Notes- Ch. 11 DNA and the Language of Life
11.2 Nucleic Acids store information
A. People involved with discovering DNA’s structure
1. Rosalind Franklin & Maurice Wilkins –1950, photographs of the
DNA molecule using X-ray crystallography which showed the shape
to be a helix
2.Erwin Chargaff – 1951, proved that the % of A = T and % of G = C
3. James Watson & Francis Crick – 1953, used data from the other
scientists and built models to finally figure out the exact structure of
DNA…. 1962 won the nobel prize in Medicine/Physiology
B. DNA’s Structure- see drawing
1. DNA Structure
a.
DNA is a double helix (twisted ladder) made of subunits called
nucleotides
b.
4 different nucleotides Adenine, Thymine, Guanine, Cytosine
*Nucleotides are made of 3 parts
5-carbon sugar (deoxyribose)
phosphate group
nitrogenous base
c.
The sides of the double helix are made of alternating sugars
and phosphate groups, the rungs of the double helix are
made of nitrogenous bases
Complementary Base Pairs in making the double helix
A pairs with T
G pairs with C
11.3 DNA REPLICATION
DNA replication is the process of copying the DNA molecule.
During DNA copying, the two strands of the double helix separate.
Each single strand acts as a “negative” or “template” for producing a
new, complementary strand.
A. Easy steps for DNA Replication.
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Enzymes are protein molecules that catalyze chemical reactions in a
cell – usually any protein ending in “ase” is an enzyme
1. DNA replication begins at specific sites called the
origins
2. DNA helicase unwinds and separates the two
strands of original DNA molecule
3. DNA polymerase 3 adds complementary
nucleotides to each separated strand
4. DNA polymerase 1 checks for correct pairing of
nucleotides (looks for mutations) and fixes any
mispaired nucleotides
11.4 and 11.5 Protein Synthesis
Transcription
DNA
Translation
mRNA
Nucleus
Protein
Cytoplasm
A. Transcription is the making of single stranded mRNA from a DNA
strand within the nucleus of a cell.
During transcription, RNA nucleotides base-pair one-by-one
with DNA nucleotides on one of the DNA strands (called the template
strand). RNA polymerase links the RNA nucleotides together.
Base pairing is the same as during DNA replication, except that
RNA has uracil instead of thymine: the U in RNA pairs with A in DNA
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B. EASY STEPS OF TRANSCRIPTION (in the nucleus of the cell)
1. DNA double helix unwound and separated by RNA polymerase
2. RNA polymerase adds RNA nucleotides together, making a single
strand of mRNA which is complementary to 1 strand of the DNA
3. mRNA is processed (modified) before leaving the nucleus
Transcription
C. TRANSLATION
First you must know the players in translation
a. ribosome – cellular organelle where translation occurs
b. transfer RNA (tRNA) – carries amino acids to the ribosome
c. amino acids – subunits which are linked together to make a
polypeptide
d. polypeptide – another name for a protein or can be linked
together with other peptide chains to make a large protein
e. codon – 3 nucleotide sequence on the mRNA
f. anticodon – 3 base sequence on the tRNA which recognizes
the codon on the mRNA
g. P site – location on the ribosome that holds the tRNA which
is carrying the growing polypeptide chain
h. A site – location on the ribosome that holds the tRNA
carrying the next amino acid to be added to the growing
chain
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D. Easy steps of translation
1. mRNA leaves the nucleus and is transported to the ribosome
where translation takes place
2. the ribosome holds onto the mRNA, the mRNA codon AUG is
located in the P site of the ribosome
3. tRNA carries an amino acid to the P site of the ribosome
4. another tRNA carries the next amino acid to the A site of the
ribosome
5. the two amino acids are joined together with a peptide bond
6. the tRNA in the P site leaves
7. the ribosome moves along the mRNA until the next codon is
located in the A site
(the tRNA which was located in the A site is now in the P site
and is holding the peptide chain)
8. a tRNA carries the next amino acid to the A site
9. the new amino acid is joined to the peptide chain with ade bond
(the polypeptid peptie now is made of 3 amino acids)
10. the tRNA in the P site leaves
11. the process continues until a stop codon appears in the A site
of the ribosome
12. the polypeptide is now complete
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E. THE GENETIC CODE : RULES FOR TRANSLATION
1. a codon is a three-base “word” that codes for one
amino acid
2. to determine the amino acid coded for by an
mRNA codon use the genetic code
3.the genetic code is universal – all species use the
same genetic code, the same 20 amino acids are used in all
living organisms
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11.6 Mutations can change the meaning of genes
A. How mutations affect genes
1. A mutation is any change in the nucleotide sequence of
DNA.
2.Two categories of mutations
a. base substitutions – replacement of one nucleotide with
another
i. this can change the protein or not change the
protein
b. base insertions or base deletions – addition of an extra
nucleotide or subtracting a nucleotide
i. usually have more effect on the protein than a
substitution
B. What causes mutations?
1. Mistakes during DNA replication can cause mutations
2. Mutagens – physical or chemical agents that cause
mutations
a. Physical mutagens – high energy radiation, X-rays,
Ultraviolet light
b. Chemical mutagens – chemicals that are similar to
DNA bases and cause incorrect base-pairing
C. Mutations are often harmful but may alter a protein in a way that is
beneficial
1. if a mutation is present in the gametes of an organism it is
passed on the offspring
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