Transcript Chapter 10 Nucleic Acids and Protein synthesis DNA

CHAPTER 10
NUCLEIC ACIDS AND
PROTEIN SYNTHESIS
 DNA = DeoxyriboNucleic Acid – stores and
transmits the genetic information that tells cells
which proteins to make and when to make them.
 DNA is made up of 2 long chains of
NUCLEOTIDES.
 Nucleotide = Sugar (deoxyribose) + Phosphate +
Nitrogen Base. See fig. 10-1 on pg. 185.
 4 Nitrogen Bases = Adenine, Guanine, Cytosine,
and Thymine.
 Purines – (have 2 carbon rings) = adenine and
guanine
 Pyrimidines – (have 1 carbon ring) = thymine and
cytosine. See fig. 10-2 on page 186.
 James Watson and Francis Crick – (1953) –
suggested the “double Helix” model for the
structure of DNA. See fig. 10-3 on pg. 186.
 Rosalind Franklin and Maurice Wilkins – took
x-ray pictures of DNA crystals (x-ray
crystallography). This helped confirm
Watson/Crick’s idea.
 1962 Nobel Prize in Medicine – went to
Watson, Crick, and Wilkins. This has been
called the discovery of the century.
Unfortunately, Rosalind Franklin died in 1958
and her work was not recognized.
Structural Details of DNA:
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Sugar/Phosphate Backbone is bonded covalently.
Nitrogen bases face toward center of helix.
DNA is “double stranded”.
Bases on 1 strand face bases on the other strand.
Weak hydrogen bonds form between the bases, holding the 2
strands together.
 Purines ALWAYS pair with pyrimidines. This keeps the pairs of
bases, between the uprights of the DNA ladder, at a uniform
length.
 Complementary Base Pairing Rules :
- Adenine always bonds with Thymine.
- Cytosine always bonds with Guanine.
NOTE: The base sequence on 1 strand is an exact “complement” of
the sequence on the other strand. See fig. 10-3 on pg. 186.
QUESTION – If one strand has the following sequence:
AGTCCATTGAAC, what would the complementary sequence be
on the other strand??
 In cell division, the ability of DNA to make
exact copies of itself is
important……Understanding base sequences
led to ideas of how DNA might copy itself.
 Replication – Process of copying DNA.
Steps of DNA Replication:
(See fig. 10-5, pg. 188)
 2 nucleotide chains separate at the “Replication
Fork”. NOTE – Helicase enzymes break
hydrogen bonds between bases to “unzip” DNA.
 DNA Polymerases – bind to the separate chains
of nucleotides (1 nucleotide at a time). The
polymerases build a new complimentary chain of
nucleotides. NOTE – New strands are built for
BOTH of the unzipped DNA chains.
 At the end of replication, there are 2 identical
copies of the original DNA molecule. Each DNA
is made up of 1 chain from the ORIGINAL DNA
and 1 NEWLY MADE chain.
 NOTE – DNA replication happens simultaneously at
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many points along the molecule. It does NOT begin
at 1 end and proceed to the other.
NOTE – Replication occurs in OPPOSITE directions
on the 2 strands.
DNA Replication is very accurate:
- Approximately 1 error in 10,000 paired nucleotides
occurs.
- DNA proofreading and repair enzymes cuts the
error rate to 1 per 1 Billion nucleotides.
Proofreading and Repair Enzymes – Scan DNA for
errors, Chemically “snip” them out and “glue” in the
correct sequences.
Mutation – a change (error) in the nucleotide
sequence….may have no effect or may have serious
consequences. Caused by a variety of agents
including chemicals, radiation, UV light from sun.
RNA
 RNA = Ribonucleic Acid – SINGLE stranded. It
moves genetic information from the nucleus
to the site for protein synthesis in the cytosol.
 RNA is similar to DNA but:
- It is SINGLE STRANDED.
- The sugar is RIBOSE instead of Deoxyribose.
- URACIL replaces thymine.
3 Types of RNA:
(Each has a different job)
 Messenger RNA – (mRNA) – nucleotides are in a
single uncoiled chain. It carries genetic
information from the DNA in the nucleus to the
cytosol.
 Transfer RNA (tRNA) – single chain folded into a
cloverleaf shape. Binds to specific amino acids.
See fig. 10-8 on pg. 194.
