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UNIT 3: Molecular Genetics Chapter 5: The Structure and Function of DNA Chapter 6: Gene Expression Chapter 7: Genetic Research and Biotechnology.

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Presentation on theme: "UNIT 3: Molecular Genetics Chapter 5: The Structure and Function of DNA Chapter 6: Gene Expression Chapter 7: Genetic Research and Biotechnology."— Presentation transcript:

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2 UNIT 3: Molecular Genetics Chapter 5: The Structure and Function of DNA Chapter 6: Gene Expression Chapter 7: Genetic Research and Biotechnology

3 Chapter 5: The Structure and Function of DNA What are the structures and functions of DNA and RNA? Scientists now know that DNA, shown here in its uncondensed form, carries genetic information that defines many of an organism’s traits (including behaviours) and its predisposition for certain diseases. UNIT 3 Chapter 5: The Structure and Function of DNA

4 5.1 DNA Structure and Organization in the Cell Before DNA was established as the genetic material in cells, scientists knew: UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 there was a connection between chromosomes and inherited traits the genetic material had to control the production of enzymes and proteins the genetic material had to be able to replicate itself with accuracy and still allow mutations to occur

5 Identifying DNA as the Material of Heredity: Griffith Frederick Griffith’s experiments with Streptococcus pneumoniae in 1928 established DNA as the material of heredity. Two forms of the bacterium were used: UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 The S-strain was highly pathogenic but could be made non-pathogenic by heating it. The R-strain was non- pathogenic. Continued…

6 Identifying DNA as the Material of Heredity: Griffith Griffith discovered that mice died after being injected with a mixture of heat-killed S-strain and living R-strain bacteria. UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 Griffith called this phenomenon the transforming principle because something from the S- strain transformed the R-strain into deadly bacteria.

7 Identifying DNA as the Material of Heredity: Avery, MacLeod, and McCarty Canadian-American microbiologist Oswald Avery and his group built on Griffith’s work to identify the molecules in the S-strain that caused the transformation. Avery, MacLeod, and McCarty prepared identical extracts of the heat-killed S-strain and subjected each extract to one of three enzymes: UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 one that destroyed proteins one that destroyed RNA, and one that destroyed DNA. Continued…

8 Identifying DNA as the Material of Heredity: Avery, MacLeod, and McCarty Each enzyme-treated extract was then mixed with live R-strain cells. The only extract that did not allow transformation of the R-strain to the pathogenic S-strain was the one treated with the DNA-destroying enzyme. Avery and his colleagues concluded that DNA was the hereditary material. UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1

9 Identifying DNA as the Material of Heredity: Hershey and Chase In 1952, Americans Alfred Hershey and Martha Chase ruled out protein as the hereditary material. Their experiments used T2 bacteriophages, which consist of nucleic material surrounded by a protein coat. Hershey and Chase used two different radioactive isotopes to track each molecule ( 35 S for proteins and 32 P for DNA). UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 Continued…

10 Identifying DNA as the Material of Heredity: Hershey and Chase In their first experiment: UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 a virus with DNA radioactively labelled with 32 P was allowed to infect bacteria. After agitation and separation, radioactivity was found in the bacteria pellet but not in the liquid medium. Continued…

11 Identifying DNA as the Material of Heredity: Hershey and Chase UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 a virus with its protein coat radioactively labeled with 35 S was allowed to infect bacteria. After agitation and separation, radioactivity was found in the liquid medium but not in the bacteria pellet. The results proved that viral DNA held the genetic material needed for viruses to reproduce. In their second experiment:

12 Determining the Chemical Composition and Structure of DNA DNA was discovered in 1869 by Fredrich Miescher. By isolating the nuclei of white blood cells, he extracted an acidic molecule he called nuclein. In the early 1900s, Phoebus Levene isolated two types of nucleic acid: RNA and DNA. In 1919, he proposed that both were made up of individual units called nucleotides. Each nucleotide was composed of one of four nitrogen- containing bases, a sugar, and a phosphate group. UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1

