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Molecular Biology of the Gene: (Molecular Genetics)
Chapter 10 Molecular Biology of the Gene: (Molecular Genetics)
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1. Nucleic Acid Structure
DNA and RNA are polymers of nucleotides DNA is a nucleic acid Made of long chains of nucleotide monomers DNA polynucleotide A C T G Sugar-phosphate backbone Phosphate group Nitrogenous base Sugar O O– P CH2 H3C N H Nitrogenous base (A, G, C, or T) Thymine (T) Sugar (deoxyribose) DNA nucleotide
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DNA has four kinds of nitrogenous bases A, T, C, and G
H3C Thymine (T) Cytosine (C) Adenine (A) Guanine (G) Purines Pyrimidines
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Nitrogenous base (A, G, C, or U)
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– P CH2 H C N OH Uracil (U) Sugar (ribose) Key Hydrogen atom Carbon atom Nitrogen atom Oxygen atom Phosphorus atom
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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
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The structure of DNA Consists of two polynucleotide strands wrapped around each other in a double helix Twist
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Hydrogen bonds between bases Hold the strands together
Each base pairs with a complementary partner A with T, and G with C G C T A O OH –O P – O H2C O– HO CH2 Hydrogen bond Base pair Ribbon model Partial chemical structure Computer model
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2. DNA REPLICATION DNA replication depends on specific base pairing
Starts with the separation of DNA strands Then enzymes use each strand as a template To assemble new nucleotides into complementary strands A T C G Parental molecule of DNA Both parental strands serve as templates Two identical daughter molecules of DNA Nucleotides
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DNA replication is a complex process, which occurs during “S” Phase of the eukaryotic cell cycle.
Due in part to the fact that some of the helical DNA molecule must untwist G C A T Replication Fork
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DNA replication: A closer look
Begins at specific sites on the double helix Origin of replication Two daughter DNA molecules Parental strand Daughter strand Bubble
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At the end of each replication bubble is a replication fork, a Y-shaped region where new DNA strands are elongating Helicases are enzymes that untwist the double helix at the replication forks Single-strand binding protein binds to and stabilizes single-stranded DNA until it can be used as a template Topoisomerase corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands
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Each strand of the double helix Is oriented in the opposite direction
HO OH A C G T 2 1 3 4 5 5 end 3 end
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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 3 5 Daughter strand synthesized continuously Daughter strand synthesized in pieces Parental DNA DNA ligase DNA polymerase molecule Overall direction of replication
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THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN
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
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3. Gene Transcription The DNA of the gene is transcribed into RNA
Which is translated into the polypeptide Figure 10.6A DNA Transcription RNA Protein Translation
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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
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DNA molecule Gene 1 Gene 2 Gene 3 DNA strand Transcription RNA Codon
Translation Polypeptide RNA Amino acid Codon A C G U Gene 1 Gene 2 Gene 3 DNA molecule
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The genetic code is the Rosetta stone of life
Nearly all organisms Use exactly the same genetic code
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An exercise in translating the genetic code
U Transcription Translation RNA DNA Met Lys Phe Polypeptide Start condon Stop condon Strand to be transcribed
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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 G U
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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
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Transcription of a gene
RNA polymerase DNA of gene Promoter DNA Terminator Area shown In Figure 10.9A Growing RNA Completed RNA polymerase Figure 10.9B 1 Initiation 2 Elongation 3 Termination
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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
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4. Translation: Gene to Protein
Transfer RNA molecules serve as interpreters during translation Translation Takes place in the cytoplasm
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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
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Is attached to the other end
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
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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
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The subunits of a ribosome
Hold the tRNA and mRNA close together during translation tRNA-binding sites Large subunit Next amino acid to be added to polypeptide Growing polypeptide tRNA mRNA- binding site mRNA Small subunit Codons
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An initiation codon marks the start of an mRNA message
Start of genetic message End
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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 C A U G P site
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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
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Each addition of an amino acid
Occurs in a three-step elongation process Amino acid Polypeptide P site A site Anticodon mRNA Codons 1 Codon recognition mRNA movement Stop codon 2 Peptide bond formation New Peptide bond 3 Translocation
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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
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Elongation continues Until a stop codon reaches the ribosome’s A site, terminating translation
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Review: The flow of genetic information in the cell is DNARNAprotein
The sequence of codons in DNA, via the sequence of codons Spells out the primary structure of a polypeptide
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Summary of transcription and translation
DNA Transcription mRNA mRNA is transcribed from a DNA template. 1 RNA polymerase Amino acid Translation Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP. 2 Enzyme ATP tRNA Anticodon Initiator tRNA Large ribosomal subunit Initiation of polypeptide synthesis The mRNA, the first tRNA, and the ribosomal subunits come together. 3 Start Codon Small ribosomal subunit mRNA New peptide bond forming Growing polypeptide 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 The ribosome recognizes a stop codon. The poly-peptide is terminated and released. Termination Stop codon
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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 T A Normal hemoglobin Mutant hemoglobin DNA G U Sickle-cell hemoglobin Normal hemoglobin DNA Glu Val mRNA
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THE IGR EDC ATA TET HEB IGB ADR AT
1. Point Mutations a. A gene mutation is a change that takes place within a single gene. Point mutations are mutations that involved one base pair and may include ... substitutions, insertions or deletions of nucleotides. The sentence below contains all three-letter words but has experienced a mutation. Can you find the mutation? THE IGR EDC ATA TET HEB IGB ADR AT What kind of mutation did you find? Now what does the sentence say?
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Substituting, inserting, or deleting nucleotides alters a gene
With varying effects on the organism Normal gene mRNA Base substitution Base deletion Missing Met Lys Phe Gly Ala Ser Leu His A U G C Protein
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Viral DNA may become part of the host chromosome
MICROBIAL GENETICS Viral DNA may become part of the host chromosome Viruses Can be regarded as genes packaged in protein
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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
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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
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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 cycle Lysogenic cycle OR Bacterial chromosome Phage DNA circularizes 1 7 2 4 3 5 6
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Many viruses cause disease in animals
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 Membranous envelope RNA Protein coat Glycoprotein spike
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Steal a bit of host cell membrane as a protective envelope
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 VIRUS Glycoprotein spike Viral RNA (genome) Protein coat Envelope Plasma membrane of host cell Entry 1 Uncoating 2 Viral RNA (genome) RNA synthesis by viral enzyme 3 Protein synthesis 4 RNA synthesis (other strand) 5 mRNA Template New viral genome New viral proteins Assembly 6 Exit 7
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Plant viruses are serious agricultural pests
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
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Emerging viruses threaten human health
CONNECTION Emerging viruses threaten human health Colorized TEM 50,000 Colorized TEM 370,000
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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
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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 1 2 3 4 5 6
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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) Sex pili Mating bridge Donor cell (“male”) Recipient cell (“female”)
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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
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Bacterial plasmids can serve as carriers for gene transfer
Are small circular DNA molecules separate from the bacterial chromosome
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Plasmids can serve as carriers For the transfer of genes
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 F factor (integrated) Recipient cell
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