EDEN COLLEGE
DURBAN
LIFE SCIENCES GRADE 12
THE NUCLEUS AND NUCLEIC ACIDS
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Most plant and animal cells have a well-defined nucleus
surrounded by a double, permeable nuclear membrane made up
of proteins and phospholipids
Ribosomes occur on the outer membrane and these are
concerned with protein synthesis.
The nucleoplasm is the matrix, in which various substances are
suspended.
There may be one or more nueleoli [singular = nueleolus] in the
nucleus. These contain proteins and nucleic acids found within the
nucleus and they manufacture RNA and are concerned with
protein synthesis.
Chromosomes are long threads made of DNA and some protein.
They are the carriers of GENES which are responsible for our
hereditary traits. When a cell Is not dividing the chromosomes are
not visible but are in a very tangled mass, the chromatin network
LABEL THIS DIAGRAM AND GIVE IT AN APPROPRIATE HEADING
Nucleic acids (DNA and RNA)
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FUNCTIONS OF THE NUCLEUS
1
Controls metabolic activities of the cell by dictating the type of
proteins to be manufactured.
Carries hereditary traits and allows the transmission of these from
parent to offspring.
2
During cell division, the chromatin is visible as chromosomes. Chromosomes
are the control centres of the nucleus
They are rod-shaped, seen clearly during cell division. They consist of a core
of DNA, surrounded by a protein matrix (chromatid)
The DNA (DeoxiriboNucleic Acid) is a double
helix, which means it is like a twisted ladder.
DNA is made of building-blocks called
nucleotides
You have heard of GENES (not JEANS!). A
gene is a segment of DNA and it controls the
making of specific proteins in the body,
making that body unique and different. Let us
look at how this happens.
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3
THE KEY TO LIFE
GUILTY OR NOT GUILTY???
DID YOU KHOW THAT.....
A DNA profile of a person can be made from these
sources!
Police and forensic scientists use this information in
criminal investigations!
This is known as DNA fingerprinting. No two
people except identical twins have the same DNA
DNA STRUCTURE
DNA [deoxyribose nucleic acid] is known as the key to life. It is the
hereditary material found in the chromosomes as well as other parts of
the cell e.g. mitochondria.
It is a very long giant molecule made up of smaller units called
NUCLEOTIDES. Watson & Crick were awarded the Nobel Prize for working
out its structure [although a South African woman was just pipped at the
post (1953-1962)].
DNA is a double strand twisted into a spiral called a HELIX like a twisted
rope-ladder].
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Each side is made up of alternating SUGARS [deoxyribose] and
PHOSPHATES.
Forming the 'rungs' are PAIRED NITROGENOUS BASES;
 ADENINE pairs with THYMINE A
 GUANINE pairs with CTYOSINE G
T
C
These pairs are linked by weak hydrogen bonds.
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ONE sugar + ONE PHOSPHATE + ONE BASE = A NUCLEOTIDE.
Many nucleotides = one gene
Thousands of genes may occur on one DNA (chromosome) strand.
A
NUCLEOTIDE
5
Cytosine and Thymine nitrogenous bases
have a single ring.
Guanine and Adenine
have two rings
6
A two ringed base always links to a one ringed base
Adenine always joins to
Thymine
Guanine always joins to
Cytosine
A very simplified representation of a
short portion of a DNA molecule to
show the arrangement of the
nucleotides
Keep in mind that this molecule
actually occurs, as a double helix
structure
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SEE WHAT YOU KNOW
Make a simplified drawing of a short DNA strand showing THREE PAIRS OF
BASES. Label as fully as possible; draw a ring around ONE NUCLEOTIDE
and NAME THIS NUCLEOTIDE.
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COMPARISON BETWEEN D.N.A. AND R.N.A.
DNA = Deoxyribo Nucleic Acid RNA = Ribo Nucleic Acid
1.
Similarities
1.1
1.2
2.
Both are made of four types of nitrogenous base
Both are composed of nucleotides
DIFFERENCES
RNA
DNA
1
Double helix
1
Single strand (not a helix)
2
2
Adenine (A), Uracil (U), Cytosine (C)
and Guanine (G) are the
nitrogenous bases
3
4
5
Adenine (A), Thymine (T),
Cytosine (C) & Guanine
(G) are the nitrogenous
bases
Can REPLICATE itself
Deoxyribose sugar
Found in nucleus
3
4
5
6
Has hydrogen bonds
6
Does not replicate itself
Ribose sugar
Formed in nucleus (mRNA) and
cytoplasm (tRNA)
No hydrogen bonds
Functions of D.N.A and R.N.A.
The drawing below
nucleotides:
D.N.A.
mR.N.A.
tR.N.A.
Proteins
summarises the functions of the
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DNA.
1.
2.
Controls all the cell’s Metabolic
processes by controlling the
synthesis of Enzymes (special
proteins) which control the cell’s
chemical reactions
(metabolism)
Hereditary information (genes) is
found in the D.N.A. Genes,
represented by a section of
D.N.A. control the synthesis of
proteins, which give a cell its
structural individuality.
