1. Chapter 17: From Gene to Protein

2. DNA makes RNA makes Protein
Summary of Transcription and Translation
A triplet code in DNA is transcribed into a codon sequence in mRNA
This pre-RNA is processed in the nucleus
The codon sequence is then translated into an amino acid sequence at the ribosome

3. DNA makes RNA makes Protein
If the strand of DNA triplets to be transcribed is: 5’ – AAA TAA CCG GAC – 3’
Then the strand of mRNA codons that forms is: 3’ – UUU AUU GGC CUG – 5’
The tRNA anticodon strand complementary to the mRNA strand is:
AAA UAA CCG GAC

4. Transcription Transcription is the process by which DNA makes RNA
mRNA: messenger; carries messages directly from DNA to cytoplasm
tRNA: shaped like cloverleaf; carries amino acids to the mRNA at the ribosome
rRNA: ribosomal; makes up ribosome

5. 3 Stages: initiation, elongation & termination
RNA polymerase recognizes and binds to DNA at the promoter region (facilitates transcription of a gene)
A collection of proteins called transcription factors recognize a key area within the promoter called the TATA box and mediates the binding of RNA polymerase to DNA
Transcription factors + RNA polymerase + promoter = transcription initiation complex

6. 3 Stages: initiation, elongation & termination
Now that RNA polymerase has attached to the promoter, transcription of the DNA template can begin

7. 3 Stages: initiation, elongation & termination
RNA polymerase adds nucleotides to the 3’ end of a growing chain
RNA polymerase pries the two strands of DNA apart and attaches RNA nucleotides according to the base pairing rules: C with G, A with U
Stretch of DNA transcribed into mRNA molecule is called a transcription unit

8. 3 Stages: initiation, elongation & termination
Each unit consists of triplets of bases called codons (AAU, CGA), which code for specific amino acids

9. 3 Stages: initiation, elongation & termination
Final stage of transcription
Elongation continues for a short distance after the RNA polymerase transcribes the termination sequence (AAUAAA)
mRNA is now cut free from the DNA template

10. RNA Processing
Pre-RNA that has just formed cannot be shipped to the ribosome until it is processed by a series of enzymes
A 5’ cap consisting of modified guanine nucleotide is added to the 5’ end and helps RNA bind to the ribosome

11. RNA Processing
A poly (A) tail, consisting of a string of adenine nucleotides, is added to the 3’ strand
Protects RNA from degradation, helps ribosomes attach to RNA, facilitates release of RNA into cytoplasm
Noncoding regions of mRNA called introns or intervening sequences are removed by snRNPs and spliceosomes
Allows exons, or expressed sequences, to only leave the nucleus

12. Translation
Translation is the process by which the codons of an mRNA sequence are changed into an amino acid sequence
Amino acids are present in a pool in the cytoplasm
They are carried by tRNA molecules to the codons of the mRNA strand at the ribosome according to base pairing rules

13. Translation
One end of the tRNA molecules holds the specific amino acid; the other end bears a nucleotide triplet called the anticodon
Energy for this process is provided by GTP
AUG is the start codon; UAA, UGA and UAG are the stop codons

14. Translation
Some tRNA molecules have anticodons that can recognize two or more different codons
This happens because pairing rules for the third base of a codon are not as strict as they are for the first two bases ; this is called wobble
Example: UCU, UCC, UCA and UCG all code for serine

15. Initiation, Elongation & Termination
Initation
mRNA attaches to a subunit of the ribosome
The first codon is always AUG, which codes for methionine
Elongation
tRNA brings amino acids to the ribosome and a polypeptide chain is formed

16. Initiation, Elongation & Termination
The ribosome reaches one of three termination or stop codons
A release factor breaks the bond between the tRNA and the last amino acid of the polypeptide chain
Polypeptide is then freed and mRNA is broken down

17. Gene Mutation
Mutations are changes in genetic material that occur spontaneously and randomly
Mutagenic agents include toxic chemicals and radiation
Genetic disorders or hereditary diseases occur when mutations have adverse effects in somatic cells

18. Gene Mutation
Mutations occurring in gametes can be passed on to offspring and change the gene pool of a population
Mutations are the raw material for natural selection

19. Point Mutation Simples mutation caused by a base-pair substitution
Example:
THE FAT CAT SAW THE DOG
THE FAT CAT SAW THE HOG
Remember wobble
If the final base in a sequence is different, it may not call for a harmful mutation

20. Insertion or Deletion Deletion = loss of a letter
Insertion = addition of a letter
Both result in a frameshift
Example: Deletion
THE FAT CAT SAW THE DOG
THF ATC ATS AWT HED OG

21. Insertion or Deletion Example: Insertion
THE FAT CAT SAW THE DOG
THE FTA TCA TSA WTH EDO G
A mutated polypeptide is formed or no polypeptide is formed
When point mutations or frameshifts change a codon within a gene into a stop codon, a missense or nonsense mutation occurs