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Chapter 11 DNA: The Molecule of Heredity What is DNA? • Deoxyribonucleic Acid – sugar is deoxyribose • Carries complete instructions for manufacturing proteins • Genetic information that determines an organisms traits • DNA produces proteins • Proteins are enzymes • Enzymes control chemical reactions in body Structure of DNA • Very long polymer made of repeating nucleotides – Nucleotides = simple sugar, a phosphate group, and a nitrogen base • Four possible nitrogen bases – Adenine (A) – Guanine (G) – Thymine (T) – Cytosine (C) • Phosphate group of one nucleotide binds with deoxyribose sugar of adjacent nucleotide – These form the backbone of the chain • Nitrogen bases stick out like rungs of a ladder up the chain • # of Adenine is always equal to the # of Guanine • # of Thymine is always equal to the # of Cytosine Watson & Crick • In 1953 proposed DNA made of 2 chains of nucleotides joined together via hydrogen bonds at the nitrogen bases Double Helix • Nucleotides match up with their complementary base pairs • Adenine opposite Thymine • Guanine opposite Cytosine • Looks like rungs of a ladder • Ladder twists forming a spring-like form called a double helix Importance of Nucleotide Sequencing • Sequence of nucleotides forms a unique code for each organism • Used to determine whether two people are related • The closer the relationship between two organisms, the greater the similarities in the order of DNA nucleotides Replication of DNA • Before Mitosis/Meiosis cells make a copy of their DNA • An enzyme breaks the hydrogen bonds between nitrogen bases and helix unzips forming two strands • Free nucleotides pair with their complementary base pairs • Results in formation of two new DNA molecules 11.2 From DNA to Protein • Genes to Proteins – There are an estimated 80,000 genes in a human cell – Proteins are polymers of amino acids – The sequence of nucleotides determines the protein – Proteins form complex 3D shapes that become key cell structures and regulators of cell function Actin & Tubulin form the cytoskeleton Transcription • In the nucleus, enzymes make an RNA copy of a DNA strand • Role of RNA? – Protein synthesis – RNA takes instructions from DNA on how to assemble the proteins – There are Three types of RNA – What is RNA? RNA – Ribonucleic Acid • Structure differs from DNA in three ways – RNA is a single strand – Sugar in RNA is ribose – RNA replaces Thymine with Uracil and base pairs with Adenine mRNA – messenger RNA • Brings information from DNA to cytoplasm rRNA – ribosomal RNA • Clamps onto mRNA and assembles amino acids in correct order tRNA – Transfer RNA • Transports amino acids to the ribosome to be assembled into proteins • Composed of approx. 80 nucleotides • Each tRNA molecule is specific for one amino acid • Shaped like a T with an amino acid on one end and three nucleotides on the other end, the nucleotides are the complementary base pairs to mRNA – they are referred to as an Anticodon The Genetic Code • A code is used to convert the language of mRNA into proteins • There are 20 different amino acids; however, mRNA contains only 4 types of bases (A-U-G-C) – A group of three nucleotides codes for one amino acid CODON • The order of nitrogen bases in mRNA determines type and order or amino acids in a protein • There are 64 possible combinations; therefore, there are 64 different mRNA codons • The codon below codes for Alanine • Some codons don’t code for amino acids, they give instructions – UAA – is the stop codon, it tells where the protein stops • One codon gives instructions as well as code for an amino acid – AUG – is the start codon, as well as the Methionine codon, it tells where the protein starts • Amino acids can have more than one codon; however, an individual codon only codes for one amino acid – Leucine has 6 codons – UUG codes for Leucine only • All organisms use the same genetic code for amino acids and assembling proteins Translation • Process of converting mRNA into proteins • mRNA leaves nucleus and enters cytoplasm; ribosomes attach to mRNA • tRNA brings the first amino acid to the mRNA • Anticodon binds to mRNA • Ribosome slides down the mRNA chain to the next codon and a new tRNA molecule brings the next amino acid • The new tRNA binds and the first tRNA is released • This process continues until a stop codon is reached • Translation ends • Amino acid strand is released from the ribosome, it twists and forms complex 3-D structures and becomes protein 11.3 Genetic Changes Mutation: A change in DNA • Mutations are changes in a DNA sequence that also change the protein it codes for • Can affect reproductive cells and altered gene will be part of offspring’s genetic makeup – Can produce new trait – May result in a malfunction of a protein, resulting in structural or functional problems – In rare cases, mutation can be positive, make an organism faster or stronger • In non-reproductive cells, a mutation would not be passed to offspring, it would only affect the individual • Damage to a gene may impair cell functions – Cause it to lose its ability to work – Cause it to divide uncontrollably Point Mutations • A change in a single base pair – This changes the entire structure of the protein Frameshift Mutation • A single base is added or deleted – Results in a shift in translation causing every codon after addition or deletion to be out of position by one Chromosomal Mutations • Can occur in different ways – Chromosome can break off during mitosis or meiosis – Chromosome can break and rejoin incorrectly • Attach backwards • Join wrong chromosome • Common in plants Causes of Mutations • Spontaneous mutations – Mutations that just seem to happen (mistakes during base pairing) • Environmental – Mutations that are caused by a mutagen (an agent that causes a change in DNA) – Radiation (X-Rays, UV Light) – Chemicals (dioxins, asbestos, formaldehyde) Repairing DNA • Organisms have enzymes that proofread DNA and replace incorrect nucleotides with the correct ones • the exposure to a mutagen the the chance that the mistake will not be corrected Esa es toda la gente