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Chapter 10 – Biology Honors
DNA
DNA is often called
the blueprint of life.
In simple terms,
DNA contains the
instructions for
making proteins
within the cell.
Why do we study DNA?
We study DNA for
many reasons, e.g.,
its central
importance to all life
on Earth,
medical benefits
such as cures for
diseases,
better food crops.
DNA by the numbers
Each cell has about 2 m of
DNA.
The average human has 75
trillion cells.
The average human has
enough DNA to go from the
earth to the sun more than 400
times.
DNA has a diameter of only
0.000000002 m.
The earth is 150 billion m
or 93 million miles from
the sun.
Chromosomes and DNA
Our genes are on
our chromosomes.
Chromosomes are
made up of a
chemical called
DNA.
How did we find out that it was DNA
that carried information?
Use of viruses; bacteriophages
(phages) infecting E.coli bacteria
Hershey-Chase experiments
The Shape of the Molecule
DNA is a very
long polymer.
The basic shape
is like a twisted
ladder or zipper.
This is called a
double helix.
Introduction to nucleotides
Nucleotides are nitrogen-containing organic substances
that form the basis of the nucleic acids DNA and RNA.
All nucleotides contain the following three groups:
a nitrogena phosphate
group
containing base
a pentose
sugar
In DNA the sugar is deoxyribose, whereas in RNA the
sugar is ribose.
Bases
There are five bases, split into two types:

adenine (A) and
guanine (G) are
purine bases.
A

thymine (T),
cytosine (C) and
uracil (U) are
pyrimidine bases.
T
G
C
DNA contains A, G, T and C, whereas RNA contains
A, G, U and C.
U
Identifying bases
Base pairing rules
Formation of nucleotides
Formation of polynucleotides
Determining the structure of DNA
The double-stranded structure
of DNA was determined in 1953
by the American biologist
James Watson and the British
physicist Francis Crick.
X-ray diffraction studies by
British biophysicist Rosalind
Franklin strongly suggested that
DNA was a helical structure.
The Austrian chemist Erwin
Chargraff had earlier showed
that DNA contained a 1:1 ratio
of pyrimidine:purine bases.
Structure of DNA
DNA Replication
Before a cell can divide, in order to
reproduce a new cell, it must
duplicate its DNA.
This ensures that each new cell will
have a complete set of DNA to carry out
its life functions.
DNA Replication
This process of making more DNA
from an existing strand is known as
DNA replication.
Replication is carried out by a series
of enzymes.
4 main enzymes - helicase, primase,
DNA polymerase, ligase
DNA Replication:
The Process
1.
2.
3.
Helicase (enzyme) separates or “unzip”the 2
strands of the double helix, by breaking
hydrogen bonds.
Primase insert the appropriate base pairs on
each side of the template strand, starting at
specific spots, then DNA polymerase extends
the nucleotide sequence all the way down the
strand.
Ligase “proofreads” the bases inserted to
make sure they were paired correctly and fills in
gaps in the phosphate-sugar backbone.
DNA replication animation
Protein Synthesis
(A completely different process
than replication, involving DNA!)
DNA leads the production
of proteins
Making proteins involves 2
complex processes:
Part 1: Transcription – “To write”
Part 2: Translation – “to change
form and interpret”
REMEMBER!
DNA codes for proteins, which are
made in ribosomes! The proteins are
what do the work!
Why proteins?
The reason proteins are made is because
most enzymes are proteins.
Proteins (specifically enzymes) control every
biochemical process in an organism.
Proteins direct synthesis of lipids,
carbohydrates, and nucleotides.
Proteins are also responsible for cell structure
and movement.
DNA transcription
DNA contains a set of instructions
coded in letter sequences of A, T, G,
C.
The first step in decoding DNA is to
copy part of the message into mRNA
(messenger ribonucleic acid).
RNA is a nucleic acid that acts as a
messenger between DNA and
ribosomes.
Structure of RNA
RNA, like DNA,
consists of a long
chain of nucleotides.
