1. Chapter 17 From Gene to Protein

2. 17.1 – Genes specify proteins via transcription & translation
Gene Expression
DNA directs the synthesis of proteins (or RNA)
Includes translation & transcription
Proteins are the links between genotype and phenotype
One gene-one polypeptide hypothesis
Each gene codes for a polypeptide
Can be a protein or part of a protein

3. Messenger RNA (mRNA) In Eukaryotes In Prokaryotes
Produced during transcription
Carries the genetic message of DNA to the protein making machinery of the cell (ribosome)
In Eukaryotes
Transcription results in pre-mRNA, which undergoes RNA processing to yield the final mRNA
In Prokaryotes
Transcription directly makes mRNA
Transcription & Translation occur at the same time

5. Transcription Synthesis of RNA using DNA as a template
Occurs in the nucleus
Only one strand of DNA is transcribed (called template strand)
The mRNA produced is a complementary strand
The mRNA base triplets are called codons
Written in the 5’ to 3’ direction

6. The genetic code is redundant
More than one codon codes for the 20 Amino Acids
Read based on a consistent reading frame
Groups of 3 must be read in the correct groupings in order for translation to be successful

7. All 64 codons were deciphered by the mid-1960s
Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation

8. Translation Production of a polypeptide chain using mRNA
Occurs at the ribosomes
The instructions for the PP chain are written as a triplet code

10. The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals
Genes can be transcribed and translated after being transplanted from one species to another

12. 17.2 – Transcription is the DNA-directed synthesis of RNA
RNA polymerase
Enzyme that separates the two DNA strands
Connects the RNA nucleotides as they base-pair
Can add RNA nucleotides only to the 3’ end so it elongates in the 5’ to 3’ direction
Uracil replaces thymine

13. Promoter Terminator DNA sequence that RNA polymerase attaches
DNA sequence that signals the end of transcription

14. Transcription unit Entire stretch of DNA that is transcribed into RNA
May code for a polypeptide or an RNA such tRNA or rRNA

15. 3 stages of transcription
1) Initiation
2) Elongation
3) Termination

16. 1) Initiation In bacteria In Eukaryotes Transcription Factors
RNA polymerase recognizes & binds to the promoter
In Eukaryotes
RNA polymerase II cannot bind to the promoter without supporting help from proteins known as transcription factors
Transcription Factors
Assist the binding of RNA polymerase to the promoter, & the initiation of transcription

17. Transcription initiation complex
The whole complex of RNA polymerase II & transcription factors
A promoter called a TATA box is crucial in forming the initiation complex in eukaryotes

19. 2) Elongation
RNA polymerase moves along the DNA (untwists the double helix)
10 to 20 bases at a time
RNA nucleotides are continually added to the 3’ end of the growing chain
40 nucleotides per second
As the complex moves down the DNA strand, the double helix re-forms with the new RNA molecule straggling away from the DNA template

21. 3) Termination
RNA transcript is released & the polymerase detaches upon transcribing a terminator sequence in the DNA

23. 17.3 – Eukaryotic cells modify RNA after transcription
Modifications to RNA after transcription:
Adding a 5’ cap & a poly-A tail
Facilitate the export of mRNA from the nucleus
Help protect mRNA from degradation by enzymes
Facilitate the attachment of the mRNA to the ribosome

24. RNA splicing (in Eukaryotic Cells)
Large portions of the newly made RNA strand are removed – called INTRONS
The ones left behind are called EXONS & are spliced together by a spliceosome

26. Special RNA called small nuclear RNA (snRNA) aid the spliceosomes
Play a role by catalyzing the excision of the introns & joining the exons
When RNA is an enzyme it is called a RIBOZYME

27. 17.4 – Translation is the RNA-directed synthesis of a polypeptide
Utilizes mRNA, tRNA, & rRNA
tRNA
Transfers AA from a pool of AA in the cytoplasm to a ribosome
The ribosome accepts the AA & adds it into a growing PP chain
Each tRNA is specific for an AA
One one end, it binds to the AA & the other end has a triplet called an anticodon which allows it to pair with a codon on an mRNA

29. Codon – mRNA triplet (there are 64)
mRNa is read codon by codon & one AA is added to the chain for each codon read
The rules for base-pairing between the third base of a codon & the corresponding tRNA anticodon are not as strict as DNA & mRNA so it is called a wobble

30. rRNA complexes with proteins
Forms the 2 subunits that form ribosomes
Ribosomes have 3 binding sites for tRNA
P-site – holds the tRNA that carries the growing PP chain
A-site – holds the tRNA that carries the AA that will be added next
E-site – exit site for tRNA

32. 3 stages of Translation
1) Initiation
2) Elongation
3) Termination

33. Initiation
A) a small ribosomal subunit binds to mRNA in a way that the first codon of the mRNA strand (AUG) is placed in the proper position
B) tRNA with the anticodon UAC (carries the AA methionine), hydrogen bonds to the first codon (proteins called initiation factors aid)
C) Large subunit of ribosome attaches
Allows the tRNA with methionine to attach to the P- site
The A-site will now be available for the next tRNA with the 2nd AA

35. 2) Elongation A) Codon Recognition B) Peptide bond formation
The codon in the A-site is matched by the incoming tRNA anticodon
B) Peptide bond formation
The incoming AA in the A-site forms a peptide bond with the existing chain of AA held in the P-site
Catalyzed by an rRNA (ribozyme)
C) Translocation
Occurs when tRNA in the A-site is moved to the P-site & the tRNA in the P-site is moved to the E-site
A-site is now clean and is ready for another AA

37. 3) Termination A stop codon in the mRNA is reached & translation stops
A protein called release factor binds to the stop codon & the PP is freed from the ribosome
PP’s then will fold to assume their specific shape
May be modified further to make them functional
The destination of the protein is determined by the sequence of about 20 AA’s at the leading end of the PP chain (signal peptide)

39. 17.5 – Point mutations can affect structure & function
Point mutation are alterations of just one base pair – 2 basic types:
1) Base-pair substitutions
The replacement of one nucleotide & its complementary base pair in the DNA with another pair of nucleotides
Missense – enable the codon to still code for an AA although it may not be the correct one
Nonsense – change a regular AA codon into a stop codon

42. 2) Insertions & deletions
Additions & loses of nucleotide pairs in genes
If they interfere with the codon groupings they can cause a frameshift mutation
Causes the mRNA to be read incorrectly

45. Mutagens
Substances or forces that interact with DNA in ways that cause mutations
X-rays & chemicals