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The Molecular Basis of Inheritance

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1 The Molecular Basis of Inheritance
Chapter 16 Notes The Molecular Basis of Inheritance

2 Concept 16.1 Once Morgan showed that genes are located on chromosomes, DNA and proteins became the candidates for the genetic material. - at the time nucleic acids seemed too uniform to be responsible for the multitude of possible traits.

3 Concept 16.1 The role of DNA in heredity was first studied by using bacteria and viruses. Griffith (1928) was studying streptococcus pneumonia. - used 2 strains (pathogenic and harmless)

4 Concept 16.1

5 Concept 16.1 - Griffith found that when he killed the pathogenic bacteria with heat and then mixed the cell remains with living bacteria of the harmless strain, some of the living cells were converted into the pathogenic form.

6 Concept 16.1 Transformation: a change in genotype and phenotype due to the assimilation of external DNA by a cell. Avery (1944) purified various chemicals from the heat killed bacteria to recreate Griffith’s experiment. Only DNA worked.

7 Concept 16.1 More evidence was found from viruses that infect bacteria. A virus is basically DNA enclosed by a protective coat of protein. Bacteriophages (or phages): viruses that infect bacteria

8 Concept 16.1

9 Concept 16.1

10 Concept 16.1 Hershey and Chase performed experiments showing that DNA is the genetic material of T2 (a type of phage). - experiment that shows that only DNA enters E. coli during infection.

11 Concept 16.1

12 Concept 16.1 The monomer of nucleic acids are nucleotides
- each consists of 3 parts: a nitrogenous base, a pentose sugar called deoxyribose, and a phosphate group.

13 Concept 16.1 - the base can be adenine (A), thymine (T), cytosine (C), or guanine (G). It was found that even though the DNA varies from one species to another, the amounts of bases are in characteristic ratios

14 Concept 16.1

15 Concept 16.1 Watson and Crick are credited with finding that DNA is formed of two complementary strands called a double helix. - in each rung, a purine (A and G) will bind to a pyrimidine (T and C) - A binds to T, G binds to C

16 Concept 16.1

17 Concept 16.1

18 Concept 16.2 During DNA replication, base pairing enables existing DNA strands to serve as templates for new complementary strands Three theories for DNA replication: conservative, semiconservative, dispersive

19 Concept 16.2

20 Concept 16.2 It was found that the semiconservative model was the actual model used by cells. - the first step is separation of the strand - secondly, each parental strand serves as a template for a complementary strand

21 Concept 16.2

22 Concept 16.2

23 Concept 16.2

24 Concept 16.2

25 Concept 16.2 A series of enzymes carries out the steps of DNA replication Origins of Replication: place where replication begins - specific sequence of DNA - may be more than one on a DNA strand - forms a replication fork

26 Concept 16.2

27 Concept 16.2 DNA Polymerase: enzyme that catalyzes the elongation of new DNA DNA strands line up in an antiparallel arrangement 5’ ’ 3’ ’

28 Concept 16.2

29 Concept 16.2 DNA polymerase adds nucleotides only to the free 3’ end of a growing DNA strand. A new DNA strand can elongate only in the 5’  3’ direction. - leading strand: DNA strand made by this mechanism; works toward the replication fork

30 Concept 16.2 To elongate the other strand, polymerase must work in the direction away from the replication fork. This is the lagging strand - Okazaki fragment - DNA ligase: joins Okazaki fragments to make a single DNA strand

31 Concept 16.2

32 Concept 16.2

33 Concept 16.2 Other proteins:
- Primase: joins RNA nucleotides to form the primer - Helicase: unwinds the DNA before replication can begin

34 Concept 16.2

35 Concept 16.2

36 Concept 16.2 Enzymes proofread DNA during its replication and repair damage in existing DNA. - DNA polymerase proofreads its work - Mismatch repair: special enzymes fix incorrectly paired nucleotides. - nuclease; nucleotide excision repair

37 Concept 16.2

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