1. Transcription and Translation
Protein Synthesis
Transcription and Translation
AP Biology
Unit 2

2. Flow of Genetic Information
All living organisms use DNA to synthesize RNA to make proteins
Same two-step process:
Transcription  Translation
Some antibiotics inhibit protein synthesis in bacteria.
Ex. Neomycin (the antibiotic in Neosporin) interferes with the process of translation)

3. Genes and Chromosomes DNA is organized into chromosomes
Humans have 46 chromosomes in each cell.
Genes are “coding” regions of DNA
Each gene is the code for how to make a specific protein.
Human chromosomes are made up of
DNA
Histone proteins that DNA is wound around

4. Structure of DNA The carbons in the 5C sugar each have a number
Start to the right of the oxygen and go around clockwise
phosphate
Sugar
Base 5 1
This gives the nucleotide 2 distinct ends
5’ end (closer to carbon 5)
3’ end (closer to carbon 3) 4 3 2

5. A way to remember it: Human Nucleotide 
Phosphate
Base

6. Nucleic Acid Structure
DNA is double stranded
Hydrogen bonds between bases
A pairs with T
C pairs with G
Note that there are 2 H bonds between A and T, 3 H bonds between C and G
The strands are antiparallel
One strand runs 5’-3’
The other runs 3’-5’
Image taken without permission from

7. Question… Why can’t the DNA strands be parallel (both running 5’-3’)?
This wouldn’t allow the bases to be near each other to hydrogen bond.

8. Transcription DNA is transcribed into 3 kinds of RNA
mRNA = messenger RNA (the RNA code used to make protein)
tRNA = transfer RNA (participates in translation)
rRNA = ribosomal RNA (part of ribosomes)
RNA Polymerase is the enzyme that transcribes the DNA into RNA

9. Initiation How transcription starts
RNA Polymerase recognizes a promoter sequence on the DNA
RNA Polymerase binds to the promoter
DNA is unwound to start transcription
What kind of bonds are being broken to unwind/separate the strands of DNA?
Hydrogen bonds

10. Promoter Sequences
In prokaryotes, RNA Polymerase must find these sequences:
+ 1 is the first base in the RNA (where the actual transcription of DNA starts from) 5’ 3’ 5’ 3’

11. Eukaryotic Promoter Sequences
In eukaryotes, the RNA polymerase must find the following sequences:
Eukaryotic genes can also have enhancer sequences to help RNA polymerase bind
We’ll talk about these a little later– don’t worry about them right now 

12. What do you think this diagram shows about transcription?
Bases changed to…

13. Promoters
In order for RNA Polymerase to recognize it, the promoter sequences
Must be the correct sequence of bases (small changes OK)
Must be correctly spaced apart
If these conditions aren’t met, RNA Polymerase can’t bind to the DNA and no transcription occurs.

14. Elongation How the RNA strand is built
RNA Polymerase matches the appropriate (complementary) nucleotides to the DNA template strand
Template strand = the actual strand RNA Polymerase uses to build RNA
Coding (Nontemplate) strand = not used for building RNA, but has the same sequence as the RNA.

15. Building the RNA The RNA Polymer grows in a 5’-3’ direction
RNA Polymerase only adds new nucleotides on to the 3’ end.
Considering this, in what direction must the template strand of DNA be running?
3’-5’ (since it is building its complement) 5’ 3’ 5’ 3’

16. Question …
In terms of the sequence, how will the RNA differ from the sequence of the coding strand in the DNA?
T’s are replaced with U’s

17. Termination How transcription of RNA ends
RNA Polymerase recognizes a termination signal on the DNA template
Usually a long string of A’s or a series of A’s and T’s
RNA Polymerase falls off the DNA template
Stability of mRNA is minutes  hours (depends on type of cell and RNA)
In some cases, a helper protein pulls the transcript away.

18. Question…
How do the specific chemical properties of the termination sequence cause termination to occur?
There are only 2 hydrogen bonds between A and T/U
With a string of A’s and U’s, there are much fewer bonds to hold the DNA template and RNA together  they separate  transcription ends

19. Translation Using the mRNA code to create the appropriate protein.
Occurs in the cytoplasm/on the rough ER
Sequence of 3 nucleotides codes for a particular amino acid = codon
64 different codons

20. Question… Why can’t 1 or 2 nucleotides code for an amino acid?
Not enough combinations to code for all 20 amino acids
With 1 nucleotide  only 4 possibilities (A, C, G, U)
With 2 nucleotides  only 4 x 4 = 16 possibilities (AA, AU, AC, AG, CC, …)

21. The codon table

22. Amino acid attached here
tRNA
Amino acid attached here
tRNA brings the correct amino acid to match with the mRNA codon
Each tRNA holds a specific amino acid and has a particular anticodon.
Aminoacyl tRNA synthetases are enzymes that attach the correct amino acids to the tRNA

23. Question…
For the anticodon shown in the diagram, what would the complementary codon on the mRNA be?
5’ UUC 3’
Which amino acid is attached to this tRNA?
Phe

24. Ribosomes Made up of 2 subunits Composed of rRNA and protein
Not specific to any particular protein– can be used to translate any RNA into protein
Workbench for translation – holds mRNAs and tRNAs in the correct positions to assemble protein.

25. Ribosomes 3 sites on the ribosome
A site = where tRNA first binds to mRNA
P site = where the amino acid is added on to the polypeptide chain
E site = exit site

26. Translation Begins with the Start codon = AUG
Codes for methionine (Met)
Not the same thing as +1

27. Translation
Ribosome moves along mRNA in a 5’->3’ direction, catalyzing the translation of the mRNA into protein
breaks bond between tRNA and amino acid
creates a new peptide bond to link it to polypeptide chain

28. Question…
How does the mRNA know if it is correctly matched to the tRNA?
Hydrogen bonding between the bases is correct

29. Stopping Translation Ribosome is released when a stop codon is reached
UAA, UAG, UGA = stop codons (don’t code for any tRNA anticodons)
A release factor binds to the mRNA instead
Ribosome breaks apart, mRNA and protein are released

30. Summary of Protein Synthesis
In Eukaryotes
How is it different in prokaryotes?

31. Why is this important?
1. Changes in the DNA sequence will lead to changes in the transcribed _________.
2. This results in a different codon which may code for a different ______________.
3. A different ___________ means a different R group.
4. A different R group may have different chemical properties.
5. These different chemical properties may lead to a different protein ____________.
6. A different protein structure may affect its _________!
7. See how this is all starting to connect! Exciting!!! 

32. Why is this important?
1. Changes in the DNA sequence will lead to changes in the transcribed RNA.
2. This results in a different codon which may code for a different amino acid.
3. A different amino acid means a different R group.
4. A different R group may have different chemical properties.
5. These different chemical properties may lead to a different protein structure.
6. A different protein structure may affect its function!
7. See how this is all starting to connect! Exciting!!! 

33. microRNAs and RNAi
Small, single stranded RNA molecules (miRNAs and siRNAs)
microRNA = miRNA
Small interfering RNA = siRNA
Bind to complementary sequences in mRNA molecules
Can control the expression of (translation of) specific RNA molecules

34. Question… How will microRNAs disrupt translation?
Block translation by creating a physical road block
RNA-RNA binding also marks the mRNA for degradation
Called RNAi = RNA interference