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
Use the guided notes sheet to make notes on
the Primary – Tertiary structure of proteins on
page 29 and then Fibrous and Globular
proteins on page 31.
All students will be able to describe the structure of DNA
All students should be able to outline how DNA replicates
All students could explain the function of the enzymes
involved in replication
DNA is made up of a chain of
nucleotides
 Each nucleotide is comprised of
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 a pentose sugar (deoxyribose in DNA,
ribose in RNA)
 a phosphate
 a nitrogen containing base – adenine,
thymine, guanine, cytosine
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The phosphate
alternates with the
deoxyribose sugar to
form the backbone.
It attaches to the
OH groups at carbons
3 and 5 on the sugars
to form
phosphodiester
bonds between them
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Two complementary chains
are held together by weak
forces.
The two chains run in
opposite directions
Bases are inside the helix
because they are
hydrophobic
A forms 2 hydrogen bonds
with T
G forms 3 hydrogen bonds
with C
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Covalent bonds hold nucleotide together
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Phosphodiester bond (covalent) holds sugar
phosphate backbone together
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This bond is made by a condensation reaction
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Hydrogen bonding between complementary base
pairs holds the 2strands together
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Strands run anti-parallel – one is ‘upside down’
Happens during interphase
DNA copying needs to be accurate so the genetic
code can be passed correctly
 Helicase enzyme unwinds the helix and unzips the
strands by breaking H bonds
 The strands of DNA act as templates for the new
strands
 DNA replication is semi-conservative – what does
this mean?
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Helicase
RNA primase
DNA polymerase III
DNA polymerase I
DNA ligase
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State a role for each of four different named
enzymes in DNA replication.
(Total 6 marks)
Award [1] for any two of the following up to [2 max].
helicase;
DNA polymerase / DNA polymerase III;
RNA primase;
DNA polymerase I;
(DNA) ligase; 2 max
 Award [1] for one function for each of the named enzymes.
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helicase:
splits / breaks hydrogen bonds / uncoils DNA / unwinds DNA;
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(DNA) polymerase III:
adds nucleotides (in 5' to 3' direction) / proofreads DNA;
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(RNA) primase:
synthesizes a short RNA primer (which is later removed) on DNA;
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5.
(DNA) polymerase I:
replaces RNA primer with DNA;
(DNA) ligase:
joins Okazaki fragments / fragments on lagging strand / makes
sugar-phosphate bonds between fragments;
4 max
[6]
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Helicase holds the two strands apart
Free floating nucleotides line up with the bases on the strands through
complementary base pairing
H bonds form between bases to hold nucleotide in place
DNA polymerase catalyses condensation reaction between sugars and
phosphates to form new backbone
Now have two new strands of DNA that coil into a helix
DNA polymerase proof reads molecule to make sure no mistakes have
occurred
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The primer allows DNA polymerase III to bind and start
replication
DNA polymerase I removes the RNA primers and replaces
them with DNA
RNA primase adds short sequences of RNA to both
strands (the primer)
DNA ligase then joins the Okazaki fragments together to
form a continuous strand
One strand is replicated in a continuous manner in the
same direction as the replication fork (leading strand)
DNA polymerase III adds nucleotides to each template
strand in a 5'→3' direction
These nucleotides are initially deoxyribonucleoside
triphosphates but they lose two phosphate groups during
the replication process to release energy
The other strand is replicated in fragments (Okazaki
fragments) in the opposite direction (lagging strand)
Helicase uncoils the DNA
Helicase uncoils the DNA
RNA primase adds short sequences of RNA to both
strands (the primer)
 The primer allows DNA polymerase III to bind and start
replication
 DNA polymerase III adds nucleotides to each template
strand in a 5'→3' direction
 These nucleotides are initially deoxyribonucleoside
triphosphates but they lose two phosphate groups during
the replication process to release energy
 One strand is replicated in a continuous manner in the
same direction as the replication fork (leading strand)
 The other strand is replicated in fragments (Okazaki
fragments) in the opposite direction (lagging strand)
 DNA polymerase I removes the RNA primers and replaces
them with DNA
 DNA ligase then joins the Okazaki fragments together to
form a continuous strand
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 Explain the
significance of
complementary
base pairing. 5marks
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when DNA replicates the 2 strands separate;
each single strand acts as template for base-pair
matching;
free nucleotides of adenine bond only with thymine
nucleotides of cytosine bond only with guanine;
This copies the opposite strand of the original DNA
molecule;
replication is semi-conservative;
original order of bases is maintained;
new DNA identical to parent molecule;
significance of base-pair matching is that the information
encoded in one DNA molecule is passed to others
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
What 3 things make up a nucleotide?
