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SBI 3U January 7th, 2012 Packaging of DNA Chromatin, a combination of DNA and protein During cell division, DNA compacts into structures called chromosomes Chromosome – condensed DNA, can contain 100s1000s of genes! Fun Fact: A human body cell contains enough DNA to stretch about 2 m in length! Chromosomes Diploid number 1 homolog of each pair from each parent Extreme Chromosomes! Radiolarian – 1600 chromosomes Roundworm – 2 chromosomes DNA in Chromatin Tightly coiled around proteins called histones Nucleosomes Discovery of DNA structure Early 1950s – Rosalind Franklin discovered that DNA had the form of a spiral or helix Used x-ray crystallography Discovery of DNA structure James Watson and Francis Crick used Franklin’s findings and added to it Watson & Crick’s model: sugar-phosphate backbone on outside and subunits called nitrogenous bases on the inside The Components of DNA Long chain of subunits called nucleotides A nucleotide has 3 parts: 1. deoxyribose 2. A phosphate group 3. A nitrogeneous base; a single or double ring of carbon and nitrogen atoms The Components of DNA 4 types of nucleotides in DNA – only differ in their nitrogeneous bases: adenine, guanine, thymine & cytosine Thymine & Cytosine are pyrimidines Adenine & Guanine are purines Arrangement of nucleotides in DNA stores information -- genes Chromosomes Within the karyotype, there is one pair of chromosomes that are not identical in shape and size: sex chromosomes Female: XX Male: XY 22 pairs are autosomes Sex Linkage Discovered by Thomas Hunt Morgan Any gene located on a sex chromosome is called a sex- linked gene X-linked genes: genes on X chromosome Color blindness & hemophilia Sex-Linked Disorders Hemophilia: bleeder’s disease Bloodlines of royal British family