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Lecture 1: "Origin Of Genetic Variation"

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Biology 3466B
Yolanda Morbey

Evolution Lecture No.1: Origin Of Genetic Variation I th Monday September 10 , 2012 Evolution & Genetic Variation: -The definition of evolution is the descent with change in allele frequencies. It is a concept that deals largely with populations and not necessarily with individuals; individuals develop, populations evolve. -Within the field of evolution, genetic variation is rampant and it is one of the many distinct causes for consequences over evolutionary time. Evolution argues that mutation is a necessary phenomenon, in the sense that it is required to vary genes in populations. The Nucleotide (DNA Structure): -The nucleotide is the basic building block in the structure of DNA containing the instructions for constructing an organism. Every DNA nucleotide consists of 3 key components: a deoxyribose (5-carbon) sugar, a phosphate group and a nitrogenous base. The Nitrogenous Bases (DNA Structure): -There are 4 possible bases present in all DNA structures: adenine, guanine, cytosine, and thymine. The first two bases, adenine and guanine, are known as purines as they both contain double-ring structures. The latter, cytosine and thymine, are known as pyrimidines as they both contain single-ring structures. With regards to complementary base-pairing in DNA, adenine pairs with thymine, and guanine with cytosine. The Backbone (DNA Structure): -The backbone of the structure of DNA is synthesized by extending the 5’ carbon of a nitrogenous base and connecting it to the 3’ OH of another nitrogenous base. This connection between bases in the structure of DNA is known as a phosphodiester bond and it forms the long backbone of DNA. Once the synthesis of the ladder-like DNA structure is complete, the molecule will then twist and form an alpha- helical structure. The Double Helix (DNA Structure): -As the structure of DNA is double-stranded, double helical shape emerges from the numerous base pairings in the DNA sequence. It is important to understand that proper complementary base-pairing is what ensures stability to DNA’s helical structure. Terminology: -Gene – stretches of DNA that encode for a particular type of RNA or protein product. -Locus – the location of the gene on the chromosome. -Allele – alternative form of a gene at a given locus; varies in sequence. -Genotype – characterization of the alleles that and individual possesses. -Phenotype – characterization of the visible traits expressed by an individual. -E.g. The gene for colour in flowers has two distinct alleles: purple (p) and white (w). The possible combinations for a given flowers genotype may be pp, pw, or ww, in which their respective phenotypes would respectively coincide as purple, pink, or white. For each locus on each homologous chromosome (1 from each parent), the alleles may vary. DNA Replication: -The first step in replicating DNA is the unwinding of the double helix by enzymes such as helicase, which temporarily interrupt complementary base pairing between the two strands. Catalyzed by various DNA polymerases, the synthesis of a new strand for each template strand of DNA is underway. Once the complementary sequence is complete, the two DNA molecules twist back into their helical structure thereby concluding the process of DNA replication. -Within the process of DNA replication, several types of mutations may arise if improper base-pairing is administered. These include: point mutations (single base substitutions) as well as insertion/deletion mutations (single base additions or removals). Mitosis: -Mitosis is the cycle of somatic cell division that occurs when a somatic cell is not in Interphase. Mitosis consists of 4 primary stages: Prophase, Metaphase, Anaphase, and Telophase. Prior to Prophase, the somatic cell must replicate its entire genome in order to pursue mitotic division. When this DNA replication occurs, correction of errors in sequence is important, as not to copy faulty genetic information. -During Metaphase, the tetrads (2 pairs of homologous chromosomes) align along the equator of the cytoplasm as the spindle fibres attach to each centromere. It is during Anaphase that the spindle fibres independently assort the homologues (1 pair per cell). Telophase is when the cytoplasm commences division and two diploid somatic cells are formed as the process co
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