1. Transcription and Translation

2. Transcription and Translation
What is transcription? Explain what happens during transcription.
What is translation? Explain what happens during translation.
Explain how transcription and translation are related to DNA replication.
If you begin with a parent DNA strand of A A T G C A G T, what will the complementary mRNA strand be? (Think about it before you answer.)

6. Compare DNA and RNA DNA- Deoxyribonucleic Acid RNA- Ribonucleic Acid
Bases-cytosine, guanine, adenine and thymine
Double stranded
Function-store genetic information
4. Deoxyribose sugar
Base-Cytosine, guanine, adenine and uracil
Single stranded
Functions- See next slide
Ribose sugar

7. RNA Functions Functions-
rRNA-ribosomal RNA (makes up about 60% of ribosomal structure
mRNA-messenger RNA (record information from DNA and carry it to ribosomes)
tRNA-transfer RNA (delivers amino acids to proteins at the ribosome to extend the chains)

8. RNA nucleotide
DNA nucleotide
deoxyribose

9. Comparison of RNA and DNA sugars
Deoxyribose
Ribose

10. Compared structures of DNA and RNA

11. RNA single strand with hairpin loop
Although it looks like a double helix, it is one strand wrapped around itself. It is similar to a twisted bobby pin.

12. Reading Quiz Transcription is the process of making:
a. RNA b. tRNA c. mRNA d. rRNA
2. An intron is found in:
a. DNA b. RNA c. mRNA d. tRNA
3. The enzyme used in transcription is:
a. RNA primase b. RNA polymerase
c. DNA polymerase d. a and b are both correct
4. How many bases make a codon?
a. 1 b. 2 c. 3 d. 4
5. Translation occurs in the:
a. nucleus b. cytoplasm c. both

13. Transcription

21. Transcription (Initiation)
RNA polymerase binds to the promoter site and begins base pairing the transcription unit (area of DNA that will be transcribed
The area where transcription factors and RNA polymerase are bound to the promoter is called the Transcription initiation complex
Promoter =region of DNA where RNA polymerase attaches and initiates transcription
-Determines which strand of DNA will serve as the template

22. Transcription (Initiation)
TATA box -promoter DNA sequence
-the actual sequence is 5'-TATAAA-3'
-RNA polymerase binding site

29. Transcription (RNA strand elongation)
RNA polymerase moves along DNA template
It unwinds DNA bases at a time
RNA polymerase adds nucleotides in the 5’→3’ direction
As RNA polymerase moves along, the DNA double helix reforms
The new section of RNA ‘peels away’ as the double helix reforms

34. Transcription (termination)
Transcription stops when RNA polymerase reaches a section of DNA called the terminator
Terminator sequence = AAUAAA
Next, the RNA strand is released and RNA polymerase dissociates from the DNA
The RNA strand will go through more processing

35. Sense vs. Antisense DNA strands
The DNA double helix has two strands
Only one of them is transcribed
The transcribed strand is the antisense strand
The non transcribed strand is the sense strand
mRNA is complementary to the anitsense strand

36. Sense vs. Antisense DNA strands
F. The 5’ end of the RNA nucleotides are added to the 3’ end of the growing chain
G. RNA nucleotides are linked together in the same fashion as DNA molecules

37. Because mRNA becomes degraded by other enzymes, it must go through mRNA processing.

39. RNA Processing

40. RNA splicing (in eukaryotes)
Introns-non-coding sections of nucleic acid found between coding regions
Exons-coding regions of nucleic acids
(eventually these are expressed as amino acids)

41. RNA splicing (in eukaryotes)

44. Alternative Splicing

47. Exit Slip
Where exactly in the cell do the following processes take place?
Transcription
Translation
Distinguish between introns and exons.
Why is it important to modify the mRNA after it is produced by transcription in eukaryotic cells?

48. Translation

58. Translation Translation-forming of a polypeptide
-uses mRNA as a template for a.a. sequence
-4 steps (initiation, elongation, translocation and termination)
-begins after mRNA enters cytoplasm
-uses tRNA (the interpreter of mRNA)

59. Translation B. Ribosomes -made of proteins and rRNA
-each has a large and small subunit
-each has three binding sites for tRNA on its surface
-each has one binding site for mRNA
-facilitates codon and anticodon bonding
-components of ribosomes are made in the nucleus and exported to the cytoplasm where they join to form one functional unit

60. #8. Translation B. Ribosomes (continued)
-the three tRNA binding sites are:
1. A site=holds tRNA that is carrying the next amino acid to be added
2. P site= holds tRNA that is carrying the growing polypeptide chain
3. E site= where discharged tRNAs leave the ribosome

61. Ribosomal structure E A P Large subunit Exit site
Peptidyl-tRNA binding site
Aminoacyl-tRNA binding site 5’ 3’
mRNA
Small subunit

62. Translation C. The genetic code
Four RNA nucleotides are arranged 20 different ways to make 20 different amino acids
Nucleotide bases exist in triplets
Triplets of bases are the smallest units that can code for an a.a.
3 bases = 1 codon = 1 a.a.
There are 64 possible codes (64=43)

63. Translation C. The genetic code
Most of the 20 a.a. have between 2 and 4 possible codes
The mRNA base triplets are codons
In translation the codons are decoded into amino acids that make a polypeptide chain
It takes 300 nucleotides to code for a polypeptide made of 100 amino acids (Why?)

