[BIO] Chapter 17 - Molecular Genetics

© Hee Xin Wei (Copyrighted) Topic 1 7: Molecular Genetics

Chapter Analysis FOCUS EXAM WEIGHTAGE •Constitute to around 5% in Paper 2 in the past 5 years •commonly tested in MCQ and structured questions •tested twice in section B in the past 5 years •may be an abstract topic for some •linked to inheritance chapter © Hee Xin Wei (Copyrighted)

DNA, gene, chromosome Key Concept © Hee Xin Wei (Copyrighted)

DNA •Deoxyribonucleic acid (DNA) is a molecule that carries genetic code which is used to synthesise specific polypeptides •DNA is a double stranded molecules that are twisted around each other to form double helix structure of DNA. •The basic units of DNA is called nucleotides. nucleotide Each nucleotide consists of: -A deoxyribose sugar -A phosphate group -A base containing nitrogen There are four types of nitrogenous bases: -Adenine (A) -Guanine (G) -Cytosine (C) -Thymine (T) © Hee Xin Wei (Copyrighted)

DNA •The nucleotides polymerise to form a polynucleotide when the deoxyribose sugars of the nucleotides are joined together by phosphate groups, forming the sugar-phosphate backbone of the DNA molecule. •Double helix DNA strands are held together by hydrogen bonds between the nitrogenous bases by complementary base pairing Adenine forms 2 hydrogen bonds with Thymine Cytosine forms 3 hydrogen bonds to Guanine polynucleotide Double helix DNA Sugar phosphate backbone Complementary base pairing hydrogen bonds © Hee Xin Wei (Copyrighted)

gene & chromsome Gene •Gene is sequence of nucleotides. It forms part of a DNA molecule that is used to synthesise specific polypeptides. •This involves transcription and translation •A DNA molecule contains many genes along its length. •Eg a DNA molecule contains eye colour gene which codes for pigment protein that gives our iris colour Chromosomes •DNA is wrapped around proteins to form a chromatin fibre. •The chromatin fibres coil, condense, and shorten to form the compact structures called chromosomes during prophase of cell division © Hee Xin Wei (Copyrighted)

transcription & translation Transcription: •Transcription is the process by which the DNA template is used to make a single-stranded molecule called messenger RNA (mRNA) by complementary base pairing •There is no thymine in RNA, instead Adenine pairs with Uracil •Thymine pairs with Adenine •Guanine pairs with Cytosine, vice versa Translation: •Translation is the process by which the sequence of mRNA codons is used to make a polypeptide, which will fold into a protein •Ribosome is needed for the process Phenotype: the protein formed from the gene are responsible for every aspect of a living organism •appearance - protein that affect the pigment colour of iris •disease - gene is faulty and doesn’t produce insulin which causes Type 1 diabetes © Hee Xin Wei (Copyrighted)

Genetic Engineering Key Concept © Hee Xin Wei (Copyrighted)

Genetic Engineering Genetic Engineering •Genes may be transferred between cells via genetic engineering •Target genes may be cut off from the cells of one organism and inserted into the cells of another organism of the same or different species •The plants or animals that received the genes is called transgenic. •The transferred gene can express itself in the recipient organism. Restriction enzyme -Restriction enzyme cuts the genes at restriction sites to produce sticky ends -When genes and plasmids are cut by the same restriction enzymes, they produce complementary sticky ends, which will anneal by complementary base pairings -Restriction enzymes are essential in genetic engineering process © Hee Xin Wei (Copyrighted)

human insulin genetic engineering Transfer of human insulin gene into bacteria E.coli 1)Human insulin gene is isolated by adding restriction enzyme that cuts the gene, producing sticky ends. 2)Plasmid from E.coli is cut with the same restriction enzyme. This produces sticky ends complementary to those of the insulin gene. 3)Mix the plasmid with the DNA fragment containing the human insulin gene. The human insulin gene will bind to the plasmid by complementary base pairing between their sticky ends, forming recombinant plasmid 4)Add the enzyme DNA ligase to seal the nick. 5)Mix the recombinant plasmid with E.coli bacterium. Heat shock or electric shock is applied to open up pores on the cell surface membrane of the bacterium for plasmid to enter. The E.coli that contains this recombinant plasmid is a transgenic bacterium 6)The transgenic bacteria are placed in large fermenters under optimal conditions for growth and reproduction. Fermenter consists of: •a nutrient broth containing glucose water and salts •37°C temperature maintained by a temperature probe •optimal pH maintained by a pH probe •air supply for aeration and a stirrer to mix substances evenly. 7)At the end of fermentation, the insulin protein is extracted and purified before it can be used. © Hee Xin Wei (Copyrighted)