DNA and RNA
What does DNA look like?
What are the elements that makeup
DNA?
DNA
Structure=
String of
nucleotides (sugar,
phosphate, base)
*Adenine
*Thymine
*Guanine
*Cytosine
•purines - adenine, guanine
•pyrimidines - cytosine,
thymine
Figure 12–5 DNA Nucleotides
Purines
Adenine
Guanine
Phosphate
group
Pyrimidines
Cytosine
Thymine
Deoxyribose
Francis Crick and James Watson (1953)
Twisted
Double
Helix
Each strand of
the helix is a
chain of
nucleotides
What holds the
strands
together?
Hydrogen Bonds are
the “glue” that keeps
the two strands
together
Always Together….Great Couple
D
H
U
Y
E
Y
A
T
N
M
N
O
I
I
I
S
N
N
N
I
E
E
E
N
E
A&T
G&C
Figure 12–7 Structure of DNA
Nucleotide
Hydrogen
bonds
Sugar-phosphate
backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
How is DNA organized in a chromosome?
ONE nucleus of ONE human cell
= more than 1 meter of DNA!!!
Remember Chromatin??
What exactly is chromatin?
DNA tightly coiled around proteins
forming Chromatin which pack
together to form thick fibers.
Prokaryotic Chromosome Structure
Section 12-2
Chromosome
E. coli bacterium
Bases on the chromosome
Chromosome Structure
of Eukaryotes
Chromosome
Nucleosome
DNA
double
helix
Coils
Supercoils
Histones
How can DNA use its
double-stranded structure to
its advantage for
replication???
“Something Old…Something New”
DNA Replication
When does this occur in the cell cycle?
Template
1) Enzymes un-twist and unzip the molecule (break H
bonds between base pairs).
2) Each strand serves as a template (something “OLD”)
3) Free nitrogen bases form bonds and make complementary strands
(Something “NEW”)
4) DNA Polymerase bonds the nucleotides and proofreads the molecule
Figure 12–11 DNA Replication
New strand
Original
strand
DNA
polymerase
Growth
DNA
polymerase
Growth
Replication
fork
Replication
fork
New strand
Original
strand
Nitrogenous
bases
DNA vs. RNA


1.
2.
3.
RNA – also a long chain of nucleotides (5carbon sugar, phosphate group, nitrogenous
base)
Differences:
RNA sugar = ribose, instead of deoxyribose
RNA – usually single-stranded
RNA has uracil to replace thymine (so U
binds with A)
“Always United & Great Couple”
RNA is in charge of
assembling Amino Acids into
Proteins
From DNA(Gene) to Protein
The players:
DNA - sequences of nitrogen bases forms the genetic code
mRNA - messenger RNA - makes a copy of the DNA in the
nucleus and brings it to the rRNA
tRNA - transfer RNA - reads the mRNA and brings
specific amino acids to the rRNA
rRNA - ribosomal RNA - location of protein synthesis uses
tRNA to make proteins
Step 1: Transcription = recording the
message



Occurs in nucleus
New mRNA strand forms from one of DNA
strands (creating the message)
Let’s Practice…
Transcription Practice

Transcribe the DNA molecule below:
ATTATCGCGTAATGCTAATAGC
TAATAGCGCATTACGATTATCG
Template
mRNA
transcript
AUUAUCGCGUAAUGCUAAUAGC
Figure 12–14 Transcription
Adenine (DNA and RNA)
Cystosine (DNA and RNA)
Guanine(DNA and RNA)
Thymine (DNA only)
Uracil (RNA only)
RNA
polymerase
DNA
RNA
Step 2: Editing of mRNA
Introns are removed – non coding regions of the DNA molecule
Exons remain – sequences that will be expressed
Step 3: Translation = Protein
Synthesis



Occurs at ribosome
tRNA reads mRNA which has message from
genetic code (DNA)
Genetic code is read 3 letters at a time, so
each word is 3 bases long
Every 3 letters is a CODON

Each codon codes for a specific amino acid.
What does an Amino Acid do again?
Helps make proteins!
•We need codons for Protein Synthesis (Translation)
•They are like directions to make proteins
•Every set of directions tells you where to START and
where to STOP
•We too have these, we call them the “start and stop
codons”
Codons to remember…

START is always:

AUG

STOP is always:
 UAA
 UAG
 UGA
Translation Explained
methionine


tRNA
UAC
mRNA AUGCGCAUAACGCAU
Start
Codon
Alternate sequence:
Stop
codon
There are 20
different amino
acids to be coded
for.
There are 64
possible codons.
Start codon
Figure 12–17 The Genetic Code
Translation Practice

Make a polypeptide (chain of amino acids) chain
from the mRNA molecule
AUGAUCGCGUAUUGCUACUAG - mRNA
methionine-isoleucine-alanine-tyrosine-cysteine-tyrosine STOP
Figure 12–18 Translation
Section 12-3
Figure 12–18 Translation (continued)
Section 12-3
Mutations - changes in the DNA
sequence
Gene mutation- changes in a single gene


•
Point Mutations - substitution of one nucleotide for
another
•
Frame Shift Mutations - shifting of the genetic code due
to insertion or deletion of nucleotide
Chromosomal mutation changes in the entire
chromosome (containing many genes)
Figure 12–20 Chromosomal Mutations
Deletion
Duplication
Inversion
Translocation
Mutation Analogy
THE FAT CAT ATE THE RAT
substitution THE FAT CAT ATE THE CAT
*The letter “C” was substituted for the “R”
insertion THE FAT CAT ATE THE RAT
THC EFA TCA TAT ETH ERA T
C
Deletion
*Because the “C” was added, all other letters
shifted down, thereby changing the amino
acids that are made.
THE FAT CAT ATE THE RAT
THE FTC ATA TET HER AT
*Again, the amino acids will change b/c the “F” was
removed
Mutation Practice
AUGAUCGCGUAUUGCUACUAG - mRNA

What will the new amino acid be if the 5th
nucleotide is substituted with an adenine?
ASPARAGINE
What will the new amino acid sequence be if
a guanine is inserted between the 9th and 10th
nucleotide ?
G
GUA = VALINE
When a mutation occurs…


If the amino acid sequence is stopped early
(a STOP codon is reached) = Nonsense
If the amino acid sequence continues but the
wrong amino acids are coded for = Missense
Putting it all together

What is the amino acid sequence that forms from
the following DNA molecule? (DNA synthesis)
Template
TACTACACCGTATAACAGGGCCTAGCAACT
ATGATGTGGCATATTGTCCCGGATCGTTGA
(Transcription)
DNA TACTACACCGTATAACAGGGCCTAGCAACT
mRNA AUGAUGUGGCAUAUUGUCCCGGAUCGUUGA
(Translation)
amino acid sequence
methionine-methionine-tryptophan-histidineisoleucine-valine-proline-aspartic acid-arginine-stop