MIDDLETOWN HIGH
SCHOOL SOUTH
BIOLOGY
BOOKLET 6
NAME: _________________________________
CLASS: _____________
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LEARNING OUTCOMES
DNA Replication, Synthesis and Release of Proteins
(a) Understand the basic structure of DNA
(b) Name the two scientists that discovered the structure of DNA
(c) Name the basic sub-unit of DNA
(d) Describe the three parts of these sub-units
(e) Understand the need for DNA replication
(f) Describe the process of DNA replication
(g) Describe the similarities and differences between DNA and RNA
(h) Understand that there are two forms of RNA used in transcription and
translation
(i) Name the 4 bases in DNA
(j) Describe how complementary bases pair up
(k) Understand that there is no thymine in RNA and name the base that
replaces it
(l) Describe the process of transcription
(m)Describe the process of translation
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THE STRUCTURE OF DNA

Deoxyribonucleic acid or DNA codes for proteins in
the cell. During cell division DNA must be replicated
(copied) so that daughter cells carry an identical set
of instructions to those in the parent cell.

In this topic the structure of DNA will be described and
the method of DNA replication explained.

In 1953 the structure of the DNA molecule was
explained for the first time by two scientists, James
Watson and Francis Crick at the Cavendish
Laboratory in Cambridge.

They found that a molecule of DNA consists of two
strands of repeating units called nucleotides.

These two strands run in opposite directions to each
other forming a double helix.
DNA NUCLEOTIDES

DNA nucleotides are the basic structures that make up the double helix
of DNA.

Nucleotides are made up of three
different parts:
o Phosphate head
o Deoxyribose sugar
o One of four different bases.

The bases that are found in DNA are:
o Adenine
o Thymine
o Cytosine
o Guanine

The two strands of DNA pair up by the bases bonding together in a
certain way known as complementary base pairing.
o Adenine only bonds with thymine (A-T or T-A)
o Cytosine only bonds with guanine (C-G or G-C)
o There are equal numbers of each base pair – for example, if
there are 25% adenine bases, there will be 25% thymine bases.
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
The phosphate heads of DNA bond to the deoxyribose sugar of the
nucleotide above it. This bond is a strong, covalent bond.

The structure of DNA is like a ladder where the “rungs” are the bases
and the “uprights” are the sugar-phosphate repeats.
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Task 1: Answer the following questions.
1. What shape is the DNA molecule?
2. What type of bonding holds two DNA strands together?
3. Which two components make up a DNA molecule?
4. Name the nitrogenous base that pairs with cytosine.
5. Name the nitrogenous base that pairs with thymine.
6. What do the symbols P, S, A, T, G and C represent?
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DNA REPLICATION
The mechanism of DNA replication is said to be semi-conservative. That is,
after replication, each of the two resulting DNA molecules is composed of one
original (or conserved) strand and one new strand.
Cells divide for an organism to grow or reproduce. Every new cell needs a
copy of the DNA or instructions to know how to be a cell. DNA replicates right
before a cell divides.
Replication of DNA is a fast but complex process that requires the
involvement of:





a DNA template;
several enzymes including DNA polymerase, the enzyme that joins
new nucleotides to the growing DNA strand;
proteins;
a supply of the four types of DNA nucleotides;
energy provided by triphosphate molecules such as ATP.
Step 1 of DNA replication: Unwinding


