(a)    Functions of monosaccharides

-Monosaccharides are hexoses, pentoses or trioses.

-Hexoses such as glucose, fructose and galactose are a source of energy⁷ when oxidised.

-They are used to synthesize disaccharides and polysaccharides.

-Pentoses like ribose and deoxyribose are used to synthesize nucleic acids (DNA and RNA), Ribose is used to synthesize co-enzymes like NAD and NADP, and ATP. Ribulose is a component of Ribulose biphosphate which is the CO2 acceptor in photosynthesis. _}

-Trioses like glyceraldehyde and dihydroxyacetone are intermediates in respiration, photosynthesis and other metabolic reactions.

(b)    Isomerism in glucose

-Glucose can exist in 2 possible ring forms, alpha

(a) and beta (P) forms

-When the hydroxyl group of carbon atom one projects below the ring it forms a-glucose

-When the hydroxyl group projects above it forms p-glucose

-Both forms have the same chemical formulae but different structures.

NB: The hexoses also exhibit isomerism between fructose, glucose and galactose.

-Fructose is a structural isomer of glucose with identical chemical groups bonded to different carbon atoms.

-Galactose is a stereo isomer of glucose with identical chemical groups bonded to the same carbon atoms, but in different orientations.

(c)    Denaturation is the loss of the specific 3- dimensional shape of a protein molecule. It can be caused by some agents.

-Very high temperatures and ultra violet rays cause atoms to vibrate violently, disrupting weak H-bonds, and ionic bonds causing the protein to coagulate.

-Strong acids and alkalis disrupt ionic bonds and break peptide bonds

--Heavy metals form strong bonds with the negatively charged carboxyl groups and disrupt ionic bonds. This reduces polarity causing proteins to precipitate in solution.

-Organic solvents and detergents disrupt hydrophobic groups and disrupt H-bonds.

(ii)    Renaturation is when the protein spontaneously refolds into its original structure after denaturation.

-Occurs when conditions are suitable

-Bonds are reformed since the tertiary structure equally depends on primary structure.

(a) (i) Association of heredity with cell nucleus

-     The nucleus of cells contains the genetic material DNA. DNA carries inherited characteristics from generation to generation.

-     Avery and Co. carried out transformation of pneumococcus using DNA extracted from heat- killed smooth cells from the nucleus.

-     Mendel carried out experiments equally to prove that the genetic material is found in the nucleus and that during fertilization only the nuclei of gametes fuse to form the off-spring, thus hereditary is associated to the nucleus of cells.

(ii)    Association of heredity with chromosomes

-Griffith carried out experiments to show that the chromosomes carry genetic information. The chromosome is the structure containing the genes which are DNA in condensed form. -During cell division the chromosomes must first divide, and any changes in them causes changes in the off­springs accordingly.

(iii)    Association of heredity with DNA

-     The DNA is the major component of genes. It gives information of type of protein to be made,

hence determines how cells should grow, divide and be specialized.

-     The study of reproduction in viruses using electron microscopy and micrography has revealed that the T2 viruses injects only the DNA into the host cell and take control of all the activities of the host cell.

-     This explains that the genetic information determining heredity is information on the DNA molecule.

-     This information determines the polysaccharides, lipids and proteins synthesized within living cells and hence heredity.

-    b) (i) Principal requirements of Genetic materials

-Genetic materials must be capable of duplicating themselves and direct the formation of all the other structures in an organism.

-They must also direct the formation of structural and enzymatic proteins, which will in turn, direct the metabolism of cells and affect the development of the entire organism.

-Genetic materials must equally be stable and "hot easily altered.

(ii) How DNA satisfies requirement as genetic material.

-The DNA molecule satisfies the above requirements because it is stable with high boiling and melting points from generation to generation without changing.

-It can replicate in a semi-conservative manner, maintaining strands of the original parent.

-Has a double helix with base pairings held by H- bonds that carries information on types of polypeptides to be formed.

(a)     Definition of the genetic code.

-The genetic code is information in DNA used to determine the sequence of amino acids in a protein.  

Characteristics of the genetic code

-The genetic code is triplet i.e. three bases in DNA code for one amino acid.

(a)      Structure of DNA

-The structure of DNA was established by Watson and Crick.

-It is made up of two helical chains of polynucleotides

-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 purines of DNA having two rings are adenine and guanine while the pyrimidines with a single ring are thymine and cytosine.

-The two chains are complimentary since the sequence of bases in one chain determines that of the other.

-Pentose sugars and phosphate groups form the pentose sugar backbone at the outer corners

(a)     Properties of polysaccharides that make them convenient for food storage

-Polysaccharides have large sizes, making them insoluble in water, and do not exert any osmotic or chemical influence in cells.

-They fold into compact shapes and are easily converted to sugars by hydrolysis when required

(a) (i) Structure and functions of Starch

-Starch is a polymer of a-glucose

-It is made up of amylose and amylopectin

-Amylose has straight chains of several glucose residues linked by 1, 4 bonds.

-Amylopectin contains 1, 6 glycosidic bonds

-Serve as food storage substance in plants

-Can easily be converted to glucose and oxidised to release energy

(ii)     Glycogen

-Glycogen is a polymer of a-glucose and is the energy storage molecule in animals and many fungi.

