24.1 DNA Structure and Replication

1. 24.1 DNA Structure and Replication • Hershey-Chase Experiments (1952) • Demonstrated that DNA is the genetic material • DNA stands for Deoxyribonucleic Acid

2. Hershey-Chase Experiments

3. 24.1 DNA Structure and Replication • Structure of DNA • James Watson and Francis Crick determined the structure of DNA in 1953 • DNA is a chain of nucleotides • Each nucleotide is a complex of three subunits • Phosphoric acid (phosphate) • A pentose sugar (deoxyribose) • A nitrogen-containing base

4. 24.1 DNA Structure and Replication • Structure of DNA • Four Possible Bases • Adenine (A) - a purine • Guanine (G) - a purine • Thymine (T) - a pyrimidine • Cytosine (C) - a pyrimidine • Complimentary Base Pairing • Adenine (A) always pairs with Thymine (T) • Guanine (G) always pairs with Cytosine (C)

5. Overview of DNA Structure

6. 24.1 DNA Structure and Replication • Replication of DNA • Semi-conservative replication • Each daughter DNA molecule consists of one new chain of nucleotides and one from the parent DNA molecule • The two daughter DNA molecules will be identical to the parent molecule

7. 24.1 DNA Structure and Replication • Replication of DNA • Before replication begins, the two strands of the parent molecule are hydrogen-bonded together • Enzyme DNA helicase unwinds and “unzips” the double-stranded DNA • New complementary DNA nucleotides fit into place along divided strands by complementary base pairing. These are positioned and joined by DNA polymerase • DNA ligase repairs any breaks in the sugar-phosphate backbone • The Two double helix molecules identical to each other and to the original DNA molecule

8. Overview of DNA Replication

9. Ladder Configuration and DNA Replication

10. 24.2 Gene Expression • Gene: A segment of DNA that specifies the amino acid sequence of a polypeptide • DNA does not directly control protein synthesis, instead its information is transcribed into RNA

11. 24.2 Gene Expression • RNA (ribonucleic acid)

12. 24.2 Gene Expression • Three Classes of RNA • Messenger RNA (mRNA) • Takes a message from DNA to the ribosomes • Ribosomal RNA (rRNA) • Makes up ribosomes (along with proteins) • Transfer RNA (tRNA) • Transfers amino acids to ribosomes

13. 24.2 Gene Expression • Gene Expression Requires Two Steps: • Transcription • Translation

14. 24.2 Gene Expression • Transcription • During transcription, a segment of the DNA serves as a template for the production of an RNA molecule • Messenger RNA (mRNA) • RNA polymerase binds to a promoter • DNA helix is opened so complementary base pairing can occur • RNA polymerase joins new RNA nucleotides in a sequence complementary to that on the DNA

15. 24.2 Gene Expression • Transcription • Processing of mRNA • Primary mRNA becomes mature mRNA • Contains bases complementary to both intron and exon segments of DNA • Introns are intragene segments • Exons are the portion of a gene that is expressed • Intron sequences are removed, and a poly-A tail is added • Ribozyme splices exon segments together

16. Transcription of DNA to form mRNA

17. mRNA Processing

18. 24.2 Gene Expression • Translation • The Genetic Code • Triplet code- each 3-nucleotide unit of a mRNA molecule is called a codon • There are 64 different mRNA codons • 61 code for particular amino acids • Redundant code-some amino acids have numerous code words • Provides some protection against mutations • 3 are stop codons signal polypeptide termination

19. Messenger RNA Codons

20. 24.2 Gene Expression • Transfer RNA • tRNA transports amino acids to the ribosomes • Single stranded nucleic acid that correlates a specific nucleotide sequence with a specific amino acid • Amino acid binds to one end, the opposite end has an anticodon • the order of mRNA codons determines the order in which tRNA brings in amino acids

