1. Gene Expression and Control
Chapter 7
Part 1

2. 7.1 Impacts/Issues Ricin and Your Ribosomes
The ability to make proteins is critical to all life processes – ricin kills because it inactivates ribosomes that assemble proteins

3. 7.2 The Nature of Genetic Information
DNA carries all the genetic information needed to build a new individual
Genetic information consists of base sequences
Genes are subunits of that sequence
Gene
Part of a DNA base sequence
Specifies structure of an RNA or protein product

4. From Gene to RNA to Protein
Gene expression involves transcription (DNA to RNA), and translation (mRNA, or messenger RNA, to protein)
Gene expression
Process by which the information in a gene becomes converted to an RNA or protein product

5. Transcription
A gene’s nucleotide base sequence encodes instructions for building an RNA or protein product
A cell transcribes the base sequence of a gene into mRNA
mRNA carries a protein-building message

6. Transcription Transcription Messenger RNA (mRNA)
Process by which an RNA is assembled from nucleotides using the base sequence of a gene as a template
Messenger RNA (mRNA)
Type of RNA that has a protein-building message

7. Translation
Translation requires the participation of tRNA (transfer RNA) and rRNA (ribosomal RNA)
Translation
Process by which a polypeptide chain is assembled from amino acids in the order specified by an mRNA

8. RNA and DNA Nucleotides

9. An RNA nucleotide: guanine (G), or guanosine triphosphate
base (guanine)
3 phosphate groups
sugar (ribose)
Figure 7.2
Comparison between (A) an RNA nucleotide and (B) a DNA nucleotide.
An RNA nucleotide: guanine (G), or guanosine triphosphate
Fig. 7-2a, p. 117

10. A DNA nucleotide: guanine (G), or deoxyguanosine triphosphate
base (guanine)
3 phosphate groups
sugar (deoxyribose)
Figure 7.2
Comparison between (A) an RNA nucleotide and (B) a DNA nucleotide.
A DNA nucleotide: guanine (G), or deoxyguanosine triphosphate
Fig. 7-2b, p. 117

11. 7.3 Transcription: DNA to RNA
Base-pairing rules in DNA replication also apply to RNA synthesis in transcription, but RNA uses uracil in place of thymine

12. The Process of Transcription
In transcription, RNA polymerase binds to a promoter in the DNA near a gene
RNA polymerase
Enzyme that carries out transcription
Promoter
In DNA, a sequence to which RNA polymerase binds

13. The Process of Transcription
Polymerase moves along the DNA, unwinding the DNA so it can read the base sequence
RNA polymerase assembles a strand of RNA by linking RNA nucleotides in the order determined by the base sequence of the gene
The new mRNA is a copy of the gene from which it was transcribed

14. Transcription: DNA to RNA

15. promoter sequence in DNA
RNA polymerase
gene region
promoter sequence in DNA
Figure 7.3: Animated!
Transcription assembles a strand of RNA from nucleotides using a gene region in DNA as a template.
1 RNA polymerase binds to a promoter in the DNA. The binding positions the polymerase near a gene. In most cases, the base sequence of the gene occurs on only one of the two DNA strands. Only the DNA strand complementary to the gene sequence will be translated into RNA.
Fig. 7-3a, p. 118

16. RNA DNA winding up DNA unwinding
Figure 7.3: Animated!
Transcription assembles a strand of RNA from nucleotides using a gene region in DNA as a template.
2 The polymerase begins to move along the DNA and unwind it. As it does, it links RNA nucleotides into a strand of RNA in the order specified by the base sequence of the DNA. The DNA winds up again after the polymerase passes. The structure of the “opened” DNA at the transcription site is called a transcription bubble, after its appearance.
Fig. 7-3b, p. 118

17. direction of transcription
Figure 7.3: Animated!
Transcription assembles a strand of RNA from nucleotides using a gene region in DNA as a template.
3 Zooming in on the gene region, we can see that RNA polymerase covalently bonds successive nucleotides into an RNA strand. The base sequence of the new RNA strand is complementary to the base sequence of its DNA template strand, so it is an RNA copy of the gene. Figure It Out: After the guanine, what is the next nucleotide that will be added to this growing strand of RNA?
Answer: Another guanine (G)
Fig. 7-3c, p. 119

