DNA, RNA and
Protein Synthesis
TAKS Review
Spring 2012
Structure of DNA
• DNA is deoxyribonucleic acid
• DNA is a large molecule that has
subunits called nucleotides.
• The nucleotides come together to
make the DNA molecule in the
shape of a double helix (looks like
a twisted ladder).
Nucleotides
• Nucleotides are the subunits that
make up DNA and they have 3
major parts:
1. A sugar molecule (deoxyribose
sugar)
2. A phosphate group
3. A nitrogen base
Nitrogen bases
There are 4 kinds of nitrogen bases:
Adenine (A)
Guanine (G)
Thymine (T)
Cytosine (C)
DNA Structure
• DNA is a double helix (twisted ladder)
• The handrails of the ladder are made of
phosphate and sugar
• The rungs of the ladder are made of
nitrogen bases.
Bonding
• Sugar and phosphate bond using covalent
bonds (strong) and nitrogen bases bond
using hydrogen bonds (weak)
• Nitrogen bases are specific: Adenine (A)
always attaches to Thymine (T); and
Cytosine (C) always attaches to Guanine
(G)
• (Think apples in trees, cars in garages)
DNA Replication
A review:
A cell will make an exact copy of itself
during a process called mitosis
Before the cell can copy itself it must copy
all of it’s DNA so that both daughter cells
have the same number of chromosomes.
DNA replication takes place in 3
steps
• 1. DNA unwinds at the hydrogen bonds
• 2. nucleotides are added to the exposed
nitrogen bases. It follows base pairing
rules. (A –T and C-G)
• 3. The DNA winds back up producing 2
new strands.
What’s the complementary strand?
• If one side is CAG, what is the
complementary strand?
• Answer: GTC
• If one side is AAC, what is the
complementary strand?
• Answer: TTG
RNA vs DNA
Characteristic
RNA
DNA
Strands
1
2
Sugar molecule Ribose
Deoxyribose
Bases
A, U, C, G
A, T, C, G
Found
Nucleus and
cytoplasm
Nucleotide
Nucleus only
Building block
Nucleotide
Kinds of RNA
• There is only one kind of DNA, but
there are 3 kinds of RNA.
– Messenger RNA (mRNA)
– Transfer RNA (tRNA)
– Ribosomal RNA (rRNA)
Gene Expression
DNA
Transcription RNA
Translation
(double(singlestranded)
stranded)
http://onlinetc.its.brooklyn.cuny.edu/Core81/chap5.html#jump2
Protein
(amino acid
chain)
Central Dogma of Molecular
Biology
Transcription= taking DNA and making
an RNA copy of it. Uses complementary
nucleotides. C=G, G=C, T=A, A=U
http://www.wappingersschools.org/RCK/staff/teacherhp/johnson/visualvocab/mRNA.gif
The Genetic Code
• Remember that messenger RNA – mRNAis a copy of the DNA. It carries instructions
for making a protein.
• The instructions (nucleotides) have to be
translated into proteins (amino acids).
• A sequence of three nucleotides is called a
codon, and it corresponds to an amino acid.
• We use a codon chart for translation
•
Genetic Mutations
Mutation
• A mutation is any change in a genetic code
(DNA).
• It may not change the resulting amino acid chain
and is called a silent mutation
• EXAMPLE: Notice how the resulting a.a.
sequence is the same in spite of the change:
–
–
–
–
–
–
Original DNA:
Resulting mRNA:
Resulting amino acid:
Mutated DNA:
Resulting mRNA
Resulting amino acid:
CAA CCC AAA
GUU GGG UUU
Val – Gly - Phe
CAA CCC AAG
GUU GGG UUC
Val – Gly – Phe
Point Mutation or Substitution Mutation
• A point mutation is a change in one nucleotide. It
can be silent or it can cause a change in one
amino acid
• Can be devestating if the resulting amino acid has
a STOP codon inserted as a result
–
–
–
–
–
–
–
–
–
Original DNA:
Resulting mRNA:
Resulting amino acid:
Mutated DNA:
Resulting mRNA
Resulting amino acid:
Mutated DNA:
Resulting mRNA:
Resulting amino acid:
ATG CCC AAA
UAC GGG UUU
Tyr – Gly - Phe
ATG ACC AAA
UAC UGG UUU
Tyr – Trp – Phe
ATT CCC AAA
UAA GGG UUU
Stop
Insertion and Deletion Mutations
• Insertion – addition of one or more nucleotides
• Deletion – deletion of one or more nucleotides
• Insertion and deletion mutations are almost
always devastating because it will cause a frameshift to occur.
• Imagine if a sentence of 3-letter words lost a
letter?
• Original sentence: The cat and dog are fat.
• A mutation occurs that deletes the c in cat.
• Mutated sentence: Thc ata ndd oga ref at.
• The same things happen to DNA when it mutates
Frame Shift Mutation
•
•
•
•
•
Original DNA:
Resulting mRNA:
Resulting amino acid:
Mutated DNA:
Resulting mRNA:
• Resulting amino acid:
CAT AGC TAG GAT
GUA UCG AUC CUA
Val–Ser-Ile-Leu
CAA GCT AGG AT
GUU CGA UCC UA
Val-Arg-Ser-?
Mendel’s Theory and Studying
Heredity
The Flower
Mendelian Theory of Heredity
• 2 sets of chromosomes = 2 copies of each
gene
• Alleles—alternative forms of a gene
– Purple vs. white flowers
• Dominant allele—trait that is expressed or
seen.
• Recessive allele—trait that is not seen. To
be seen, must be present in 2 copies.
Terms and Notations
•
•
•
•
Dominant alleles = capital letters
Recessive alleles = lower case letters
Dominant allele is always written first
Homozygous individual—alleles are the
same (EX: PP or pp)
• Heterozygous individual—alleles are
different (EX: Pp)
Terms and Notations Continued
• Genotype—set of alleles, or genes.
• Phenotype—physical appearance of a trait.
Genotype Phenotype
PP
Pp
pp
purple flowers, homozygous dominant
purple flowers, heterozygous
white flowers, homozygous recessive
Punnett Squares
• Diagram that predicts the outcome of a
genetic cross
• Top represents one parent, side the other
• Letters in the box indicate the possible
genotypes of the offspring.
Example Punnett Square
¼=YY homozygous dominant
2/4=Yy heterozygous
¼=yy homozygous recessive
Y
Yy—heterozygote
Y
y
YY
Yy
Yy
yy
Yy—heterozygote
y
Inheritance of Traits
• Pedigree—family history that shows how a
trait is inherited over several generations.
• Helps identify carriers of genetic disorders
• Carriers—individuals who are
heterozygous for a genetic disorder but do
not show symptoms—can pass the mutant
allele to their offspring
Example Pedigree
Male
Male with disorder
Female
Female with disorder