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2025 • Biology • BIOMOLECULES, DNA & PROTEIN SYNTHESIS

33. (a) Define the terms Transcription' and 'translation', indicating why these processes are necessary for an organism's survival.

(b) What do you understand by DNA finger printing

(c) Discuss the structural and metabolic roles of lipids in the body. (4,10, 6 mks)

(a) Definition and importance of transcription and translation

-Transcription is the process by which a complementary mRNA copy is male from a specific region or cistron of the DNA molecule which codes for the polypeptide.

-Translation is the process by which specific sequences of amino acids are linked together in accordance with the codons in mRNA.

-Both processes are involved in protein synthesis. Proteins are required as enzymes :o control metabolic pathways, for growth, repair, as hormones, in muscle contraction, carriage of oxygen, as antibodies and in blood clotting.

(b) DNA Fingerprinting

-This is the analysis and use of DNA patterns from body tissues such as blood, saliva or semen in order to establish somebody's identity.

-Human DNA contains repetitive pieces called minisatellites which are distributed in a unique way in every individual.

-Someone's distribution ‘pattern is ascertained using a probe.

- The probe is prepared from ³²P labelled viral DNA broken to sticky-ended fragments using restriction endonucleases.

- The same enzyme is used to cut human DNA extracted from blood, semen or hair.

- The fragments are separated by electrophoresis and blotted to a thin nitrocellulose filter.

- The filter is then washed with a solution containing the probe, when both types of DNA adhere by their sticky ends.

-The positions of the radioactive complexes are finally determined by placing the filter on an x- ray film.

-Radiations emitted by the hybrid DNA causes dark bands to appear on the film.

-The banding pattern is different for every human other than identical twins.

-Minisatellites are inherited from both pe/ents

-The genetic fingerprint of each individual is different.

-It is applied in forensic science and disputed parenthood.

-Phospholipids form part of membranes

-Gives buoyancy to some cells

-Surround vital organs like kidneys and prevent shock

-Waxes offer protection and water-proofing

Metabolic role of lipids

-Triglycerides are oxidized to release energy. Have high H-content.

-Steroids form many animal hormones e.g. oestrogen, testosterone which affect processes in the body

-Cholesterol is raw material used for the synthesis of Vitamin D.

2025 • Biology • BIOMOLECULES, DNA & PROTEIN SYNTHESIS

34. (a) What are the qualities of water that enable it to be an important biological molecule?

(b) Briefly account for the solubility of proteins and the existence of isoelectric points (12,8 mks)

(a) Qualities of water that qualify it as an important biological molecule

-Water is a liquid at room temperature, which allows it to provide a liquid milieu in and around cells.

-It is a universal solvent, most chemical reactions in cells occur in water and it serves as the medium of transport. It takes part in most chemical reactions. It combines with many molecules to form hydrated molecules.

-Water has high surface tension and forms a skin that can support some organisms, so they move on water e.g. pond scatters.

-Water is wet, sticks to polar surfaces and has low viscosity. This allows capillarity for its easy transport in tubes.

-It has a very high specific heat capacity which allows it to maintain internal temperatures and keeps aquatic temperatures stable.

-Has high latent heat of vaporization and therefor helps in cooling.

-Has high latent heat of fusion, preventing easy freezing.

-Water is colorless and transparent and allows easy passage of light to aquatic life.

-It is denser than air and therefore supports and disperses aquatic organisms and reproductive structures.

-Water is difficult to compress and so forms a hydrostatic skeleton in worms and turgid plants.

-It has high tensile strength and water columns do not break or pull apart, making it easier for its transport within living organisms.

(b) Solubility of proteins

-Globular proteins are generally soluble in water because they have polar R groups jolting outwards from the molecule. These attract water molecules.

-Some globular proteins of high molecular mass form colloidal suspensions in water.

-Fibrous proteins are insoluble in water. This is due to existence of a large number of non-polar

hydrophobic R groups on the exterior of the molecule.

Isoelectric point

-This is the pH value at which the net electric charge of a molecule in a solution is zero.

-Each protein exists as an amphoteric ion carrying electrostatic charges at a precise pH.

-This pH is called the isoelectric point of the protein.

-If the protein has a large number of amino groups in its side chains, the isoelectric point is above the pH 7.

-Proteins with a large number of carboxyl groups have their isoelectric points at pH below 7.