Each nucleotide is
made up of a
phosphate group, 5carbon sugar
(ribose), and a
base.
MAJOR DIFFERENCES
between RNA and DNA
DNA
1) Deoxyribose sugar
2) Double stranded
3) Bases A, G, C, T
RNA
1) Ribose - sugar
2) Single strand
3) Bases A, G, C, U.
RNA contains Uracil
in place of
thymine.
Examples of different
forms of RNA
Different forms of RNA are used
depending on the job.
All have same chemical structure
and make up.
– mRNA : messenger RNA
– tRNA : transfer RNA
– rRNA : ribosomal RNA
PROTEIN SYNTHESIS: Part 1
TRANSCRIPTION
In order to get the DNA message to
the ribosomes, which make the
proteins, we need to copy the
message onto a RNA strand. (WHY??)
TRANSCRIPTION - process which a
molecule of DNA is copied into a
complementary strand of RNA. “writing
information on an mRNA strand”
Transcription Process
1) Helicase attaches to special places
on DNA strand, which causes the
DNA strands to “unzip”.
2) RNA polymerase pairs nucleotides
with ONE side of the DNA strand, to
create a single-stranded mRNA
molecule. (U substitutes for T)
Transcription Process (cont.)
3)
4)
Special spaces on the DNA strand, called promoters,
signal the RNA polymerase where to start reading and
base pairing on one side of the DNA.
When the RNA polymerase reaches a specific terminator
sequence it stops and detaches from the DNA strand.
*These are 3 letter sequences called START and STOP codons.
Codons – 3 letter base segments on the mRNA
5) DNA strand “zips” back together and mRNA is now free to
leave the nucleus and head towards the ribosomes in the
cytoplasm to make proteins.
Transcription process
Protein Synthesis: Part 2
Translation
Once the mRNA leaves the nucleus and
arrives at the ribosomes, translation
begins.
Translation involves “translating” or
decoding of mRNA sequence into a
polypeptide (protein chain)
This process involves an elaborate system
of 2 types of RNA molecules, tRNA
(transfer RNA) and rRNA (ribosomal RNA).
Translation (continued)
tRNA - single strand of
RNA that loops back on
itself.
There are 20 different
tRNA molecules for each
of the 20 different amino
acids.
Polypeptide - many amino acids held
together in a chain by peptide
bonds. (PROTEIN CHAIN)
In order to decode the information,
mRNA is broken up into “words”or
codons that can be read by the
ribosomes.
This is how the ribosomes know which
amino acid to link to the protein
chain!
Translation: The Process
1. First, the mRNA begins to move through the
ribosomes, getting read codon by codon. (3 letter
sequence of mRNA). The start signal to begin
reading is always the START CODON sequence
AUG!
2. Each codon is matched up with the 3 letter
sequence of tRNA called an ANTICODON.
3. The tRNA anticodon will bind to AUG (anticodon UAC) carrying the first amino acid of the
polypeptide, methionine.
Translation process (continued)
1. Next, the ribosome moves down the
mRNA to the next codon. The next tRNA
binds and attaches its amino acid to the
first one, with a peptide bond.
2. This continues down the mRNA strand,
until the ribosome reaches one of the
STOP codons (UAA, UAG, UGA).
Then, mRNA and polypeptide are
released from the ribosome.
TRANSLATION PROCESS ENDS!
CH. 10 Animations
(under Helpful websites on
class website)
Ribosomes
Ribosomes are made
of 2 subunits. Each
subunit is made up of
ribosomal RNA and
protein. They come
together during this
process surrounding
the mRNA strand.
Amino Acid Genetic codes
P. 192 in textbook (Lists codon = mRNA sequence)
NOTICE!
Some codons code for the same amino acid.
(CUA, CUG, CUU, CUC all code for the
amino acid - leucine)
Specific codons
– UAA, UAG, UGA are “STOP” codons. They do
NOT code for an amino acid. They are like
periods at the end of a sentence and signify the
end of a protein chain.
– AUG is always the “START” codon and will
start every protein with a Methionine (Met).