18. Gene transcription details

19. Three Genes, Many RNA Polymerases
Many polymerases can transcribe a gene region at the same time

20. Pre-mRNA transcript processing

21. Transcription

22. Transcription

23. 7.4 RNA Players in Translation
Three types of RNA are involved in translation: mRNA, rRNA, and tRNA
mRNA produced by transcription carries protein-building information from DNA to the other two types of RNA for translation

24. mRNA and the Genetic Code
The information in mRNA consists of sets of three nucleotides (codons) that form “words” spelled with the four bases A, C, G, and U
Codon
In mRNA, a nucleotide base triplet that codes for an amino acid or stop signal during translation

25. mRNA and the Genetic Code
Sixty-four codons, most of which specify amino acids, constitute the genetic code
20 amino acids in proteins; most have more than one codon
Genetic code
Sixty-four mRNA codons; each specifies an amino acid or a signal to start or stop translation

26. The Genetic Code

27. Animation: Genetic code

28. Translating mRNA to Amino Acids

29. rRNA and tRNA – the Translators
Ribosomes and transfer RNAs (tRNA) interact to translate an mRNA into a polypeptide
Ribosomes consist of two subunits of rRNA and structural proteins
Ribosomal RNA (rRNA)
A type of RNA that becomes part of ribosomes

30. Ribosomes
During translation, one large and one small ribosomal subunit (rRNA) converge as a ribosome on an mRNA
rRNA reads the mRNA and acts as an enzyme to form peptide bonds between amino acids, assembling them into a polypeptide chain

31. A Ribosome

32. tRNA
tRNAs deliver amino acids to ribosomes in the order specified by mRNA
Transfer RNA (tRNA)
Type of RNA that delivers amino acids to a ribosome during translation

33. tRNA Each tRNA has two attachment sites Anticodon
An anticodon that can base-pair with a codon
A site that binds to the kind of amino acid specified by the codon
Anticodon
Set of three nucleotides in a tRNA
Base-pairs with mRNA codon

34. tRNA for Tryptophan

35. amino acid attachment site
anticodon
amino acid attachment site
Figure 7.7
tRNA. (A) Models of the tRNA that carries the amino acid tryptophan. Each tRNA’s anticodon is complementary to an mRNA codon. Each also carries the amino acid specified by that codon. (B) During translation, tRNAs dock at an intact ribosome. Here, three tRNAs are docked at the small ribosomal subunit (the large subunit is not shown, for clarity). The anticodons of the tRNAs line up with complementary codons in an mRNA (shown in red).
Fig. 7-7a, p. 121

36. Structure of a tRNA

37. 7.5 Translating the Code: RNA to Protein
Translation, the second part of protein synthesis, occurs in the cytoplasm of all cells
Translation is an energy-requiring process that converts the protein-building information carried by an mRNA into a polypeptide

38. Three Stages of Translation
Initiation
mRNA joins with an initiator tRNA and two ribosomal subunits
Elongation
Ribosome joins amino acids delivered by tRNAs in the order specified by mRNA codons
Termination
Polymerase encounters a stop codon; mRNA and polypeptide are released; ribosome disassembles

39. Elongation

40. start codon (AUG) initiator tRNA first amino acid of polypeptide
peptide bond
Stepped Art p

41. Polysomes
In cells making a lot of protein, many ribosomes may simultaneously translate the same mRNA
Polysome
Cluster of ribosomes that are simultaneously translating an mRNA

42. Translation in Eukaryotes
Transcription 1 2
ribosome subunits
RNA transport
tRNA
5 Polysomes
3 Convergence of RNAs
mRNA
4 Translation
Figure 7.8: Animated!
Translation in eukaryotes. 1 In eukaryotic cells, RNA is transcribed in the nucleus. 2 Finished RNA moves into the cytoplasm through nuclear pores. 3 Ribosomal subunits and tRNA converge on an mRNA. 4 A polypeptide chain forms as the ribosome moves along the mRNA, linking amino acids together in the order dictated by the mRNA codons. 5 Many ribosomes can translate an mRNA at the same time.
polypeptide
Fig. 7-8, p. 122

43. Animation: Translation

44. The major differences between prokaryotic and eukaryotic protein synthesis

45. Overview of transcription and translation