1. Chapter 17
From Gene to Protein

2. Connection Between Genes and Proteins
Transcription – synthesis of RNA using DNA Template
Messenger RNA is transcribed from DNA with the help of RNA polymerase; separates DNA and inserts the complementary base pairs to the 3’ end.
The genetic code to produce proteins contain adenine, uracil, guanine, and cytosine.
Transcription begins at initiation and ends at the termination region
Transcription factors- DNA binding proteins that bind to specific DNA sequences at the promoter to let RNA polymerase bind to the promoter so transcription can begin.
TATA box – nucleotide sequence in eukaryotes at the promoter that RNA polymerase II recognizes and binds to the DNA so transcription can begin.

3. Connection Between Genes and Proteins (Continued)
Elongation of the RNA strand
RNA polymerase II moves along the DNA exposing 10 nucleotides at a time inserting the appropriate bases to the 3’ end
Eukaryotic mRNA grows at nucleotide bases per second; as the strand elongates, it peels away fro the DNA template
Many RNA polymerase II enzymes may be working on the same DNA template
Mass production of proteins in a series.
Termination of Transcription
RNA polymerase II recognizes a termination sequence, usually AAUAAA in the mRNA strand
Prokaryote RNA is ready for translation, eukaryote RNA must be processed further.

4. Modification of mRNA in Eukaryotes
Alteration of pre-mRNA ends
Pre-mRNA – primary transcript that will be processed to functional mRNA
5’ cap – modified guanine nucleotide (guanine triphosphate that is added to the 5’ end
Protects growing mRNa from hydrolytic enzymes
Help ribosome subunits to recognize the 5’ end and the leader sequence (unstranslkated strand from the 5’ end to the start codon
Poly (A) Tail – sequence of adenine nucleotides to the 3’ end
Inhibits degradation in cytoplasm
Facilitate attachment to the small ribosomal subunit
Facilitate export of mRNA from nucleus to cytoplasm
Protects stop codon

5. Split Genes and RNA Splicing
Introns – noncoding sequences of mRNA
Exons – coding strands of mRNA
RNA Splicing – removal of introns and splicing of exons to form a mature mRNa strand that will move into the cytoplasm
Introns contain short boundary sequences that signal RNA splicing sites
Small nuclear ribonucleoproteins (snRNP’s)- proteins found in the nucleus that participate in Rna splicing; called “snurps”
Spliceosome – molecular complex that catalyzes splicing reactions that involve snurps
Cuts the 5’ and 3’ ends to splice exons together

6. Split Genes and RNA Splicing (Continued)
Ribozymes – RNA molecules that can break and from covalent bonds to produce mature rRNA and tRNA
Evolutionary importance of Introns
Cutting of introns and the splicing of exons may regulate export into the cytoplasm
Introns may allow a single gene to direct the synthesis of different proteins
All introns may be removed from a transcript in one case; but in another, one or more introns may be left in to be translated; each resulting protein may be different
With introns in, coding sequences have a higher recombination frequency than if introns were always left out

7. The Synthesis of Protein
Translation – RNA directed synthesis of a polypeptide
Transfer RNA (tRNA) – carries the anticodon to the mRNA and the amino acid
Obtains amino acid from cytoplasm’s pool
Recognizes the correct codon of mRNA
3’ bind the amino acid
Single-stranded with 80 base pairs that is double stranded at regions by hydrogen bonding between complimentary nucleotide sequences
Wobble Position – ability of a tRNA molecule to base pair with two or three different mRNA codons due to the 5’ end of the anitocodon

8. The Synthesis of Protein (Continued)
Ribosomes – coordinate the pairing of tRNA anticodons to mRNA codons
Two subunits, large and small, that are separated when not involved in synthesis
During synthesis, the small subunit is on the bottom
Ribosomes are constructed by the nucleolus
Ribosomes contain three bonding sites
P site – holds tRNA carrying the growing polypeptide chain
A site – holds next tRNA with the next amino acid
E site – discharges tRNA’s

9. The Synthesis of Protein (Continued)
Initiation
Small ribosome subunit binds to mRNA and initiator tRNA carrying methionine and the anticodon UAC (start codon)
Large subunit binds to the small u nit producing a translation complex.
5’ cap helps bind the leader to the small subunit in eukaryotes
Initiator tRNA binds to the P site and a site is ready for the next tRNA
Elongation
Codon Recognition – mRNA codon in the A site forms hydrogen bonds with the entering tRNA
Peptide bond formation – peptide bond is formed between the polypeptide in the P site and the new amino acid in the A site
Translocation – A site tRNA is moved to the P site, P site tRNA is moved to E site where it is detached
Termination
Termination Codon – triplet of UAA, UAG, and UGA that signal the end of translation
These codons have nothing to do with termination sequences on DNA during transcription of mRNA.
Hydrolysis of the peptide chain from the P site tRNA releases the protein from the ribosome

10. The Synthesis of Protein (Continued)
Mutations
Point – one or two base pairs
Substitutions – a base pair is replaced by another base pair according to base pairing rules
Silent Substitution – base pair substitution may transform one codon into another that codes for the same amino acid
Conservative Substitution – base pair substitution that may transform one codon into anther that codes for a different amino acid, but it has no effect
May produce an improved protein that will enhance mutant organisms and descendants
Frameshift Mutation – insertion of deletion of a base pair that causes the reading frame to shift
Causes premature termination
May cause nonfunctional protein unless the mutation is near the end of the gene

11. The Synthesis of Protein (Continued)
Mutagens
Cause errors in DNA replication, repair, or recombination of chromosomes that result in substitution and frameshift mutations
Radiation
Base Analogues – chemicals that mimic DNA bases, but base pair incorrectly