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DNA, RNA and Protein
A. The Structure of DNA (Deoxyribonucleic Acid)
1. DNA is
double
stranded and
in the shape
of a double
helix.
2. The monomers of DNA are called nucleotides: made of a
sugar (deoxyribose), a phosphate group and a nitrogen base
Phosphate group
Nitrogen
Base
Deoxyribose Sugar
3. There are four types of DNA bases and they pair
specifically:
Adenine
Thymine
with
Guanine
with
Cytosine
DNA Looks Like a Ladder
The alternating sugar and
phosphates form the sides
of a ladder and the nitrogen
bases form the rungs. The
Two sides of the ladder are
held together with hydrogen
bonds
4. If the sequence of one strand is
known, the other strand is known
A T C G G C T A T C G A
T A G C C G A T AG C T
DNA Works as a Code
1. Codon- a three nucleotide sequence that
codes for a single amino acid
2. With 4 different types of nucleotides in
triplet codes, 64 combinations are possible
a) 61 of the codons code for 20 amino acids
b) 3 codons signal a stop- Identifies the end of a
protein
3. The genetic code is exact. If a sequence
of DNA is known, the resulting amino
acid chain (protein) is known
Amino
Acid
DNA codons
Serine
TCT, TCC, TCA, TCG, AGT,
AGC
ATT, ATC, ATA
Tyrosine
TAT, TAC
Leucine
CTT, CTC, CTA, CTG, TTA,
TTG
Tryptophan
TGG
Valine
GTT, GTC, GTA, GTG
Glutamine
CAA, CAG
Phenylalan
ine
TTT, TTC
Asparagine
AAT, AAC
Methionine
ATG
Histidine
CAT, CAC
Cysteine
TGT, TGC
Glutamic
acid
GAA, GAG
GCT, GCC, GCA, GCG
Aspartic
acid
GAT, GAC
Isoleucine
Alanine
Glycine
Proline
Threonine
GGT, GGC, GGA, GGG
Lysine
AAA, AAG
CCT, CCC, CCA, CCG
Arginine
CGT, CGC, CGA, CGG, AGA,
AGG
ACT, ACC, ACA, ACG
Stop codons
TAA, TAG, TGA
DNA Replication
C. DNA Replication
1. The structure of DNA explains how it replicates
a) DNA “unzips” down the middle
b) Free floating nucleotides pair up with both sides of the
DNA molecule
A
T
C
G
A
T
A
G
C
T
Original
AT
TA
CG
GC
AT
A
T
C
G
A
T
A
G
C
T
AT
TA
CG
GC
AT
Exactly Copied DNA Strands
DNA Replication
2. This process
produces two exact
DNA molecules
(Chromosomes)
that are the same
3. DNA never leaves
the nucleus
If DNA never leaves the
nucleus, how can the
DNA message get to the
site of protein production,
the ribosome?
Answer:
The
DNA message is copied to
RNA during the process of
Transcription
How do
RNA and
DNA differ?
D. The Structure of RNA
1. RNA is single stranded
2. The sugar in RNA is Ribose, not
deoxyribose as in DNA
3. The DNA nucleotide thymine is
replaced by the RNA nucleotide
Uracil
E. RNA’s Functions: Two
Types, Two Jobs
1. m-RNA (messenger RNA) delivers the
copied DNA from the nucleus to the
Ribosome- the site of protein synthesis
2. t-RNA (transfer RNA) picks up specific
amino acids in the cytoplasm and delivers
them to the ribosome
F. Steps in Protein Synthesis
1. DNA molecule unzip where the desired gene is located
2. Free floating RNA nucleotides pair with the DNA strand
forming m-RNA (Transcription)
3. The m-RNA leaves the nucleus and goes to a ribosome
4. A specific t-RNA delivers a specific amino acid to the
ribosome (Translation)
5. The m-RNA codon matches with the t-RNA anticodon
6.
bringing the amino acid into its proper place
When the next amino acid is in place, the two are joined in a
condensation reaction
7.
The process is repeated until a stop code is read and a
complete protein is formed
G. Mutation- change in the genetic code
1. Gene Mutation or Point Mutation- a
nucleotide base is added, subtracted or
changed to produce a change in the amino
acid sequence of a protein
Genetic Code with RNA Codons
A change in a single base in the DNA strand will result in
a change in the m-RNA strand and the resulting protein
Normal Hemoglobin
DNA
RNA
GGA CTC CTC CCU GAG GAG
5
Amino Acids
6
7
Proline Glutamic Acid Glutamic Acid
Sickle Cell Hemoglobin
GGA CAC CTC CCU GUG GAG
Proline
Valine
Glutamic Acid
2. Chromosome Mutationinvolves a change in many genes
a) Deletion- part of a
chromosome is lost
b) Inversion- part
of a chromosome is
flipped around
c) Translocation- part
of a chromosome is
added to another
chromosome
3. Somatic & Germ Mutations
a) Somatic mutations: change that
occurs in body cells. Affects only the
individual. Ie. cancer
b) Germ Mutations- changed in the
genetic code of gametes that will affect the
individuals offspring
4. Mutagens- substances capable of
causing damage to DNA
5. Most mutations are harmful
6. Frame Shift Mutations
A) An insertion or deletion that results in the
reading frame being shifted.
B) All codons following the mutation will be
changed.
C) Example:
THE RED DOG ATE THE CAT
HER EDD OGA TET HEC AT