Download DNA’s Discovery and Structure

DNA’s Structure
 DNA
is composed of nucleotides
 DNA
contains four nitrogen bases:
adenine(A) cytosine(C) guanine(G) thymine (T)
 A & G are purines
C & T are pyrimidines
DNA’s Structure

DNA is a double helix
-2 strands of DNA nucleotides joined to form a
“twisted ladder”
-the strands are held by hydrogen bonds between
the nitrogen bases
Chargaff’s Rule

The nitrogen bases form the “steps” on the
DNA ladder by complementary pairing
A == T
C == G
T == A
G == C
A == T
T == A
Sugarphosphate
backbone
A always pairs with T
C always pairs with G
DNA Replication – What and Why

Replication = DNA making copies of itself
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DNA must be copied before a cell can divide
Each new cell will have
a complete set of DNA
3 Models of Replication
3 Models of Replication:
Semi-conservative Replication
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Each strand in a DNA molecule
is used as a template to build
a new strand using
complementary base pairing
Results in new molecule
with one original DNA strand
and one new strand
DNA Replication: Process

Step 1: Replication begins when the enzyme
DNA helicase opens the DNA forming
replication bubbles
DNA Replication: Process

The nitrogen bases on the
original DNA strands are
exposed in the replication
bubbles.
 the new DNA strands are
built on the exposed
nitrogen bases
DNA Replication
 The ends of the replication bubbles known as the
replication fork is where replication begins
DNA Replication – DNA Polymerase
Step 2: The enzyme DNA polymerase
brings new nucleotides to the
replication fork
- it pairs them according to base
pairing rules A pairs with T
C pairs with G
DNA Replication- Orientation

5’
The DNA polymerase reads
the original DNA strands from
3’- 5’
- as a result, the new DNA
strands are built from 5’-3’
3’
3’
5’
DNA Replication – Leading Strand

Leading strand- is built
toward the replication fork
–the replication of the leading
strand is continuous
- as helicase opens the
molecule, DNA polymerase
can continue to add new
nucleotides
DNA Replication – Lagging Strand

Lagging strand- replication
begins at the replication
fork.
- As the new DNA strand
elongates, it grows away
from the replication fork
DNA Replication- The Big Picture
Each Bubble has 2 Forks – each fork has a leading and
lagging strand
DNA Replication

The process continues until 2 complete copies
of the DNA are produced

Each copy of the DNA contains one strand of
DNA from the original DNA molecule and one
new strand that was produced by replication
Replication Animation
DNA Replication
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Step 3 : Termination = DNA polymerase
reaches the end of the DNA segment and the
replication process stops
Step 4 : Repair Process Takes place
–
Enzymes fix any mistakes in the replication process
DNA Replication
Reasons For DNA
Replication
• DNA replication must be done before a cell
reproduce.
• DNA replication is essential in passing on of genetic
material from one generation to the next.
• It is essential in cellular reproduction, growth/repair,
and adaptation due to genetic mutations.
Why Is This Process So
Accurate?
There are several reasons :
1- One half the DNA serves as a template for DNA to
copied from, therefore the copying process is
relatively simple.
2- The way the base pairs pair up chemically and the
space in which they have to fit in force the
nitrogenous base pairs to always be a purine and
pyrimidine.
3- The proofreading process fixes most errors in the
process and will delete any problems in the DNA
sequence.
Protein Synthesis
From DNA to Protein
Protein Synthesis

Protein Synthesis is the process that
cells use to produce protein.
- it involves 2 distinct phases
Transcription – occurs in the nucleus
involves the creation of mRNA
Translation – occurs in the cytoplasm
at a ribosome – the protein recipe is
“read” and the correct protein is made
Function of DNA:
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
controls the function of cells
contains recipes for proteins.
-Proteins are
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Enzymes to run chemical reactions
Hormones
Numerous tissues and structures
Proteins are chains of amino acids.
amino acid + amino acid + amino acid = protein
The order of amino acids determines protein
shape
Shape determines function
DNA recipe consists of the order of amino acids
for each protein
- the recipes are known as genes
DNA contains
recipes for
all of the proteins
in living things
-these recipes are
called genes
Recipe has to get from DNA to the
ribosome which builds the protein
Transcription: makes a copy of the
protein recipe
This is necessary because:
 DNA cannot leave the nucleus!!!
 Proteins are made on ribosomes in
the cytoplasm.
mRNA provides the solution


