REVIEW OF DNA


DNA stands for
Deoxyribonucleic
Acid.
DNA contains the
genetic code and the
“working
instructions” for a
cell.
Structure of DNA


DNA, which is a nucleic acid, is made up of
monomers known as nucleotides.
Parts of a DNA nucleotide
- 5 carbon sugar called
Deoxyribose
- Phosphate group
- Nitrogen Base –
Adenine (A),
Thymine (T),
Guanine (G),
Cytosine (C)
Bonding



Hydrogen bonds form
between the nitrogen
bases of two strands.
These hydrogen bonds
occur between 2 specific
nitrogen base pairs: A – T
and G – C
Memory Helper for
base pairing rules:
straight letters pair
together (A – T) and
curved letters pair
together (G – C)
DNA Replication
 DNA replicates during S
phase of interphase.
This occurs before cell
division (mitosis).
 DNA replicates in the
nucleus, while in the
form of chromatin.
 DNA replicates according
to Chargaff’s base pairing
rules : A – T, G – C
DNA Replication – The Process
 Replication takes place in a 2 Step Process:
1. An enzyme breaks the hydrogen bond that is
between the nitrogen bases, unzipping DNA.
The DNA molecule is unzipped in several places ~
like a “broken zipper”.
 2. After DNA is unzipped,
a second enzyme, called
DNA polymerase helps
bind free nucleotides to
the exposed nitrogen
bases. They bind
according to Chargaff’s
rules, so A – T, G – C.
 This continues along
both strands of DNA in
both directions.
• The end result is 2
identical DNA
molecules.
• Each new DNA
molecule consists of
one original strand
and one newly
formed strand.
• These 2 DNA
molecules are the
sister chromatids that
undergo mitosis.
Remember this???
RNA
 RNA stands for Ribonucleic acid
 RNA carries out the instructions of DNA by
protein synthesis.
 DNA is too large (double stranded) to leave the
nucleus, so RNA (which carries the code of DNA),
leaves the nucleus through the nuclear pores and
help make proteins at ribosomes out in the
cytoplasm.
Structure


RNA, which is a nucleic acid, is
made up of monomers known
as nucleotides.
Parts of a RNA nucleotide
 5 carbon sugar called
ribose
 Phosphate group
 Nitrogen Base –
Adenine (A), Uracil (U),
Guanine (G), Cytosine
(C).
RNA is a single helix
molecule.
Nucleotide
Compare & Contrast of DNA & RNA
Characteristic
DNA
RNA
Sugar
Deoxyribose
Ribose
Base
Thymine
Uracil
Structure
Double Helix
Single Helix
Types of RNA

There are 3 forms of RNA involved in carrying out the
genetic instructions of DNA:
1. mRNA – messenger RNA; It carries the instructions
(message) from DNA in the nucleus to the Ribosomes.
2. tRNA – transfer RNA; It carries (transfers) the amino
acids to the ribosome according to the message of
mRNA.
3. rRNA – ribosomal RNA; It is part of the structural
component of the ribosomes; it also produces enzymes
needed to bond the amino acids together to form the
proteins.
Protein Synthesis
Protein synthesis occurs in 2 steps:


The genetic material contained in DNA must be
transcribed (rewritten) onto an RNA molecule.
The message contained in RNA must be translated
(converted) into a specific molecule.
Transcription (DNA transferred to RNA)
 Transcription takes
place in the nucleus
and allows for the
genetic code of DNA
to be carried out to
the ribosome. This
happens in G1 of
interphase.
(Remember G1 where
normal cell activities
take place?)
Steps of Transcription
1. The enzyme,
RNA
polymerase,
unzips part of
the DNA
molecule.
2. RNA polymerase
adds
nucleotides
(A,U,G,C)
according to
Chargaff’s rules
(A – U, G – C),
and RNA is
synthesized.
Initially mRNA has too many nucleotides, some code for
proteins and some do not.
 Nucleotides in mRNA are classified as introns or exons.
Introns – not required for protein synthesis – they are
removed from mRNA.
Exons – required for protein synthesis – kept and
bonded together once the introns are removed.
“EX”ONS = “EX” pressed
“IN”TRONS = “IN” between the good stuff
3. Then the new mRNA strand breaks apart from DNA,
leaves the nucleus through the nuclear pores and goes
to the ribosomes, the site of protein synthesis.
There are 2 important ways
that transcription differs from
replication:
 Only one side of
the DNA molecule
is copied in
transcription, both
sides are copied in
replication.
 In transcription,
RNA is used, so the
nitrogen base that
pairs with adenine
is uracil, in DNA
replication,
adenine pairs with
thymine.
 http://www.johnkyrk.com/DNAtranscription.html
 RNA being made! - transcription
Translation (RNA transferred to Proteins)
 Translation takes
place in the
cytoplasm at
ribosomes. The
message is carried
by mRNA, it is
interpreted by
tRNA, and the
final product is a
protein.
Codon
 Codon = a group of 3 nucleotides on mRNA that codes for
a specific amino acid.
 A codon can be made up of any combination of the 4
nucleotides (A, U, G, C), therefore there are up to 64 possible
codons. (b/c 4 x 4 x 4 = 64) Ex: AAA, AAU, AAG, AAC
 Many different codons represent the same amino acid b/c there
are 64 possible codons, and only 20 amino acids.
 Amino acids are the building blocks (or monomers) of
proteins. Even though there are only 20 amino acids, there are
many, many proteins. It is the number and arrangement of the
amino acids that makes every protein unique.
 Start codon: AUG –
starts the making of the
amino acid chain.
 Stop codon: signal to
stop the making of the
amino acid chain.
 Genetic Code is universal,
therefore the codons
codes for the same amino
acid in all organisms.
The mRNA message is translated by the ribosome using
these codons.
1st letter
2nd letter
3rd
letter
 Practice: AUG _________
CGG _________
GAA _________
UCA _________
GAG _________
AAU _________
tRNA
 The tRNA
molecule
(which is out
in the
cytoplasm)
has an amino
acid on one
end and an
anticodon
on the other
end.
 Anticodon – an anticodon is a group of 3 nucleotides
on tRNA that pairs up to the codon on mRNA.
 Acts as a binding site - The anticodon binds to the
mRNA codon according to base pairing rules (A-U, G-C)
and insures that the proper amino acid is brought to
the ribosome.
 tRNA - transfers the amino acids specified by the
mRNA codon to the inside of the ribosomes for protein
synthesis. The amino acid is then transferred to the
growing polyopeptide chain. (polypeptide chain =
becomes a protein)
Building of a protein





The mRNA molecule slides through the
ribosome one codon at a time.
The specified amino acids are carried to the
ribosome by tRNA.
As another tRNA arrives at the ribosome, the
tRNA already there is bumped off, but leaves its
amino acid behind.
This process continues and a chain of amino
acids forms until a stop codon is reached.
Then the ribosome releases the amino acid
chain, which coils and folds to form a protein.
Steps for translation:
• mRNA attaches to the
•
•
•
•
•
•
ribosome
mRNA message in the form
of codons is read by the
ribosome.
Amino acids are made as the
mRNA is being read
tRNA transfers these amino
acids to the inside of the
ribosome
Amino acid chains form in
the ribosome
Stop codon is reached
Protein (amino acid chain) is
released and tRNA and
mRNA is released.
 http://learn.genetics.utah.edu/units/basics/transcribe
/
 Suggested Study Questions
 Pp 315 (3-10, 12-16, 18-21, 23)