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DNA Definition Deoxyribonucleic acid (DNA) – molecule that contains genetic information that directs the activities of cells. DNA contains the instructions cells use to make proteins. Helps make bones, muscle, new cells, etc. History Rosalind Franklin - took x-ray pictures of DNA molecules which helped to determine its shape. History James Watson and Francis Crick built the first DNA model and are usually given credit for determining its shape. Structure Monomers called NUCLEOTIDES. 3 parts: sugar (deoxyribose) phosphate nitrogen base Structure There are four different DNA nitrogen bases: Purines - adenine and guanine (double rings) Pyrimidines - cytosine and thymine (single rings) Structure The nitrogen bases always bond to the deoxyribose molecule (sugar) of the nucleotide. Purines Adenine Guanine Phosphate group Pyrimidines Cytosine Thymine Deoxyribose Structure Nucleotides bond together to form a double stranded DNA molecule. DNA’s structure looks like a twisted ladder This structure is called the double helix. The deoxyribose and phosphate form the “rails,” of the ladder. Nitrogen bases bond together to form the “rungs,” of the ladder. Structure Nucleotide Sugarphosphate backbone Key Adenine (A) Thymine (T) Cytosine (C) Guanine (G) Chargaff’s Rule Chargaff's Rule: Adenine (A) = Thymine (T) Guanine (G) = Cytosine (C) Nitrogen Base Pairing Rules ADENINE always bonds to THYMINE CYTOSINE always bonds to GUANINE Chemical Structure The nitrogen bases are bonded with weak hydrogen bonds to hold the two DNA strands together. The structure and shape of the nitrogen bases allows them to always bond to their complement Scientist Check Point The picture to the right shows an x-ray diffraction of DNA. The x-ray diffraction of DNA led to the idea that DNA — What Scientist Took this picture? Why are Watson and Crick famous? Draw 2 strands, four nucleotides each: Remember, each nucleotide consists of sugar, phosphate, base! Label the sugars “S”, and the phosphates “P” … notice a pattern Notice: opposite directions! ? ? ? ? The Genetic Code The order of the nucleotides in creates a code used to make proteins. The arrangement of nitrogen bases (A-TG-C) determines the amino acids used to construct proteins Like the alphabet makes words How Does It All Fit??? All your DNA (end-to-end, from one cell) is about 2 meters long! One cell’s nucleus is about 2 micrometers in diameter! So, how can 2m of DNA fit into 2µm of space? Supercoiling! 1. DNA wraps around histone DNA + histone = nucleosome proteins 2. Nucleosomes form a coil Supercoiling! 3. Coils coil again … This is a SUPERCOIL! Supercoiling! A Chromosome Is Supercoiled DNA Supercoiling! Got It? So, Again, What is A Chromosome? A chromosome is supercoiled DNA Making Connections