2Section 8.2: Structure of DNA Since the 1920’s scientists have known that DNA is a very long polymer (chain of repeating units).The small units that make up DNA are called nucleotides.Remember, nucleotides are made up of 3 parts:A phosphate group5-carbon sugar called deoxyriboseA nitrogen base (Guanine, Cytosine, Thymine, or Adenine)
3Section 8.2: Structure of DNA To give you an idea of the size, one molecule of DNA contains about a billion nucleotides.For a long time scientists believed that organisms were made up of equal amounts of four different types of nucleotides.For example: humans were 25% Guanine, 25% Thymine, 25% Adenine, and 25% Cytosine.
4Section 8.2: Structure of DNA By 1950, Erwin Chargaff changed the thinking on DNA. Chargaff studied several different organisms and found that the same 4 bases are in all organisms, but the proportion of the bases varied.He found that in all organisms thatthe amount of adenine=the amount of thyminethat the amount of guanine=the amount of cytosine.A=T & C=G became known as Chargaff’s Rule
5Section 8.2: Structure of DNA Thymine and cytosine are single ring structures called pyrimidinesAdenine and guanine are double ring structures called purines
6Section 8.2: Structure of DNA Remember, it can’t just be a pyrimidine bonding with a purine…..it is more specific than that…. (A) always bonds with (T) and (G) always bonds with (C).
7Section 8.2: Structure of DNA Finally, in the early 1950’s, a complete understanding of DNA was finally coming into focus.Rosalind Franklin was studying DNA at the time.Franklin took x-ray photographs of DNA and it showed it to be in an “X” form.
8Section 8.2: Structure of DNA At the same time James Watson and Francis Crick were studying DNA and saw Franklins work.They both cam to the conclusion that the picture, if put in 3-dimension, the “X” form would be twisted on itself like a spiral staircase (helix).
9Section 8.2: Structure of DNA Watson and Crick found the sugar and phosphates were the outside backbone of the molecule and the nitrogen bases were on the inside.In 1953, Watson and Crick published their DNA double helix model.It shows a two-stranded molecule wrapped around each other held together by hydrogen bonds between adjacent bases.
10Section 8.2: Structure of DNA The model shows the two strands interwinded with each other.It also shows the complimentary bases paired.The back ribbon-like part is the phosphates and 5 carbon sugar deoxyribose.Because of their unique structures, adenine can only bond with thymine and cytosine with guanine!
11Section 8.3: DNA Replication Watson and Crick’s model was also important because it suggested a way DNA could be replicated.Both scientists suggested that because of the base pairing rules (A-T & C-G), each strand could serve as a template to make a copy of the other strand.This process is called DNA replication.
12Section 8.3: DNA Replication DNA replication insures that every cell has a complete set of identical genetic information.Enzymes and other proteins do the actual work of DNA replication.A group of enzymes called DNA polymerases guide this 3 step process.
13Section 8.3: DNA Replication The process of DNA replication can be described in 3 steps:Enzymes begin to “unzip” the double helix. This means the hydrogen bonds between the nitrogen bases are broken. When these hydrogen bonds are broken, the two strands separate and each individual base is exposed. Like unzipping a suitcase, it proceeds in two directions at the same time.
15Section 8.3: DNA Replication The process of DNA replication can be described in 3 steps: 2) One by one, free floating nucleotides pair with their exposed complimentary case. DNA Polymerase bond the nucleotides together to make a new strand * Each strand is a template to make the other strand.
17Section 8.3: DNA Replication The process of DNA replication can be described in 3 steps: 3) Two identical molecules of DNA are the end result. Each molecule is made up of one new strand and one old strand. This is called semi-conservative replication. *Because of semi-conservative replication, something amazing happens! What is it??
19Section 8.3: DNA Replication DNA replication happens over and over again in every cell in your body.This process also happens remarkably fast, about 50 nucleotides per second.The only way it gets done is that replication occurs at multiple places on DNA at one time!
20Section 8.3: DNA Replication There is also a built in proofreading system.This system corrects any mis-paired nucleotides.The error rate is about 1 out of 1,000,000,000 because of the proofreading!
21Section 8.4: Transcription of DNA Francis Crick defined the Central Dogma of biology after the discovery of the structure of DNA.This states that information flows in one direction, from DNA RNA Proteins.The central dogma involves 3 processes:Replication: of DNA strandsTranscription: converts DNA messages into RNA languageTranslation: interprets RNA language into a string of amino acids called polypeptides. These polypeptides working together make up proteins.
22Section 8.4: Transcription of DNA In prokaryotic cells (bacteria), all 3 processes occur in the cytoplasm at the same time.In eukaryotic cells, replication and transcription occur in the nucleus and translation occurs in the cytoplasm at different times.RNA or Ribonucleic Acid acts as a link between the DNA in the nucleus and protein synthesis in the cytoplasm.
23Section 8.4: Transcription of DNA RNA is similar to DNA in that it is a chain of nucleotides made up of sugar, phosphates, and nitrogen base.You can think of RNA as a temporary copy of DNA that is used and then destroyed.RNA, while similar to DNA, differs in 3 significant ways:The sugar in RNA is ribose sugar which has oxygenRNA contains the base Uracil instead of ThymineRNA is only a single strand
24Section 8.4: Transcription of DNA By definition, transcription is the process of copying a sequence of DNA to produce a complimentary strand of RNA.RNA strands only copy the segment of DNA it needs to make a specific gene.During transcription, the whole DNA code is not copied. Only the code for the specific gene needed is copied.
