1. DNA Transcription & Translation

2. *How do genes work?
We’ve seen how DNA is replicated, but still haven’t learned exactly how genes work!
The first step in understanding how genes work is to know how to “decode” the DNA.

3. Decoding DNA The decoder: RNA (Ribonucleic acid)
Long stranded like DNA, except:
Ribose instead of Deoxyribose
Uracil instead of Thymine
Single stranded instead of double stranded

5. Types of RNA
The main job of all RNA types is protein synthesis (creation)
3 types: (*You will learn what each does later)
Messenger RNA Transfer RNA Ribosomal RNA
mRNA tRNA rRNA

6. Protein Creation DNA holds the instructions for making proteins
Proteins do the cell’s work
Two steps required for protein synthesis (creation)
TRANSCRIPTION
TRANSLATION

7. Transcription
TRANSCRIPTION is the process by which a DNA gene is “rewritten” into RNA

8. Transcription A gene is only a segment of DNA
Only the gene is transcribed, not all of the DNA (like replication)
DNA
Transcription
mRNA

9. Transcription 3 phases: Initiation (“the beginning”)
Enzymes recognize appropriate gene in DNA
Elongation (“the middle”)
Enzymes copy gene into mRNA
Termination (“the end”)
Enzymes unbind from DNA

12. Why re-write the instructions?
DNA is like the “master plan” for the construction of a house
Too important to bring to the work site and risk being damaged or lost
RNA is like an inexpensive, disposable copy of the master plan
Used at the work site (the ribosomes)

13. Transcription INITIATION
RNA POLYMERASE (RP) recognizes a promoter on the DNA sequence
PROMOTER: base sequence that signals the start of a gene
DNA is split by RP and one strand is used as a template

14. Animation #1
Animation #2

15. Transcription
ELONGATION:
RP makes an mRNA copy of the gene

16. Transcription TERMINATION RP reaches TERMINATOR SEQUENCE
Signals it to unbind and stop transcription

18. Transcription
After transcription is done you end with one mRNA copy of a gene
Some of the mRNA must be “spliced out” (removed) before we translate it

19. mRNA Processing
Pieces of the mRNA that don’t contain genes (called INTRONS) are removed by a SPLICEOSOME

20. mRNA Processing
Pieces of DNA that do code for genes (called EXONS) are left and pieced together

21. The Genetic Code
The mRNA strand is then used to make proteins in the process of TRANSLATION

22. The Genetic Code The “language” of mRNA is known as the genetic code
The 4 “letters”
(A-G-C-U) are used to write “words” that correspond to different amino acids

23. The Genetic Code “Words”are written using 3 letters
Every 3 letters = 1 codon
Every 1 codon = 1 amino acid

24. The Genetic Code
Because there are four different bases, there are 64 (43) different codon combinations
Certain codons code for the same amino acid

25. Translation Shortly after mRNA is transcribed, translation begins.
The sequence of bases in mRNA serves as instructions for the order in which amino acids should be joined

26. Translation
Translation: The decoding of an mRNA message and creation of a polypeptide chain (protein)
RIBOSOMES are the factories that assemble the proteins

27. Translation INITIATION mRNA in the cytoplasm attaches to a ribosome
ELONGATION
The ribosome “reads” the codons and attaches the proper amino acids
Stop codon
Start codon

28. Transfer RNA
Transfer RNA brings the correct amino acids for the ribosome to use
Has an ANTI – CODON on one end, and an amino acid on the other
Anti-codon has a sequence complementary to an mRNA codon

30. Translation TERMINATION
The ribosome continues along until it reaches a stop codon
The amino acid chain is then released and allowed to fold into a protein
Protein then performs its job within the cell

31. Translation
Translation can occur at many different spots along the mRNA strand, creating many proteins at one time.

32. Let’s Try it ourselves
Transcribe and translate the following DNA sequence:
TAC GCA TGG AAT
AUG CGU ACC UUA

34. Translation Animations
Transcribe and Translate a Gene
Why do fireflies glow?

35. Let’s try it ourselves
Transcribe and translate the following DNA sequence:
TAC GCA TGG AAT
AUG CGU ACC UUA
Met – Arg – Thr – Leu

36. Mutations Every now and then cells make mistakes.
Inserting incorrect bases
Deleting bases
Adding extra bases
Mistakes that cause a change in genetic information are called mutations.

37. Point Mutations
Point mutations are mutations that affect one nucleotide.
3 different types:
Substitution
Insertion
Deletion

38. Substitution
Occurs when the wrong base is added to a growing nucleotide chain
Original DNA: TAC GCA TGG AAT
Original mRNA: AUG CGU ACC UUA
Original Protein: Met – Arg – Thr - Leu
Substitution
Original DNA: TAC GTA TGG AAT
Original mRNA: AUG CAU ACC UUA
Original Protein: Met – His – Thr - Leu

39. Insertion
Occurs when an extra base is added to a growing nucleotide chain
Original DNA: TAC GCA TGG AAT
Original mRNA: AUG CGU ACC UUA
Original Protein: Met – Arg – Thr - Leu
Insertion
Original DNA: TAT CGC ATG GAA T
Original mRNA: AUA GCG UAC CUU A
Original Protein: Ile – Ala – Tyr – Leu

40. Deletion When a base is deleted from a growing nucleotide chain
THE FAT CAT ATE THE RAT
TEF ATC ATA TET HER AT

41. Frameshift Mutations Mutations that cause a shift in the reading frame
Insertion
Deletion
More than one amino acid gets changed downstream
TACGCATGGAAT
TATCGTATGGAAT

42. Original DNA: TAC GCA TGG AAT Mutated DNA: TAC GCC TGG AAT
Silent Mutations
DNA changes that do not cause a change in the amino acid chain
Original DNA: TAC GCA TGG AAT
Mutated DNA: TAC GCC TGG AAT
mRNA: AUG CGG ACC UUA
Protein: Met – Arg – Thr - Leu