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DNA. Understanding DNA Frederick Griffith – bacteriologist, who experimented with virulent (disease causing) and nonvirulent bacteria in 1928. His experiment.

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Presentation on theme: "DNA. Understanding DNA Frederick Griffith – bacteriologist, who experimented with virulent (disease causing) and nonvirulent bacteria in 1928. His experiment."— Presentation transcript:


2 Understanding DNA Frederick Griffith – bacteriologist, who experimented with virulent (disease causing) and nonvirulent bacteria in His experiment showed that nonvirulent bacteria could acquire the ability to cause disease from virulent bacteria.

3 Transformation – the transfer of genetic material from one organism to another. - bacteria taking up foreign DNA.

4 Avery Identifies the Agent of Transformation Oswald Avery – a biologist who was interested in Griffith’s experiment, but wanted to identify the substance that made nonvirulent bacteria virulent. 14 Years after Griffith’s experiments, Avery conducted his experiments. Oswald T. Avery, M.D

5 1 st experiment: his team extracted DNA from smooth colonies & added it to rough colonies. Smooth colonies grew from the rough colonies. 2 nd experiment: his team took pneumonia bacteria and destroyed lipids, RNA, carbohydrates, and proteins inseparate set-ups. The transformation still happened. However, when he destroyed the DNA in the cells, the transformation did not occur.

6 Hershey & Chase Confirm DNA is the Genetic Material Alfred Hershey & Martha Chase – concluded DNA is hereditary material. By radio-labeling sulphur in one culture, they could tag the path of proteins and not DNA, because there is no sulphur in DNA and there is sulphur in the amino acids methionine and cysteine. By radio-labeling phosphorous, the opposite effect could be achieved. DNA could be traced and not protein, because there is phosphorous in the phosphate backbone of DNA and none in any of the amino acids. Cultures could be grown in each of these two ways and the phage purified away from the host bacteria, resulting in one culture in which only the phage protein was labeled, and one culture in which only the phage DNA was labeled.

7 Supernatant – the liquid above the solid particles after centrifuging.

8 Structure of DNA  Composed of 2 long chains of repeating nucleotides.  Each nucleotide has 3 parts: 1.) Deoxyribose – sugar molecule 2.) Phosphate group – consists of phosphorus atom surrounded by oxygen atoms. 3.) Nitrogen-Containing Base – molecule that contains a nitrogen atom.

9 Pyrimidines – single ringed nitrogen bases Purines – double ringed nitrogen bases.


11 The Double Helix In 1953, James Watson and Francis Crick proposed that DNA is composed of two nucleotide chains that wrap around each other to form a double spiral – double helix. Rosalind Franklin’s X-ray photographs of DNA indicated that DNA is a helix with a sugar-phosphate backbone.

12 Watson and Crick (and Wilkins) WatsonCrick Wilkins

13 In 1951, Watson attended a lecture by Franklin on her work to date. She had found that DNA can exist in two forms, depending on the relative humidity in the surrounding air. This had helped her deduce that the phosphate part of the molecule was on the outside. Watson returned to Cambridge with a rather muddy recollection of the facts Franklin had presented, though clearly critical of her lecture style and personal appearance. Based on this information, Watson and Crick made a failed model. It caused the head of their unit to tell them to stop DNA research. But the subject just kept coming up.

14 Franklin, working mostly alone, found that her x-ray diffractions showed that the "wet" form of DNA (in the higher humidity) had all the characteristics of a helix. She suspected that all DNA was helical but did not want to announce this finding until she had sufficient evidence on the other form as well. Wilkins was frustrated. In January, 1953, Wilkins showed Franklin's results to Watson, apparently without her knowledge or consent. Crick later admitted, "I'm afraid we always used to adopt -- let's say, a patronizing attitude towards her." By 1962, when Watson, Crick, and Wilkins won the Nobel Prize for physiology/medicine, Franklin had died. The Nobel Prize only goes to living recipients.

15 Erwin Chargaff ( ) - Chargaff's observation that in the base composition of DNA the quantity of adenine equaled the quantity of thymine and the quantity of guanine equaled the quantity of cytosine. In the early 1950’s he had made his discoveries about base pairing and shared his findings with Watson and Crick in 1952.

16 Complementary Base Pairing Base pairs are connected by hydrogen bonds. Cytosine only bonds with Guanine. Connected by 3 hydrogen bonds. Adenine only bonds with Thymine. Connected by 2 hydrogen bonds.

17 Replication of DNA Replication – process of copying DNA. Happens in a 5’ to 3’ direction Replication Fork – the point at which the two nucleotide chains separate. Helicase – an enzyme that separates DNA strands before replication. DNA Polymerase – an enzyme that binds to separated strands of DNA and assembles each strands complement in replication.

18 (a)During DNA replication, helicase enzymes separate DNA’s two chains of nucleotides. (b) DNA polymerases bind to the separated chains of nucleotides. One nucleotide at a time, the enzyme constructs a new complementary chain of nucleotides. (c) After replication, 2 copies of the original DNA molecule strand exist. Each DNA molecule is made of one chain of nucleotides from the original DNA molecule and one new chain of nucleotides.


20 Leading Strand – the template strand of the DNA molecule that has the replication fork moving in the 3’ to 5’ direction. Lagging Strand – the template strand of the DNA molecule that is oriented so that the replication fork moves in a 5’ to 3’ direction. Okazaki Fragments - short molecules of single stranded DNA formed on the lagging strand of DNA as it replicates.

