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DNA: Structure and Function

DNA: Structure and Function. Unit 7. Recall:. DNA is a nucleic acid and made of nucleotides Nucleotides contain a sugar, phosphate, and a base In DNA, the bases are adenine, cytosine, guanine, and thymine. What is DNA?. Deoxyribonucleic acid The “blueprint” of the cell

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DNA: Structure and Function

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  1. DNA: Structure and Function Unit 7

  2. Recall: • DNA is a nucleic acid and made of nucleotides • Nucleotides contain a sugar, phosphate, and a base • In DNA, the bases are adenine, cytosine, guanine, and thymine

  3. What is DNA? • Deoxyribonucleic acid • The “blueprint” of the cell • Stores genetic information • Instructions for making proteins

  4. How did we discover DNA? • Frederick Griffith experiment - 1928

  5. How did we discover DNA? • Avery, MacLeod, and McCarty experiment - 1944

  6. How did we discover DNA? • Hershey and Chase experiment – 1952

  7. What is the structure of DNA? • Shape = double helix (“twisted ladder”) • The “backbone” is made of the sugar and phosphate groups • The “rungs” of the ladder are the bases

  8. What is the structure of DNA? • The sugar-phosphate backbone is polar, which means it has a positive and negative end • It is also hydrophilic • Bases attach 5’ to 3’ • Each strand runs in the opposite direction = antiparallel

  9. What is the structure of DNA? • Between the sugars and phosphates in the back bone, there are strong covalent bonds • Between the bases in the middle, there are weak hydrogen bonds that are easily broken

  10. How did we discover the shape of DNA? • James Watson and Francis Crick – 1953 • Used a photograph from Rosalind Franklin, given to them by her assistant, Maurice Wilkins • Because she died of cancer, she was not awarded the Nobel prize in 1962

  11. Chargaff’s Rule • Adenine always pairs with thymine • Cytosine always pairs with guanine • Question: If a strand of DNA is 15% adenine, how much thymine, cytosine, and guanine does it contain?

  12. Structure of DNA – Levels of Complexity • Nucleotides = building blocks • Strand = covalently linked nucleotides in a linear fashion • Double helix = two strands twisted together • Chromosome = association of DNA with proteins to organize strands into a compact structure • Genome = complete complement of an organism’s genetic material

  13. How does DNA replicate? • DNA replicates semi-conservatively • The two strands unwind and one “parent” strand joins with one “daughter” strand to make two new double helixes

  14. How does DNA replicate? • Strands are unwound and separated by DNA helicase • Sites of separation and replication = replication forks

  15. How does DNA replicate? • New nucleotides are brought in and attached to the parent strand  A pairs with T, C pairs with G • The new nucleotides are joined to the parent strand by DNA polymerase • Usually, there are no mistakes, but sometimes they do occur • Mutation = change in a DNA sequence that affects genetic information

  16. What does DNA do? • Instructions for making proteins • DNA is too big to leave nucleus  it copies the information into RNA, which can leave the nucleus • Transcription = copying the information from DNA to RNA • Translation = using the information in the RNA to make proteins • Where do you think translation happens? • Ribosomes

  17. Recall: • Amino acids are the building blocks of proteins • During protein synthesis, amino acids are joined together in long polypeptide chains, which are folded into proteins

  18. DNA vs. RNA • DNA = double stranded, RNA = single stranded • DNA = uses deoxyribose as sugar, RNA = uses ribose as sugar • DNA = uses A, T, C, G as bases, RNA = uses A, C, G, and U (uracil) as bases

  19. Types of RNA • mRNA = messenger RNA, carries information from DNA to ribosome • tRNA = transfer RNA, carries anti-codons and amino acids to ribosome to be assembled into proteins • rRNA = ribosomal RNA, makes up the ribosomes, formed in nucleolus

  20. Codon vs. anticodon • Codon = String of three nucleotides on mRNA that codes for a specific amino acid • Anticodon = Group of three bases on tRNA that are complementary to an mRNA codon

  21. Steps of protein synthesis • 1. DNA is transcribed into RNA. • 2. RNA leaves the nucleus and goes to the ribosome. • 3. RNA passes through the ribosome while tRNA brings anti-codons with amino acids to the matching codons. • 4. The amino acids are joined together in the ribosome into a long polypeptide chain  making proteins. • http://www.youtube.com/watch?v=983lhh20rGY&feature=related

  22. 1. DNA is transcribed into RNA • RNA polymerase separates DNA and binds to a single strand • It uses the strand as a template to assemble nucleotides into a strand of mRNA • How does RNA polymerase “know” where to start and stop? • Promoters = specific base sequences that signal where to start transcription • There are also signals in the DNA to stop transcription

  23. 2. RNA leaves the nucleus and goes to the ribosome. • mRNA leaves through the pores in the nucleus • Sometimes, it needs to be “edited” • Intron = sequence of nucleotides that does not code for proteins, “junk” DNA • Exon = sequence of nucleotides that does code for proteins • Introns get cut out, exons get spliced together

  24. 3. RNA passes through the ribosome while tRNA brings anti-codons with amino acids to the matching codons.

  25. 3. RNA passes through the ribosome while tRNA brings anti-codons with amino acids to the matching codons.

  26. 4. The amino acids are joined together in the ribosome into a long polypeptide chain  making proteins.

  27. How does a codon determine what amino acid is used?

  28. What if something goes wrong? • Two kinds of mutations – gene mutations and chromosomal mutations • Chromosomal mutations are much more severe because they involve changes in the number or structure of entire chromosomes  more genetic information • Gene mutations can still be very significant  two main kinds: point mutations and frameshift mutations

  29. What if something goes wrong? • Point mutations = occur at a single point in DNA sequence, one base is substituted for another • Ex. The quick brawn fox jumps over the lazy dog • Frameshift mutations = a base is inserted or deleted  much more severe change, because it affects every codon that follows • Ex. The quick brwnfoxjumpsoverthelazydog

  30. What causes mutations? • Chromosomal mutations occur during early embryonic development • Gene mutations can occur during early embryonic development or later in life during cell replication • Mutations can be caused by alcohol, drugs, smoking, exposure to radiation (tanning = UV radiation exposure), etc.

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