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Nucleic Acids and Protein Synthesis Chapter 7 DNA 7-1 The Genetic Code • What is the structure that carries genetic information? • Scientists wondered this for years, and through many experiments, they figured out exactly what it was. Griffith and Transformation • Frederick Griffith wanted to know how bacteria made people sick. • He isolated 2 separate strains of pneumonia bacteria from mice. – Only 1 of these caused pneumonia – The disease-causing strain of bacteria grew into smooth colonies (S-bacteria) – The harmless strain produced colonies with rough edges (R-bacteria) • The Experiment – Griffith injected mice with S-bacteria. • Mice died from pneumonia – Mice injected with R-bacteria • Mice never got sick – Heat-killed S-bacteria and then injected it into mice • Mice survived. – Mixed heat-killed S-bacteria with R-bacteria and injected them into mice. • Mice got sick and died. – The dead bacteria somehow passed on its diseasecausing ability to the harmless bacteria. – Transformation: The process by which one strain of bacteria is changed by a gene or genes from another strain of bacteria. Avery and DNA • Decided to repeat Griffith’s experiments, but also wanted to figure out which part of the bacteria was responsible for the transformation. • They had it down to being a protein, carbohydrate, or DNA. • Made a juice from heat-killed S-bacteria. • In different experiments, they introduced enzymes that destroyed proteins, carbohydrates, and DNA. • When the proteins and carbohydrates were destroyed, transformation occurred. • When DNA was destroyed, transformation did not occur – DNA is the structure that passes on genetic information. Hershey-Chase Experiment • Scientists still doubted that DNA contained the genetic info. It was too simple. – They thought it was protein. • Hershey and Chase worked with viruses, specifically bacteriophages. – Bacteriophage: A virus that infects bacteria • A bacteriophage is made of only 2 things: DNA and Protein • To figure out which component was passed on, they tagged the viruses with phosphorus-32 and sulfur-35. – Proteins don’t contain phosphorus – DNA doesn’t contain sulfur. • If P32 was found in the bacteria, it’s the DNA that is transferred. If S35, it’s the protein. • Results: P32 was found in the bacteria. – DNA is definitely the genetic material. The Structure of DNA • What is the basic unit of a nucleic acid? – A nucleotide. • Made of 3 parts: – 5-carbon sugar (deoxyribose)-Side of ladder – Phosphate Group- Side of ladder – Nitrogenous Base » There are 4 bases • Adenine (A), Cytosine (C), Guanine (G), Thymine (T) • These are placed into 2 groups: • Purines: 2 rings in the structure. • Adenine, Guanine • Pyrimidines: 1 ring in structure • Cytosine, Thymine Chargaff’s Rules • Discovered that in DNA samples, percentages of guanine and cytosine are almost equal to one another. Adenine and Thymine percentages were also similar. [A]=[T], [C]=[G] X-Ray Diffraction • Rosalind Franklin used x-ray diffraction to get a general shape of the DNA molecule. – Aimed a powerful x-ray beam at concentrated DNA samples. • Results: – DNA is spring-shaped (helix) – Since it looked like an x, there are 2 helices (Double Helix) – Nitrogenous bases are in the middle Making the Model • Watson and Crick used Chargaff’s rules and Franklin’s images to create a 3-D model. • It resembled a spiral staircase with nitrogenous bases in the middle. • Base-Pairing: Adenine goes with Thymine, Guanine goes with Cytosine. • Held together with hydrogen bonds – 2 bonds between A and T – 3 bonds between G and C DNA and Chromosomes • DNA length – The DNA in a single human cell measures out to between 1-2 meters. – The total length of all the DNA in a human is 2.0 x 1013 meters. • That’s 20,000,000,000,000 meters – At that distance, you would be able to travel to the sun and back almost 70 times. » Distance from Earth to the Sun = 93,000,000 miles. • Chromosome Structure – Chromosomes are made of both DNA and proteins. – Chromatin: Consists of DNA tightly wound around proteins. • Histones: The proteins – When the DNA and several histones are wound together, a nucleosome is created. • Nucleosomes wind together to create chromosomes. DNA Replication • Each strand of DNA can be used as a template to create another strand. – The strands are complimentary. – If you only had one strand, you could figure out the base sequence to the other side. • Practice: Construct the complimentary strand to the following base sequence: ACCGATATGCAAT • In prokaryotes, DNA replication begins as a single point and then moves in both directions until replication is complete. • In Eukaryotes, replication occurs in hundreds of places on a single strand. • Replication Fork: The sites where separation and replication occur. – Duplicating DNA • Replication: The process of copying DNA • During DNA replication, the DNA molecule separates into 2 strands, and then produces 2 new complimentary strands following the rules of base pairing. Each strand of the double helix of DNA serves as a template, or model, for the new strand. • How DNA Replication occurs – DNA Helicase (an enzyme) “unzips” the DNA molecule. • Done by breaking the hydrogen bonds between the nitrogenous bases. – DNA Polymerase joins individual nucleotides to produce