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BIOL 308
Dragana Miskovic

STUDY QUESTIONS: LEC 8-9 1. The human gametes have about 3 billion bp of DNA in their chromosomes. a. Express the size of the diploid genome in kb (kilo bases; 1kb = 1000bp) The diploid genome will have about 6 billion bp of DNA in chromosomes. Thus the diploid genome is about 6 million kb in size. b. If the entire DNA was in relaxed B-DNA form, what would be the average length of a chromosome expressed in cm? Since there are 46 chromosomes in human diploid cells, each chromosome would have approximately 150 million bp. In B-DNA conformation, where 10 base pairs are 34 nm long in the helix, this would result in an unrelaxed chromosome length of approximately 51 mm. 150Mbp = 150 X 10 bp6 6 6 Length = 150 X 10 bp X 0.34 nm = 51 X 10 nm = 51 mm c. On average, how many complete turns would be in each chromosome? There would be 15 million turns per chromosome on average, as there is one complete turn per 10 bp in B-DNA. d. If there are around 30-40,000 genes in a human gamete, how many genes are there in an average chromosome? On average, since there are 30-40000 genes per 23 chromosomes, there would be 1300- 1750 genes per chromosome on average (30,000/23 to 40,000/23) 2. Define homologous chromosomes. Homologous chromosomes are those that have the same genes located at the same loci, and synapse in meiosis, but may have different alleles of these genes. For example, diploid cells will have 2 copies of chromosome 1. One will come from the father and one from the mother, with some differences due to recombination. Due to the different alleles, the chromatids will not be sister chromatids as they are not completely identical. 3. Define non-homologous chromosomes. Non-homologous chromosomes are those that do not have the same genes located at the same loci. This is seen in humans between different chromosomes, i.e. chromosome 1 and 22 which are totally different size and would contain different genes in different sequences. They also don’t pair together at metaphase of meiosis and don’t carry alleles for the same genes. 4. How many homologous chromosomes are there in a germ cell of a woman? Of a man? A woman’s germ cell would contain 23 pairs of homologous chromosomes, as there are 22 autosomal chromosomes and since females have two X chromosomes (which are the same size and contain the same genes at the same loci), these will also count as homologous chromosomes. Men will have only 22 pairs of homologous chromosomes, since they have X and Y sex chromosomes, which differ greatly and have different sizes and different genes at different locations. 5. Distinguish between homologous chromosomes and sister chromatids. Sister chromatids are identical DNA molecules that make up one chromosome, and are the result of DNA replication in the S phase. Seen in mitosis and meiosis II, and they are separated in equational division. They separate into 2 identical cells in mitosis, and 2 gametes in meiosis. Homologous chromosomes are separate chromosomes that have the same genes on the same loci, but may have different copies – i.e. they are not exactly the same as each other. They pair together at metaphase during meiosis. They are only formed in meiosis I, and their separation leads to the reduction in chromosomal count from 2n to 1n. 6. What is the purpose of cell division in Prokaryotes? In Eukaryotes? In prokaryotes  solely for reproduction and continuation of the species. In eukaryotes  reproduction, organism tissue growth (in higher organisms), and tissue repair (i.e. skin and bone repair). 7. Distinguish between DNA replication and cell division. DNA replication is a part of interphase during the S phase, when DNA chromatids are replicated to form identical sister chromatids which can then be separated in cell division to provide nuclear material for two cells. Cell division is different from this because it involves other processes of reproduction, including biomass production, division of the newly replicated DNA, and cytokinesis to form new cells. It occurs during cytokinesis and it is the physical cytoplasmic division and results in two complete cells. 8. Distinguish between reason/purpose for/of mitosis and reason/purpose for/of meiosis? Mitosis  solely to reproduce an exact copy of the original cell – the DNA content is maintained, and allows for tissue growth. This is why mitosis occurs throughout an organism’s lifespan. Meiosis  solely required for gamete production for sexual reproduction. It allows for unique haploid gametes to be formed, which can combine to form a zygote. This allows for maintaining variability while still preserving the genetic information of an organism. This is important because variability is important for survival if life and environmental conditions change. 9. How is variability of genetic information attained by meiosis and fertilization? - Crossing over of tetrads and recombination during prophase of meiosis I, paternal and maternal
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