 Ribosomal RNA – (rRNA) – most abundant form.
Its structure is “globular”. It makes up the
ribosomes where proteins are put together.
TRANSCRIPTION
 Transcription – process by which genetic
information (genetic “code”) is copied from
the DNA to RNA. OCCURS IN THE
NUCLEUS. Read pages 191 – 192. Continued
on next slide………..
Steps of Transcription:
 RNA Polymerase = transcription enzyme – starts
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transcription by binding to “PROMOTER REGIONS” ( these
regions have lots of A-T base pairs) on the DNA.
Promoter Region – marks the beginning of the DNA portion
that will be transcribed
RNA polymerase attaches to 1 strand of unzipped DNA and
begins pasting together complementary RNA nucleotides
to form a strip of RNA. NOTE – Base pairing rules are the
same as in DNA replication, EXCEPT URACIL REPLACES
THYMINE!!
Transcription continues until the RNA polymerase reaches a
DNA region called the “TERMINATION SIGNAL” = specific
sequence of nucleotides that marks the end of a gene or
genes.
NOTE – All 3 types of RNA (mRNA, tRNA, and rRNA) are
transcribed this way.
Protein Synthesis
 Protein Synthesis = production of proteins.
NOTE – all 3 types of RNA will be at work
here.
Protein Structure &
Composition: Key Points
 Proteins are polymers made up polypeptide
chains (chains of amino acids – “aa” from here on
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out).
There are 20 different aa’s that make up
proteins.
Proteins may consist of 100’s or 1,000’s of the 20
different aa’s arranged in a particular sequence.
The aa sequence determine how a polypeptide
will bend and twist to form the 3-D structure of a
protein.
The 3-D structure determines how a protein will
function.
 Genetic Code – the link between the nucleotide
sequence in DNA and the aa sequence in
proteins.
 Codon – a combination of 3 nucleotides in
mRNA. Each codon codes for a specific aa. See
table 10-1 on pg. 194.
 Facts About the Genetic Code:
- several codons code for each aa.
- Start Codon = AUG – engages a ribosome to
start translation (also codes for the aa –
Methionine).
-Stop Codon - = UAA, UAG, UGA – cause the
ribosome to stop translating.
 Translation – process of assembling aa chains
from information encoded in mRNA. After
mRNA is made, it leaves the nucleus and
migrates to a ribosome in the cytosol – THIS
IS THE SITE FOR PROTEIN SYNTHESIS!!
 tRNA – bonds to and transports an aa to the
ribosome = “aa taxi cab”. See fig. 10-8 on pg.
194.
 Anticodon – sequence of 3 nucleotides that is
complementary to pairs with a corresponding
mRNA codon. If ACA is the codon, then UGU
would be the anticodon.
 Ribosome – site for protein assembly. It can float
freely in the cytosol or can be attached to the
endoplasmic reticulum (ER). It is made up of
rRNA and proteins that hold it together.
- free floating ribosomes make proteins to be
used WITHIN the cell.
- ribosomes attached to the ER make proteins to
be exported OUT OF THE CELL.
 3 binding sites on a ribosome – (Key to
Translation):
- 1 site that anchors the mRNA that is going to
be translated.
- 2 sites that hold the tRNA “taxi cabs”.
SEE fig. 10-9 on pg. 195.
Steps of Translation =
(Protein Synthesis)
See fig. 10-9, pg. 195
 1. Ribosome attaches to the start codon (AUG) = methionine on
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the mRNA transcript. Therefore, the 1st aa in every protein chain
is Methionine.
2. As the ribosome move s along the mRMA, each mRNA codon
is sequentially paired with its tRNA anticodon (aa taxi cab).
3. aa’s are hooked to a growing polypeptide chain In the order
specified by mRNA.
4. As each aa is added, the ribosome moves ahead 3 nucleotides
(1 codon) on the mRNA transcript.
5. Eventually the ribosome reaches a stop codon….this ends
translation.
6. mRNA is released from ribosome and polypeptide chain
(protein) is complete.
 NOTE :
- Several ribosomes may simultaneously
translate the same mRNA. As soon as 1
ribosome is out of the way, another follows
behind.
- Translated polypeptide chain = PRIMARY
structure of a protein. As it folds and joins
other chains, it becomes the functional
structure of a completed protein.
- mRNA breaks down rapidly. Its parts are
recycled and used in the future.