21 In reality, the DNA molecule does NOT “unzip” from one end to the other, but separates at several locations along its length at sites called replication forks. These forks are formed by DNA Helicase and move in both directions – until they meet up with another replication bubble.

22 In eukaryotes, there are hundreds or thousands of origin sites along the giant DNA molecule of each chromosome. A replication bubble expands laterally, as DNA replication proceeds in both directions. Eventually, the replication bubbles fuse (center), and synthesis of the daughter strands of DNA is complete (bottom).

23 Accuracy & Repair Mutation – a change in the nucleotide sequence. Repair enzymes - proofread DNA and repair errors in the nucleotide sequence. Types of mutations Inversion – a piece of DNA breaks off spins around and reattaches to the same chromosome. Translocation – a piece of DNA breaks off and reattaches to another chromosome.

24 More Mutations Substitution – one base in the DNA sequence is substituted for another. THE FAT CAT ATE THE RAT THE FAT HAT ATE THE RAT Insertion – an extra base pair is put into a DNA sequence. THE CAT CAN EAT THE BAT THE CAT TCA NEA TTH EBA T Deletion – a base pair is removed from a DNA sequence. THE DOG CAN SEE FAR THE OGC ANS EEF AR

25 Mutations can belong to one of 2 main categories: Point mutations – these mutations involve only one base and may or may not result in a change in the protein produced by the DNA sequence. Frame shift mutations – these mutations not only change one base in a DNA sequence, but as a result, also shifts bases and changes the type of proteins being produced at several locations.

26 RNA Ribonucleic Acid is responsible for the movement of genetic information from the DNA in the nucleus to the site of protein synthesis in the cytosol. Structure of RNA  Composed of Ribose sugars. (DNA contained Deoxyribose)  Contains Uracil, a nitrogen-containing pyrimidine base, that replaces thymine.


28 TYPES OF RNA Messenger RNA (mRNA) – consists of RNA nucleotides in the form of a single uncoiled strand. mRNA carries genetic information from the nucleus to the cytosol. Transfer RNA (tRNA) – consists of a single chain of about 80 nucleotides folded into a hairpin shape that binds to specific amino acids. tRNA carries amino acids from the cytoplasm to the ribosomes. There are about 45 varieties. mRNA

29 Ribosomal RNA (rRNA) – most abundant form of RNA – rRNA consists of RNA nucleotides in a globular cluster. rRNA is joined together by proteins make up ribosomes – What do ribosomes do? rRNA Amino Acid NucleusmRNA tRNA

30 Transcription Transcription - the process by which genetic information is copied from DNA to RNA. RNA Polymerase – the primary transcription enzyme. Produces RNA by copying specific regions of DNA. Promoter - a nucleotide sequence on a DNA molecule that, when attached to an RNA polymerase molecule, will initiate transcription of a specific structural gene.

31 Repressor Protein – a protein that inhibits a specific gene from being expressed. Termination Signal (point)- a specific sequence of nucleotides that marks the end of a gene sequence be copied.


33 Introns – are sections of a structural gene that do not code for amino acids and therefore are not translated into proteins. Exons – are the sections of a structural gene that, when expressed, are translated into proteins.

34 Both introns and exons are transcribed to form pre- mRNA. Enzymes cut out the introns and join the remaining exons together, forming mRNA.

35 Enhancer Control Enhancer - a region adjacent to a eukaryotic gene that must be activated if the gene is to be expressed. Transcription Factors – one of the additional proteins bound to enhancers and RNA polymerase that regulate transcription.

36 Many enhancers are located far away from the gene they activate. Transcription factors facilitate transcription by binding to the enhancer and to the RNA polymerase. Bending of the DNA strand brings the enhancer close to the RNA polymerase and the transcription factors associated with it, enabling transcription to begin.

37 Transcripts – the different types of RNA molecules produced during transcription, include mRNA, tRNA and rRNA.

38 During transcription, RNA polymerase binds to the promoter of a specific gene. Then a complementary copy of the gene’s DNA base sequence is made using RNA nucleotides, thus forming the mRNA strand. Following transcription, mRNA moves through the pores of the nuclear membrane into the cytosol of the cell, where it will direct the synthesis of proteins.

39 Protein Synthesis Recall proteins consists of a specific sequence of amino acids linked together by peptide bonds. There are 20 amino acids that can make up a protein. Genetic Code – triplets of nucleotides in mRNA that determines the sequence of amino acids in proteins. Codon – a group of three mRNA nucleotides that codes for a specific amino acid.

40 mRNA codons

41 mRNA Codon Wheel Start Codon (AUG) – engages a ribosome to start translating a mRNA molecule. Stop Codons (UAA, UAG, UGA) – cause the ribosome to stop translating an mRNA.

42 Translation Translation – the process of converting the genetic code in RNA into the amino sequence that makes up a protein. Transfer RNA (tRNA) – the type of RNA that carries amino acids from the cytoplasm to the ribosomes. Anticodon – a region of tRNA consisting of 3 bases complementary to the codon of mRNA.


44 During translation, amino acids are assembled from information encoded in mRNA. As each mRNA codon is sequentially paired with its tRNA anticodon, tRNA adds a specific amino acid to the growing polypeptide chain. Example Problem: Given the DNA sequence: TAC GGT CTC AGC ACT mRNA AUG CCA GAG UCG UGA (start) (stop) tRNA UAC GGU CUC AGC ACU Amino Acids methionine, proline, glutamic acid, serine, stop (start)

45 DNARNA Protein TranscriptionTranslation

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