-It is easily converted to glucose in mammals under the influence of the hormone insulin. The glucose is oxidised to release energy.

-Glycogen is made up of many branches, it forms granules in cells associated to the smooth endoplasmic reticulum.       

(iii)     Cellulose

-Cellulose is a polymer of P-glucose in plants

-It forms part of the structure of the cell wall.

-The chains cross-link with each other by H-bonds which gives the molecule high tensile strength.

-Prevents cells from bursting when they absorb water.

-Can be digested by cellulase producing microbes to release glucose

-Used commercially to make cotton goods, paper and sellotape.

(b)     Differences between cellulose and glycogen

(a)    A zwitterion is an ion with two poles, both positive and negative poles. The amino group can accept H⁺, forming a positive charge while the carboxyl group can donate H⁺ resulting to a negative pole at that end.

Why amino acids act as buffers

-An Amino acid acts as a buffer, because it is amphoteric.

- It has a carboxyl group which can donate a proton, making the molecule negative in an alkaline solution.

-The amino group gives basic properties and can take up protons, giving the molecules a positive* charge in an acid solution.

-The molecule ionises at a particular ph.

-The ability to donate or receive protons causes amino acids to behave as buffers, i.e. resist changes in pH on addition of acids or bases.

(b)      Fibrous proteins result from the secondary structure containing long parallel polypeptide chains that cross-link forming fibres. It is physically tough and insoluble in water e.g. collagen, silk and keratin.

-Globular proteins result from the tertiary structure. The polypeptide chains tightly fold. They are soluble e.g. enzymes, antibodies and hormones.

-Conjugate proteins are fibrous but soluble in water e.g. fibrinogen.

(c)    (i) Structural

-Proteins form part of connective tissues like bone, tendons and cartilage e.g. collagen.

-Forms surface of skin, feathers, nails, hair and horn e.g. keratin

-Protects nucleic acids of viruses e.g. viral protein coat

-Forms, elastic connective tissues of ligaments e.g. elastin.

(a) (i) Semi conservative replication

-In semiconservative replication, the DNA strands unwind and each strand serves as a template for the replication.

-So both of the new copies of DNA contain half of the old DNA and half of newly synthesized DNA -Many enzymes are necessary to successfully replicate DNA. Topoisomerase is used to unwind the DNA strand. By cutting one strand, the tension in the supercoil is released.

-Next helicase is used to prevent the strand from recoiling.

-DNA polymerase HI then travels across the strand, from the five carbon (5‘, or the fifth carbon on the phosphate group) to the third carbon (3', or the third carbon on the phosphate group). DNA ^polymerase III places the complementary nucleotide on the strand.

-     So adenine would be put opposite of thymine (and the reverse), and guanine would be opposite of cytosine (and the reverse).

-     Ligase is then used to reattach the strands with their new nucleotides.

-     Throughout the entire process single-strand binding proteins are used to keep the DNA- stable so that DNA polymerase can work.

-     DNA polymerase can only work in the 5' to 3' directions. The reason behind this is because 3' is more stable than 5‘ when attaching new nucleotides. If DNA polymerase ran in the other direction, then there is the risk that the phosphate group could break off.

-     So, when DNA polymerase is working it only goes in the 5' to 3' direction.

-However, there are two sides, and one runs in the 5' to 3¹ directions, but the other runs from 3' to 5'. So, while DNA polymerase can work continuously in the 5' to 3' direction it has to work in sprats on the 3' to 5' direction, which are called Okazaki fragments.

-     The 5' to 3' is called the leading strand while the other is called the lagging strand.

-     DNA ligase helps to close the gaps between the molecules. This is called the semi-conservative replication since each newly formed strand maintains one of the original parent strands.

Parental DNA double helix

(ii) Conservative replication

-In conservative replication, the two strands of DNA do not unwind as a brand-new copy is produced and the old molecule remains intact.

-One of the newly formed DNA maintains parental strands while the other has two strands all different from the parent strand.

two DNA double helix

(b) Evidence for semi conservative replication of DNA

-Cultures of E. coli with a single circular chromosome were grown for many generations in a medium containing the heavy isotope of nitrogen ¹⁵N.

-The entire DNA was labelled with ¹⁵N.

(a)   Reasons why starch molecules are similar but proteins of several kinds    

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

-A protein molecule can be composed of 20 kinds of amino acid monomers.

-These can be arranged in chains of an almost infinite number of different sequences and lengths, producing many different kinds of proteins.

(b)    How the DNA molecule replicates

-The enzyme helicase causes the unwinding of the double helix.

-DNA polymerase binds to one of the single strands and starts to move along the strand.

-Each time it meets the next base on the DNA, free nucleotides approach the DNA strand, and the one with the correct complimentary base forms hydrogen bonds with the base in the DNA.

-The free nucleotide is held in place by the enzyme until it binds the preceding, nucleotide, thus extending the new strand of DNA.

-The movement of DNA polymerase is in the 5’ to 3' direction only.

-To start copying the other strand, the gap between the strands is closed by the enzyme called DNA ligase.