13 The Chemical Composition of the Nucleotides, DNA, and RNA UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 DNA has the nucleotides adenine (A), guanine (G), cytosine (C), and thymine (T). RNA has the nucleotides adenine (A), guanine (G), cytosine (C), and uracil (U). In later years, other scientists confirmed and extended Levene’s work. DNA and RNA are both made up of a combination of four different nucleotides. Nucleotides are often identified by referring to their bases: Continued…

14 The Chemical Composition of the Nucleotides, DNA, and RNA UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 The general structure of a DNA nucleotide includes a phosphate group, a deoxyribose sugar group, and a nitrogen-containing base. Nucleotides in RNA have the same basic structure, except a ribose sugar group is used. The sugar groups differ by a hydroxyl group at the 2′ carbon. Both DNA and RNA contain the same purine bases and the cytosine pyrimidine base. However, thymine is only present in DNA, and uracil is only present in RNA.

15 Chargaff’s Rule: Closing in on the Structure of DNA UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 There is variation in the composition of nucleotides in different species. Regardless of the species, DNA maintains certain nucleotide proportions. That is, the amount of A and T nucleotides are equal and the amount of C and G nucleotides are equal. This constant relationship is known as Chargaff’s rule. Erwin Chargaff was inspired by Avery, MacLeod, and McCarty’s work on DNA and launched a research program to study nucleic acids. By the late 1940s, he had reached two conclusions: Continued…

16 Chargaff’s Rule: Closing in on the Structure of DNA UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 In DNA, the percent composition of adenine is the same as thymine, and the percent composition of cytosine is the same as guanine.

17 Pauling Discovers a Helical Structure for Proteins UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 In 1951, Linus Pauling discovered that many proteins have helix- shaped structures. Scientists, including James Watson and Francis Crick, used this information when deducing the structure of DNA.

18 Franklin Determines a Helical Structure for DNA UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 DNA has a helical structure. The nitrogen bases are on the inside of the DNA helix, and the sugar-phosphate backbone is on the outside. In the early 1950s, Rosalind Franklin and Maurice Wilkins used X-ray diffraction to analyze DNA samples. Franklin captured high-resolution photographs and, using mathematical theory to interpret them, determined the following: (A) Rosalind Franklin was a British chemist who was hired to work alongside Maurice Wilkins at the X-ray diffraction facilities at King’s College. (B) In the diffraction image of DNA that she produced, the central x-shaped pattern enabled researchers to infer that DNA has a helical structure.

19 Watson and Crick Build a Three- Dimensional Model for DNA UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 a twisted ladder, which they called a double-helix. The sugar- phosphate molecules make up the sides or “handrails” of the ladder, and the bases make up the “rungs” of the ladder by protruding inwards. In the early 1950s, Watson and Crick began working on a description of the structure of DNA using the results and conclusions of their peers. In 1953, they published a paper that proposed a structure with the following features: Continued…

20 Watson and Crick Build a Three- Dimensional Model for DNA UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 The distance between the sugar-phosphate backbones remains constant over the length of a molecule of DNA. An A nucleotide on one strand always sits across from a T nucleotide (and C across from G) in order to maintain constant distance. These are called base pairs. Different sequences of base pairs can exist, which accounts for the differences between species.

21 The Modern DNA Model: The DNA Double Helix UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 The double helix is composed of two polynucleotide strands that twist around one another. Each strand has a backbone of alternating phosphate groups and sugars. The distance between the sugar- phosphate backbones in each strand is constant. The bases of each nucleotide are attached to each sugar and face inward. Structural features of DNA: Continued…

22 The Modern DNA Model: The DNA Double Helix UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 The two strands are complementary due to complementary base pairing of A with T and C with G. Hydrogen bonds link each complementary base pair. Each strand has a 5 ′ end and a 3 ′ end. The 5 ′ and 3 ′ come from the numbering of the carbons in the deoxyribose sugar. The two strands are antiparallel, where the 5 ′ end from one strand is across from the 3 ′ end of the complementary strand. The sequence of a DNA strand is written in the 5 ′ to 3 ′ direction.

23 The DNA Double Helix UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1

24 The Structure and Organization of Genetic Material UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 how DNA is organized in the cell, and how its genome (the total genetic material of an organism) is arranged into distinct functional regions on DNA To relate DNA’s primary structure (how nucleotides are linked) and secondary structure (how two strands of nucleotides form a double helix) to DNA function, it is necessary to consider: The functional unit of DNA is a gene, which is a specific sequence of DNA that codes for proteins and RNA molecules. The majority of DNA in an organism’s genome does not contain genes and, instead, has non-coding regions. These regions may contain regulatory sequences, which are sections of DNA that regulate the activity of genes.