21. Transfer RNA: Amino Acid Carrier

22. 24.2 Gene Expression • Ribosome and Ribosomal RNA • Ribosome has a binding site for mRNA and for 2 tRNAs • Facilitate complementary base pairing • Ribosome moves along mRNA and new tRNAs come in and line up in order • This brings amino acids in line in a specific order to form a polypeptide • Several ribosomes may move along the same mRNA • Multiple copies of a polypeptide may be made • The entire complex is called a polyribosome

23. Polyribosome Structure and Function

24. Overview of Gene Expression

25. 24.2 Gene Expression • Translation Requires Three Steps • Initiation (requires energy) • Elongation (requires energy) • Termination

26. Initiation

27. Elongation

28. Termination

29. Summary of Gene Expression

30. 24.2 Gene Expression • Genes and Gene Mutations • A gene mutation is a change in the sequence of bases within a gene. • Gene mutations can lead to malfunctioning proteins in cells.

31. 24.2 Gene Expression • Genes and Gene Mutations • Causes of Mutations • Errors in replication • Rare • DNA polymerase “proofreads” new strands and errors are cleaved out • Mutagens • Environmental influences • Radiation, UV light, chemicals • Rate is low because DNA repair enzymes monitor and repair DNA • Transposons • “jumping genes” • Can move to new locations and disrupt sequences

32. Transposon

33. 24.2 Gene Expression • Types of Mutations • Frameshift Mutations • One or more nucleotides are inserted or deleted • Results in a polypeptide that codes for the wrong sequence of amino acids • Point Mutations • The substitution of one nucleotide for another • Silent mutations • Nonsense mutations • Missense mutations

34. 24.3 DNA Technology • The Cloning of a Gene • Cloning: Production of many identical copies of an organism through some asexual means. • Gene Cloning: The production of many identical copies of a single gene • Two Ways to Clone a Gene: • Recombinant DNA • Polymerase Chain Reaction

35. Cloning of a Human Gene / Recombinant DNA

36. 24.3 DNA Technology • Using Recombinant DNA Technology • Restriction enzymes breaks open a plasmid vector at specific sequence of bases “sticky ends” • Foreign DNA that is to be inserted is also cleaved with same restriction enzyme so ends match • Foreign DNA is inserted into plasmid DNA and “sticky ends” pair up • DNA ligase seals them together

37. Restriction Enzymes and Stick Ends

38. 24.3 DNA Technology • Polymerase Chain Reaction • Amplifies a targeted DNA sequence • Requires DNA polymerase, a set of primers, and a supply of nucleotides • Primers are single stranded DNA sequences that start replication process • Amount of DNA doubles with each replication cycle • Process is now automated

39. 24.3 DNA Technology • DNA Fingerprinting • Permits identification of individuals and their relatives • Based on differences between sequences in nucleotides between individuals • Detection of the number of repeating segments (called repeats) are present at specific locations in DNA • Different numbers in different people • PCR amplifies only particular portions of the DNA • Procedure is performed at several locations to identify repeats

40. DNA Fingerprints

41. 24.3 DNA Technology • Biotechnology • Biotechnology uses natural biological systems to create a product or to achieve a goal desired by humans. • Transgenic organisms have a foreign gene inserted into their DNA

42. 24.3 DNA Technology • Transgenic Bacteria • Medical Uses: Production of Insulin, Human Growth Hormone, Tissue Plasminogen Activator, Hepatitis B Vaccine • Agricultural Uses: Bacteria that protects plants from freezing, bacteria that protect plant roots from insects • Environmental: Bacteria that degrade oil (clean up after oil spills), bacteria that remove sulfur from coal

43. 24.3 DNA Technology • Transgenic Plants • Plants have been engineered to secrete a toxin that kills insects • Plants have been engineered to be resistant to herbicides

44. 24.3 DNA Technology • Transgenic Animals • Fish, cows, pigs, rabbits and sheep have been engineered to produce human growth hormone in order to increase size of the animals

45. Transgenic Animals