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Structure, Replication & Protein Synthesis
DNA
DNA (deoxyribonucleic acid) is the
hereditary material for all living things.
 contains the codes for constructing
proteins in the body, including the
various enzymes.
 Made up of repeating units called
nucleotides.

DNA
Nucleotides are composed of a
phosphate group, a deoxyribose, and a
nitrogen base.
 The nitrogen bases are

 Adenine
 Thymine
 Cytosine
 Guanine
Nucleotide
DNA Structure


The structure of DNA
was determined in the
1953 by James
Watson and Francis
Crick.
They proposed that
DNA is made from
two chains that wrap
around each other in
the shape of a double
helix. Kind of like a
winding spiral
staircase
DNA Structure



Watson and Crick used
findings from other
scientists to develop the
final version of the DNA
model.
Rosalind Franklin and
Maurice Wilkins were
researchers that had taken
X-ray diffraction
photographs of DNA
crystals.
These photographs helped
to reveal the double helix 3D structure of DNA
DNA Structure
Another scientist that
paved the way for
Watson and Crick
was a biochemist
named Erwin
Chargaff.
 Chargaff set out to
see whether there
were any differences
in DNA among
different species.

Chargaff concluded that almost all DNA
no matter what organism it comes from
has certain properties.
 He also concluded that the amount of
adenine (A) approximately equals the
amount of thymine (T), and the amount
of guanine (G) approximately equals the
amount of cytosine (C). In other words
 (A + G) = (T + C)

Chargaff’s Rule
DNA Structure

With this information from Rosalind
Franklin, Chargaff and other scientists,
Watson and Crick were able to develop
the model of DNA and win the Nobel
Prize in 1962.
DNA is packaged into
chromosomes
DNA Replication
The process by which DNA is copied.
 Why do you think DNA needs to be
copied?

DNA Replication
The process by which DNA is
copied.
 Why do you think DNA needs to be
copied?
 Before a cell divides by mitosis,
meiosis, or binary fission it must
copy its DNA so that each new cell
will have the genetic information to
survive.

DNA Replication

Steps
 Unzip
 String on new nucleotides
 Linking and Release of two new strands
DNA Replication

The first thing that must happen to copy
of double strand of DNA is …
DNA Replication – Step 1
The double helix must
be unzipped.
 The protein called
Helicase is the
UNZIPPER
 The weak hydrogen
bonds are broken
between base pairs by
helicase

The Y shaped region that is formed
when the two strands are separated is
called the REPLICATION FORK.
 Remember that the two DNA strands run
anti-parallel.

DNA Replication – Step 2
Proteins call DNA Polymerase add
complementary nucleotides to each of
the parent strands.
 A’s bond with T’s and C’s bond with G’s
 Each parent strand acts as a template
for the two new strands.

DNA Replication – Step 2
Proteins called DNA Polymerase add
complementary nucleotides to each of
the parent strands.
 One of the strands is copied
continuously from the 3’ end to the 5’
end.
 The is called the LEADING STRAND

DNA Replication – Step 2
The other DNA strand is
copied discontinuously
from the 5’ to the 3’
direction. Making little
unfinished strands.
 These are called Okazaki
fragments
 Short bits of DNA that
make up the lagging
sttrand

DNA Replication – Step 3
DNA polymerase finishes replicating and
fall of off the strands
 DNA ligase links the short unfinished
strands of DNA to create one long
strand.

Semi - Conservative

DNA replication is called semi
conservative because each daughter
DNA consists of half of the parental DNA
and half of the new DNA.
Mutations
What is a mutation?
 Mistakes in DNA replication

 Like if and adenine pairs with a cytosine
instead of a thymine.
 The bases sequence of the newly formed
DNA molecule is different from that of the
original DNA molecule.
 DNA polymerase acts as a proofreader to
prevent most mutations. It can change the
wrong base and correct it with the correct
one.
Mutations

What causes mutations?
 Random
 Chemicals (like carcinogens in cigarettes)
 Radiation (from the sun)
Protein Synthesis

Remember this!!!
DNA
mRNA
Protein
 DNA is transcribed to mRNA.
mRNA takes the code for a gene out of
the nucleus to a ribosome where a
protein is made.
 This is called the CENTRAL DOGMA

RNA
DNA is a huge molecule. But it holds
important information. But cannot leave
the nucleus.
 RNA is a nucleic acid that can hold a
small amount of information.
 mRNA is a small molecule that can
leave the nucleus!
 mRNA is not the only type of RNA.

RNA
What does RNA stand for?
 Ribonucleic Acid
 It is a nucleic acid just like DNA but…..

RNA
There are some
differences.
 Contains a
phosphate, a sugar
(ribose) and a base
(A, U, C, G)
 U is for uracil. There
is no Thymine in RNA

RNA
RNA is single
stranded instead of
double stranded.
 There are three types
of RNA

 mRNA
 rRNA
 tRNA
Protein Synthesis
Now we are going to make a protein!
 The first step in protein synthesis is…

TRANSCRIPTION
Transcription
The process by which the genetic
instructions in a specific gene are
rewritten into an RNA molecule
 Takes place in the nucleus
 RNA polymerase is the protein that
catalyses this process.

Transcription
A section of DNA serves as a template
for an mRNA molecule.
 Except instead of Adenine pairing with
Thymine Adenine pairs with Uracil.

 A --- U
 C --- G
Transcription
Translation
Every three nucleotides on an
mRNA molecule is called a
codon
 Each codon codes for an
amino acid
 Amino Acids are the building
blocks of proteins
 Remember 21 amino acids?

Translation
Is the decoding of mRNA into a string of
amino acids (protein)
 All three types of RNA are involved in
translation.

 mRNA carries the info from DNA out of the
nucleus.
 rRNA makes up the ribosomes where proteins
are made
 tRNA carries the amino acids one by one to the
ribosome to be linked together to form a protein.