8a) DNA and Genomics table 9744 2018

Genetics and Inheritance (9744) DNA replication, transcription, translation & mutations 2018 Prepared by: Mrs Selvamani Nair Raffles Institution (Yr 5-6) 1 Nucleic acid RNA (mRNA, tRNA & rRNA) DNA Pentose (five carbon) sugar Ribose Deoxyribose Nitrogenous bases Purines (2 rings) Adenine & Guanine Adenine & Guanine Pyrimidines (1 ring) Cytosine & Uracil Cytosine & Thymine Complementary base pairing occurs between Adenine & Uracil ( 2 H bonds) Cytosine & Guanine ( 3 H bonds) Adenine & Thymine ( 2 H bonds) Cytosine & Guanine ( 3 H bonds) Structure Single Stranded Double Stranded Found in (location) Cytoplasm, Nucleus Nucleus Structure of a nucleotide (a nucleoside + phosphate group = nuceloside monophosphate) Structure of RNA and DNA Evidence for semi-conservative replication Nitrogenous base: attached to C1 Phosphate group: attached to C5 OH group attached to C3: involved in phosphodiester bond formation If H is attached to C2  deoxyribose sugar If OH is attached to C2  ribose sugar In DNA * A:T = 1:1 and C:G = 1:1 * (A+G) = (C+T) i.e. no.of purines = no.of pyrimidines * Purines: 2 rings (baby’s are pure & go googooGAGA) * Pyrimidines: 1 ring (CUT food on dinner plate) * Constant width between sugar phosphate backbone = 2nm * 2 strands are anti-parallel: one strand runs in the 5’ to 3’ direction, while the other strand runs in the 3’ to 5’ direction DNA is said to have directionality * 1complete turn of the double helix has 10 base pairs and spans a distance of 3.4nm * 1 DNA molecule is made up of 2 strands of DNA 3 hypotheses for DNA replication mechanism: 1) Semi-conservative replication both strands separate by the breaking of hydrogen bonds and each strand acts as a template for the synthesis of a new strand through complementary base pairing. Thus each DNA molecule formed is a hybrid consisting of 1 original strand and 1 newly synthesized strand. 2) Conservative replication 2 parental strands re-associate after acting as templates, thus restoring the original double helix. The other DNA molecule consists of 2 newly synthesized strands. 3) Dispersive replication Parental DNA molecule is fragmented and dispersed. Daughter molecules are madeup of a mixture of old and newly synthesized parts. (a) A stock of parental E. coli were grown for many generations in 15N medium as the only source of nitrogen until 15N wa s incorporated into the nitrogenous bases of all bacterial DNA. (b) The E.coli containing 15N-15N were then transferred into a medium containing only 14N. The transferred E. coli were allowed to divide once and were then collected. The DNA extracted and centrifuged in CsCl were all hybrid ( 14N- 15N) DNA . This excluded conservative replication in which no hybrids form. (c) Some of these cells were then allowed to divide once more . The DNA extracted and centrifuged in CsCl were half hybrid (14N-15N) DNA and half “light” ( 14N-14N) DNA. This excluded dispersive replication in which no pure 14N-14N can be obtained.

Genetics and Inheritance (9744) DNA replication, transcription, translation & mutations 2018 Prepared by: Mrs Selvamani Nair Raffles Institution (Yr 5-6) 2 Gene Mutations A gene mutation is an alteration in the sequence of nucleotides which may change the sequence of amino acids in a polypeptide chain. This may change the 3D shape of the protein, affecting the protein function and subsequently affect the characteristics (phenotype) of the organism. Type of mutation Substitution Inversion Insertion Deletion Description Replacement of one nucleotide by another A segment of nucleotides separates from the allele and rejoins at the original position but is inverted One or several nucleotides are inserted into a sequence One or several nucleotides are removed from a sequence Result of mutation 1 codon changed 1 or more codons changed Shifts reading frame from point of mutation Shifts reading frame from point of mutation Effect on protein Minor/Major Minor / Major, depending on whether a frameshift occurs Usually Major Usually Major If the number of nucleotides inserted or deleted are a multiple of three, there will change the primary sequence but a frame shift will not result. 1. Frame-shift mutation:  due to insertion or deletion of a number of nucleotides that is not divisible by 3. Hence due to the triplet code, this would disrupt the reading frame and produce a different and non-functional polypeptide 2. Silent mutation:  is a point mutation that does not change the amino acid sequence in a polypeptide  it can occur in the either coding or non-coding regions  due to the degeneracy of the genetic code, more than one codon can code for the same amino acid, and hence even if the mutation occurs in the coding sequence of a gene, the same polypeptide will be synthesized  if the mutation occurs in the non-coding region, the same polypeptide will be synthesised. 3. Missense mutation  is a point mutation in which a single nucleotide change results in a codon that codes for a different amino acid  if the new amino acid has similar biochemical properties (e.g. charge, size) to the one that was replaced, the mutation is said to be conservative  if the new amino acid has different biochemical properties (e.g. charge, size) to the one that was replaced, the mutation is said to be non-conservative 4. Nonsense mutation  is a point mutation which results is a premature stop codon (UAG, UAA, UGA), causing the polypeptide to be truncated and non-functional Example of a disease due to a substitution mutation: Name of disease Sickle-cell anaemia Protein affected Beta-globin chain of haemoglobin (From HbA to HbS) Description of change Change in DNA : CTC to CAC (substitution) Change in mRNA : GAG to GUG Change in amino acid : glutamate to valine Effect of the change Charged and hydrophilic glutamate changed to non-polar and hydrophobic valine in HbS. At low oxygen concentrations, HbS undergoes a conformation change which will cause the hydrophobic patches on different HbS to stick together. This polymerization of HbS results in the formation of abnormal, rigid, rod-like fibres. Shape of red blood cell distorted – sickle shaped. Effects of disease Sickle red blood cells are more fragile and break easily. This results in shortage of red blood cells and poor oxygen transport. This leads to anaemia, lack of energy and heart failure. Sickle red blood cells may also lodge in small blood vessels and interfere with blood circulation. This will lead to organ damage. Chromosomal aberrations  Chromosomal aberrations can be due to variation in (A) chromosomal structure: 1. A deletion removes a chromosomal segment. 2. A duplication repeats a chromosomal segment. 3. An inversion reverses a segment within a chromosome. 4. A translocation moves a segment from one chromosome to another, non-homologous one. Chromosomal deletions and duplications can result in phenotypic abnormalities due to the reduced or additional genes respectively. Chromosomal inversions and reciprocal translocations can result in disease although the amount of g