Gene Structure


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Gene Structure

  1. 1. MIC210 BASIC MOLECULAR BIOLOGY Lecture 2 Gene Structure By SITI NORAZURA JAMAL (MISS AZURA) 03 006/ 06-483 2132
  2. 2. OUTLINE 1. 2. 3. 4. 5. DNA structure and function Genes and genome The genetic code and mutation DNA denaturation and renaturation Comparative genomics
  3. 3. It’s all in the DNA • Genetic material found in every living cell • Contains information to make proteins
  4. 4. 1. DNA Structure & function • Polymer made of 4 nucleotides : A, C, T, G • polynucleotide eg. A-G-T-C-C-A-A-G-C-T-T…. • DNA is double stranded - Double-helix • Complementary base pairing • Anti-parallel
  5. 5. Nucleotides – the building blocks of DNA Nucleotide = Sugar + base + phosphate Nucleoside = Sugar + base Deoxyribose sugar  5 carbons Phosphate (PO4) group at carbon no. 5 Hydroxyl (OH) group at carbon no. 3 scarr/Fg10_10rt.gif
  6. 6. Fig. 16-1
  7. 7. Building a Structural Model of DNA: Scientific Inquiry • After most biologists became convinced that DNA was the genetic material, the challenge was to determine how its structure accounts for its role • Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure • Franklin produced a picture of the DNA molecule using this technique Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  8. 8. Fig. 16-6 (a) Rosalind Franklin (b) Franklin’s X-ray diffraction photograph of DNA
  9. 9. • Franklin‟s X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical • The X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases • The width suggested that the DNA molecule was made up of two strands, forming a double helix Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
  10. 10. Fig. 16-7 5 end Hydrogen bond 3 end 1 nm 3.4 nm 3 end 0.34 nm (a) Key features of DNA structure (b) Partial chemical structure 5 end (c) Space-filling model
  11. 11. How one nucleotide can be joined to another through the 5’-PO4 (5 prime phosphate) and the 3’-OH (3 prime hydroxyl)
  12. 12. Many nucleotides can be joined in such a way to form a polynucleotide chain  a single stranded DNA
  13. 13. The Double Helix The complete DNA molecule is made up of two complementary strands in antiparallel directions
  14. 14. 5’ 3’ 3’ 5’ Complementary base-pairing • A always pair with T • C pairs with G Antiparallel • each strand of DNA has a “direction” • at one end, the terminal carbon atom in the backbone is the 5‟ carbon atom • at the other end, the terminal carbon atom is the 3‟ carbon atom • therefore each DNA strand has a 5’ and a 3’ end • in a double helix, the two strands are always antiparallel The length (or size) of a DNA molecule is measured in basepairs (bp) 1 kilobase (kb) = 1,000 bp 1 Megabase (Mb) = 1,000,000 bp
  15. 15. Chromosomes and DNA DNA is packaged in the form of chromosomes in the nucleus of a cell. The chromosomes contain DNA tightly wounded around proteins.
  16. 16. 2. Genes and genomes The Genome - complete set of DNA for an organism A comparison of genomes Organism Amount of DNA No of Chromosomes No of genes 3 Gb 46 100,000 Yeast 13 Mb 16 6,000 E.coli 4.5 kb 1 1,000 Human
  17. 17. A Gene • a specific DNA sequence that contains genetic information • information required to make a specific type of protein • that information is stored in the sequence on the „sense‟ strand • we say that a gene encodes a protein • thus a DNA molecule can contain many genes • the gene sequence is always written 5‟  3‟ 5’ATGCTTGGACGTGATGACATTGGAGGA... 3’TACGAACCTGCACTACTGTAACCTCCT... ‘sense’ ‘antisense’
  18. 18. 3. The Genetic Code and mutation – how the DNA (or mRNA) sequence is translated into the amino acid sequence of a protein
  19. 19. Reading frames • There are 3 ways to read a gene sequence – reading frames • Each reading frame will give a different result • Only one reading frame is correct (usually)
  20. 20. Gene mutation • Information coded in the DNA sequence is used to make proteins • If the DNA sequence is changed, what will happen? • A change in the genetic information is called a mutation. The outcome depends on the nature of the „change‟. • 3 types of DNA sequence mutations
  21. 21. Substitution mutations -change of 1 base -AAC ATA ACG CCG CGA GAT GAA – Asn Ile Thr Pro Arg Asp Glu a. Silent mutation - AAC ATC ACG CCG CGA GAT GAA – b. Missense mutation - AAC ATA AAG CCG CGA GAT GAA – c. Nonsense mutation - AAC ATA ACG CCG TGA GAT GAA – What happens to the amino acid sequence? What happens to the protein?
  22. 22. Frameshift mutations a. Deletion - AAC ATC AC CCG CGA GAT GAA – b. Insertion - AAC AATA AAG CCG CGA GAT GAA –
  23. 23. Mutation can also happen due to -changes in long DNA sequences -changes in the structure of genes/ chromosomes -changes in the number of genes/ chromosomes -e.g. Mutations leading to cystic fibrosis
  24. 24. 4. DNA denaturation and renaturation
  25. 25. Denaturation – breaking up of the double helix molecule • • • • • hydrogen bonds broken by heating to > 96oC (and also other conditions) Tm = the melting point of a DNA molecule depends on the GC content higher %GC  higher Tm Renaturation - if the denatured DNA strands are allowed to cool slowly • the can re-anneal with each other and regain the double helix structure • reannealing by complementary base-pairing
  26. 26. 5. Comparative genomics • Comparison of whole genome sequences provides a highly detailed view of how organisms are related to each other at the genetic level. How are genomes compared and what can these findings tell us about how the overall structure of genes and genomes have evolved? • Purpose / benefits?