RNA.
1.
Serves as a messenger for
D.N.A. in the cytoplasm
mR.N.A. = Messenger R.N.A.
2.
Serves to interpret the
coded message of mR.N.A.
tR.N.A. = Transfer R.N.A. or
Transcription R.N.A.
Protein Synthesis
The drawings below help us to understand how DNA and RNA are
involved in protein synthesis. Study them and then refer to them when
reading the explanation after the drawings:
Part one: Transcription
1.
Part of the DNA in the nucleus unwinds.
2.
Free nucleotides in the nucleoplasm line up, by complimentary
base pairing and a messenger RNA (mRNA) forms.
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3.
The mRNA carries the triplet codon from the nucleus into the
cytoplasm, where it moves across a ribosome, the site of protein
synthesis. The codon is three consecutive nitrogenous bases of the
DNA, which will code for a specific amino acid. Sometimes we call it
the triplet codon, as each has three bases.
4.
In the cytoplasm there are tRNA molecules each with three free
bases (the anticodon) and each tRNA attaches to it, a specific
amino acid.
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5.
Remember that proteins differ from each other by having different
sequences of amino acids. So, protein synthesis determines the
amino acid order or sequence
Part two: Translation
t RNA with
specific
anticodon (agg)
and a specific
amino acid
(serine)
Growing
polypeptide chain
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5.
6.
As the mRNA moves along the ribosome, tRNA molecules align
themselves next to the mRNA.
The tRNA (transcription RNA) has three nitrogenous bases exposed.
We call this the anticodon, because it complements the bases of
the codon.
Each tRNA has a specific anticodon and a specific amino acid in
the cytoplasm attaches to it.
The tRNA, with its specific amino acid, lines up by complimentary
base pairing (e.g., the auc codon of the mRNA will have the uag
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7.
8.
9.
anticodon tRNA line up in that position. This tRNA attaches the
amino acid, alanine).
The amino acid attached to the tRNA now detaches and joins the
growing polypeptide chain. This tRNA will return to the cytoplasm,
and pick up another identical amino acid.
The order of bases of the DNA (the codon) is replicated in the mRNA
and the tRNA will bring the correct amino acid onto the polypeptide
chain. Therefore, the DNA (the segment of DNA is called a gene) will
code for the type of protein by determining the sequence of amino
acids.
All living organisms have the same essential 20 amino acids in or
bodies, but it’s the order of the amino acids that makes human
protein different from that of the cow. This is why you can eat
protein from a cow and from plants, because you will use the amino
acids. The DNA will ensure that human types of protein are
manufactured in the cell where they are needed.
MUTATION – A CHANGE IN DNA
A change or mistake in the DNA sequence is called a mutation.
Generally occurs during the cell processes that copy genetic material
and pass it from one generation to next.
These processes are usually accurate to ensure genetic continuity in
both new cells and offspring.
However, sometimes mistakes can occur.
Changes in the DNA base sequence is referred to as gene mutations.
The result is a different set of codons and a different protein.
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DNA REPLICATION
1.
The weak hydrogen bonds holding the complementary base pairs
together break as a result of enzyme action.
2.
Two single strands now result.
3.
Each single strand picks up free nucleotides from the nucleotide
pool in the nucleoplasm and becomes double again.
4.
Since Thymine can only combine with Adenine, and Guanine with
Cytosine, the two new double strands are identical to each other as
well as to the original strand.
See the drawings explaining the process below.
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As you can see in the drawings below, DNA replication takes place in the
chromosomes, during cell division (mitosis):
The chromosome divides into two
chromatids each with a DNA
double helix.
This replication ensures that when
the chromosome splits into 2
daughter chromosomes (or
chromatids) each has a copy of
the replicated DNA which will
now be found in each daughter
cell produced in mitosis.
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THE NUCLEUS AND NUCLEIC ACIDS - CONSOLIDATION
TOTAL: 80 MARKS
QUESTION ONE
1.1
Which of the following is correct?
present in DNA
A uracil
B de-oxyribose
C single strand
D 4 different nucleotides
present in RNA
thymine
ribose√√
double strand
5 different nucleotides
1.2
One of the nucleotides present in mRNA has the composition
A adenine- ribose- phosphate√√
B uracil- de-oxyribose- phosphate
C thymine- ribose- phosphate
D guanine-deoxyribose -phosphate
1.3
mRNA template is
A
translated from protein B
C
translated into DNA
D
transcribed into protein
transcribed from DNA√√
1.4
The approximate number of different amino acids that have been
found in proteins is …
A 3
B 4
C 20√√ D 64
1.5
In which of the following pairs, are BOTH strands of nucleic acid
complementary to strand X?
