CHAPTER 10:
DNA,RNA & Protein Synthesis
I. Discovery of DNA
• Scientist originally believed PROTEINS
would be the molecules which contained
hereditary information.
• Some scientists who did experiments that
proved DNA had genetic information:
– 1. Fredrick Griffith
– 2. Oswald Avery
– 3. Hershey & Chase
4. James Watson &
Francis Crick
• In 1953 DNA structure discovered
• Double Helix model.
– (2 chains of DNA)
– Showed how DNA could replicate.
• Relied on work of other scientists:
– Rosalind Franklin & Maurice Wilkins took
X-ray photos of DNA structure
– ( Franklin died 1958 before Watson & Crick
received Nobel Prize. 1962.)
II. DNA structure
A. Double Helix shape is
formed by base pairs
attached to a sugarphosphate backbone.
-
B. Parts of Nucleotides
1. 5 carbon sugar in DNA- deoxyribose
(in RNA= ribose)
2. phosphate group
3. nitrogenous bases ( there are 4 different
ones)
• Adenine
• Guanine
• Cytosine
• Thymine
(in RNA- no thymine- Uracil is the base)
C. How Chemical Bonds hold
DNA together
• Covalent bondsbetween sugar &
phosphates of 2
nucleotides
• Hydrogen bondsbetween complementary
nitrogenous bases –
www.molecularstation.com/dna/dna-structure/
D. Base Pairing Rules
• Adenine always bonds
with Thymine (AT)
• Guanine always bonds
with Cytosine (GC)
• Note: 3 hydrogen bonds: GC
– 2 Hydrogen bonds: AT
Must have 1 purine (a 2 ring shape) plus 1
pyriomidine (1 ring shape) in each pair or the
would not “fit” inside ladder of DNA
• 2. Purines (A, G) double C ring
• 3. Pyrimidines (T, C) single C ring
III. DNA Replication
A. Is the process by which DNA is copied in
a cell before a cell divides by mitosis,
meiosis or binary fission.
B. Steps in Replication
1. Helicase enzymes -
separate hydrogen bonds
in strands – create
replication fork
2. Attach- DNA polymerase
enzyme -adds nucleotides
3. Release –DNA
polymerase enzyme – now
have 2 identical DNA
strands
oak.cats.ohiou.edu/.../Heredity/Heredity.htm
C. Errors
• DNA replication is very accurate.
• Errors occur ~ 1 in 1 billion paired
nucleotides.
• “Proofreading” enzyme checks for “spelling”
errors.
*If a mistake does occur- new DNA is different:
• Mutation- a change in the nucleotide
sequence of a DNA molecule.
• Caused by chemicals, radiation,UV rays.
• Mutations can be favorable
• - or harmful. (example- cancer)
IV Protein Synthesis
A. Flow of Information (DNA – RNA - Proteins)
Before protein can be synthesized, the instructions in
DNA must first be copied to another type of nucleic
acid called messenger RNA.
Then -a group of 3 nucleic acids codes for an amino
acid & it is built at the ribosomal RNA with help
from the transfer RNA
RNA differs from DNA in the
following ways:
• RNA is single stranded while DNA is
double stranded.
• RNA has a sugar called ribose while
DNA has a sugar called deoxyribose.
• RNA has the nitrogenous base uracil
while DNA has the base thymine.
B. 3 types RNA:
1. messenger RNA(mRNA)
2. transfer RNA (tRNA)
3. ribosomal RNA (rRNA)
1. Messenger RNA, or mRNA.
carries the code for building a protein from the
nucleus to the ribosomes in the cytoplasm. It acts
as a messenger.
2. Transfer RNA or tRNA.
picks up specific amino acids in the cytoplasm &
brings them into position on ribosome where
they are joined together in specific order to
make a specific protein.
3. Ribosomal RNA or rRNA
place for protein synthesis
C. Steps in Transcriptionmaking RNA
1. RNA polymerase (enzyme)–
binds to promoter area on
DNA
2. Nucleotides added & joined by
the enzyme (RNA polymerase)
3. Termination signal- stopRNA polymerase releases both
DNA & new RNA molecules
www.smartdraw.com/.../dna_transcription.htm
D. Proteins
• Review of protein structure
-recall that proteins are made of
amino acids joined together with
peptide bonds
-there are 20 different amino acids,
the order they are joined determines
the structure & function of the
proteins.
-proteins can be very large,
complicated molecules
mRNA codons for specific amino acids
• Each 3 nucleotide sequence (letters) in
mRNA encodes for 1 specific amino acid,
or a “start” or “stop” signal.
• Each 3 nucleotide group is called a codon.
• The genetic code- means the rules that
relate how a particular sequence of
nitrogenous bases corresponds to a particular
amino acid.
teachline.ls.huji.ac.il/72693/weekly2.html
E. Steps in Translation
1.
Initiation- tRNA & mRNA join together.
(The codon is on the mRNA, the “anticodon” is on tRNA)
The tRNA has an amino acid attached to it)
2. Elongation- continued as ribosome moves the
distance of 1 codon on mRNA
3. Elongation is built with new tRNAs attaching each
amino acid as it reads the codons on the mRNA.
4. Termination- ribosome reaches “stop” codon on
the mRNA
5. Disassembly – each piece is free.
(see sequence page 208-209 in textbook)
Remember:
• Replication- copying DNA from DNA
• Transcription- making RNA from DNA
• Translation- making proteins