Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Molecular Biology Of The Gene Chapter 10

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Sabotage Inside Our Cells A saboteur –Lies low waiting for the right moment to strike

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Viruses are biological saboteurs –Hijacking the genetic material of host cells in order to reproduce themselves Viruses provided some of the earliest evidence –That genes are made of DNA

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings THE STRUCTURE OF THE GENETIC MATERIAL 10.1 Experiments showed that DNA is the genetic material The Hershey-Chase experiment showed that certain viruses reprogram host cells –To produce more viruses by injecting their DNA Figure 10.1A Head Tail Tail fiber DNA 300,000 

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings The Hershey-Chase experiment Phage Bacterium Radioactive protein DNA Phage DNA Empty protein shell Radioactivity in liquid Pellet Centrifuge Batch 1 Radioactive protein Batch 2 Radioactive DNA Radioactive DNA Centrifuge Pellet Radioactivity in pellet Figure 10.1B Mix radioactively labeled phages with bacteria. The phages infect the bacterial cells. 1 Agitate in a blender to separate phages outside the bacteria from the cells and their contents. 2 Centrifuge the mixture so bacteria form a pellet at the bottom of the test tube. 3 Measure the radioactivity in the pellet and the liquid. 4

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Phage reproductive cycle Figure 10.1C 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.

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings DNA polynucleotide A C T G T Sugar-phosphate backbone Phosphate group Nitrogenous base Sugar A C T G T Phosphate group O O–O– O O P CH 2 H3CH3C C C C C N C N H H O O C O O H C H H H C H Nitrogenous base (A, G, C, or T) Thymine (T) Sugar (deoxyribose) DNA nucleotide 10.2 DNA and RNA are polymers of nucleotides DNA is a nucleic acid –Made of long chains of nucleotide monomers Figure 10.2A

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings DNA has four kinds of nitrogenous bases –A, T, C, and G C C C C C C O N C H H O N H H3CH3C H H H H N N N H O C HH N H C N N N N C C C C H H N N H C C N C H N C N HC O H H Thymine (T)Cytosine (C) Adenine (A) Guanine (G) Purines Pyrimidines Figure 10.2B

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings RNA is also a nucleic acid –But has a slightly different sugar –And has U instead of T Nitrogenous base (A, G, C, or U) Phosphate group O O–O– OOPCH 2 H C C C C N C N H H O O C O O H C H H OH C H Uracil (U) Sugar (ribose) Key Hydrogen atom Carbon atom Nitrogen atom Oxygen atom Phosphorus atom Figure 10.2C, D

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 10.3 DNA is a double-stranded helix James Watson and Francis Crick –Worked out the three-dimensional structure of DNA, based on work by Rosalind Franklin Figure 10.3A, B

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings The structure of DNA –Consists of two polynucleotide strands wrapped around each other in a double helix Figure 10.3C Twist

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Hydrogen bonds between bases –Hold the strands together Each base pairs with a complementary partner –A with T, and G with C Figure 10.3D GC TA AT G G C C A T GC T A T A AT AT GC A T O O OH –O–O P O O –O–O P O O O P – O O P O O O OH H2CH2C H2CH2C H2CH2C H2CH2C O O O O O O O O P O–O– O–O– O–O– O–O– HO O O O P P P O O O O O O O O T A G C C G AT CH 2 Hydrogen bond Base pair Ribbon model Partial chemical structure Computer model

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings DNA REPLICATION 10.4 DNA replication depends on specific base pairing DNA replication –Starts with the separation of DNA strands Then enzymes use each strand as a template –To assemble new nucleotides into complementary strands Figure 10.4A A T C G G C A T T A AT C G G C A T T A A T C G G C A T T A A T C G G C A T AT C G A C T A Parental molecule of DNA Both parental strands serve as templates Two identical daughter molecules of DNA Nucleotides

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings DNA replication is a complex process –Due in part to the fact that some of the helical DNA molecule must untwist Figure 10.4B GC A T GC AT C G A G A C G C G C G T A G C T A T A A T T A C G C G C G T A G C T A T A A T T A T C T

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 10.5 DNA replication: A closer look DNA replication –Begins at specific sites on the double helix Figure 10.5A Origin of replication Two daughter DNA molecules Parental strand Daughter strand Bubble

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Each strand of the double helix –Is oriented in the opposite direction Figure 10.5B P P P P P P P P HO OH A C G T T C G A end3 end 5 end

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Using the enzyme DNA polymerase –The cell synthesizes one daughter strand as a continuous piece The other strand is synthesized as a series of short pieces –Which are then connected by the enzyme DNA ligase Figure 10.5C Daughter strand synthesized continuously Daughter strand synthesized in pieces Parental DNA DNA ligase DNA polymerase molecule Overall direction of replication

