Replication of DNA
To be able to explain:
• the semi-conservative mechanism of DNA
replication, including the role of DNA polymerase.
• How DNA acts as a genetic code by controlling the
sequence of amino acids in a polypeptide.
• That codons for amino acids are triplets of
nucleotide bases.
• Role of nucleic acids in protein and enzyme
synthesis
• The structure of RNA.
Function of DNA
• DNA is the genetic material, and genes are made of
DNA.
• DNA therefore has two essential functions: replication
and expression.
• Replication means that the DNA, with all its genes,
must be copied every time a cell divides.
• Expression means that the genes on DNA must control
characteristics.
• Characteristics are controlled by genes through the
proteins they code for.
Replication - DNA Synthesis
• DNA is copied, or replicated, before
every cell division.
• Two new strands are built up by
complementary base-pairing onto the
two old strands.
Replication
• The enzyme helicase unwinds and unzips DNA
• The enzyme DNA polymerase joins the new
nucleotides to each other
• DNA replication can takes a few hours.
• Bacteria can reproduce so fast is that they have a
relatively small amount of DNA.
• In eukaryotes replication is speeded up by taking
place at thousands of sites along the DNA
simultaneously
SEMICONSERVATIVE
REPLICATION
Original
polynucleotide
strand
New
polynucleotide
strands
Structure in relation to function
• sequence of bases so code for
information storage
• long molecule so large amount of
information stored
• complementary pairing so information can
be replicated
• double helix makes molecule stable so
prevents code being corrupted
• chains held together by weak hydrogen
bonds chains so chains can unzip for
replication / transcription
RNA
• RNA is a nucleic acid like DNA, but with
4 differences:
– RNA has the sugar ribose instead of
deoxyribose
– RNA has the base uracil instead of
thymine
– RNA is usually single stranded, but can
fold into 3-dimentional structures, like
proteins.
– RNA is usually shorter than DNA
Messenger RNA (mRNA)
• mRNA carries the "message" that codes for a
particular protein from the nucleus (where
the DNA master copy is) to the cytoplasm
(where proteins are synthesised).
• It is single stranded and just long enough to
contain one gene only.
• It has a short lifetime and is degraded soon
after it is used.
Ribosomal RNA (rRNA)
• rRNA, together with proteins, form
ribosomes
• rRNA is coded for by numerous genes in
many different chromosomes.
• Ribosomes free in the cytoplasm make
proteins for use in the cell.
• Ribosomes attached to the RER make
proteins for export.
Transfer RNA (tRNA)
• tRNA is an “adapter” that matches amino
acids to their codon.
• tRNA is only about 80 nucleotides long, and
it folds up by complementary base pairing to
form a looped clover-leaf structure.
• At one end of the molecule there is always the base
sequence ACC, where the amino acid binds.
• On the middle loop there is a triplet nucleotide
sequence called the anticodon.
• There are 64 different tRNA molecules, each with a
different anticodon sequence complementary to the
64 different codons.
• The amino acids are attached to their tRNA molecule
by specific aminoacyl tRNA synthase enzymes.
Comparison of DNA and RNA
DNA
Sugar
Bases
Shape
Where found
Deoxyribose
ATCG
Double helix
In nucleus
Very stable
Permanent
Types
One
Concentration
Constant
Molecular mass V large
Stability
Permanence
RNA
Ribose
AUCG
Single strand
Nucleus and
cytoplasm
Less stable
Temporary
Three – m t and r
Variable
smaller
How structure is related to function
• Function
DNA
1. To pass on hereditary information
2. To control production of proteins
Structure
F
How structure is related
Double helix is
stable
1
Maintains information store
Bases act as code
2
2
Order acts as instruction
4 bases
2 strands
connected by
hydrogen bonds
Helix encloses
hydrogen bonds
Long, large
molecule
Folds into coil
Allow for a triplet code
1&2 Allows unzipping for copying
1
Stability
1&2 Holds a lot of information together
1
Compact shape
m RNA
Function 1. Translation
Structure
Function How structure is related
Can pass through nuclear
Single strand
pore
Bases as
codon
Act as code for amino
acids
tRNA –
pick up their specific amino acids
from the cytoplasm
Transcription - RNA Synthesis
• DNA never leaves the nucleus, but proteins
are synthesised in the cytoplasm
• A copy of each gene is made to carry the
“message” from the nucleus to the
cytoplasm.
• This copy is mRNA, and the process of
copying is called transcription.
t RNA
Structure
20 forms
Anticodon
Function 1. Amino acid
activation
Function How structure is related
For 20 amino acids
To pair with codon on
m RNA
• Only one strand of RNA is made.
• The DNA stand that is copied is called the
template or sense strand because it contains
the sequence of bases that codes for a
protein.
• The other strand is just a complementary
copy, and is called the non-template or
antisense strand.
• The new nucleotides are joined to each
other by strong covalent
phosphodiester bonds by the enzyme
RNA polymerase.
• The introns are cut out and the exons are
spliced together by enzymes.
• The result is a shorter mature RNA
containing only exons.
• The introns are broken down.
• The mRNA diffuses out of the nucleus
through a nuclear pore into the cytoplasm.
Translation - Protein Synthesis
• A ribosome attaches to the mRNA at an
initiation codon (AUG).
• A codon is a sequence of three
nucleotides
• The ribosome encloses two codons.
1.
•First the mRNA attaches itself to a ribosome (to the small subunit).
•Six bases of the mRNA are exposed.
•A complementary tRNA molecule with its attached amino acid
(methionine) base pairs via its anticodon UAC with the AUG on the
mRNA in the first position P.
•Another tRNA base pairs with the other three mRNA bases in the
ribosome at position A.
•The enzyme peptidyl transferase forms a peptide bond between the two
amino acids.
•The first tRNA (without its amino acid) leaves the ribosome.
Translation 2
The ribosome moves along the mRNA to the next codon (three
bases).
The second tRNA molecule moves into position P.
Another tRNA molecule pairs with the mRNA in position A bringing
its amino acid.
A growing polypeptide is formed in this way until a stop codon is
reached.
End of Translation
A stop codon on the mRNA is reached and this signals the ribosome
to leave the mRNA. A newly synthesised protein is now complete!
Translation - animation