64. Translation C. The genetic code (continued)
61 of 64 codons code for a.a.
Codon AUG has two functions
-codes for amino acid methionine (Met)
-functions as a start codon
mRNA codon AUG starts translation
The three ‘unaccounted for’ codons act as stop codons (end translation)

65. Translation D. How it works DNA (antisense) A C C A A A C C G
mRNA (transcription)
U G G U U U G G C
polypeptide (translation)
Trp - Phe Gly-

66. Translation E. More on tRNA
tRNA is transcribed in the nucleus and must enter the cytoplasm
tRNA molecules are used repeatedly
Each tRNA molecule links to a particular mRNA codon with a particular amino acid
When tRNA arrives at the ribosome it has a specific amino acid on one end and an anticodon on the other
Anticodons (tRNA) bond to codons (mRNA)
p. 304 (red book)

67. = tRNA diagrams Where the a.a. attaches
Although we draw tRNA in a clover shape it’s true 3-D conformation is L-shaped.
Hydrogen bonds
Anticodon
tRNA diagrams

68. Translation (Initiation)
A. Initiation
1. Brings together mRNA, tRNA (w/ 1st a.a.) and ribosomal subunits
2. Small ribosomal subunit binds to mRNA and an initiator tRNA
-start codon= AUG
-start anticodon-UAC
-small ribosomal subunit attaches to 5’ end of mRNA

70. #9. Translation (Initiation)
B. Initiation
2. (continued)
-downstream from the 5’ end is the start codon AUG (mRNA)
-the anticodon UAC carries the a.a. Methionine
3.After the union of mRNA, tRNA and small subunit, the large ribosomal subunit attaches
4. Initiation is complete

71. Translation (Initiation)
B. Initiation
5. The intitiator tRNA and a.a. will sit in the P site of the large ribosomal subunit
6. The A site will remain vacant and ready for the aminoacyl-tRNA

72. Translation (Initiation)

73. Translation (Elongation)
Amino acids are added one by one to the first amino acid (remember, the goal is to make a polypeptide)
Step 1- Codon recognition
mRNA codon in the A site forms hydrogen bonds with the tRNA anitcodon
Step 2- Peptide bond formation
The ribosome catalyzes the formation of the peptide bonds between the amino acids (the one already in place and the one being added)
The polypeptide extending from the P site moves to the A site to attach to the new a.a.

74. Translation (Translocation)
The tRNA w/ the polypeptide chain in the A site is translocated to the P site
tRNA at the P site moves to the E site and leaves the ribosome
The ribosome moves down the mRNA in the 5’→3’ direction

75. Translation (Termination)
Happens at the stop codon
Stop codons are UAA, UAG and UGA
-they do not code for a.a.
C. The polypeptide is freed from the ribosome and the rest of the translation assembly comes apart
D. Animation (you move it)
E. Animation (you watch it)
F. Animation (McGraw-Hill)

76. Gene expression Jacob and Monad (1961)
-studied control of protein synthesis in E. coli and lactose digesting enzymes
-found that E. coli do not produce lactose digesting enzymes when grown in a medium without lactose
-when bacteria were placed in a lactose environment, enzymes were found within minutes

77. Gene expression B. Genes can be switched on or off as necessary
-a gene that is ‘on’ will be transcribed
-in E.coli, the enzyme lactase will be produced if the gene is ‘on’
-if the gene is ‘off’ mRNA will not be created and translation can not occur

78. Gene expression C. The operon model -proposed by Jacob and Monad
-explains how genes switch on and off
-operon=promoter, operator and structural genes
-lac operon is found in E.coli

79. Gene expression
D. The lac operon

80. Gene expression D. The lac operon (no lactose)
-lactose is absent, repressor is active, operon is off, no mRNA is produced, RNA polyermase cannot bind because it is blocked by the repressor that has bound to the operator

81. Gene expression D. The lac operon (lactose is present)
-lactose is present, repressor is inactive, operon is on, mRNA is transcribed, RNA polymerase binds to operator
-an isomer of lactose binds to the repressor and changes its shape
-this prevents it from binding to the operator
-lactase is produced

82. #11. Compare Transcription in Eukaryotes and Prokaryotes
Link