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Genes: Structure, Replication, and Expression

This chapter explores the structure and replication of genes, as well as the process of gene expression through transcription and translation. It covers the organization of DNA in cells, DNA replication, and the importance of reading frames in gene coding. The chapter also discusses the structure of genes that code for proteins, tRNA, and rRNA, and provides an overview of transcription in bacteria.

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Genes: Structure, Replication, and Expression

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  1. Chapter 12 Genes: Structure Replication and Expression

  2. Replication: During mitotic division information is duplicated by DNA replication and is passed on to next generation daughter cells has exavcyt replica of the parent DNA

  3. Role of DNA in Protein synthesis DNA and protein synthesis involves: Transcription- yields a ribonucleic acid (RNA) copy of specific genes Translation- uses information in messenger RNA (mRNA) to synthesize a polypeptide. Protein synthesis is assisted by RNA(tRNA) and ribosomal RNA (rRNA)

  4. Nucleic Acids

  5. Nucleic Acid StructureDeoxyribonucleic Acid (DNA) polymer of nucleotides contains the bases adenine, guanine, cytosine and thymine sugar is deoxyribose molecule is usually double stranded

  6. DNA is a double-stranded molecule twisted into a helix (think of a spiral staircase). • Each spiraling strand, comprised of a sugar-phosphate backbone and attached bases, is connected to a complementary strand by non-covalent hydrogen bonding between paired bases. • The bases are adenine (A), thymine (T), cytosine (C) and guanine (G). A and T are connected by two hydrogen bonds. G and C are connected by three hydrogen bonds.

  7. DNA Structure – Two Complementary Strands base pairing Adenine (purine) and thymine (pyrimidine) pair by 2 hydrogen bonds Guanine (purine) and cytosine (pyrimidine) pair by 3 hydrogen bonds major and minor grooves form when the 2 strands twist around each other

  8. Nucleic Acid StructureRibonucleic Acid (RNA) polymer of nucleotides contains the bases adenine, guanine, cytosine and uracil sugar is ribose most RNA molecules are single stranded

  9. RNA Structure three different types which differ from each other in function and in structure messenger RNA (mRNA) ribosomal RNA (rRNA) transfer RNA(tRNA)

  10. The Organization of DNA in Cells In most bacteria DNA is a circular, double helix further twisting results in supercoiled DNA in bacteria the DNA is associated with basic proteins help organize the DNA into a coiled chromatin like structure

  11. DNA Replication

  12. DNA Replication complex process involving numerous proteins which help ensure accuracy the 2 strands separate, each serving as a template for synthesis of a complementary strand synthesis is semi-conservative; each daughter cell obtains one old and one new strand

  13. DNA Replication bidirectional from a single origin of replication

  14. DNA replication (arrows) occurs in both directions from the origin of replication in the circular DNA found in most bacteria.

  15. Rolling Circle Replication some small circular genomes (e.g., viruses and plasmids) replicated by rolling-circle replication Animation illustrating DNA replication by complementary base pairing

  16. Genes

  17. Gene Structure Gene the basic unit of genetic information also defined as the nucleic acid sequence that codes for a polypeptide, tRNA or rRNA linear sequence of nucleotides codons are found in mRNA and code for single amino acids reading frame organization of codons such that they can be read to give rise to a gene product

  18. Importance of Reading Frame Figure 12.16

  19. Genes that Code for Proteins template strand directs RNA synthesis promoter is located at the start of the gene is the recognition/binding site for RNA polymerase functions to orient polymerase leader sequence is transcribed into mRNA but is not translated into amino acids Shine-Delgarno sequence important for initiation of translation

  20. Genes that Code for Proteins The Coding Region: begins with the DNA sequence from 3´-TAC-5´ produces codon AUG which codes for N-formylmethionine, a modified amino acid used to initiate protein synthesis in bacteria ( check fig.) coding region ends with a stop codon, immediately followed by the trailer sequence which contains a terminator sequence used to stop transcription

  21. Bacterial Gene Structure

  22. Genes That Code for tRNA and rRNA • tRNA/rRNA genes have promoter (recognition/binding site for RNA polymerase), leader (is transcribed into mRNA), coding region, spacer and trailer regions (contains a terminator sequence used to stop transcription) • during maturation process. leader, spacer, and trailer removed during maturation process Figure 12.19a:

  23. rRNA genes have promoter, leader, coding, spacer, and trailer regions spacer and trailer regions may encode tRNA molecules Figure 12.19b:

  24. Fig. 12.20 m

  25. Transcription

  26. Transcription RNA is synthesized under the direction of DNA RNA produced has complementary sequence to the template DNA three types of RNA are produced mRNA carries the message for protein synthesis tRNA carries amino acids during protein synthesis rRNA molecules are components of ribosomes

  27. Transcription in Bacteria… Definitions to understand protein synthesis: in most bacterial RNA polymerases: Holoenzyme can begin transcription> What is Holoenzyme??* the core enzyme is composed of 5 chains and catalyzes RNA synthesis the sigmafactor has no catalytic activity but helps the core enzyme recognize the start of genes *holoenzyme = core enzyme + sigma factor only the holoenzyme can begin transcription

  28. Transcription in Bacteria…. • Transcription in Bacteria is catalyzed by a single RNA polymerase. • a reaction similar to that catalyzed by DNA polymerase for DNA syntehsis. • ATP,GTP,CTP and UTP are used to produce a complementary RNA copy of the template DNA sequence

  29. http://www.vidoemo.com/yvideo.php?i=M2FWVDJEcWuRpVGJ0QTg&replication-transcription-and-translation=http://www.vidoemo.com/yvideo.php?i=M2FWVDJEcWuRpVGJ0QTg&replication-transcription-and-translation=

  30. Transcription Process

  31. Transcription Initiation Promoter site where RNA polymerase binds to initiate transcription & is not transcribed

  32. Transcription Elongation after binding, RNA polymerase unwinds the DNA transcription bubble produced moves with the polymerase as it transcribes mRNA from template strand within the bubble a temporary RNA:DNA hybrid is formed

  33. Coupled Transcription and Translation in Prokaryotes

  34. Proteins

  35. The Genetic Code mRNA sequence is translated into amino acid sequence of polypeptide chain (process = translation). an understanding of the genetic code is necessary before translation is studied.

  36. Organization of the Code code degeneracy up to six different codons can code for a single amino acid sense codons the 61 codons that specify amino acids stop (nonsense) codons the three codons used as translation termination signals do not encode amino acids

  37. Translation

  38. Translation Translation of mRNA into protein: synthesis of polypeptide is directed by sequence of nucleotides in mRNA Ribosome: 70S ribosomes = 30S + 50S subunit site of translation polyribosome (polysome) – complex of mRNA with several ribosomes

  39. Translation of mRNA into protein: • Three phases: • Initiation • Elongation • Termination

  40. During translation, the mRNA is "read" according to the genetic code which relates the DNA sequence to the amino acid sequence in proteins • Each group of three base pairs in mRNA constitutes a codon, and each codon specifies a particular amino acid (hence, it is a triplet code). • The mRNA sequence is thus used as a template to assemble—in order—the chain of amino acids that form a protein.

  41. Transfer RNA (tRNA) and Amino Acid Activation The tRNA molecules are adaptor molecules—they have one end that can read the triplet code in the mRNA through complementary base-pairing, and another end that attaches to a specific amino acid attachment of amino acid to tRNA is catalyzed by aminoacyl-tRNAsynthetases

  42. The translation of mRNA begins with the formation of a complex on the mRNA (Fig. below). • First, three initiation factor proteins (known as IF1, IF2, and IF3) bind to the small subunit of the ribosome. • This preinitiation complex and a methionine-carrying tRNA then bind to the mRNA, near the AUG start codon, forming the initiation complex.

  43. The Ribosome

  44. Methionine (Met) is the first amino acid incorporated into any new protein, however, it is not always the first amino acid in translation of protein. • In many proteins, methionine is removed after translation.

  45. The large ribosomal subunit binds to this complex, which causes the release of IFs (initiation factors) once the initiation complex is formed on the mRNA • The large subunit of the ribosome has three sites at which tRNA molecules can bind: • The A (amino acid) site is the location at which the aminoacyl-tRNA anticodon base pairs up with the mRNA codon, ensuring that correct amino acid is added to the growing polypeptide chain.

  46. The P (polypeptide) site is the location at which the amino acid is transferred from its tRNA to the growing polypeptide chain. • Finally, the E (exit) site is the location at which the "empty" tRNA sits before being released back into the cytoplasm to bind another amino acid and repeat the process.

  47. The initiator methionine tRNA is the only aminoacyl-tRNA that can bind in the P site of the ribosome, and the A site is aligned with the second mRNA codon. • The ribosome is thus ready to bind the second aminoacyl-tRNA at the A site, which will be joined to the initiator methionine by the first peptide bond.

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