2025 • Biology • BIOMOLECULES, DNA & PROTEIN SYNTHESIS

35.     (a) Briefly describe the formation of the following biological molecules

(i)     Maltose (ii) Dipeptide (iii) Triglyceride

(b)     What are the properties of sugars?

(c)     Why is water so important to plants?

(12, 4,4 mks)

(a) (i) Formation of maltose

-Maltose is a disaccharide formed by condensation reactions between two a-glucose sub units.

-The two molecules become joined as one of them releases a hydrogen atom and the other a hydroxyl group.

-The bond formed is called a glycosidic bond.

+          H₂O

Formation of Maltose

(ii)     Formation of a Dipeptide

-They are formed from the condensation of two amino acid molecules.

-During the polymerization reaction, the amino group of one amino acid joins to the carboxyl group of another.

-The bond formed between them is called a peptide bond

Formation of a Dipeptide

(iii)     Formation of a Triglyceride

-      Three fatty acid molecules combine with a molecule of glycerol to form a triglyceride.

-      The carboxyl groups of the fatty acids combine with the hydroxyl groups of the glycerol in condensation reactions called esterification.

-      The bond formed is called an ester bond.

Formation of a triglyceride

(b)     Properties of sugars

-Sugars are monosaccharides and disaccharides having a sweat taste.

-They are small molecules which are soluble in water.

-They are crystallizable molecules

-All monosaccharides, maltose and lactose are reducing sugars

-Sucrose is a non-reducing sugar

(c)    Importance of water to plants

-Water is used for photosynthesis and protein synthesis

-Cools down the temperature of the leaves when it is lost during transpiration.

-Maintains the turgidity of some cells.

-Translocation of substances occurs in water.

-Dissolves soil mineral salts before absorption

-     Seeds require water for germination

-     Water helps in the dispersal of some seeds.

-     Motile garnets of lower plants like mosses, liver worts and ferns swim chemotactically in water to fertilize the ovum in the archegonium

-     The elimination of some waste substances occur in water-Water is implicated in stomata opening and closure.

2025 • Biology • BIOMOLECULES, DNA & PROTEIN SYNTHESIS

36. Give biological explanations to the following (a) Animal fats seem to be solid at room: temperatures while plant oils are liquid.

(b)    Starch molecules are much the same while there are millions of types of proteins

(c)    Viruses can disappear and reappear several years after.

(d) In DNA, thymine pairs only with adenine and not with cytosine or guanine.

(e)    The genetic code is triplet. (4x5 mks)

(a)    Why animal fats are solid and plant oils liquid at room temperature

-Animal fats are saturated and their fatty acids contain no double bonds. They can therefore pack together tightly at room temperature and solidify. -Plant oils are unsaturated. Their fatty acids contain double bonds, which cause kinks that prevent them from packing together and solidify.

(b)    Why starch molecules are similar and proteins of different kinds

Every starch molecule is a polymer of glucose monomers, which are all the same (a-glucose), so starch molecules differ only in length.

-A protein molecule can be composed of 20 different types of amino acid monomers. These can be arranged in chains of an almost infinite

number of different sequences and length, producing Many different types of proteins, amino acids

(c)   Why viruses can disappear and reappear several years later.

The virus may be able to® insert its genes into a cells DNA and remain latent within the cell as a provirus. From time to time the provirus may begin reproducing complete viruses, causing disease symptoms.

(d)    Why thymine pairs only with adenine

A single pyrimidine, such as thymine must pair with a double ringed purine purine, such as adenine.

-Two pyrimidines such as adenine and cytosine would not be large enough to reach across the double helix.

-Thymine specifically pairs with adenine and not guanine because thymine thymine and adenine have complimentary chemical side groups that form hydrogen bonds.

(e)    Why the genetic code is triplet

Twenty amino acids are used in building proteins. There are only four types bf nucleotides in DNA, with each of the four bases of DNA. A one base code could specify only four amino acids. A two base code could specify 4² or 16 amino acids. In a triplet code, 4³ or 64 combinations of bases are possible, more than enough to code for 20 amino acids.

2025 • Biology • BIOMOLECULES, DNA & PROTEIN SYNTHESIS

37.       (i) Define mutation. State its causes and briefly explain why some mutations are harmful while others are not.

(11) Which type of mutation, base deletion or substitution has the greatest effect on the organism?