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN 10.6 The DNA genotype is expressed as proteins, which provide the molecular basis for phenotypic traits The information constituting an organism’s genotype –Is carried in its sequence of its DNA bases A particular gene, a linear sequence of many nucleotides –Specifies a polypeptide

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings The DNA of the gene is transcribed into RNA –Which is translated into the polypeptide Figure 10.6A DNA Transcription RNA Protein Translation

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Studies of inherited metabolic disorders in mold –First suggested that phenotype is expressed through proteins Figure 10.6B

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 10.7 Genetic information written in codons is translated into amino acid sequences The “words” of the DNA “language” –Are triplets of bases called codons The codons in a gene –Specify the amino acid sequence of a polypeptide

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings DNA strand Transcription Translation Polypeptide RNA Amino acid Codon A AA C C GG C A AA A U UU G G CC G U UU U Gene 1 Gene 2 Gene 3 DNA molecule Figure 10.7

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 10.8 The genetic code is the Rosetta stone of life Nearly all organisms –Use exactly the same genetic code UUG

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings An exercise in translating the genetic code Figure 10.8B TA CTTCAAAATC AT GAAGTTTTAG AU G AAGU UUUAG Transcription Translation RNA DNA Met LysPhePolypeptide Start condon Stop condon Strand to be transcribed

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 10.9 Transcription produces genetic messages in the form of RNA A close-up view of transcription RNA polymerase RNA nucleotides Direction of transcription Template Strand of DNA Newly made RNA T C A T CC A A T T G G C C A A TT GGAT G U C AUCCA A U Figure 10.9A

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings In the nucleus, the DNA helix unzips –And RNA nucleotides line up along one strand of the DNA, following the base pairing rules As the single-stranded messenger RNA (mRNA) peels away from the gene –The DNA strands rejoin

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Transcription of a gene RNA polymerase DNA of gene Promoter DNA Terminator DNA Area shown In Figure 10.9A Growing RNA Completed RNA RNA polymerase Figure 10.9B 1 Initiation 2 Elongation 3 Termination

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Eukaryotic RNA is processed before leaving the nucleus Noncoding segments called introns are spliced out –And a cap and a tail are added to the ends Exon Intron Exon Intron Exon DNA Cap Transcription Addition of cap and tail RNA transcript with cap and tail Introns removed Tail Exons spliced together mRNA Coding sequence Nucleus Cytoplasm Figure 10.10

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Transfer RNA molecules serve as interpreters during translation Translation –Takes place in the cytoplasm

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings A ribosome attaches to the mRNA –And translates its message into a specific polypeptide aided by transfer RNAs (tRNAs) Amino acid attachment site Hydrogen bond RNA polynucleotide chain Anticodon Figure 10.11A

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Each tRNA molecule –Is a folded molecule bearing a base triplet called an anticodon on one end A specific amino acid –Is attached to the other end Amino acid attachment site Anticodon Figure 10.11B, C

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Ribosomes build polypeptides A ribosome consists of two subunits –Each made up of proteins and a kind of RNA called ribosomal RNA tRNA molecules mRNA Small subunit Growing polypeptide Large subunit Figure 10.12A

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings The subunits of a ribosome –Hold the tRNA and mRNA close together during translation Large subunit mRNA- binding site Small subunit tRNA-binding sites Growing polypeptide Next amino acid to be added to polypeptide mRNA tRNA Codons Figure 10.12B, C

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings An initiation codon marks the start of an mRNA message Start of genetic message End Figure 10.13A

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings mRNA, a specific tRNA, and the ribosome subunits –Assemble during initiation Met Initiator tRNA 1 2 mRNA Small ribosomal subunit Start codon Large ribosomal subunit A site U A CA U C A U G P site Figure 10.13B

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation Once initiation is complete –Amino acids are added one by one to the first amino acid

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Each addition of an amino acid –Occurs in a three-step elongation process Polypeptide P site mRNA Codons mRNA movement Stop codon New Peptide bond Anticodon Amino acid A site Figure Codon recognition 2 Peptide bond formation 3 Translocation

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings The mRNA moves a codon at a time –And a tRNA with a complementary anticodon pairs with each codon, adding its amino acid to the peptide chain

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Elongation continues –Until a stop codon reaches the ribosome’s A site, terminating translation

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Review: The flow of genetic information in the cell is DNA  RNA  protein The sequence of codons in DNA, via the sequence of codons –Spells out the primary structure of a polypeptide

