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DNA, RNA Genes • A segment of a chromosome that codes for a protein. – Genes are composed of DNA Three Properties of Genes: • They carry information from one generation to the next. • They determine the inherited characteristics of the organism. • They are easily copied because genetic information must be copied (replicated) every time a cell divides. The Structure of DNA: • DNA is a long molecule composed of nucleotide units. • A nucleotide unit contains: – Deoxyribose sugar – Phosphate group – One of four nitrogen bases: • • • • Adenine Guanine Cytosine Thymine The Structure of DNA is a Double Helix • This was determined by Watson and Crick. • Backbone of helix – sugar and phosphate molecules. • Rungs of ladder – nitrogenous bases. • The strands are held together by hydrogen bonds. Chargaff’s Rule for Base Pairs: • Adenine pairs with thymine. • Cytosine pairs with guanine. • Adenines = Thymines • Cytosines = Guanines Chromosomes in Prokaryotic Cells: • Prokaryotic cells have no nucleus. • They have one ring of DNA or RNA in the cytoplasm. Chromosomes in Eukaryotic Cells: • Eukaryotic cells have 1,000 times as much DNA as prokaryotes. • It is packed tightly in the nucleus of the cell. • DNA is a long molecule – the nucleus of a human cell contains 1 meter (3 feet) of DNA. • Chromosomes are composed of DNA wrapped tightly around proteins called histones. DNA Replication • DNA must be copied before the cell can divide. • The DNA molecule separates – each strand acts as a template for a new strand. • This process results in two DNA molecules. • Each molecule has one original strand and one new strand. How is RNA different from DNA? • DNA – Carries the recipe for the protein. – Double stranded. – Located in the nucleus. – Contains the sugar deoxyribose. – Contains the nitrogen bases cytosine, guanine, adenine and thymine. • RNA – Makes the protein from the recipe. – Single stranded. – Located in the cytoplasm. – Contains the sugar ribose. – Contains the nitrogen bases cytosine, guanine, adenine, uracil. How much DNA do we have in our bodies? • If all the DNA in your body was tied together, it would stretch to the sun and back 600 times! Three Types of RNA • mRNA – messenger RNA – A gene is a recipe for a protein – mRNA copies the recipe for the protein. – It carries a copy of these instructions from the nucleus to the cytoplasm. • rRNA – ribosomal RNA – Makes up the ribosomes. • tRNA – transfer RNA – Transfers amino acids that are coded for by the mRNA recipe from the cytoplasm to the ribosomes. Protein Synthesis – Two Steps • Transcription – The recipe is written (in the nucleus). • Translation – The recipe is translated into the language of proteins (amino acids). Transcription • To transcribe means “to write”. • DNA strands separate. • One strand serves as a template and is copied by mRNA. • The DNA recipe is written in 3 base segments called triplets. • One triplet codes for one amino acid. • mRNA writes the recipe for a protein in its own language. • In the language of RNA, C pairs with G, A pairs with U. • This process takes place in the nucleus. • Proteins are composed of amino acids. • The language of mRNA instructions is called the genetic code. • Each word of the coded message is 3 bases long. • This 3 base segment is called a codon. • Each codon codes for one amino acid (20 amino acids). Practice • DNA triplets: – GGG – CCC – CGC – CTT – AGG – TAT • What would the mRNA codons be for these triplets? • Which amino acids would be specified for by these codons? Translation • To translate means to “change to another language”. • The language of proteins is amino acids. • mRNA attaches to the ribosomes with the recipe for the protein. • The tRNA molecule with the anticodon to the mRNA codon brings the amino acid called for by the recipe to the ribosomes. • Amino acids are attached in a polypeptide chain until the recipe for the protein is complete. • Translation takes place in the cytoplasm. Practice • Write the anticodons to the following codons: • CGC – GGA – AAU – UAU – ACA - GAU Why are proteins important? • All enzymes are proteins. • Every chemical reaction in the body requires an enzyme to make it go. • Enzymes make some reactions go 1 million times faster than they would go without the enzyme. • Proteins regulate the rate of growth of an organism. • Most structures in the body are made of proteins. – – – – Muscles Hair Organs Skin • Proteins regulate the pattern of growth of an organism. • Proteins are the key to almost everything living cells do. Mutations • A change in a gene or chromosome. • Some mutations result in no change in the organism. • A few mutations cause favorable changes in an organism. • In humans, mutations usually cause harm. • Cystic fibrosis, sickle cell anemia, Down’s syndrome. Gene Mutations • Point mutations – occur at a single point in the DNA sequence. • Some point mutations simply substitute one nucleotide for another. • Frameshift mutation – results in a much bigger change because a nucleotide is inserted or deleted. • Every amino acid that follows the point of insertion or deletion is affected. Frameshift Mutations Chromosomal Mutations • Involve changes in the number or structure of chromosomes. Human Genome Project • The complete recipe list for all of the proteins needed to make a human. • Our DNA contains more than a billion base pairs. • The genome was mapped in 2000. • We were found to have about 20,000 genes that code for proteins. • Worms have 19,098! • Fruit flies have 13,602! • Yeast has 6,023! • Less than 2% of the genome actually codes for proteins. • 97% of the genome has no known function (junk DNA).