(b)       Describe the roles of mRNA, tRNA, ribosomes and amino acids in making a polypeptide.         (5,5,10 mks)

(a)        (i) Definition, causes of, and reasons why some mutations are harmful and others are not

-A mutation is a change in the nucleotide sequence of DNA.

-Some mutations are spontaneous, but many are caused by X-rays, ultraviolet length and chemical substances. Most mutations are harmful because they alter protein amino acid sequences and impair the functions of proteins.

-Some mutations do not alter amino acid sequence.

-Others lead to an improved protein or one new capability that enhances an organism's success.

-Mutations create genetic diversity that makes evolution by natural selection possible.

(ii)Effects of base substitution and deletion

A base substitution can result in no change to a protein or most often a change in one amino acid. Effects on the organism may be minimal.

-A base deletion alters the sequence of triplet groupings in a gene, causing a drastic change in the amino acid sequence downstream from the deletion. This can have a profound effect on the protein and the organism

(b)       Roles of mRNA, tRNA, ribosomes and amino acids in polypeptide synthesis

-Messenger RNA (mRNA) is transcribed from a gene and carries the instructions for the making a particular polypeptide. It contains the codons on which anticodons of tRNA are attached during translation

-A ribosome is the site of translation, the process the process of making a polypeptide according to the mRNA message.

-Transfer RNAs act as translators, each kind matching a particular codon in the mRNA. They contain the enzyme amino acyl-tRNA synthetase -The ribosomes read the mRNA one codon at a time and tRNAs deliver their amino acids, which are-added to the polypeptide chain one at a time. -Amino acids have hydroxyl and carboxyl groups which combine in a condensation reaction through peptide bonds. Several of them combine to form polypeptides. Some have Sulphur, positive/negative charges and slightly positive/negative charges at their R groups that form Sulphur bridges, ionic bonds and hydrophobic interactions that hold the final structures of the proteins formed from the polypeptides.

2025 • Biology • BIOMOLECULES, DNA & PROTEIN SYNTHESIS

38.(a) Outline the structure and composition of nucleic acids that adapts it for replication

(b) Discuss the importance of energy being released in a controlled way within cells.

(10,10 mks)

(a) Structure and composition of the DNA in relation to replication

-     The basic structural unit of the DNA molecule is the nucleotide, made up of a pentose sugar, phosphate group and one of the four nitrogenous bases adenine, cytosine, thymine and guanine. These bases pair in a complimentary fashion, adenine with thymine and cytosine with guanine.

-     The sequence of nucleotides is the primary structure of DNA.

-     The sugar and phosphate groups form the backbone of the molecule while the complimentary bases are located on the inside and held together by weak hydrogen bonding.

-     Two nucleotides are linked to form two chains which are complimentary⁷ to each other, and form a double helix. The helices are anti-parallel and wound round each other. Their sequences run in opposite directions.

-     The bases are arranged in a linear manner which forms the bases of the genetic code, having enough redundancy to code for all natural amino acids.

-     DNA replication starts with the breaking of the weak hydrogen bonds between the complimentary bases.

-     This exposes two naked strands, each of which effectively acts as a template, allowing complementation of the freely available nucleotides.

-     New daughter DNA molecules are thereby built ^UP-

b) Importance of controlled release of energy within cells

-Energy is required for the metabolism of all cells, which must be able to grow, maintain and repair themselves, reproduce and respond to stimuli.

-     Energy is needed for cellular work, mechanical work of muscle contraction, electrical transmission of impulses and active transport to move molecules against concentration gradients.

-     It is also used for the synthesis of new molecules for growth, or for energy storage

-     Random energy release or sudden release of large amounts of heat would be inappropriate to the order essential to the human body, which is homeothermic.

-Cells contain effective control mechanisms, which minimize the energy lost as heat, by storing energy in chemical bonds or complex organic molecules, such as adenosine triphosphate.

-     Since metabolism consists of both catabolic and anabolic reactions, each must be controlled in such a way that couples both reactions

-     The cell has control mechanisms that ensure that the energy available for particular reactions to proceed in the required direction and the correct time are available. Uncontrolled release of energy would alter this balance, since most chemical reactions are reversible.

-     Lack of control in the energy⁷ released would also endanger the cell.

2025 • Biology • BIOMOLECULES, DNA & PROTEIN SYNTHESIS

39. (a) Describe the Watson and Crick structure of DNA.