a DNA molecule. • Also, it proofreads each DNA strand to eliminate mistakes. – DNA Ligase binds the DNA strand back together. RNA and Protein Synthesis 7-2, 7-3 • Gene: Coded DNA instructions that control the production of proteins within the cell. – Well how are proteins made from the genes? • Protein Synthesis is divided into 2 stages. – Transcription – Translation • The first step (transcription) is to copy part of the nucleotide sequence from DNA into RNA, which then makes the proteins • RNA= ribonucleic acid – The assembly of amino acids into proteins is controlled by RNA The Structure of RNA • Very similar to DNA structure, but 3 differences. – The sugar is Ribose – RNA is usually single-stranded – Instead of having Thymine, it has Uracil Types of RNA • 3 main types – mRNA – rRNA – tRNA • mRNA= messenger RNA – Copies the instructions from genes in DNA. – Serves as the messenger from the DNA to the rest of the cell. • rRNA= ribosomal RNA – Proteins are synthesized/assembled by ribosomes – Ribosomes are made of rRNA and other proteins. • tRNA= transfer RNA – Transfers amino acids to the ribosome to be assembled into proteins. Transcription • Transcription: Process of copying part of the nucleotide sequence of DNA into a complimentary sequence of RNA. • Occurs in the nucleus. • Works very similarly to DNA replication. – RNA Polymerase works a lot like DNA Polymerase • RNA polymerase binds to DNA and separates the DNA strands. RNA polymerase then uses 1 strand of DNA as a template from which nucleotides are assembled into strands of RNA. • How does the RNA polymerase know where to start and stop copying? – RNA polymerase doesn’t bind to DNA just anywhere. • The enzyme only binds to a region called a promoter, which has a specific sequence or “start codon.” – Promoters: Signals in DNA that indicate to the enzyme where to bind to make RNA. • Similar signals in DNA cause transcription to stop. RNA Editing • When RNA is made from DNA, there is a lot of editing that needs to be done. – RNA Polymerase incorrectly copies 1:10,000 nucleotides. • DNA contains sequences of nucleotide that do not code for proteins. (Introns) • The DNA sequences that do code for proteins are called Exons. – These are expressed in protein synthesis. – When the RNA is being created from DNA, both introns and exons are copied. – Introns must be cut out and then the exons are spliced together. – The information that is left from the exons can then be translated into a protein. The Genetic Code • The properties/characteristics of proteins are determined by the order in which amino acids are joined together to make a polypeptide – Polypeptide= A long chain of amino acids. • The genetic code is read 3 letters at a time – Each 3-letter word is called a codon – Codon: consists of 3 consecutive nucleotides that specify a single amino acid. • 64 possible codons. • 1 start codon • 3 stop codons • What is the start codon? Stop codons? Crack the code: • UCGCACGGU Translation • Translation: Process of decoding an mRNA message into a protein – Takes place on ribosomes Step 1: messenger RNA • After transcription, mRNA leaves the nucleus and enters the cytoplasm. Step 2: • mRNA attaches to a ribosome. • mRNA moves through the ribosome’s AP site, the codon sequence is read, and tRNA brings in the matching amino acid, which is added to the polypeptide. tRNA • Each tRNA molecule carries only 1 kind of amino acid • Each tRNA has 3 unpaired bases, which are complimentary to an mRNA codon= anticodon Step 3 • Peptide bonds are formed between the amino acids in the AP site. • The ribosome moves on to the next codon, the first tRNA exits the ribosome, and the bond between the amino acid and the tRNA is broken. • A new tRNA enters the empty A site. Step 4 • Polypeptide chain grows until a stop codon is reached. • When this happens, translation ends and the polypeptide folds into a new, useable protein. Summary • DNA contains a code for the creation of proteins. – This happens through a series of steps. • DNA needs to be transcribed into a readable code – Transcribe: To write out in another language. • The newly transcribed code then needs to be translated, or interpreted, so a protein can be made. Protein Synthesis Overview • Crash Course • The Hippies Genetic Mutations Genetic Mutations Intro. • Cells sometimes make mistakes when they are copying their own DNA. – Mutation: An accidental change in the genetic material. – 2 main categories for mutations: • Gene Mutations: Produce changes in a single gene. • Chromosomal Mutations: Produce changes in whole chromosomes. Gene Mutations • Point Mutation: Involves changes in one or a few nucleotides. – Occur at a single point in the DNA sequence. – Types: Substitutions, Insertions, and deletions. • Substitutions are not that bad for the protein. – Ex: ATT CGT ACA GAT ATT CGT ACC GAT • Insertions and deletions can ruin the protein by causing the entire sequence to shift, changing all of the amino acids. – Frameshift Mutation – Ex: ATT CGT ACT GAT ATT CTA CTG AT (Deletion) ATT CGG TAC TGA T (Insertion) Chromosomal Mutations Significance of Mutations • Most mutations have no real affect on an organism. • When protein structure is altered, most of the time it is harmful – Cancer, fatality • Sometimes these are beneficial and lead to genetic variation and evolution.