Molecular Biology of the Gene

2. Molecular Biology of the Gene Chapter 10

3. Molecular Biology • DNA and how it serves as the molecular basis of heredity • Structure of DNA • How it replicates • How DNA controls the cell • DNA and protein synthesis • How DNA can change

4. Viruses • Share some characteristics of living organisms • Genetic material in the form of nucleic acids • Not considered alive because it is not cellular and cannot reproduce on its own • Basically a nucleic acid wrapped in a protein coat • Host provides most of the tools and raw material for viral multiplication

5. Viruses • Fools the cell into taking it into nucleus • Can be dormant in the cell or replicate • Uses the cells own molecules and organelles to replicate the virus • Virus production eventually bursts the cell to release viruses

6. Viruses and DNA Structure • By the 1940’s, scientists knew that eukaryotic cells had proteins and DNA • Thought proteins were the material of genes • It was not until experiments like the Hershey-Chase experiment were performed that scientists were convinced otherwise • Used radioactive isotopes to track which type of molecule was transferred to infected cells in order to replicate the virus

7. Phage attaches to bacterial cell. Phage injects DNA. Phage DNA directs host cell to make more phage DNA and protein parts. New phages assemble. Cell lyses and releases new phages. Bacteriophages • Bacterial phages or phages (feed on bacteria) • Consist solely of DNA and protein

8. Agitate in a blender to separate phages outside the bacteria from the cells and their contents. Centrifuge the mixture so bacteria form a pellet at the bottom of the test tube. Measure the radioactivity in the pellet and liquid. Mix radioactivelylabeled phages with bacteria. The phages infect the bacterial cells. 1 2 3 4 Radioactiveprotein Emptyprotein shell Radioactivityin liquid Phage Bacterium PhageDNA DNA Batch 1Radioactiveprotein Centrifuge Pellet RadioactiveDNA Batch 2RadioactiveDNA Centrifuge Radioactivityin pellet Pellet Hershey-Chase Experiment

9. Phosphate group Nitrogenous base Sugar Nucleotide Sugar-phosphate backbone Polynucleotide DNA • Deoxyribonucleic acid • DNA is a polymer or polynucleotide, made of long chains of nucleotides • Phosphate group, nitrogenous base and a sugar • Sugar, phosphate backbone

10. Nitrogenous base(A, G, C, or T) Phosphategroup Thymine (T) Cytosine (C) Adenine (A) Guanine (G) Pyrimidines Purines Thymine (T) Sugar(deoxyribose) DNA nucleotide DNA • DNA has four kinds of bases, A, T, C, and G • Nucleotides joined by covalent bonds • O- is shared between phosphate group of one and sugar of another

11. Nitrogenous base(A, G, C, or U) Phosphategroup Uracil (U) Sugar(ribose) RNA • RNA is also a nucleic acid • RNA has a slightly different sugar • Ribose • RNA has U instead of T • A, U, G, C

12. DNA’s Double-Stranded Helix • James Watson and Francis Crick worked out the three-dimensional structure of DNA • Saw that t he 3D structure was a helix with nitrogenous bases stacked one on top of another • Also decided that it had to be a double helix based on the diameter of the helix • Worked together to deduce a double helix that would conform to the chemical properties of DNA

13. Twist DNA Structure • The structure of DNA consists of two polynucleotide strands wrapped around each other in a double helix

14. Hydrogen bond Partial chemical structure Structure of DNA • Hydrogen bonds between bases hold the strands together • Each base pairs with a complementary partner • A pairs with T • G pairs with C

15. Structure of DNA • Base-pairing effects the side-by-side combinations of nucleotides, but there are no restrictions on the sequence of nucleotides up and down the strand • By showing the structure of DNA we can see how the genetic information must be encoded in the nucleotide sequence • Also explains some aspects of genetic inheritance

16. A A Nucleotides Parental moleculeof DNA Both parental strands serveas templates Two identical daughtermolecules of DNA DNA Replication • In DNA replication, the strands separate • Enzymes use each strand as a template to assemble the new strands • Free nucleotides are attached to make two strands

17. DNA Replication • The DNA has to unwrap and untwist in order to replicate • It also must copy both strands at the same time in order to prevent the ‘unzipped’ strands from reattaching • Nucleotides are added at a very rapid rate

18. Parental strand Origin of replication Daughter strand Bubble Two daughter DNA molecules DNA Replication • DNA replication begins at specific sites • Origins of replication • Starts in both directions • DNA strands open up a bubble as ‘Daughter Strands’ are formed • Eventually bubbles merge

19. 5 end 3 end P P P P P P P P 3 end 5 end DNA Replication • Each strand of the double helix is oriented in the opposite direction • This is important because the enzymes that add the nucleotides only work in one direction • 5’ ---> 3’ end

20. 3 DNA polymerasemolecule 5 5 end Daughter strandsynthesizedcontinuously Parental DNA 5 3 Daughter strandsynthesizedin pieces 3 P 5 5 P 3 DNA ligase Overall direction of replication DNA Replication • DNA polymerases work from 5’ 3’ and add nucleotides to the daughter strands in pieces • After the polymerases add the nucleotides an enzyme called a DNA ligase then ties the strand pieces together to form a continuous strand