Name the 4 bases
Which bases pair up?
What forms the back bone of the DNA?
In which direction does the DNA polymerase add
new nucleotides?
What bonds hold the sugar and phosphate
together?
What bonds hold the 2 strands of DNA together?
What enzyme adds the new nucleotides to the ?
DNA replication is ______--______
What unwinds and unzips the helix?
The DNA double helix contains major and minor
grooves on its outer diameter, which
expose chemical groups that can form hydrogen
bonds
 The DNA of eukaryotes associates with proteins
called histones
 DNA is wound around an octamer of histones
(146 bases and 1.65 turns of the helix per
octamer)
 The octamer and DNA combination is secured to
a H1 histone, forming a nucleosome
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Nucleosomes serve two main functions:
 They protect DNA from damage
 They allow long lengths of DNA to be packaged
(supercoiled) for mobility during mitosis / meiosis
When supercoiled, DNA is not accessible for
transcription
 Cells will have some segments of DNA
permanently supercoiled (heterochromatin) and
these segments will differ between different cell
types
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7.1.4 Distinguish between unique or single copy
genes and highly repetitive sequences in nuclear
DNA
7.1.5 State that eukaryotic genes
contain introns and exons
Intron: A non-coding sequence of
DNA within a gene (intervening sequence)
that is cut out by enzymes when RNA is
made into mature mRNA
Exon: The part of the gene which codes for
a protein (expressing sequence)
•Eukaryotic DNA contains introns but
prokaryotic DNA does not
(a) two genetically identical nuclei/daughter cells formed during mitosis (so hereditary information in DNA can be
passed on);
two copies of each chromosome/DNA molecule/chromatid needed;
helicase unwinds the DNA/double helix;
to allow the strands to be separated;
helicase separates the two (complementary) strands of DNA;
by breaking hydrogen bonds between bases; [4 max]
(b) DNA replication is semi-conservative;
DNA is split into two single/template strands;
nucleotides are assembled on/attached to each single/template strand;
by complementary base pairing;
adenine with thymine and cytosine with guanine / A with T and C with G;
strand newly formed on each template strand is identical to other template strand;
DNA polymerase used; [5 max]
Marks may be awarded for any of the above points if clearly presented in a well-annotated diagram.
(c) hydrogen bonds between nucleotides of opposite strands/complementary bases/adenine and thymine and
cytosine and guanine;
covalent bonds between nucleotides within strands/between sugar/deoxyribose and phosphate;
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7. (a) Draw a labelled diagram to show how
two nucleotides are joined together in a
single strand of DNA. [3]
Objectives:
7.3.1 – State that transcription is carried out in a
5’  3’ direction
 7.3.2 – Distinguish between the sense and
antisense strands of DNA
 7.3.3 – Explain the process of transcription in
prokaryotes, including the role of the promoter
region, RNA polymerase, nucleoside
triphosphates and the terminator.
 7.3.4 – State that eukaryotic RNA needs the
removal of introns to form mature mRNA
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Similar to replication in that it is carried
out in the 5’ to 3’ direction.
Helicase not involved
RNA polymerase separates the 2 strands
of DNA
RNA polymerase only allows synthesis of
RNA in the 5’-3’ direction
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Promoter (start)  Transcription unit 
Terminator (stop)
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The terminator is a sequence of nucleotides
that, when transcribed, causes the RNA
polymerase to detach from the DNA.
The transcript that carries the code of the
DNA is called messenger RNA (mRNA).
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Transcription continues beyond the terminator fro a number
of nucleotides. Eventually it is released from the DNA strand.
NTPs (Nucleoside triphosphates) containing 3 phosphates
and the 5-carbon sugar ribose, are paired with the
appropriate bases of the antisense strand.
 mRNA is synthesis with the help of RNA polymerase and the
release of 2 phosphates from NTP.
 This is often referred to as ELONGATION.
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Eukaryotic DNA is different from prokaryotic DNA
in that within the protein coding regions there are
stretches of non-coding DNA.
 These regions are called introns. To make a
functional mRNA strand from eukaryotes, the
introns are removed.
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Prokaryotic mRNA does not require processing
because no introns are present.
 Use the worksheet provided to
annotate and make notes as the
animation progresses.
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A. Sigma factor helps to transport RNA
polymerase to promoter region
B. In eukaryotes the introns are removed
from the mRNA leaving only the exons.