Messenger ribonucleic acid
mRNA is a copy of the protein recipe
that can leave the nucleus
mRNA – messenger RNA
mRNA
is a copy of the recipe for a
protein. It is a copy of a gene
- it can leave the nucleus
- takes the recipe to the ribosome
where it is converted to a protein
mRNA carries the recipe from DNA
to the ribosomes
Meet mRNA:
RNA has three structural differences
from DNA
 Structure of RNA
 1. Sugar is ribose
 2. Single strand
 3. Uracil replaces
thymine as a
base pair
Transcription: Initiation
The Process Begins
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The enzyme RNA polymerase
finds the beginning of a protein
recipe called the promotor
- promotor = a series of
nucleotides that indicate the
start of a protein recipe
The RNA polymerase opens the
DNA molecule at the promotor
Transcription: Initiation
 The
RNA polymerase uses one DNA
strand as a template to build the
mRNA
- only one of the DNA strands
contains the protein recipe
- the strand with the recipe is the
template strand
- the strand without the recipe is
the non-template strand
- it is not copied
Transcription: Elongation
Building the mRNA Molecule
 RNA
polymerase brings RNA
nucleotides to the template strand
-pairs them with their
complements on the original
DNA molecule
-this follows the base pairing rules
except that uracil replaces
thymine
- Adenine on DNA is paired with
Uracil (U) on the new mRNA
Transcription: Elongation


The RNA polymerase reads the
template strand in the 3’ to 5’
direction
RNA polymerase builds the mRNA in
the 5’ to 3’ direction
Transcription: Termination
The Process Ends


the RNA polymerase continues to
add new nucleotides until it reaches
the terminator
- the terminator is a sequence of
nucleotides that indicates the
end of the recipe
the mRNA drops off the DNA
-this is pre-mRNA it needs further
processing before it can be translated
Processing pre-mRNA
 Pre-mRNA
contains sections of nucleotides
called introns
-introns are sections of mRNA that don’t
contain information needed to build the
protein
-they are extras and must be removed
before the protein can be built
 Pre-mRNA also contains sections called
exons
-these contain the protein recipe and are
joined to form the finished or mature
mRNA
Summary
1.
2.
3.
4.
5.
DNA contains recipe for
protein – can’t leave
nucleus
RNA polymerase opens
DNA molecule at recipe
RNA polymerase builds
a complementary
mRNA copy of the
protein recipe
pre-mRNA is processed
and the introns are
removed
mRNA takes recipe to
ribosome outside
nucleus
Vocabulary
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Transcription
Gene
mRNA
Ribose
Uracil
RNA polymerase
Promotor
Template Strand
Non-Template Strand
Terminator
intron
exon
Translation
From mRNA to Protein

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There are twenty different amino
acids that build proteins
There are 64 different
triplets/codons
Each amino acid is coded for by
more than one triplet/codon
The Players
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mRNA:messenger RNA
- carries protein recipe from the
nucleus
tRNA: transfer RNA
-brings amino acids to the
ribosome
Ribosome: the site of protein
synthesis
- made of rRNA (ribosomal RNA )and
Protein
The Process of Translation
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mRNA takes recipe to
the ribosome in
cytoplasm
ribosome attaches to
the mRNA
Translation
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The ribosome moves along the mRNA
until it reaches the “Start” codon
Start codon = AUG signals the start
of the recipe
AUG also codes for the amino acid
methionine
The process of Translation cont.
•A molecule of transfer RNA brings the
amino acid called for by the mRNA to the
ribosome
•transfer RNA = tRNA
The process of Translation cont.
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A second tRNA
bringsthe second
amino acid to the
ribosome
The amino acids
are joined
together to begin
the protein
The process of Translation
concluded
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The ribosome moves over 1 codon
and another tRNA molecule brings
another amino acid
The process continues until the stop
codon on the mRNA is reached
-the stop codon = the end of the
protein recipe
Meet tRNA

