1. Molecular Basis of Inheritance
Chapter 16
Molecular Basis of Inheritance

2. History of DNA 1928 – Frederick Griffith 2 strains of bacterium
Pathogenic – disease causing
Nonpathogenic – harmless
Heat killed pathogenic bacteria & mixed with nonpathogenic – some living cells became pathogenic
This new trait of pathogenicity was inherited by all descendents of transformed bacteria
Transformation – modern definition – a change in genotype & phenotype due to assimilation of external DNA by a cell

3. 1944 – Oswald Avery
Transforming agent was DNA
1952 – Alfred Hershey & Martha Chase
Proved it was DNA, not protein, that was the hereditary material in viruses
Protein contains sulfur, which was left outside the host
DNA contains phosphorus, which was injected into host cell
1940’s - Erwin Chargaff
Equal amounts of Adenine & Thymine / Cytosine & Guanine
Chargaff’s Rules – A pairs w/ T & C pairs w/ G

4. Structure 1950’s – Maurice Wilkins & Rosalind Franklin
X-ray crystallography – Photo 51
Showed double helix structure
James Watson & Francis Crick
Built model of DNA using other people’s research
Nitrogen bases inside, sugars & phosphates on outside
A-T C-G
Adenine & Guanine are purines – 2 carbon rings
Cytosine & Thymine are pyrimidines – 1 carbon ring

5. Purine + purine = too wide
Pyrimidine + pyrimidine = too narrow
Purine + pyrimidine = width consistent w/ x-ray data

6. Chemical structure - Nucleotides

7. A & T – 2 H bonds
C & G – 3 H bonds
DNA model showed how replication was possible and how it happened.
5’ & 3’ ends indicate which carbon is on end of strand.
Bases will only fit together if ends are opposite

8. Semi-Conservative Replication
Replication – DNA produces new molecules
Semi-conservative – each newly produced strand will contain
1 old strand – original
1 new strand – new nucleotides

9. Important Enzymes in Replication
DNA Polymerase – adds nucleotides, does not start process
DNA Ligase – joins Okazaki fragments
Helicase – untwists DNA
Primase – can start RNA from scratch, responsible for primer
Topoisomerase – relieves strain caused by untwisting
Nuclease – DNA cutting enzyme
Telomerase – catalyzes lengthening of telomeres in eukaryotic germ cells

10. Chapter 17
From Gene to Protein

11. Central Dogma of Biology
Garrod (1909) – genes dictate phenotypes through enzymes that catalyze specific reactions in the cell
Beadle & Tatum
One gene - one enzyme (not all enzymes are proteins)
One gene – one polypeptide – newly stated
Central Dogma of Biology
DNA RNA Protein

12. RNA vs. DNA DNA RNA Double stranded Deoxyribose
Bases – Adenine, Thymine, Cytosine, Guanine
Longer
RNA
Single stranded
Ribose
Bases – Adenine, Uracil, Cytosine, Guanine
Shorter

13. Ribonucleic Acid 3 types of RNA
Ribosomal – part of structures called ribosomes
Transfer – carry amino acids to the ribosome and put them in the correct sequence
Messenger – carries a copy of DNA message to the ribosome to direct the production of proteins

14. Transcription RNA Polymerase
Attaches to the promoter region of DNA
It opens the DNA between the bases
Moves down the DNA adding RNA nucleotides that are complementary to the DNA
At the termination sequence, the RNA polymerase releases the mRNA
In prokaryotic cells, the mRNA will immediately make protein. Why?
In eukaryotic cells, the mRNA will be modified first

15. DNA – A C G T T A A C G G C T A T C G
RNA –
Series of DNA bases dictates series of RNA bases, which dictates series of amino acids, which determines protein structure and function

16. Translation Specific jobs of RNA
tRNA – transfers amino acids to ribosome
80 nucleotides long
Base pairing makes it cloverleaf shape
Aminoacyl – tRNA synthase is enzyme that attaches a specific amino acid to a tRNA.
ATP energy used
Covalent bond
The anticodon matches the codon (3 base sequence) on mRNA
The third base in the anticodon can sometimes match more than one base (called wobble) on the codon so fewer than 61 tRNA are needed – 45 or so)

17. rRNA – part of the structure of ribosomes – the part near the action
mRNA – carries code for sequence of amino acids in polypeptide (hundreds of nucleotides)
rRNA – part of the structure of ribosomes – the part near the action
catalyzes condensation reactions to attach amino acids
Prokaryotic ribosomes are smaller & slightly different, so some antibiotics inhibit bacterial translation but not eukaryotic

18. 3 stages of translation Initiation
Small ribosome subunit binds to 5’ end of mRNA and initiator tRNA (AUG codon). The small subunit scans until it finds the AUG codon and the tRNA hydrogen bonds with it
Large subunit attaches to complete the initiation complex
Energy used
tRNA is in P site

19. Elongation – amino acids are added one by one (less than 1/10 sec per amino acids in prokaryotes)
tRNA with correct anticodon comes into Asite & hydrogen bonds with mRNA
Peptide bond forms between new amino acid & the carboxyl end of growing polypeptide
Catalyzed by rRNA – now the polypeptide attached to A site
Ribosome translocates the tRNA in the A site to the P site by moving the mRNA down by one codon. The empty tRNA is now in the E site & leaves

20. Termination
When a stop codon reaches the A site, a protein called a release factor binds to the stop codon
The release factor hydrolyzes the polypeptide from the tRNA (adds a H2O) which releases it
Translation complex comes apart

21. Modification of mRNA after transcription (in eukaryotes)
The transcription product is called the primary transcript
Alteration of mRNA ends
5’ end has a GTP cap attached
3’ end has a poly-A tail
These both seem to help mRNA get out of the nucleus & protect it from degradation
They also help the ribosome attach to 5’ end

22. Splicing
Primary transcript approx 8000 bases, but only about 1200 are needed to code for a. acids
Noncoding segments are introns
Coding segments are exons
The introns are cut out and the exons are spliced together to make functional mRNA
Spliceosomes splice & cut
Some introns serve as the enzyme (ribozyme) to cut themselves out
The introns may explain how one gene can code for more than one polypeptide if alternate splicing occurs

23. Signal Mechanism – proteins that will become part of the endomembrane system or those to be exported (secreted) will be synthesized on bound ribosomes
The protein that needs to be put into the ER has a signal peptide (20 aa sequence)
A signal recognition particle recognizes the signal particle and brings the protein/ribosome to the ER to a pore where there is a receptor
Polypeptide synthesis continues & the finished protein ends up in the rough ER lumen or as a protein in the ER membrane.
Free ribosomes in cytosol manufacture proteins that will do their work in the cytosol