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In the 1940s, most scientists thought that protein was the genetic material because :. there are many varieties of proteins they are very specific nobody knew much about nucleic acids DNA seemed too uniform to account for so many genetic traits
In the 1940s, most scientists thought that protein was the genetic material because:
R is non-pathogenic
R cells could “become” S cells!
The R cells had been “transformed” into S cells by acquiring genetic material from the S cells.
They now give rise to new S cells when they reproduce.
The process by which genetic material is transferred to bacteria is called TRANSFORMATION.
Hershey and Chase’s experiments showed that DNA was the genetic material of a virus (phage) that infects bacteria.
A phage consists of DNA with a protein coat around it.
They labeled protein with 35S and DNA with 32P
Bacterial cells were
infected and then
washed to get rid of
the part of the phage
that remained outside
They wanted to know whether the phages inserted DNA or protein into bacteria.
# of adenines = # of thymines
# of cytosines = # of guanines
crystallography picture taken by Rosalind Franklin, Watson & Crick deciphered the double helical structure of DNA
Dark blue = original (parent) DNA
Light blue = new (daughter) DNA
Origins of Replication– where replication begins (prokaryotes have 1, eukaryotes have many) Replication proceeds in both directions.
DNA polymerase adds nucleotides to the new strand of DNA at the replication fork.
Primase lays down
an RNA primer to
which DNA pol
The primer is later
replaced with DNA.
5’ end = P
3’ end = OH
DNA polymerase can only add on to the 3’ end! So new DNA can ONLY be made 5’ 3’
Direction of Replication
elongate continuously in the 5’ 3’ direction.
But the lagging strand has to grow in short segments = Okazaki fragments
DNA Pol I replaces the RNA primer
Mistakes are made in DNA replication, but they’re usually repaired quickly. DNA pol can recognize the mistake, go back, and fix it.
At the 5’ ends of the lagging strands on each chromosome, the RNA primer can’t be replaced.
Each time the chromosome is replicated, it gets shorter since single stranded DNA is unstable.
Prokaryotes don’t have this problem since they have circular DNA.
Eukaryotes have telomeres at the ends-TTAGGG sequences- that are repeated 100-1000 times. They don’t code for anything, so the DNA can replicate many times before an actual gene is affected.
Telomerase can lengthen telomeres, but it’s not present in most of our cells (only in germ cells that produce gametes).
In fact, telomerase has been found to be present in cancerous cells