21. How do our genes become our inherited traits? • An organisms genotype is the heritable information contained in the DNA • The phenotype is the organisms specific traits • The molecular basis of the phenotype lies in the proteins of various functions • DNA specifies the synthesis of proteins, each gene codes for a specific protein

22. Protein synthesis from DNA • The gene does not directly build the protein, it is like the blue-print of a plan • Other molecules do the actual building • DNA in the nucleus of the cell makes RNA, this is called transcription • The RNA then exits the nucleus and goes into the cytoplasm where it is read and made into a protein, this is called translation

23. DNA TRANSCRIPTION NUCLEUS RNA TRANSLATION CYTOPLASM Protein Protein synthesis from DNA

24. The Language of DNA • DNA and RNA are large polymers of single monomer nucleic acids • These nucleotides differ only in their nitrogenous bases • The “words” of the DNA “language” are triplets of nitrogenous bases called codons • The codons in a gene specify the amino acid sequence of a polypeptide • Series of amino acids make proteins

25. Gene 1 Gene 3 DNA molecule Gene 2 DNA strand TRANSCRIPTION RNA Codon TRANSLATION Polypeptide Amino acid

26. The Language of DNA • Virtually all organisms share the same genetic code • Not all of the codons code for an amino acid, some code instructions for translation

27. RNA polymerase DNA of gene Promoter DNA Terminator DNA Initiation Elongation Area shownin Figure 10.9A Termination GrowingRNA Completed RNA RNApolymerase Transcription of RNA • In transcription, the DNA helix unzips • RNA nucleotides line up along one strand of the DNA following the base-pairing rules • The single-stranded messenger RNA peels away and the DNA strands rejoin

28. Exon Intron Exon Intron Exon DNA TranscriptionAddition of cap and tail Cap RNAtranscriptwith capand tail Introns removed Tail Exons spliced together mRNA Coding sequence NUCLEUS CYTOPLASM Messenger RNA • The kind of RNA that encodes amino acids is called messenger RNA • Noncoding segments called introns are spliced out • A cap and a tail are added to the ends • Transported out of the cytoplasm to be translated into proteins

29. Amino acid attachment site Anticodon Transfer RNA • In the cytoplasm, a ribosome attaches to the mRNA and translates its message into a polypeptide • The process is aided by transfer RNAs • AA’s are already there • tRNA’s match AA’s to the proper codon • Structure has specific sites for recognition and attachment

30. Next amino acidto be added topolypeptide Growingpolypeptide tRNA mRNA Codons Ribosomes Build Polypeptides • Coordinate the functions of the mRNA and the tRNA • Actually make polypeptides • The assembly line for reading mRNAs and assembling proteins from AA’s

31. Initiator tRNA A site P site Startcodon mRNA Translation • Caps help bind the mRNA to the ribosome • Sspecific start codon signals the tRNA to start building the polypeptide • tRNA’s bring in the next AA’s to empty site, strand is attached to new AA, and old tRNA is dropped off

32. Amino acid Polypeptide Asite P site Anticodon mRNA 1 Codon recognition mRNAmovement Stopcodon Newpeptidebond Peptide bond formation 2 3 Translocation Translation • The mRNA moves a codon at a time relative to the ribosome • A tRNA pairs with each codon, adding an amino acid to the growing polypeptide • A stop codon stops translation and releases the polypeptide and mRNA is released

33. TRANSCRIPTION DNA 1 Stage mRNA is transcribed from a DNA template. mRNA RNApolymerase Amino acid TRANSLATION Stage Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP. 2 Enzyme tRNA Initiator tRNA Anticodon Stage Initiation of polypeptide synthesis 3 Largeribosomalsubunit The mRNA, the first tRNA, and the ribosomal subunits come together. Start Codon Smallribosomalsubunit mRNA

34. Newpeptidebondforming Growing polypeptide Stage Elongation 4 A succession of tRNAs add their amino acids to the polypeptide chain as the mRNA is moved through the ribosome, one codon at a time. Codons mRNA Polypeptide 5 Stage Termination The ribosome recognizes a stop codon. The poly-peptide is terminated and released. Stop Codon

35. Normal hemoglobin DNA Mutant hemoglobin DNA mRNA mRNA Normal hemoglobin Glu Sickle-cell hemoglobin Val Genetic Mutations • Any change in the nucleotide sequence of DNA is called a mutation • Alternate forms of alleles may be mutations • May be a change in a large sequence of DNA or in a single nucleotide pair • The change of a single DNA nucleotide causes sickle-cell disease

36. NORMAL GENE mRNA Protein Met Lys Phe Gly Ala BASE SUBSTITUTION Met Lys Phe Ser Ala Missing BASE DELETION Met Lys Leu Ala His Types of Mutations

37. Virus’ • Enter into the cell via fusing with the plasma membrane • Inject their RNA • RNA either replicates itself • Or synthesizes the protein that it codes for • New viral proteins are made • Proteins surround the newly formed RNA and make a new virus that is shipped out of the cell

38. The AIDS Virus • Enters the cell by means of fusing with the plasma membrane and injecting its RNA into the cytoplasm • Retrovirus- when the RNA enters the cell it gets transcribed into DNA • Inserts itself into the nuclear genome • Now can act just like a normal plant cell gene and make its own proteins