Overview: The Flow of Genetic Information

1. Overview: The Flow of Genetic Information • The information content of DNA is in the form of specific sequences of nucleotides • The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins • Gene expression is the process by which DNA directs protein synthesis, includes two stages: • Transcription • Translation

2. Basic Principles of Transcription and Translation • Transcription is the synthesis of RNA under the direction of DNA • Transcription produces messenger RNA (mRNA) • Translation is the synthesis of a polypeptide, which occurs under the direction of mRNA • Ribosomes are the sites of translation DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell

3. DNA RNA protein • In an eukaryotic cell, the nuclear envelope separates transcription from translation • Eukaryotic RNA transcripts are modified through RNA processing to yield finished mRNA before leaving the nucleus • The finished mRNA instructs the ribosomes in the cytoplasm of the cell to make proteins • Cells are governed by a cellular chain of command: DNA mRNA protein

4. DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell Nuclear envelope DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA Ribosome TRANSLATION Polypeptide Eukaryotic cell

5. During transcription, a DNA strand called the template strand provides a template for ordering the sequence of nucleotides in an RNA transcript (mRNA) • During translation, the mRNA base triplets, called codons, are read in the 5 to 3 direction • Each codon specifies the amino acid to be placed at the corresponding position along a polypeptide

6. Gene 2 DNA molecule Gene 1 Gene 3 5¢ 3¢ DNA strand (template) TRANSCRIPTION 5¢ 3¢ mRNA Codon TRANSLATION Protein Amino acid

7. Second mRNA base Third mR NA base (3¢ end) First mRNA base (5¢ end)

8. Transcription is the DNA-directed synthesis of RNA: Molecular Components of Transcription • RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart, adds and binds the RNA nucleotides • RNA synthesis follows the same base-pairing rules as DNA, except uracil substitutes for thymine

9. Transcription • The three stages of transcription: • Initiation • Elongation • Termination • The DNA sequence where RNA polymerase attaches is called the promoter. • The stretch of DNA that is transcribed is called a transcription unit.

10. Promoter Transcription unit Elongation Non-template strand of DNA 5 3 3¢ 5¢ DNA Start point RNA nucleotides RNA polymerase Initiation RNA polymerase 5¢ 3¢ 5¢ 3¢ Template strand of DNA RNA tran- script Unwound DNA 3¢ 3¢ end Elongation Rewound DNA 5¢ 5¢ 3¢ 3¢ 3¢ 5¢ 5¢ RNA transcript Termination Direction of transcription (“downstream”) 5¢ Template strand of DNA 5¢ 3¢ 3¢ 5¢ 5¢ 3¢ Newly made RNA Completed RNA transcript

11. Eukaryotic cells modify mRNA after transcription • The outcome of the transcription process in eukaryotic cells is the pre-mRNA • Enzymes in the eukaryotic nucleus modify pre-mRNA before it is dispatched to the cytoplasm, through the pores of the nucleus, as mRNA

12. Translation is the RNA-directed synthesis of a polypeptide: Molecular Components of Translation 3¢ Amino acid attachment site • A cell translates an mRNA message into protein with the help of transfer RNA (tRNA) • Molecules of tRNA are not identical: • Each carries a specific amino acid on one end • Each has an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA 5¢ Hydrogen bonds Anticodon Two-dimensional structure

13. 3¢ tRNA Amino acid attachment site 5¢ Hydrogen bonds Anticodon Two-dimensional structure Amino acid attachment site 5¢ 3¢ Hydrogen bonds 3¢ 5¢ Anticodon Anticodon Three-dimensional structure Symbol used in this book

14. Ribosomes • Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis • The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA)

15. Ribosomes • A ribosome has three binding sites for tRNA: • The P site holds the tRNA that carries the growing polypeptide chain • The A site holds the tRNA that carries the next amino acid to be added to the chain • The E site is the exit site, where discharged tRNAs leave the ribosome

16. Ribosomes P site (Peptidyl-tRNA binding site) A site (Aminoacyl- tRNA binding site) E site (Exit site) E P A Large subunit mRNA binding site Small subunit Schematic model showing binding sites

17. LE 17-16c Amino end Growing polypeptide Next amino acid to be added to polypeptide chain E tRNA mRNA 3¢ Codons 5¢ Schematic model with mRNA and tRNA

18. Translation (Building a Polypeptide) • The three stages of translation: • Initiation • Elongation • Termination • All three stages require protein “factors” that aid in the translation process

19. Ribosome Association and Initiation of Translation • The initiation stage of translation brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits Large ribosomal subunit P site Met Met Initiator tRNA GTP GDP A E mRNA 5¢ 5¢ 3¢ 3¢ Start codon Small ribosomal subunit mRNA binding site Translation initiation complex

20. Ribosome Association and Initiation of Translation • First, a small ribosomal subunit binds with mRNA and a special initiator tRNA • Then the small subunit moves along the mRNA until it reaches the start codon (AUG) • Proteins called initiation factors bring in the large subunit so the initiator tRNA occupies the P site

21. Initiation of Translation Large ribosomal subunit P site Met Met Initiator tRNA GTP GDP A E mRNA 5¢ 5¢ 3¢ 3¢ Start codon Small ribosomal subunit mRNA binding site Translation initiation complex

22. Amino end of polypeptide E 3¢ mRNA P site A site Ribosome ready for next aminoacyl tRNA 5¢ GTP 2 2 GDP • Polypeptide • Elongation E E P A P A GDP GTP E P A

23. Termination of Translation • Termination occurs when a stop codon in the mRNA reaches the A site of the ribosome • The A site accepts a protein called a release factor • The release factor causes the addition of a water molecule instead of an amino acid • This reaction releases the polypeptide, and the translation assembly then comes apart

24. Release factor Free polypeptide 5¢ 3¢ 3¢ 5¢ 5¢ Stop codon (UAG, UAA, or UGA) When a ribosome reaches a stop codon on mRNA, the A site of the ribosome accepts a protein called a release factor instead of tRNA. Termination of Translation 3¢ The release factor hydrolyzes the bond between the tRNA in the P site and the last amino acid of the polypeptide chain. The polypeptide is thus freed from the ribosome. The two ribosomal subunits and the other components of the assembly dissociate. From DNA to Protein http://www.youtube.com/watch?v=D3fOXt4MrOM

25. Polysomes • Several ribosomes can translate an mRNA at the same time, forming what is called a polysome. • Polysomes make it possible to produce many polypeptides simultaneously from a single mRNA.

26. Concept 17.7: Point mutations can affect protein structureandfunction • Mutations are changes in the genetic material of a cell or virus • Point mutations are chemical changes in just one base pair of a gene • The change of a single nucleotide in a DNA template strand leads to production of an abnormal protein

27. Wild-type hemoglobin DNA Mutant hemoglobin DNA 3¢ 5¢ 3¢ 5¢ mRNA mRNA 5¢ 3¢ 5¢ 3¢ Normal hemoglobin Sickle-cell hemoglobin