C. RNA polymerases polymerises RNA in the
5’  3’ direction
D. NTPs are pairs with the appropriate DNA
bases and 2 phosphates are released to
provide energy for the pairing
E. In prokaryotes the terminator region of the
F. RNA polymerase binds to DNA at promoter
region
 A. Sigma factor helps to transport RNA
polymerase to promoter region
 C.RNA polymerases polymerises RNA in the
5’  3’ direction
 D. NTPs are pairs with the appropriate DNA
bases and 2 phosphates are released to
provide energy for the pairing
E. In eukaryotes the introns are removed from
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7.4.1 Explain that each tRNA molecule is
recognised by a tRNA-activating enzyme that
binds a specific amino acid to the tRNA using
ATP for energy
7.4.2 Outline the structure of ribosomes,
including protein and RNA composition, large
and small subunits, three tRNA binding sites
and mRNA binding sites
7.4.3 State that translation consists of
initiation, elongation, translocation and
•Ribosomes are made of protein (for stability) and
ribosomal RNA (rRNA - for catalytic activity)
•They consist of two subunits:
• The small subunit contains an mRNA binding site
• The large subunit contains three tRNA binding sites an aminacyl (A) site, a peptidyl (P) site and an exit
(E) site
•Ribosomes can be either found freely in the cytosol or
bound to the rough ER (in eukaryotes)
•Ribosomes differ in size in eukaryotes and prokaryotes
(eukaryotes = 80S ; prokaryotes = 70S)
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Translation occurs in four main steps:
Initiation: Involves the assembly of an active
ribosomal complex
Elongation: New amino acids are brought to
the ribosome according to the codon
sequence
Translocation: Amino acids are translocated
to a growing polypeptide chain
Termination: At certain "stop" codons,
translation is ended and the polypeptide is
A. Translation is initiatied by formation of an
initiation complex consisting of the smaller
ribosomal subunit, the first amino acid-tRNA and
mRNA.
 B. The ribsome dissociates into the smaller and
larger subunits and the messenger RNA and protein
are released.
C. The larger ribosomal subunit then joins the complex.
Proteins called initiation factors are involved but aren’t
shown in the diagram.
 D.Elongation of the polypeptide is terminated when
the ribsome reachesa codon that does not for an
amino acid, called a stop codon.
E. The 70s ribosome has 2 sites to which transfer RNAcarrying amino acids can bind. One is called the
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A. Translation is initiatied by formation of an
initiation complex consisting of the smaller
ribosomal subunit, the first amino acid-tRNA and
mRNA.
 C. The larger ribosomal subunit then joins the
complex. Proteins called initiation factors are
involved but aren’t shown in the diagram.
 E. The 70s ribosome has 2 sites to which transfer
RNA-carrying amino acids can bind. One is called the
peptidyl or P site and the other is called the acceptor
or A site.
 H. The amino acid carried by the transfer RNA in the
P site is then joined to the amino acid carried by the
transfer RNA that just entered the A site.
 F. The ribsome now advances a distance of one
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7.4.6 Explain the process of translation,
including ribosomes, polysomes, start codons
and stop codons
7.4.7 State that free ribosomes synthesise
proteins for use primarily within the cell, and
that bound ribosomes synthesise proteins
primarily for secretion or for lysosomes
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Initiation:
The small ribosomal subunit binds to the 5' end of
mRNA and moves along until it reaches the start codon
(AUG)
Next, the appropriate tRNA molecule binds to the
codon via its anticodon (according to complementary
base pairing)
Finally, the large ribosomal subunit aligns itself to the
tRNA molecule at its P-site and forms a complex with
the small ribosomal subunit
A second tRNA molecule pairs with the next codon
in the ribosomal A-site
 The amino acid in the P-site is covalently attached
via a peptide bond to the amino acid in the A-site
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The ribosome moves along one codon
position, the deacylated tRNA moves into the
E-site and is released, while the tRNA bearing
the dipeptide moves into the P-site
Another tRNA molecules attaches to the next
codon in the newly emptied A-site and the
process is repeated
The ribosome moves along the mRNA
sequence in a 5' - 3' direction, synthesising a
polypeptide chain
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Elongation and translocation continue until
the ribosome reaches a stop codon
These codons do not code for any amino
acids and instead signal for translation to
stop
The polypeptide is released and the ribosome
disassembles back into subunits
The polypeptide may undergo posttranslational modification prior to becoming
a functional protein
Ribosomes floating freely in the cytosol produce
proteins for use within the cell
 Ribosomes attached to the rough ER are primarily
involved in producing proteins to be exported from the
cell or used in the lysosome
 These proteins contain a signal recognition peptide on
their nascent polypeptide chains which direct the
associated ribosome to the rough ER
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