25Section 8.4: Transcription of DNA The process is helped along by RNA polymerases, which are enzymes that bond nucleotides to make an RNA strand.There are 3 basic steps to transcription:
26Section 8.4: Transcription of DNA There are 3 basic steps to transcription:1) RNA polymerase recognizes the transcription start site for a specific gene. A large transcription complex (RNA polymerase and other proteins) assembles on the DNA strand and begins to unwind the DNA segment needed.
27Section 8.4: Transcription of DNA There are 3 basic steps to transcription:2) RNA polymerase, using only one strand of DNA, strings together complimentary strand of RNA nucleotides. RNA follows the same base pairing rules as DNA, however, RNA contains the base uracil, not thymine. As the RNA strand is made, the DNA helix zips back up behind it.
28Section 8.4: Transcription of DNA There are 3 basic steps to transcription:3) Once the entire gene has been transcribed, the RNA strand detaches completely from the DNA. RNA polymerase recognizes the end of the gene and transcription is stopped.
29Section 8.4: Transcription of DNA Transcription produces 3 major types of RNA…not all RNA molecules code for proteinsMessenger RNA (m-RNA)- the molecule that carries the transcribed message from DNA to the ribosomes to make proteins.Ribosomal RNA (r-RNA)- forms part of the ribosomesTransfer RNA (t-RNA)- brings amino acids from the cytoplasm to the ribosomes to put the protein together.
30Section 8.4: Transcription of DNA The transcription process is similar to replication of DNA.Both occur in the nucleus, catalyzed by enzymes, unwind DNA, and produce complimentary base pairs.However, the end results of replication and transcription is very different.
31Section 8.5: TranslationTranslation is the process that converts a mRNA message into a protein.The language of nucleic acids is A,C,T,G, & U’s, but the language of proteins is amino acids (remember these are the monomers of proteins)The A,C,T,G, &U’s of a nucleic acid are in a very specific order.Every 3 letters is a triplet, or Codon.
32Section 8.5: TranslationA codon is a 3 nucleotide sequence that codes for a specific amino acid.Scientist believe it is every 3 nucleotides because that gives enough possible nucleotide combinations to cover all 20 amino acids.Amino acids are generally coded for by more than one possible codon.For example: CUU, CUA, CUG, UUA, & UUG are all codes for Leucine, one of the 20 amino acids
33Section 8.5: TranslationSo codons are every 3 nucleotides, so every 3 nucleotide makes another amino acid.
34Section 8.5: TranslationThere are two other types of codons other than the ones that code for proteins.There are 3 stop codons (UUA, UAG, & UGA).These codons signal the stopping of an amino acid sequence.There is also one start codon (AUG) that triggers the start of translation.AUG also codes for an amino acid (Methionine), so translation always starts with this amino acid.
35Section 8.5: TranslationIf, during translation, a codon is read wrong or one nucleotide is incorrect, this could affect the whole protein!The genetic code is shared by almost all organisms.For example: UUU codes for Phenylalanine in humans, a cactus, yeast, or an armadillo.
36Section 8.5: TranslationThis makes most scientist believe that all living organisms gave rise from a common ancestor.It also means scientists can use a gene from one organism in different organisms.So , how do we get a protein made from the instructions mRNA carries to the ribosomes?This process is called translation.Translation actually has many steps, requires a lot of energy, and is a complicated process.
37Section 8.5: TranslationWe will try to summarize translation in 3 steps:The codons on the mRNA reach the ribosomes and attracts a complementary tRNA molecule that carries an amino acid. The tRNA anticodon pairs with the mRNA codon.
38Section 8.5: TranslationWe will try to summarize translation in 3 steps:2) The ribosome helps form bonds between amino acids. It then breaks the bond between the tRNA and amino acid.
39Section 8.5: TranslationWe will try to summarize translation in 3 steps:3) The ribosome continues to pull the mRNA strand through the ribosome until all the codons are read and matched with their tRNA anticodon and amino acids. The amino acids are then all connected to make a protein.
40Section 8.5: TranslationBelow is an example of transcription and translation working together to make a protein.
41Section 8.7: Mutations So, what happens when something goes wrong? A mutation occursA mutation is a change in an organisms DNA.There is many different types of mutations.Mutations usually happen during DNA replication (usually affects a single gene)Or during meiosis (affects entire chromosomes)
42Section 8.7: Mutations Gene Mutations: Point Mutations: this type of mutation happens when one nucleotide is substituted for anotherACTG is copied as TGAA (last nucleotide is mispaired)Frameshift Mutation: involves a deletion or insertion
43Section 8.7: Mutations Chromosomal Mutations: These type of mutations affect parts or a whole chromosome.Deletion: Part of a chromosome is missingDuplication: part of a chromosome is copied twiceInversion: part of a chromosome switches placesTranslocation: pieces of one chromosome moves to a non-homologous chromosome.
45Section 8.7: MutationsAlthough there are many types of mutations, the affect isn’t always bad!Silent mutations are changes in an organisms DNA that do not change anything.Also, only mutations that happen in gametes (sperm & egg) affect an organisms offspring.If a mutation occurs in a body cell, that mutation only affects that organism.
46Section 8.7: MutationsMutagens are agents in the environment that cause mutations.They speed up replication errors and your body’s proofreading system.Examples: UV light, chemicals in cigarettes, and other chemicals.