Animals with international agricultural importance: Donkey 31 Asian buffalo 24 Camelids 37 Asian Elephant 28 Some laboratory/companion animals: Cat 19 Rabbit 22 Rat 21 Dog 39 Mouse 20 Hamster 22 Ferret 17 Guinea Pig 32 2007 version – page 100
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A. Nuclear proteins and chromosomal structure The proteins in chromatin fall into two
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This note was uploaded on 03/23/2009 for the course ANSCI 1110 taught by Professor Brucecurrie during the Fall '08 term at Cornell.

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14 - C THE NUCLEUS CHROMOSOMES AND EXPRESSION OF THEIR...

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