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Polypeptide Transcription DNA mRNA RNA polymerase Amino acidTranslation tRNA Enzyme Anticodon ATP Initiator tRNA Large ribosomal subunit Start Codon Codons mRNA Stop codon Small ribosomal subunit Growing polypeptide New peptide bond forming mRNA Figure Summary of transcription and translation mRNA is transcribed from a DNA template. 1 Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP. 2 Initiation of polypeptide synthesis The mRNA, the first tRNA, and the ribosomal subunits come together. 3 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. 5 The ribosome recognizes a stop codon. The poly-peptide is terminated and released. Termination

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Mutations can change the meaning of genes Mutations are changes in the DNA base sequence –Caused by errors in DNA replication or recombination, or by mutagens C TTC AT Normal hemoglobin Mutant hemoglobin DNA GAAGUA Sickle-cell hemoglobin Normal hemoglobin DNA Glu Val mRNA Figure 10.16A

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Substituting, inserting, or deleting nucleotides alters a gene –With varying effects on the organism Normal gene mRNA Base substitution Base deletion Missing MetLys Phe Gly Ala Met Lys PheSerAla Met Lys Leu AlaHis AUGA A G U U U G G C GC A AUGA A G U U U A G C GC A AUGA A G U U GGCG CA U U Protein Figure 10.16B

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings MICROBIAL GENETICS Viral DNA may become part of the host chromosome Viruses –Can be regarded as genes packaged in protein

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings When phage DNA enters a lytic cycle inside a bacterium –It is replicated, transcribed, and translated The new viral DNA and protein molecules –Then assemble into new phages, which burst from the host cell

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings In the lysogenic cycle –Phage DNA inserts into the host chromosome and is passed on to generations of daughter cells Much later –It may initiate phage production

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Phage reproductive cycles Lysogenic bacterium reproduces normally, replicating the prophage at each cell division Phage DNA inserts into the bacterial chromosome by recombination New phage DNA and proteins are synthesized Phages assemble Cell lyses, releasing phages Phage Attaches to cell Phage DNA Phage injects DNA Many cell divisions Prophage Lytic cycleLysogenic cycle OR Bacterial chromosome Phage DNA circularizes Figure

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Membranous envelope RNA Protein coat Glycoprotein spike Figure 10.18A CONNECTION Many viruses cause disease in animals Many viruses cause disease –When they invade animal or plant cells Many, such as flu viruses –Have RNA, rather than DNA, as their genetic material

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Some animal viruses –Steal a bit of host cell membrane as a protective envelope –Can remain latent in the host’s body for long periods Glycoprotein spike Envelope Protein coat Viral RNA (genome) VIRUS Plasma membrane of host cell Viral RNA (genome) Template New viral genome Exit mRNA 7 New viral proteins Figure 10.18B Entry 1 Uncoating 2 RNA synthesis by viral enzyme 3 Protein synthesis 4 RNA synthesis (other strand) 5 Assembly 6

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings CONNECTION Plant viruses are serious agricultural pests Most plant viruses –Have RNA genomes –Enter their hosts via wounds in the plant’s outer layers Protein RNA Figure 10.19

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings CONNECTION Emerging viruses threaten human health Colorized TEM 50,000  Colorized TEM 370,000  Figure 10.20A, B

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings The AIDS virus makes DNA on an RNA template HIV, the AIDS virus –Is a retrovirus Envelope Glycoprotein Protein coat RNA (two identical strands) Reverse transcriptase Figure 10.21A

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Inside a cell, HIV uses its RNA as a template for making DNA –To insert into a host chromosome Viral RNA RNA strand Double- stranded DNA Viral RNA and proteins CYTOPLASM NUCLEUS Chromosomal DNA Provirus DNA RNA Figure 10.21B

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Bacteria can transfer DNA in three ways Bacteria can transfer genes from cell to cell by one of three processes –Transformation, transduction, or conjugation DNA enters cell Fragment of DNA from another bacterial cell Bacterial chromosome (DNA) Phage Fragment of DNA from another bacterial cell (former phage host) Phage Sex pili Mating bridge Donor cell (“male”) Recipient cell (“female”) Figure 10.22A–C

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Once new DNA gets into a bacterial cell –Part of it may then integrate into the recipient’s chromosome Recipient cell’s chromosome Recombinant chromosome Donated DNA Crossovers Degraded DNA Figure 10.22D

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Bacterial plasmids can serve as carriers for gene transfer Plasmids –Are small circular DNA molecules separate from the bacterial chromosome

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Plasmids can serve as carriers –For the transfer of genes Plasmids Colorized TEM 2,000  Cell now male Plasmid completes transfer and circularizes F factor starts replication and transfer Male (donor) cell Bacterial chromosome F factor (plasmid) Recombination can occur Only part of the chromosome transfers F factor starts replication and transfer of chromosome Origin of F replication Bacterial chromosome Male (donor) cell F factor (integrated) Recipient cell Figure 10.23A–C