(b)    Give five structural differences between DNA and RNA

(c)     Explain the following terms as applied in DNA replication: (i) Template (ii) Semiconservative replication (iii) Continuous replication (iv) Discontinuous replication

(7,5,2,2,2,2mks)

(a)    Watson and Crick structure of DNA

-     The DNA molecule is, made up of two helical chains of polynucleotides

-     The two polynucleotides are held by pairing of bases by hydrogen bonds between purine and pyrimidine nitrogenous bases.

-Adenine always pairs with thymine’ while guanine pairs with cytosine.

-The two chains are complimentary since the sequence of bases in one chain determines that of the other due to base pairing complementarity -Pentose sugars and phosphate groups form the phosphate sugar backbone at the outer corners ’and the nitrogenous bases are arranged at right angles.

-     The chains are anti-parallel and run in opposite directions. One chain runs from the 5' to the 3' end while the other runs from the 3' to the 5' end.

-     The polynucleotide chains coil around each other forming a double helix.

(b)    Structural differences between DNA and RNA

DNA

RNA

Pentose sugar lacks oxygen in carbon atom 2 (deoxyribose)

Pentose sugar has oxygen in carbon atom 2 (ribose)

Double stranded

Single stranded

Only one type

Three types tRNA, rRNA, mRNA

Larger molecule

Smaller molecule

Contains nitrogenous bases adenine, thymine, guanine and cytocine

Contains the bases adenine, uracile, guanine and cytocine

(c)      (i) Template

-     This is one of the strands of DNA used during transcription to form messenger RNA required for protein synthesis.

-     It is a section of one strand that act as pattern to produce a new strand called messenger RNA (mRNA)

-     The two strands of DNA formed after separation of the two polynucleotide chains during replication are called templates, used for the assembly of a new complementary chain

(ii) Semi conservative replication

This results when DNA replicates forming two molecules in which each newly formed strand maintains one of the original parent strands.

(iii)    Continuous replication

This is replication that resi strand can be copied continue that DNA polymerase me direction as the unwinding ere

(iv)    Discontinuous replication

This type of replication occurs when DNA polymerase moves to the other strand different from the template and read it from its 5' to 3' end, away from the unwinding enzyme.

-Small gaps are left since polymerase cannot join the 3' end of one newly synthesized strand to the 5' end of the next, resulting in discontinuous replication

-It requires the enzyme DNA ligase to close the gaps created

2025 • Biology • CYTOLOGY & HISTOLOGY

2.    (a) What is meant by a cell?

(b)    Describe how the structure of each of the following is related to the functions it performs.

(i)   a parenchyma cell (ii) a tracheid

(c)    Why does specialization of some plant cells often result in the loss of their ability to divide?   (4,12, 4 mks)

(a)    Definition of a cell

A cell is the basic unit of structure and function in living organisms. It is bound by an outer membrane and contains definite organelles. A cell comes from pre-existing cells by division and can carry out all the activities of life including metabolism.

(b)    (i) Relationship between structure and functions of parenchyma

-Parenchyma has a thin cellulose wall which provides a pathway for water and mineral salts.

-The cells are unspecialized and serve as parking tissues. When turgid, they become tightly packed and provide support for the organs in which they are found,

-Air spaces exist between their cells which allow gaseous exchange to occur with the external environment.

-Their cells are metabolically active and provide sites for metabolic activities.

-They have large vacuoles and serve in food storage e.g. potato tubers where they store starch -Plasmodesmata between adjacent cells allow lateral transportation of substances between parenchyma cells.

(ii) Tracheids

-Tracheids have dead cells at maturity.                    -.

-This prevents obstruction or usage of water being transported.

- It has pits for horizontal transportation of water

- The walls of their cells are lignified which enables them give support to other plant organs.

- Cells have tapering end walls which overlap, giving additional mechanical strength.

- Make a good labelled diagram of the tracheid.

(c)    Why specialization sometimes leads to loss of the ability of plant cells to divide

-Specialisation may lead to loss of some cell organelles like the nucleus. This may restrict metabolic activities and prevent synthesis of new materials and make them loss the ability to divide. Also, specialisation may cause the introduction of new materials like lignin, or suberine which blocks transportation of substances causing cells to loss their ability to divide.

2025 • Biology • CYTOLOGY & HISTOLOGY

3. (a) Explain why (i) Xylem and (ii) Phloem are referred to as compound tissues.