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each molecule of tRNA carries
a specific type of amino acid
- each tRNA molecule can
only carry one type of
amino acid
The tRNA has a group of 3
nucleotides at the base called the
anticodon
How does tRNA know which
amino acid goes where?
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The anticodon on tRNA is complementary
to a mRNA codon
the amino acid that a tRNA molecule
carries is the amino acid that the
complementary mRNA codon codes for
Example:
mRNA codon = GAC = aspartic acid
tRNA anticodon = CUG carries only
aspartic acid
Vocabulary
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tRNA
Triplet
Codon
Anticodon
Start codon
Stop codon
THE DNA CODE
The Key to Protein Synthesis
The Question of DNA

DNA stores information to build proteins in
sequences of nucleotides
- DNA nucleotides contain one of 4 nitrogen
bases A T C G
- there are 20 different amino acids used to
build protein
Problem: How to code for 20 amino acids
with only 4 nitrogen bases?
Solution: Use groups of 3 nucleotides to code
for each amino acid
Why are 3 nucleotides required?
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Using one nucleotide can only code for 4
amino acids
Using pairs of nucleotides produces 16
combinations – can code for 16 amino acids
Using groups of three nucleotides produces
64 different combinations – can code for all
20 amino acids
- several different groups can each code
for the same amino acid
THE DNA CODE
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The DNA code is:
- universal to all living things
-the groups of nucleotides code for the
same amino acid in all living things
3 DNA nucleotides = Triplet
- one triplet = one amino acid
Examples – TCA = Serine
CTG = Aspartic Acid
THE CODE CONTINUED

3 mRNA nucleotides = Codon
- codon is the complement of a triplet
- codon codes for the same amino acid
as the triplet it is complementary to
Example:
DNA triplet = CTG = Aspartic Acid
mRNA codon = GAC = Aspartic Acid
Triplet
Codon
Transcription
Amino Acid
Translation
DNA
Triplet
mRNA
Codon
Amino
Acid
CTG
GAC
Aspartic
Acid
CGC
GCG
Alanine
THE CODON CHART – FROM CODON TO AMINO
ACID
Codon = UGC
Cysteine
Codon = CAC
Histidine
Codon = AAA
Lysine
Codon = GCG
Alanine
Mutations

Hollywood’s images of
mutation
Mutations

Actual Mutations
in fruit flies
What is a mutation?
A mutation is any change in a cell’s DNA
 A mutation can occur in an individual gene
- results in a single changed protein
- cystic fibrosis a mutation in the
protein that makes a type of ion
channels in cell membrane
- bacterial resistance to antibiotics is an
example of a beneficial gene mutation

What is a mutation continued

A mutation can occur in a chromosome
- a chromosome contains many genes
- chromosomal mutations affect many
proteins
Examples: Down Syndrome
Edward’s Syndrome
Cri-du-Chat
What Causes Mutations?
Can be caused by mutagens- a physical or
chemical cause of mutation. Examples: UV
light, radiation, drugs, and benzene.
 Mutagens are often also carcinogens –
anything that causes cancer
 Can be natural, random events.
- mutations occur in 1/100,000 DNA
replications
 Mutations do not have to be bad (evolution)

Point Mutations
A single nucleotide is altered. Can
change one amino acid in a protein
 Milk – Mile
 GGACAATCA
GGACCATCA

proline -valine-serine
proline-glycine-serine
Frameshift Mutations

A nucleotide is either inserted or deleted
from a gene.
-all of the triplets from the point of
mutation onward will be changed
Frameshift Mutations Insertion

An insertion occurs when a nucleotide is
added to a gene
Example: A nucleotide is inserted
The fat cat ate the rat
The faa tca tat eth era t
-the extra nucleotide shifts all of the
triplets that follow
Frameshift Mutations Deletions

A deletion occurs when a nucleotide is
removed from a gene.
Example: A nucleotide is removed
The fat cat ate the rat
Thf atc ata tet her at
Insertion
 GGACAATCA
proline -valine-serine
Deletion
 GGACAATCA
proline -valine-serine
GCGACAATCA
arginine-cysteine-stop
GGAAATCA
proline-leucine
Vocabulary
Mutation
 Mutagen
 Carcinogen
 Point mutation
 Frameshift mutation
- insertion
- deletion