DNA is wound up into a double helix normally. The first stage of
replication is for the section of DNA that is to be copied to unwind.
This results in the two DNA strands being parallel to each other.
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Step 2 of DNA replication: Unzipping
The sequence of bases in a strand of DNA determines the
amino acids produced and that will eventually form
proteins.
In order to copy this section of DNA, the bases must be
exposed so that the base sequence can be copied.
Step 3 of DNA replication: Complementary base pairing
Complementary bases begin adding into both sides of the DNA
Adenine binds with Thymine, Cytosine binds with Guanine.
The enzyme DNA Polymerase is the enzyme that controls this process.
This diagram only shows one
side of the original DNA strand
being replicated for simplicity.
Step 4 of DNA replication: Hydrogen bonding between base pair
Weak hydrogen bonds form between complementary base pairs
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Step 5 of DNA replication: Strong covalent bonds forming between
phosphate and sugar groups
A strong chemical bond is formed between the sugar of one nucleotide and
the phosphate of the next one in the chain  sugar-phosphate backbone.
This is controlled by the enzyme DNA polymerase.
The arrows point to the strong bond between the dexoyribose sugar
of one nucleotide and the phosphate group of the next one.
Step 6 of DNA replication: Proof reading
The final job of the Polymerase is to proofread the nucleotides after they are
added and to remove any that are incorrectly paired.
There are now two identical copies of a DNA strand that are made from the
one original strand of DNA.
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SUMMARY OF DNA REPLICATION
Complete the summary by writing in the missing words.
1. The original DNA strand _____________
2. Weak hydrogen bonds break, exposing the DNA bases. This is known
as ______________
3. A new strand of DNA starts to build up by free nucleotides pairing up
with its _______________ base.
4. The base adenine pairs up with _______________ and
_____________ pairs up with cytosine.
5. Once bases have paired up with each other, ____________ bonds
start to form between them.
6. Strong _____________bonds start to form between the
______________ and __________________________ of the
nucleotides.
7. Once this is complete, the new DNA strands are ______________ to
make sure that there are no errors in the new DNA strands.
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Task 2: Answer the following questions.
1. The diagram shows DNA during replication. Base H represents thymine and
base M represent guanine. Which letters represent the base cytosine?
A
B
C
D
J and K
J and L
N and P
N and R
2. A section of a DNA molecule contains 300 bases. Of these bases, 90 are
adenine. How many cytosine bases would this section of DNA contain?
A
B
C
D
60
90
120
180
3. 56. If ten percent of the bases in a molecule of DNA are adenine, what
is the ratio of adenine to guanine in the same molecule?
A
B
C
D
1:1
1:2
1:3
1:4
4. If a DNA molecule contains 8000 nucleotides of which 20% are adenine, then
the number of guanine nucleotides present is
A
B
C
D
1600
2000
2400
3200
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5. How many adenine molecules are present in a DNA molecule of 2000 bases,
if 20% of the base molecules are cytosine?
A
B
C
D
200
300
400
600
6. The replication of part of a DNA molecule is represented in the diagram.
(a)
Name the nucleotide component R and the base S.
R _______________________________
S _______________________________
(b)
Name the type of bond labelled X.
X _______________________________
(c)
Explain why DNA replication must take place before a cell divides.
______________________________________________________
______________________________________________________
______________________________________________________
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RNA REPLICATION

RNA is a type of nucleic acid called ribonucleic acid:

The RNA nucleotides are joined to form a single strand.

The nucleotides are joined by a bond between the sugar of one
nucleotide and the phosphate of the next.

RNA differs from DNA in the following ways:
RNA is single stranded while DNA is double stranded.
RNA has a sugar called ribose while DNA has a sugar called deoxyribose.
RNA has the base uracil while DNA has the base thymine.

There are two types of RNA that need to be known for this topic. The first is
messenger RNA, or mRNA. This is formed in the nucleus of the cell.

It rewrites the sequence of bases of a section of DNA in a process
called transcription.