A 1 and 3 B 2 and 4√√ C 1 and 2
D
3 and 4
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1.6
A free transfer RNA molecule can combine with
A one specific amino acid√√
B any available amino acid
C three different amino acids
D
a chain of amino acids
1.7
The number of bases present in one codon is
A
1.8
1
B
2
C
3√√
D
4
DNA molecules isolated from a rat cell and a human cell are
found to differ in the order of their …
A bases only√√
B
sugars only
C phosphates only
D
bases, sugars and phosphates
Items 1.9, 1.10 and 1.11 refer to the
possible answers in the list opposite:
1
2
3
4
5
DNA molecules
tRNA molecules
mRNA molecules
amino acids
ribosomes
1.9
Which of these must be present in the largest number for
successful synthesis of a large protein molecule to occur?
A 1
B 3
C 4√√
D 5
1.10
Which of these controls the order in which amino acids are
added to a growing protein chain?
A
1.11
2
3√√
C
4
D
5
On which of these are anti-codons present?
A 1
1.12
B
B
2√√ C
3
D
5
If each amino acid weighs 100 mass units, what is the weight (in
mass units) of the protein molecule synthesised from a mRNA
molecule which is 600 bases long?
A 2000
B 6000
C 20 000√√
D 60 000
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1.13 (i) and (ii) refer to the following table list of possible answers.
A
B
C
D
heart cells
liver cells
spermatozoa
red blood corpuscles
i)
What is the correct identity of
cell type X? Give a reason for
your answer.
(2)
D√ red Blood cells have no
nucleus√
ii)
What is the correct identity of
cell type Y? Give a reason for
your choice.
(2)
C√ Sperm cells are haploid√
1.14
The sugar present in DNA is
A
C
1.15
ribose
B sucrose
ribulose D de-oxyribose√√
The structure of one nucleotide is shown. Which of the following
shows two nucleotides correctly joined together?
B√√
[32]
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QUESTION TWO
This diagram represents a small portion of a DNA molecule about to act
as a template for the formation of m-RNA
2. 1
Make a diagram of the m-RNA that will be formed from this DNA
and LABEL.
(5)
Drawing of part of an mRNA molecule
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2.2
2.3
What are the parts 1, 2 and 3 collectively called?
Nucleotide√
Name parts 1 to 5.
1 – Ribose sugar√
2 – Cytosine nitrogenous base√√
3 – Phosphate group√ 4 - Uracil√√
5 - Guanine√
(1)
(7)
[13]
QUESTION THREE
Study the diagram below which shows the process of protein synthesis.
3.1
3.2
3.3
3.4
Identify organelles A and B respectively.
A - Nucleus√ B - Ribosome√
Label structures C, E and F respectively.
C - mRNA√√ E - tRNA√√ F – Amino acids√√
Which stage of protein synthesis is represented at?
(a) 1 Transcription√√
(b) 2 Translation√√
Write down the anticodon that reads from left to right at D.
CAG√√√
(2)
(6)
(2)
(2)
(3)
[15]
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QUESTION FOUR
The following three diagrams (A, B and C) show an important process
that occurs before cell division.
4.1
4.2
Name the process that is being shown in the diagram.
(1)
DNA replication√
Provide a brief explanation for what is being shown in each of the
diagrams A, B and C.
(9)
A – A section√ of DNA double helix√ unwinds√
B – Free nucleotides√ from the nucleoplasm√ of the nucleus√ line
up with the two free strands√ by complimentary base pairing√.
C – Two identical DNA molecules√ are formed, identical to each
other and identical to the original one√.
This process is prior to the chromosomes√ being separated during
mitosis√
[10]
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5
The diagram below represents a portion of a nucleic acid.
5.1
Name the four nitrogenous bases which are present
(2)
t -thymine a – adenine
g – guanine
c - cytosine√√
Using the first letter of each nitrogen base, indicate on the diagram
in w hat combinations they always pair.
(2)
See diagram√√
Name two other substances, apart from the nitrogen bases, which
form part of the D.N.A. molecule.
(2)
Deoxyribose sugar√ Phosphate group√
Use the first letters of these names to label their positions in the
diagram.
(2)
See diagram√√
Make a ring around one nucleotide on the diagram.
(2)
See diagram√√
5.2
5.3
5.4
5.5
[10]
TOTAL: 80 MARKS
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6
The table below shows the DNA base triplets that code for different
amino acids.
6.1
Name the process by which mRNA is formed from a DNA template.
(1)
Transcription√.
6.2
How many mRNA codons would be involved in forming the portion
of protein shown above?
(1)
5√
Write down the sequence of the first THREE mRNA codons (from left
to right) for this portion of the protein.
(3)
GCU CAU UGG
The following is a sequence of base triplets in DNA:
6.3
6.4
(a)
If guanine, found in the first base triplet, is removed,
explain how this would affect the structure of the protein.
(2)
Frameshift mutations (base moved) 
- A mutation in which a single base is added or deleted from DNA.
(b) Name the process that occurs when the sequence of
bases in DNA changes.
(2)
Mutation
[9]