DNA
Deoxyribonucleic acid
DNA carries genetic information
• DNA determines an
organism’s traits
• How does DNA
achieve its control?
• By producing
proteins(enzymes)
• Enzymes are
important because the
control the chemical
reactions needed for
life
Structure of DNA
• Very long molecule
• Made of repeating subunits called
nucleotides:
Composition of Nucleotide
• Simple sugar- deoxyribose
• Phosphate group – PO4
• Nitrogen base- A carbon ring structure
that contains one or more Nitrogen’s
– guanine(G), Adenine(A)
– Cytosine(C) Thymine(T)
– 4 possible DNA nucleotides depending on
nitrogen bases
Nucleotides Cont
• Nucleotides join
together to form long
chains
• The phosphate group of
one nucleotide bonding
to the deoxyribose
sugar of an adjacent
nucleotide. This form
the backbone of DNA
• The nitrogen bases
stick out
DNA Nitrogen Bases
• In DNA, the amount
of adenine is always
equal to the amount
of thymine
• The amount of
guanine is always
equal to the amount
of cytosine
Base Pair Rules
• Adenine always pairs with thymine
• Guanine always bonds with cytosine
• A-G-T-C-C opposite strand would be
• T-C-A-G-G
James Watson and Francis Crick
double helix is like a
long zipper that is
twisted
How can organisms be so
different from each other if their
genetic material Is made of the
same four nucleotides
The differences result
from the
sequence of the
4 nucleotides
DNA Replication
• DNA is copied before
cell division
• Without replication, new
cells would only have
half of DNA from their
parents
• When a DNA molecule
replicates, 2 molecules
are formed
• Each molecule has 1
original strand and one
new strand
Steps of DNA Replication
1) Two nucleotide strands separate at their
base pairs. Hydrogen bonds between are
broken
2) Free nucleotides base pair with exposed
nucleotides
3) The sugar and phosphate parts of adjacent
nucleotide strands bond together to form
backbone of new strand
4) The process of replication produces 2
molecules of DNA. Each new molecule has
been newly synthesized from free
nucleotides in the cell
DNA Transcription
DNA encodes the
instructions for
making proteins
thereby controlling
the cells
RNA
• Ribonucleic acid
• Different from DNA
in 3 ways
– 1. RNA is single
stranded
– 2. Ribose-sugar
– 3. Uracil takes the
place of thymine
RNA’s JOB
RNA take
instructions from
the DNA on how a
protein should be
assembled. They
then assemble the
protein, amino acid
by amino acid until
the protein is made.
Three types of RNA
• mRNA- messenger RNA -Brings information
from the DNA in the nucleus to the cell’s
cytoplasm
• rRNA- ribosomal RNA- Ribosome's made of
ribosomal RNA clamp onto the mRNA and
use its information to assemble the amino
acids in the correct order
• tRNA- transfer RNA- transports amino acids
to the ribosome to be assembled into a
protein
Transcription- takes place in
Nucleus
1) The process of transcription begins as
enzymes unzip the molecule of DNA, just as
they do during DNA replication
2) As DNA unzips, free RNA nucleotides pair
with complementary DNA nucleotides on one
of the DNA strands
3)When the process of base pairing is
completed, the mRNA molecule breaks away
as the DNA rejoins.
4)The mRNA leaves the nucleus and enters
cytoplasm
The Genetic Code
What is the genetic code?
A code is needed to convert the
language of mRNA into the language of
proteins
Amino Acids
• Building blocks of
proteins
• 20 amino acids
• For every 1 amino acid
there are 3 nucleotides
• Each set of 3
nucleotides makes a
codon (triplet)
• There are 64 codons in
the genetic code
Genetic Code Continued
• Some codons do
not code for amino
acids, they provide
instructions for
assembling proteins
• UAG is a start
codon UAA is a
stop codon
• Genetic code is
universal
Translation: From mRNA to Protein
The process of
converting the
information in a
sequence of
nitrogen bases in
mRNA into a
sequence of amino
acids that make up
protein
Translation
• Takes place in ribosomes in the
cytoplasm
• mRNA- made in nucleus, ribosomes
attach to them
• tRNA- bring the amino acids to the
ribosomes
• Correct translation-depends on the
joining of each mRNA codon with the
correct tRNA molecule
Codon and Anticodon
•
•
•
•
Group of three amino acids-CODON
Anti-codon- opposite code
A-C-A codon
anticodon is U-G-U
Steps of Translation
1) the starting end of the mRNA strand
attaches to the ribosome
2)tRNA which are carrying a specific
amino acid , approach the ribosome
3) The tRNA anticodon pairs with the first
mRNA codon and temporarily join
Steps of Translation
4) The first code from mRNA is AUG, this
signals the start of protein synthesis
5) The ribosome slides along the mRNA
to the next codon
6) A new tRNA molecule carrying amino
acid pairs with the 2nd mRNA codon
7) When the first and 2nd amino acids are
in place, an enzyme joins them by
forming a peptide bond between them
Steps of the Translation
8) As the process continues, a chain of
amino acids is formed until the
ribosome reaches a stop codon on the
mRNA strand
tRNA--
Types of Mutations
Any change in
the DNA
sequence that
also changes the
protein it codes
for