When a new protein needs to be made by a cell, the genetic information
carried by this gene needs to be copied. This process is called transcription.
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TRANSCRIPTION
Transcription is the first step in protein synthesis. The DNA double helix
unwinds and unzips, exposing a single template strand of DNA.
Free RNA nucleotides in the nucleus bind to complementary DNA nucleotides
using the base pair rules shown in the table below.
The RNA molecule produced during transcription of DNA is called messenger
RNA (mRNA) and is a faithful transcript of the DNA template.
DNA nucleotide
RNA nucleotide
A pairs with
U
T pairs with
A
G pairs with
C
C pairs with
G
Each group of three bases (triplet of bases) in mRNA is
known as a codon.
These groups of three bases help to determine what amino
acid will be produced to make the correct protein needed.
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The diagrams below show the stages of transcription.
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Task 4: Complete the questions below about transcription
1(a)
The diagram below represents the four different nucleotides of
DNA.
The bases of the nucleotides are bonded in pairs.
(i)
Name the type of bond which links the base of one nucleotide to the base of
another.
Name _________________________
(ii)
On the diagram:
1
insert the appropriate letters or names of the four different nucleotides;
2
draw lines to show how adjacent nucleotides are bonded in a DNA molecule.
(b)
Complete the table below to show structural differences between the nucleic acids
DNA and RNA.
Type of nucleic acid
Structure
DNA
RNA
Number of strands present
Type of sugar in nucleotide
Letters of bases present
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2.
(a)
G
Part of one strand of DNA molecule used
G
G
G
C
C
G
T
C
G
C
G
The table shows the names of six amino acids together with some of their mRNA
codons.
Amino acid
(i)
mRNA codon (s)
Glycine
GGG
or
GGC
Serine
UCG
or
AGC
Proline
CCG
or
CCC
Arginine
CGG
Alanine
CGC
Threonine
CAG
Use the information to give the order of amino acids coded for by the DNA base
sequence.
__________________________________________________________
(ii)
What name is given to a part of a DNA molecule which carries the code for making
one protein?
__________________________________________________________
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3. (a) The diagram below shows some of the processes which take place during
synthesis of protein in a cell.
(i)
Name the type of bond that holds pairs of bases in DNA together.
__________________________________________________________
(ii)
Name two substances present in RNA and which are not present
in DNA.
_____________________________
_____________________________
(iii)
Name bases X and Y.
Base X ______________________
Base Y ______________________
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(v)
The diagram below shows the amino acids in part of a protein molecule.
lysine
alanine
Glutamic acid
threonine
The table shows the mRNA codons for the amino acids shown in the diagram.
Amino acid
Alanine
Glutamic acid
Lysine
Threonine
Codon
GCC
GAA
AAG
ACU
Write the sequence of bases in the DNA that would code for the sequence of amino
acids shown in the diagram.
Space for working
Answer _________________________
(b)
Sickle cell anaemia is a genetically transmitted disorder which causes abnormal
haemoglobin to be formed.
The amino acid sequence of part of a haemoglobin molecule and the same part of
an abnormal haemoglobin molecule are shown below.
Normal haemoglobin
Proline
Glutamic
Glutamic
acid
acid
lysine
Abnormal haemoglobin
Glutamic
Proline
Valine
acid
lysine
Describe the difference between the two forms of haemoglobin
Difference _________________________________________________
__________________________________________________________
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TRANSLATION
Transcription is when a section of DNA that codes for a protein is copied by
mRNA.
This piece of code is taken out of the nucleus and attaches to a ribosome so
that this “foreign language” can be translated into a protein.
Once the DNA in a gene has been
transcribed into mRNA translation
can take place.
First the mRNA molecules pass
through the nuclear pores.
Translation of mRNA into protein
takes place on ribosomes in the
cytoplasm and requires a second type
of RNA, transfer RNA (tRNA)
Translation takes place on
the ribosomes in the cytoplasm, or
found on the rough Endoplasmic
Reticulum (ER).
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CONDONS, ANTICODONS AND PROTEINS
As describe before, a codon is a group of three bases in mRNA that code for
a specific amino acid.
An anticodon is another group of three bases in tRNA that is the
complementary base sequence that also codes for the same amino acid.
Remember, in RNA the base paring rule is cytosine-guanine and adenineuracil. There is no thymine in RNA.
Example:
The mRNA sequence AUG-CAA-CCC-GAC-UCC-AGC
(6 condons)
The tRNA sequence UAC-GUU-GGG-CUG-AGG-UAG
(6 anticodons)
To find out what amino acid each anticon codes for, we use a reference
table like the one shown below:
The first antcodon in the genetic code above is UAC. This means that the
amino acid produced would be Try.
Using the table above, write down the 5 other amino acids that would be
produced. Remember to use the anticodon sequence!
Try-_________________________________
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THE STAGES OF TRANSLATION
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SUMMARY OF TRANSLATION
Translation takes place on the ribosomes in the cytoplasm, or found on the
rough Endoplasmic Reticulum (ER):






the ribosomes are the sites of protein synthesis
the mRNA strand attaches to a ribosome
tRNA molecules transport specific amino acids to the ribosome
each mRNA codon codes for a specific amino acid
the anti-codons and codons match up and form complementary base pairs
peptide bonds form between the adjacent amino acids to form the polypeptide
(protein)
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Task 5: Complete the questions below about translation
1.
The diagram shows events occurring during the synthesis of a protein that is
secreted from a cell.
(a)
(b)
(i)
Name molecule X ________________________________
(ii)
Name bond Y
________________________________
What name is given to a group of three bases on mRNA that codes for an
amino acid?
________________________
(c)
Give the sequence of DNA bases that code for amino acid Z.
Think carefully!
_________________________________________________________
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2.
The diagram shows translation of part of a mRNA molecule during the synthesis of a
protein.
(a)
Name structure Y.
___________________________________________________________
(b)
Name the types of bond shown at P and Q.
Bond P ___________________________________________________
Bond Q ___________________________________________________
(c)
Give the anticodon for the tRNA for amino acid 1.
_______________________
(d)
Describe two functions of tRNA in protein synthesis.
1
_____________________________________________________
_____________________________________________________
2
_____________________________________________________
_____________________________________________________
(e)
Genetic information for protein synthesis is in the form of a triplet code.
Explain what is meant by this statement.
__________________________________________________________
__________________________________________________________
__________________________________________________________
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