25 DNA of Prokaryotic Cells UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 In prokaryotes such as bacteria, the bacterial chromosome consists of a circular, double-stranded DNA molecule. More than one copy of the bacterial chromosome may exist in a bacterial cell. Since prokaryotes do not have a nuclear membrane, each bacterial chromosome is packed in the region of the cell called the nucleoid. The DNA of prokaryotic cells such as E. coli is packed near the centre of the cell in the nucleoid, which is not segregated by membranes from the rest of the interior of the cell. Continued…

26 DNA of Prokaryotic Cells UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 Prokaryotic DNA must be tightly packed so that it can fit in the nucleoid. DNA packing is achieved through coiling, compacting, and supercoiling. Continued… (A) The circular chromosomal DNA molecule can be compacted through (B) the formation of looped structures. (C) The looped DNA can be further compacted by DNA supercoiling. Note: the coloured balls represent proteins involved in supercoiling.

27 DNA of Prokaryotic Cells UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 DNA supercoiling is the formation of additional coils in the structure of DNA due to twisting forces. It is controlled by the enzymes topoisomerase I and topoisomerase II. Antibacterial drugs have been developed to specifically block these enzymes and inhibit bacterial survival. Some prokaryotes also have plasmids, which are small, circular or linear DNA molecules that often carry non-essential genes. Plasmids are not part of the nucleoid. They can be copied and transmitted between cells or incorporated into the chromosomal DNA and reproduced during cell division. Continued…

28 DNA of Prokaryotic Cells UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 This is a partial map of the E. coli genome. There are actually thousands of genes in the genome, very few of which are non-essential. The majority of prokaryotic genomes are composed of regions that contain either genes or regulatory sequences. Regulatory sequences are sections of DNA sequences that determine when certain genes and associated cell functions are activated. Most prokaryotes are haploid, meaning they only have one set of chromosomes and therefore carry only one copy of each gene. Their genomes carry very little non-essential DNA.

29 DNA of Eukaryotic Cells UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 Eukaryotic DNA is a double-stranded, linear molecule that is tightly compacted in the nucleus of eukaryotic cells. The total amount of DNA in eukaryotic cells is much greater than in prokaryotic cells. Therefore, eukaryotic DNA undergoes greater compacting than prokaryotic DNA. Continued… The compacting of DNA in eukaryotic cells is achieved through different levels of organization.

30 DNA Organization in Eukaryotic Cells UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 Histones: a family of proteins that associates with DNA Nucleosome: the condensed structure formed when double- stranded DNA wraps around an octamer of histone proteins NOTE: Organization as a distinct chromosome (shown in D) occurs during cell division. Otherwise, the DNA is less condensed and is called chromatin.

31 Variation in the Eukaryotic Genome UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1 The size and number of genes in the eukaryotic genome vary a great deal. There is no correlation between an organism’s complexity and genome size. There is also variation in the molecules that genes produce. Genes can code for RNA molecules, in addition to proteins. (A) Lungfish have 40 times more DNA per cell than a human cell. (B) Rice has more protein-coding genes than a human. (C) C. elegans has the same number of genes as humans but less DNA.

32 Section 5.1 Review UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.1

33 5.2 DNA Replication UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2 All life depends on the ability of cells to reproduce. Successful reproduction includes the transfer of the parent’s DNA to offspring. The life cycle of a cell is referred to as the cell cycle. For most cells, the cell cycle can be represented as in the diagram shown here. The process of copying one DNA molecule into two identical molecules is called DNA replication. DNA is copied during S phase of interphase in the cell cycle.

