BIO 207 LECTURE 3.docx

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Biology (Biological Sciences)
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Lesley Harrington

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BIOL207 B01 Lec03 2014-01-10 know the names of the different cell stages; There can be a gap before cell division occurs; During synthesis, we have 2 chromatids per chromosome; Meiosis 1  1 allele in one cell, 1 allele in another cell The amount of chemical present in one cell is important 1 gamete is 3 billion bp; upon fertilization, each zygote has 2C values of DNAupon fertilization; we double the DNAagain in S-phase. Meiosis 1  we reduce the amount of DNApresent, and the second division reduces it once more. We can reduce the amount of DNAwithout changing the amount of information. When we separate homologous chromosomes, we are changingin the amount of information Fertilization… 2 chromosome sets present 2n = 2 copies of chromosome sets; Number of homologous chromosomes changes when you go into Meiosis 1 - 1N ; then you go to 1C CHAPTER 2 cont’d A. Chromosome abnormalities; Karyogram is a photograph of all chromosomes; a. Karyotypes = photograph of a metaphase chromosome; b. Deletions, inversions, insertions, translocations, duplications of large parts of chromosomes are all possible; deletions can occur, chromosome breakage can occur due to segregation, accommodation c. Segments can be lost or duplicated; inversion…;insertion from one non-homolgous chromosome to another; translocation is the moving from one chromosome to another d. Aneuploidy = addition or subtraction of a chromosome from a homologous pair; daughter cell has one more or one fewer cell than the other. i. E.g. trisomy 21 = Down’s syndrome ii. Gene balance = biochemistry of the cell is thrown off by abnormal ratios of some gene products  affects chromosomal diseases; amount of enzymes present in a certain metabolic pathway is necessary for the regulation of that pathway; having an extra amount of a single chromosome needs more enzyme… throws off the balance B. Chromosome number, ploidy, and DNAcount; one chromosome has one extra copy or one less copy; polyploidy is common in plants; a. “n” = the number of chromosomes in a gamete; number of chromosomes in a gamete; in humans, n is 23 b. “2n” = the number of chromosomes in a zygote; 2n is the zygote  46 chromosomes; c. number of chromosomes means the total number of chromosomes you count on under the microscope or on a karyotype d. There is no such thing as 4n, 6n, 8n…we use different “n’s” to distinguish gametes vs zygotes e. Humans are 2n=46, because you can count 46 chromosomes in a human cell. Wheat is 2n=42, because you can count 42 chromosomes in a wheat cell. f. DNAcontent in a cell can be measured in base pairs, or in picograms (pg), or it can be represented as multiples of the variable “c”. g. DNAcontent changes throughout cell cycle; if gamete is 1c, fertilization doubles this to 2c, then replication doubles this to 4c. h. Note that replication and mitosis changes c-value, but not chromosome number. Fertilization and meiosis change both c-value and chromosome number. i. Many plants and some animals are polyploid j. “x” can be used to define ploidy; e.g. tetraploids are 2n=4x so there would be 4 copies of each chromosome in a karyotype, rather than 2 as in a diploid.; 2n = 4x if there are 4 copies of each chromosome; human is 2n = 2x which makes it diploid. 2n = 6x is a hexaploid  pasta wheat; can bread wheat (6x) mate with pasta wheat (4x) … different species, so they can’t fertilize each other k. polyploids may form bivalents or polyvalents, depending how closely related the chromosomes are (autopolyploids vs.
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