(b) Primus africana (Pygium) is a perennial whose bark is used to treat cancer. How would you advise a villager on the harvesting of this bark so as to ensure a greater conservation of the trees? (12, 8 mks)

(a)    (i) Reason why Xylem is a compound tissue

-Xylem is made up of four types of cells which are adapted to specific roles. A compound tissue is one with different types of cell layers. The various cells are:

-Xylem vessels: They are long tubular structures formed by the fusion of several cells end to end in a row. This gives additional strength. The walls are lignified differentially into spiral, pitted, reticulate and scalariform. Lignification gives strength which supports plants Vessels have wide lumen to conduct water and mineral salts.

-Tracheids are single elongated, lignified cells. Their mature cells are dead with empty lumens.

-Xylem parenchyma: These have thin cellulose cell walls and their mature cells are living. They function for food storage, gaseous exchange through their intercellular air spaces and radial transport of food and water.

-Xylem fibres have short narrow cells with thick walls for mechanical strength.

(ii) Phloem

The phloem functions in the translocation of food, and is made up of five different types of cells.

-Sieve tubes: They are formed by the end to end fusion of cells called sieve elements. Their walls are made of cellulose and pectic substances and their mature cells lack nuclei.

-Companion cells: They occur near sieve elements and supply them with their needs. The sieve plates are pores to allow flow of substances from one sieve element to the other.

-Phloem parenchyma: They are cells with an elongated nucleus, are living and function in storage.

-Phloem fibres provide strength to resist pressure and occur only in dicots.

-Sclereids: They give additional mechanical strength and occur in older phloem.

(b)    Ways of harvesting Bark of Pygium for greater conservation of the tree

-The villagers should avoid removing a complete ring not to block translocation of food from the leaves to the roots and otherv parts.

-They should allow enough time' for the replacement of cut regions.

-They should remove part of the bark from the main trunk and not from the branches.

-Harvesting should be done when the tree is large and mature, not from small ones and planting of trees should be encouraged.

-Time the harvesting season well, not to kill the plant

-Minimise cutting down of the whole tree

-Develop techniques to tap the content of the bark.

2025 • Biology • CYTOLOGY & HISTOLOGY

(a) Explain how the electron microscope has contributed to the existing knowledge of the cell.

(b)    Describe the fluid mosaic model of the plasma membrane.

(c)     How is the structure of the membrane suited for the movement of substances in and out of the cell?         (5,10, 5 mks)

(a)     Contributions of the electron microscope to the existing knowledge of cells.

-The electron microscope has a high magnification and resolution power.

- This enables extremely small cells together with many more organelles of the cell to be observed and studied.

- Clear distinctions can be made between the parts of an organelle and a detail structure of the cell and its components clearly seen.

-Also the molecular level of cells can now be studied employing the high resolution and magnification powers of the electron microscope.

(b)    Description of fluid mosaic model

-The fluid mosaic model of the cell membrane was proposed by singer and Nicholson.

- The membrane is made up of a fluid phospholipid bilayer which moves randomly in their layer.

-Proteins float about in the phospholipid bilayer forming a fluid mosaic pattern, making the membrane trilamella.

-Some proteins penetrate the entire membrane while others penetrate only half

-Some proteins and lipids may have short carbohydrate branches forming glycoproteins and glycolipids.

-Hydrophilic phosphate heads of phospholipids face outwards while the hydrophobic hydrocarbon tails face inwards.

-Cholesterol is present in the membrane which improves membrane flexibility, fluidity and stability.

Diagram of the fluid mosaic model of the plasma
membrane

(c)    Adaptation of cell membrane for movement of substances.

-The membrane contains proteins

-Some are channel proteins that allow an opening for simple diffusion of gases like O2 and CO2.

-Some are carrier proteins that help in the facilitated diffusion of glucose and amino acids, and in active transport.

-The membrane equally contains phospholipids, which facilitates the movement of fatty acids.

-Cholesterol plugs present restrict entry of' substances.

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DNA	RNA
Pentose sugar lacks oxygen in carbon atom 2 (deoxyribose)	Pentose sugar has oxygen in carbon atom 2 (ribose)
Double stranded	Single stranded
Only one type	Three types tRNA, rRNA, mRNA
Larger molecule	Smaller molecule
Contains nitrogenous bases adenine, thymine, guanine and cytocine	Contains the bases adenine, uracile, guanine and cytocine

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