1. GENE EXPRESSION / PROTEIN SYNTHESIS
AN OVERVIEW

2. Gene:. A segment of DNA that specifies the
Gene: A segment of DNA that specifies the amino acid sequence of a polypeptide (protein)
Remember proteins have many different functions:
Hormones
Enzymes
Antibodies
Transport
Structural
DNA does not directly control protein synthesis, instead its code is transcribed into RNA

3. 3 types
one type
in nucleus &
cytoplasm
in nucleus only
single-stranded
double-stranded
no particular shape
helix

4. Three Classes of RNA: Messenger RNA (mRNA) Ribosomal RNA (rRNA)
Takes a message from DNA to the ribosomes
Ribosomal RNA (rRNA)
Makes up ribosomes (along with proteins)
Transfer RNA (tRNA)
Carries amino acids to ribosomes

5. Gene Expression/Protein Synthesis
Gene Expression Requires Two Steps:
Transcription – making mRNA from DNA code
Translation – assembly of polypeptides based on mRNA sequence
Remember: Ribosomes = assembly site of polypeptides **

7. 1) Transcription
a segment of the DNA serves as a template for the production of mRNA
Only one strand serves as the template = the sense strand; the other strand is called the non-sense (or antisense) strand
RNA polymerase binds and unwinds the DNA strand
DNA helix is opened so complementary base pairing can occur
RNA polymerase joins new RNA nucleotides in a sequence complementary to that on the DNA
This creates mRNA! **

9. Once produced, the mRNA strand is processed
Introns (non-coding regions) are cut out
The mRNA leaves the nucleus

10. 2) Translation Takes place at the ribosomes
Ribosome has a binding site for mRNA and tRNA
mRNA attaches to and is “read by” ribosome

12. Each amino acid is coded for by 3 bases = triplet code
Each three-letter unit of mRNA = codon
There are 22 amino acids but some amino acids are specified by more than one codon
There are “start” & “stop” codons as well
This genetic code is universal
the same codons stand for the same amino acids in all living things

13. Messenger RNA Codons

14. Steps in Translation: Initiation:
mRNA, with its “start” codon (AUG) attaches to the ribosome.
tRNA’s anticodon (UAC) binds to the mRNA’s “start” codon at the first binding site (P)

15. 2. Elongation:
The next complementary tRNA molecule attaches to the next binding site (A)
Peptide bonds form between the two amino acids

16. Ribosome moves down the mRNA chain in the 3’ direction
tRNA is ejected from the E binding site
Elongation in repeated until a “stop” codon is reached

17. 3. Termination:
“stop” codon (UAA, UAG, or UGA) is at the A-binding site
A release factor protein binds to the stop codon & breaks the bond between the tRNA & the polypeptide
Released polypeptide forms a protein