Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Genes and How They Work Chapter 15 Copyright © McGraw-Hill Companies Permission required for reproduction or display

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Outline Cells Use RNA to Make Protein Genetic Code Transcription Translation Introns and Exons Bacterial and Eukaryotic Gene Expression

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Gene Expression Transcription Overview:  DNA sequence is transcribed into RNA sequence. - Initiated when RNA polymerase binds to promoter site.  Moves along DNA strand and adds corresponding complementary RNA nucleotide.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Cells Use RNA to Make Protein Types of RNA  ribosomal RNA (rRNA) is the site of polypeptide assembly during polypeptide synthesis.  Transfer RNA (tRNA) transport and position amino acids.  Messenger RNA (mRNA) direct which amino acids are assembled into polypeptides. Central Dogma  DNA  RNA  Protein

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Gene Expression Translation Overview:  Nucleotide sequence of mRNA transcript is translated into an amino acid sequence in the polypeptide. - rRNA molecule within ribosome recognizes and binds to start sequence on mRNA.  Moves along three nucleotides at a time.  Disengages at stop signal.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Genetic Code The genetic code consists of a series of information blocks called codons.  Reading frame is composed of three nucleotide blocks (Triplet). - Each codes for one amino acid.  Genetic code is universal in almost all organisms. Exception:  Mitochondria  Chloroplasts  Certain ciliates

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Transcription RNA Polymerase (Bacteria)  Only one of two DNA strands is transcribed (template or antisense strand (-)).  Non-transcribed strand is termed coding strand or sense strand (+).  In both bacteria and eukaryotes, the polymerase adds ribonucleotides to the growing 3’ end of an RNA chain. - Synthesis proceeds in 5’  3’ direction.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies 3 types of RNA polymerase enzymes in Eukaryotes. * RNA Polymerase I – rRNA * RNA Polymerase II – mRNA * RNA Polymerase III - tRNA

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Transcription Promoter  Transcription starts at RNA polymerase binding sites called promoters on DNA template strand.  Bacteria - Two six-base sequences are common - -TTGACA, -35 sequence, located 35 nucleotides upstream - - TATAAT, -10 sequence, located Eukaryotes -TATAAA, TATA box, located 25 nucleotides upstream

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Initiation  Binding of RNA polymerase to promoter is the first step.  Other eukaryotic factors bind, assembling a transcription complex. - RNA polymerase begins to unwind DNA helix. Transcription

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Transcription Elongation  Transcription bubble moves down DNA at constant rate leaving growing RNA strands protruding from the bubble. 50 nucleotides/sec.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Termination Stop sequences at the end of the gene cause phosphodiester bond formation to cease, transcription bubble to dissociate, and RNA polymerase to release DNA. -Simplest stop signal is a sequence of GC base-pairs followed by a series of AT base-pairs. Forms a GC hairpin followed by 4+/- uracil nucleotides. Fig Reason is weak bonds between U-A bonds

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Posttranscriptional Modifications in Eukaryotes: 5’ cap is attached. - 5’-5’ linkage is formed to protect RNA template from nucleases and phosphatases 3’ poly-A tail is added - the AAUAAA sequence is cleaved off and a poly A tail of 250 ribonucleotides is added to 3’ end to prevent degradation by nucleases and make the transcript a better template.

Fig. 15.2

Fig. 15.4

Fig. 15.6b

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Translation Begins when initial portion of mRNA molecule binds to rRNA in a ribosome.  tRNA molecule with complimentary anticodon, binds to exposed codon on mRNA. - Some tRNA molecules recognize more than one codon.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Translation Activating Enzymes  tRNA molecules attach to specific amino acids through the action of activating enzymes (aminoactyl-tRNA syntheases). - Must correspond to specific anticodon sequences on a tRNA molecule as well as particular amino acids.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Translation Start and Stop Signals  Start signal coded by AUG codon.  Stop signal coded by one of three nonsense codons: UAA - UAG - UGA. Initiation  Polypeptide synthesis begins with the formation of an initiation complex. - Initiation Factors

Fig d

Fig e

Fig f

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Formation of Initiation Complex Copyright © McGraw-Hill Companies Permission required for reproduction or display

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Translation Elongation  After initiation complex forms, large ribosome subunit binds, exposing mRNA codon adjacent to the initiating codon, positioning it for interaction with another amino acid-bearing tRNA molecule.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Translation Translocation  Ribosome moves nucleotides along mRNA molecule. Copyright © McGraw-Hill Companies Permission required for reproduction or display

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Translation Termination  Nonsense codons are recognized by release factors that release the newly made polypeptide from the ribosome. Copyright © McGraw-Hill Companies Permission required for reproduction or display

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Introns DNA sequence specifying a protein is broken into segments (exons) scattered among longer noncoding segments (introns). Initially, a primary RNA transcript is produced for the entire gene.  Small nuclear ribonucleicproteins (snRNPs), enzyme, associate with proteins to form spliceosome. - Excise introns and joins exons to form mature RNA.  Alternative Splicing

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Introns and Exons Copyright © McGraw-Hill Companies Permission required for reproduction or display

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies RNA Splicing During RNA processing, intron sequences are cut out of primary transcript before it is used in polypeptide synthesis.  Remaining sequences are not translated. - Remaining exon sequences are spliced together to form final processed mRNA.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Differences Between Bacterial and Eukaryotic Gene Expression Most eukaryotic genes possess introns. Individual bacterial mRNA molecules often contain transcripts of several genes. Eukaryotic mRNA molecules must be completely formed and must pass across the nuclear membrane before translation. In bacteria, translation begins at AUG codon preceded by special nucleotide sequence.

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Differences Between Bacterial and Eukaryotic Gene Expression Eukaryotic mRNA molecules have introns cut out and exons joined together before translation. Eukaryotic ribosomes are larger than prokaryotic ribosomes.

Fig. 15.6a

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Review Cells Use RNA to Make Protein Genetic Code Transcription Translation Introns and Exons Bacterial and Eukaryotic Gene Expression

Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies Copyright © McGraw-Hill Companies Permission required for reproduction or display