Scientist and Their Contributions:
1869 - Freidrich Miescher (Swiss biochemist) - isolated
material from the nucleus. He called nuclein
Other Scientists - high in phosphorus and acidic - became known as nucleic
acid
2
Kinds - DNA - Deoxyribonucleic acid
RNA - Ribonucleic acid
1928 - Frederick Griffith (English bacteriologist)
- determined that a genetic material exists by the
transformation of a non-pneumonia-causing bacterium into a pneumonia-causing
bacterium.
He injected mice with bacteria in his study.
R strain (uncoated) bacteria - did not kill mice
S strain (coated) bacteria - killed mice
heat-killed S bacteria - did not kill mice
heat-killed S mixed with living R - killed mice - conclusion - living R
picked up genetic
material from the dead S.
1944 - Oswald Avery, Colin Maclead. and Maclyn McCarty
(Rockefeller Institute in New York) -
- identified the tranforming genetic material as DNA.
1952 - Alfred Hershey and Martha Chase - conformed
that DNA is the genetic material using virus
and bacteria.
Radioactively labeled the DNA of virus which then infected bacteria cells.
The bacteria
cells were observed to contain radioactivity indicating that the viral
DNA entered the cell
rather than the protein of the virus.
1920's - P. A. Levene (biochemist) - DNA is made
up of the following chemical groups:
deoxyribose - a 5-carbon sugar
a phosphate group
four kinds of nitrogen bases - purines -adenine and guanine
pyrimidines - cytosine and thymine
~1950 - Rosalind Franklin and Maurice Wilkins (Oxford
University) -
X-ray images of DNA crystals - helix shape.
1953 - James Watson (American biochemist) and Francis
Crick (English physicist)
in Cambridge, England
Described the structure of DNA as a double helix - like a twisted ladder:
sides of ladder are made up of alternating phosphate groups and deoxyribose
rungs of ladder are made up of paired nitrogen bases:
adenine - thymine
cytosine - guanine
DNA Replication - making a copy of the DNA
This must occur before a cell divides into two cells
so that both daughter cells receive a complete
copy of the DNA.
DNA polymerase - unzips the DNA by breaking hydrogen
bonds between nitrogen bases.
DNA nucleotides are added along each separated
strand of DNA until two identical molecules
of DNA are formed. Each will have a
new and old strand of DNA nucleotides, therefore
DNA replication is said to be semi-conservative.
How is DNA the genetic material?
In the sequence of nitrogen bases, DNA contains the
instructions to make proteins. The proteins
an organism makes determines that organism's traits.
Every cell of an organism holds a complete
copy of the DNA in its nucleus. Humans have
46 chromosomes, each of which contains one
long molceule of DNA. Along one molecule of
DNA several different protein recipes (genes)
are found. Since DNA can not leave the nucleus
and proteins are made by ribosomes in
the cytoplasm of the cell, the protein instructions
must be delivered to the cytoplasm by a messenger.
This is the role of mRNA which is a movable copy
of the DNA. The ribosome reads the mRNA
and puts together the protein using amino acids which are delivered
to the ribosome by tRNA molecules.
Summary of Protein synthesis: DNA --(transcription)----> mRNA ---(translation)---> protein
Transcription
- RNA polymerase unzips the DNA and build a mRNA along one of the DNA strands.
The nitrogen bases of RNA nucleotides bind to the nitrogen bases of that
template strand:
DNA bases RNA Bases
A binds to
U
T binds to
A
C binds to
G
G binds to
C
The RNA polymerase connects all of the RNA nucleotides by forming a bond
between
the ribose of one RNA nucleotide to the phosphate group of the next nucleotide.
Once the
entire gene is transcribed into mRNA the mRNA peals off of the template
strand of DNA
and the DNA molecule zips back up.
RNA processing - often some alteration of the mRNA occurs before it travels to the cytoplasm.
intron - non-coding regions - cut out
exons - coding regions - joined together
also a guanosin cap and poly A tail are added:
G- cap - AUGCCCGUUGCGUACGAAUCGUAAAAAAAAAAAAA (poly-A tail)
Translation
- building of the protein.
Three kinds of RNA are involved:
mRNA (messenger) - carries the information on amino aicd sequence of the
protein
rRNA (ribosomal) - part of the ribosome structure togther woth proteins
tRNA (transfer) - delivers amino acid to the ribosome
Genetic Code - every 3 nitrogen bases on the mRNA is called a codon.
A codon codes
or calls for a certain amino acid (building block of protein). We
can use a codon
chart to figure out what each of the different codons means. Since
there are 4
different nitrogen bases which could be in three separate positions in
the codon,
there are 4 x 4 x 4 = 64 possible codons. But there are only 20 different
amino
acids. Some of the codons code for the same amino acid. This
prevents some
mutations (changes in the DNA base sequence) from causing changes in the
protein. A changed codon may still code for the same amino acid.
There is
one start codon (AUG) and three stop codons (UAA, UAG, UGA).
A ribosome joins with the beginning of mRNA and waits until a tRNA with
the
appropriate amino acid (the one coded for by the codon it is reading) wanders
by.
The tRNA has a "tag" called the anti-codon that is complementary to the
codon.
Only the tRNA that carries the correct amino acid would have the anti-codon
that
could bind to the codon that codes for the amino acid that it is carrying.
For example: The mRNA codon AUG codes for the amino acid methionine
which will be delived by a tRNA molecule with anti-codon UAC.
The ribosome travels the length of the mRNA reading 2 codons at a time
as tRNA
molecule deliver the correct amino acids. The ribosome joins the
amino acids
together with peptide bonds. When the ribosome encounters a stop
codon it
becomes detached from the mRNA and the chain of amino acids (protein).