34 DNA Replication UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2 In the mid-1950s, three competing models of DNA replication were proposed: The conservative model results in one new molecule and conserves the old. The semi- conservative replication model results in two hybrid molecules of old and new strands. The dispersive model results in hybrid molecules with each strand being a mixture of old and new strands. conservative model semi-conservative model dispersive model

35 Meselson and Stahl Determine the Mechanism of DNA Replication UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2 Matthew Meselson and Franklin Stahl reasoned that they could test the models if they could distinguish between parent and daughter strands of DNA. They used two different isotopes of nitrogen to label the DNA: “light” 14 N and “heavy” 15 N. These isotopes were chosen for two reasons: Nitrogen is a component of DNA and would be incorporated into newly synthesized daughter strands. “Light” and “heavy” forms of nitrogen would allow separation of different strands of DNA based on how much isotope was present. DNA with the “heavy” nitrogen would be more dense than DNA with “light” nitrogen. Continued…

36 Meselson and Stahl Determine the Mechanism of DNA Replication UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2

37 DNA Replication Phase 1: Initiation UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2 Semi-conservative replication has three phases: initiation, elongation, and termination. In initiation, a portion of the DNA is unwound to expose the bases for base pairing. Unwinding starts at a specific nucleotide sequence called the origin of replication. Circular prokaryotic DNA has a single origin of replication while linear eukaryotic DNA often has thousands. Continued…

38 Initiation UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2 Initiator proteins bind to the DNA and begin the process of unwinding. Helicase enzymes cleave the hydrogen bonds that link the complementary base pairs. Single-strand-binding proteins help to stabilize the unwound strands. Topoisomerase II (in prokaryotes) relieves strain on the double helix that is generated from unwinding. A replication bubble forms with a Y-shaped replication fork at each end.

39 Elongation: Part I UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2 Two new strands are assembled using the parent DNA as a template. DNA polymerase III catalyzes the addition of new nucleotides to create a complementary strand to the parent strand. However, it can only attach new nucleotides to the free 3′ hydroxyl end of a pre- existing chain of nucleotides. Only one parent strand has a free 3′ hydroxyl end. The strand that is synthesized continuously in the 5′ to 3′ direction from this parent strand is called the leading strand. Continued…

40 Elongation: Part I UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2

41 Elongation: Part II UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2 The lagging strand is formed in short segments away from the replication fork in a discontinuous manner. This requires primase to synthesize an RNA primer. Once the primer is attached to the parental strand, DNA polymerase III extends the strand by synthesizing DNA fragments called Okazaki fragments. DNA polymerase I removes the primers and fills in the space by extending the neighbouring DNA fragment. DNA ligase then joins the Okazaki fragments to create a complete strand.

42 Termination UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2 Termination occurs once the synthesis of the new DNA strands is complete. The two new DNA molecules separate from each other, and the replication machine is dismantled. At each replication fork lies a complex of proteins and DNA that make up the replication machinery. The image shown here is a simplified version of the molecules involved.

43 Important Enzymes in DNA Replication UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2

44 Errors During DNA Replication UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2 A human cell can copy its entire DNA in a few hours, with an error rate of about one per 1 billion nucleotide pairs. Incorrect pairing (mispairing) of bases is thought to occur as a result of flexibility in DNA structure. Continued… Errors naturally occur during replication. Mispairing of bases and strand slippage are two types of errors that cause either additions or omissions of nucleotides.

45 Errors During DNA Replication UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2 Strand slippage during DNA replication can cause the addition or omission of nucleotides in newly synthesized strands, which represent errors.

46 Correcting Errors During DNA Replication UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2 DNA polymerase I and II have proofreading abilities. These enzymes recognize and correct errors in newly synthesized strands of DNA. This method repairs about 99% of the mismatch errors that occur during replication. Mismatch repair is done by a group of proteins that can recognize and repair deformities in newly synthesized DNA that feature the mispairing of bases. Errors that remain after DNA polymerase proofreading or mismatch repair are considered mutations once cell division occurs.

47 Comparing DNA Replication in Eukaryotes and Prokaryotes UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2

48 Section 5.2 Review UNIT 3 Chapter 5: The Structure and Function of DNA Section 5.2

49 UNIT 3 Biology Connections STSE Biobanking is the collection and analysis of physical specimens from which DNA can be derived from a wide sampling of individuals. A goal of biobanking is to provide researchers with open access to millions of high-quality tissue samples collected from people around the world. While biobanks offer the potential for gaining insight into the interaction between genes, environmental factors, and disease, they also present difficult legal and ethical challenges.


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