DNA Quick Notes
People to remember: (pp. 287-292)
1. T.H. Morgan
2. Gregor Mendel
3. Fredrick Griffith
4. Hershey & Chase
5. Edwin Chargraff
6. Rosalind Franklin
7. Watson & Crick
8. Meselson- Stahl
Replication
Replication is semiconservative- __________________________________________________________p. 293
Origins of replication- Site on the chromosome where replication begins, on a bacterial chromosome, there is
only one. On a Eukaryotic chromosome there are several
Replication is initiated by a protein complex that attaches at an origin & opens up a bubble
Helicase makes the DNA unwind so this can happen
Single strand binding proteins- hold the replication forks open until the polymerase, etc can get to work
Replication fork- where the strands are actively separating & new DNA is being laid down- Y shaped, one at
either end of the “bubble”; new strands of DNA are elongating here
Primerase- lays down a short segment of RNA to get the replication of DNA started. Later the RNA
will be replaced with DNA nucleotides
DNA polymerases- the enzymes that add new nucleotides to the growing DNA strand;
The nucleotides are nucleotide triphosphates (similar to ATP only the sugar is deoxyribose)
Polymerase splits the last two Phosphates off and attaches the nucleotide to the DNA ; the last two
phosphates (called pyrophosphate) are split into two inorganic phosphate molecules- the energy derived
is used to further drive replication.
Elongation only occurs in the 5’ to 3’ direction as new nucleotides have their 5’ carbon joined with the trailing
3’ carbon of the growing strand through a phosphodiester bond.
Leading strand- where DNA elongation can occur in the same direction as the replication fork in going, it’s easy
to add DNA in the 5’ to 3; direction
Lagging strand- on the other side, short sections of DNA are added going in the opposite direction of the
replication fork, but still in the 5’ to 3’ direction
Okazaki fragments- short segments of DNA made on the lagging strand
Ligase- the enzyme that ties theses short strands together
Proofreading- polymerase reads & corrects mistakes as it goes along, but a few do escape.
Nucleotide Excision Repair (Mismatch repair)- Nuclease cuts out mismatched bases and DNA polymerase and
ligase put in and bond the correct nucleotide
Telomeres- repetitive nucleotide sequences found at the ends of eukaryotic chromosomes- protect the ends of
the cells, but erode over time.
Telomerase- enzyme that can repair the telomeres- it carries an RNA template of what the telomere
should look like.
Mistakes do happen!!
Types of mutations- insertion or deletion- both result in a “reading frame shift”
Transcription
Making a protein requires two steps- Transcription and translation.
It occurs 5’to 3’
Generally:
Promoter- DNA sequence where transcription is initiated
Terminator- DNA sequence that signals the RNA polymerase to fall off the DNA
Transcription unit- the stretch of DNA that is transcribed into m-RNA
In Prokaryotes:
1. RNA polymerase binds directly onto the gene at the promoter
2. There is only one kind of RNA polymerase
3. Termination occurs when the RNA polymerase transcribes (copies) the termination sequence. The the
polymerase complex falls off.
In Eukaryotes:
1. There are three kinds of RNA polymerase
2. Transcription factors bind the promoter & RNA polymerase II binds to them-makes the transcription
initiation complex
3. TATA box- a crucial promoter DNA sequence that signals where the transcription factors must bind to
the gene
4. Transcription happens when the “stop” codon is reached, but the RNA polymerase keeps going adding
as sequence of AUGAAA nucleotides at the end of the pre-m-RNA
5. m-RNA modification
a. a 5’ cap and a polyA tail are added to protect the pre-m-RNA
b. The pre-m RNA will be “spliced” to remove any sections that are not needed to form the protein code
6. Intron vs Exon
Introns- noncoding section of the m-RNA that is cut out before the m-RNA is used to make a
protein
Exon- sections that do code for the amino acid sequence that will become the protein, these are
expressed.
7. define:
Primary transcript
Spliceosome
Ribozymes
8. Alternative splicing- gives us more genes for less space on the chromosomes.
Translation:
When the final m-RNA is “read” by the ribosome to carefully determine what the sequence of amino acids
for the protein required is.
There are 21 Amino Acids and 64 codons.
One is the start codon AUG
Three are stop codons
All the rest code for a specific amino acid
AUG codes for methionine, too.
The ribosome:
A-site- aminoacyl site-homds the t-RNA that carries the next Amino Acid to be added to the chain
P-site- peptydyl site- holds the t-RNA that has the growing peptide chain attached
E-site- site where the t-RNA’s leave the ribosome
What happens- Read 315 to 319