Post on 04-Apr-2018
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From DNA to Proteins
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Nucleic Acids
• Nucleic acids are a type of macromoleculemade from repeating units called
nucleotides.• Each nucleotide is made of the same basic
parts:
– A sugar.
– A base that contains nitrogen.
– A phosphate (PO4) ion.
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DNA and RNA
• Each molecule hasthe same basic
structure, except the5-carbon sugar. – Ribose in RNA.
– Deoxyribose in DNA.
• RNA is also usuallysingle-stranded.
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Nitrogenous Bases
• There are five different types of nitrogenous bases that make up DNA
and RNA. – DNA: Thymine (T), Adenine (A), Cytosine
(C), and Guanine (G)
–
RNA: Uracil (U) instead of Thymine (T).• The sequence of these bases codes for
all of the proteins produced by the
body.
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DNA
• When a cell needs to produce a newprotein, only one specific piece of DNA
from one chromosome is involved.• The human genome has 23 pairs of
chromosomes.
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Insulin
• A total of 110 amino acids,coded by 330 base pairs,builds insulin.
• The gene for insulin islocated on chromosome 11.
• The DNA of chromosome 11contains about 134 million
base pairs. – 0.00025% of this chromosome
codes for insulin.
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Transcription
• Takes place in the nucleus.
• A messenger RNA molecule (mRNA)is copied from DNA by unzipping aportion of the DNA helix thatcorresponds to a gene
• One side of the DNA strand istranscribed into the appropriatesequence of A, U, C, & G.
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Transcription
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The Genetic Code
• Base pairs are translated in sets of three,called codons.
• Codons are like the “words” of the DNA“language.”
• One codon corresponds to one amino
acid (the building blocks of proteins).• Examples: – UUU = phenylalanine
– GGA = glycine
– AAA = lycine
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Cracking the Code• The genetic code is redundant but not
ambiguous;
– Not ambiguous meaning that each codon
specifies only one amino acid.
• Example: UUU always codes for phenylalanine.
– Redundant meaning that more than one
codon may signals a specific amino acid.• Example: Serine is coded by UCA, UCG, UCC, and
UCU.
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Codon chart
•
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Translation
• Takes place in the cytoplasm(ribosomes in the rough E.R.)
• Ribosomes read the codons of themRNA and instruct tRNAs (withmatching anticodons) to bring in specificamino acids to be added to the growingpolypeptide chain (protein).
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LE 17-4
DNA
molecule
Gene 1
Gene 2
Gene 3
DNA strand
(template)
3
TRANSCRIPTION
Codon
mRNA
TRANSLATION
Protein
Amino acid
3 5
5
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Insulin• The hormone insulin is
made of two protein chains,a connecting peptide, and a
signal peptide. – Signal peptides tell the
endoplasmic reticulum todirect the protein outside of
the cell. – Connecting peptides keep the
two chains inactive until theinsulin is actually needed.
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How Insulin is Built
1. The double helix of therelevant section of chromosome 11 is unzipped.
2. The mRNA molecule istranscribed using one of theDNA strands as a template.
3. The mRNA molecule is sentoutside the nucleus to therough endoplasmic reticulum.
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How Insulin is Built
4. The mRNA is received bya ribosome in the rough
E.R.5. The ribosome starts
translating the mRNA
codons into amino acids.6. The protein is built, one
amino acid at a time,
using tRNA molecules.
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How Insulin is Built
• The first 24 amino acids are a signal peptide,telling the endoplasmic reticulum not to release thehormone directly into the cytoplasm.
• The next 30 amino acids make the first part of theinsulin protein.
• The following 31 amino acids make a connectingpeptide. This keeps the two parts of insulin
separate until it is ready to be activated.
• The last 21 amino acids form the second part of theinsulin protein.
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How Insulin is Built
• The insulin is sent to the Golgi, where itis packaged.
• The insulin will be released by thepancreas when blood glucose levels aretoo high.
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Evolution of the Genetic Code
• The genetic code is nearly universal, shared by thesimplest bacteria to the most complex animals
• Genes can be transcribed and translated afterbeing transplanted from one species to another.
– Most medicinal insulin produced now comes
from cultures of bacteria.
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Mutations
• Mutations are changes in the geneticmaterial of a cell or virus
•
Point mutations are chemical changes in just one base pair of a gene.
– This may change a single amino acid in theprotein sequence, which could alter the entire
shape of the protein.
LE 17 23
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LE 17-23
Wild-type hemoglobin DNA
mRNA
3 5 5 3
5 3 3 5
Mutant hemoglobin DNA
mRNA
Normal hemoglobin Sickle-cell hemoglobin
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Types of Point Mutations
• Point mutations within a gene can bedivided into two general categories
– Base-pair substitutions
– Base-pair insertions or deletions
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Substitutions
• A base-pairsubstitution
replaces onenucleotide andits partner with
another pair of nucleotides
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Insertions and Deletions
• Insertions and deletions are additions orlosses of nucleotide pairs in a gene
•
These mutations have a disastrous effect onthe resulting protein more often thansubstitutions do
•
Insertion or deletion of nucleotides mayalter the reading frame, producing aframeshift mutation
LE 17-25
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LE 17 25
Base-pair insertion or deletion
Frameshift causing immediate nonsense
Extra U
Missing
Frameshift causing
extensive missense
Insertion or deletion of 3 nucleotides:
no frameshift but extra or missing amino acid
Missing
Stop
Stop
Amino end Carboxyl end
Stop
Wild type
mRNA
Protein
5 3
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Mutagens
• Spontaneous mutations can occur duringDNA replication, recombination, or repair
• Mutagens are physical or chemical agentsthat can cause mutations.
– Examples include nuclear radiation, UV light, X-
rays, chemicals in tobacco smoke.