EDEN COLLEGEEDEN COLLEGEDURBANDURBAN

LIFE SCIENCES GRADE 12

THE NUCLEUS AND NUCLEIC ACIDS 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)

2

FUNCTIONS OF THE NUCLEUS1 Controls metabolic activities of the cell by dictating the type of

proteins to be manufactured.2 Carries hereditary traits and allows the transmission of these

from parent to offspring.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

1

3

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.

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 STRUCTUREDNA [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)].

4

DNA is a double strand twisted into a spiral called a HELIX like a twisted rope-ladder].

Each side is made up of alternating SUGARS [deoxyribose] and PHOSPHATES.Forming the 'rungs' are PAIRED NITROGENOUS BASES;

ADENINE pairs with THYMINE A T GUANINE pairs with CTYOSINE G C

These pairs are linked by weak hydrogen bonds.

ONE sugar + ONE PHOSPHATE + ONE BASE = A NUCLEOTIDE. Many nucleotides = one gene

5

Thousands of genes may occur on one DNA (chromosome)

strand.

A NUCLEOTIDE

6

Cytosine and Thymine nitrogenous bases have a single ring.

Guanine and Adenine have two rings

A two ringed base always links to a one ringed base

Adenine always joins to Thymine

Guanine always joins to Cytosine

Model of DNA showing the Helix and arrangement of the different atoms

7

A very simplified representation of a short portion of a DNA molecule to show the arrangement of the nucleotidesKeep in mind that this molecule actually occurs, as a double helix structure

8

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.

COMPARISON BETWEEN D.N.A. AND R.N.A.DNA = Deoxyribo Nucleic Acid RNA = Ribo Nucleic Acid1. Similarities

1.1 Both are made of four types of nitrogenous base1.2 Both are composed of nucleotides

2. DIFFERENCES

DNA RNA1 Double helix 1 Single strand (not a helix)

9

2 Adenine (A), Thymine (T), Cytosine (C) & Guanine (G) are the nitrogenous bases

2 Adenine (A), Uracil (U), Cytosine (C) and Guanine (G) are the nitrogenous bases

3 Can REPLICATE itself 3 Does not replicate itself4 Deoxyribose sugar 4 Ribose sugar5 Found in nucleus 5 Formed in nucleus (mRNA) and

cytoplasm (tRNA)6 Has hydrogen bonds 6 No hydrogen bonds

Functions of D.N.A and R.N.A.

The drawing below summarises the functions of the nucleotides:

D.N.A.

mR.N.A.

tR.N.A.

Proteins

DNA. RNA.

1. Controls all the cell’s Metabolic processes by controlling the synthesis of Enzymes (special proteins) which control the cell’s chemical reactions

1. Serves as a messenger for D.N.A. in the cytoplasm mR.N.A. = Messenger R.N.A.

10

(metabolism)2. 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.

2. Serves to interpret the coded message of mR.N.A.

tR.N.A. = Transfer R.N.A. or Transcription R.N.A.

Protein SynthesisThe 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: Transcription1. 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.

11

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.

12

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

6 As the mRNA moves along the ribosome, tRNA molecules align themselves next to the mRNA.

7 The tRNA (transcription RNA) has three nitrogenous bases exposed. We call this the anticodon, because it complements the bases of the codon.

5. Each tRNA has a specific anticodon and a specific amino acid in the cytoplasm attaches to it.

6. The tRNA, with its specific amino acid, lines up by complimentary base pairing (e.g., the auc codon of the

Growing polypeptide chain

t RNA with specific anticodon (agg) and a specific amino acid (serine)

13

mRNA will have the uag anticodon tRNA line up in that position. This tRNA attaches the amino acid, alanine).

7. 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.

8. 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.

9. 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 DNAA 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.

14

DNA REPLICATION1. 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.

15

16

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.

17

THE NUCLEUS AND NUCLEIC ACIDS - CONSOLIDATION

TOTAL: 80 MARKSQUESTION ONE1.1 Which of the following is correct?

present in DNA present in RNAA uracil thymineB de-oxyribose ribose√√C single strand double strand D 4 different nucleotides 5 different nucleotides

1.2 One of the nucleotides present in mRNA has the composition A adenine- ribose- phosphate√√B uracil- de-oxyribose- phosphateC thymine- ribose- phosphateD guanine-deoxyribose -phosphate

1.3 mRNA template isA translated from protein B transcribed into protein C translated into DNA D 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 tostrand X?

18

A 1 and 3 B 2 and 4√√ C 1 and 2 D 3 and 4

1.6 A free transfer RNA molecule can combine withA one specific amino acid√√ B any available amino acidC three different amino acids D a chain of amino acids

1.7 The number of bases present in one codon isA 1 B 2 C 3√√ D 4

1.8 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 onlyC phosphates only D bases, sugars and phosphates

Items 1.9, 1.10 and 1.11 refer to the 1 DNA moleculespossible answers in the list opposite: 2 tRNA molecules

3 mRNA molecules4 amino acids5 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 2 B 3√√ C 4 D 5

1.11 On which of these are anti-codons present?

19

A 1 B 2√√ C 3 D 51.12 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

1.13 (i) and (ii) refer to the following table list of possible answers.A heart cellsB liver cellsC spermatozoaD red blood corpuscles

i) What is the correct identity of cell type X? Give a reason

foryour 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 isA ribose B sucrose C ribulose D de-oxyribose√√

20

1.15 The structure of one nucleotide is shown. Which of the following shows two nucleotides correctly joined together?

B√√[32]

QUESTION TWOThis 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)

21

Drawing of part of an mRNA molecule

2.2 What are the parts 1, 2 and 3 collectively called? (1)Nucleotide√

2.3 Name parts 1 to 5. (7)1 – Ribose sugar√ 2 – Cytosine nitrogenous base√√3 – Phosphate group√ 4 - Uracil√√5 - Guanine√

[13]

QUESTION THREEStudy the diagram below which shows the process of protein synthesis.

22

3.1 Identify organelles A and B respectively. (2)A - Nucleus√ B - Ribosome√

3.2 Label structures C, E and F respectively. (6)C - mRNA√√E - tRNA√√ F – Amino acids√√

3.3 Which stage of protein synthesis is represented at?(a) 1 Transcription√√ (2)(b) 2 Translation√√ (2)

3.4 Write down the anticodon that reads from left to right at D. (3)CAG√√√

[15]QUESTION FOURThe following three diagrams (A, B and C) show an important process that occurs before cell division.

23

4.1 Name the process that is being shown in the diagram. (1)DNA replication√

4.2 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]

5 The diagram below represents a portion of a nucleic acid.

24

5.1 Name the four nitrogenous bases which are present (2)t -thymine a – adenine g – guanine c - cytosine√√

5.2 Using the first letter of each nitrogen base, indicate on the diagram in w hat combinations they always pair.

(2)See diagram√√

5.3 Name two other substances, apart from the nitrogen bases, which form part of the D.N.A. molecule. (2)Deoxyribose sugar√ Phosphate group√

5.4 Use the first letters of these names to label their positions in the diagram. (2)See diagram√√

5.5 Make a ring around one nucleotide on the diagram. (2)See diagram√√

[10]

TOTAL: 80 MARKS

25

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√6.3 Write down the sequence of the first THREE mRNA codons (from left to right) for this portion of the protein. (3)

GCU CAU UGG 6.4 The following is a sequence of base triplets in DNA:

(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)

26

- 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]