DNA Structure and Gene Function Lecture 2

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Questions and Answers

What technique did Rosalind Franklin utilize to reveal the structure of DNA?

X-ray diffraction (correct)

Polymerase chain reaction

Electron microscopy

Sanger sequencing

Which of the following statements about Watson and Crick's model of DNA is NOT correct?

It included the concept of complementary base pairing.

It relied on data from other scientists, including Rosalind Franklin.

It explained the arrangement of nucleotides. (correct)

It proposed a double helix structure.

What do the 5’ and 3’ ends of a DNA strand refer to?

The orientation of the sugar-phosphate backbone. (correct)

The number of hydrogen bonds between base pairs.

The types of nitrogenous bases present.

The percentage of adenine and cytosine.

According to Chargaff's rule, which statement is true regarding the composition of DNA?

Adenine is equal to thymine and guanine is equal to cytosine. Signup and view all the answers

What was a key conclusion about DNA made by geneticists in the mid-20th century?

DNA serves as the genetic material of all organisms. Signup and view all the answers

Which characteristic of DNA structure did Rosalind Franklin's X-ray diffraction reveal?

The helical nature of DNA. Signup and view all the answers

In the DNA double helix, how far apart are the bases spaced?

0.34 nm Signup and view all the answers

What did the experiments conducted by Martha Chase and Alfred Hershey ultimately conclude?

DNA is the hereditary material in all living organisms. Signup and view all the answers

What was Frederick Griffith trying to understand through his experiments?

The difference between virulent and nonvirulent strains Signup and view all the answers

What was the outcome when Griffith injected heat-killed S strain with live R strain into the mice?

The mice died from the virulent S strain Signup and view all the answers

What term did Griffith use to describe the transfer of information from dead S strain cells to live R strain cells?

Transformation Signup and view all the answers

What did Avery, MacLeod, and McCarty conclude from their experiments related to transforming material?

Transforming material was DNA Signup and view all the answers

Which strain of Streptococcus pneumoniae is described as virulent in Griffith's study?

Live S strain Signup and view all the answers

What effect did DNA-digesting enzymes have on the transforming ability of the material in Avery’s experiment?

It destroyed the transforming ability Signup and view all the answers

In the context of vaccine development, what is the purpose of introducing a killed or weakened pathogen?

To stimulate an immune response without causing disease Signup and view all the answers

What was a key question raised by scientists following Griffith's and Avery et al.'s findings?

Whether proteins could carry genetic information Signup and view all the answers

Study Notes

Rosalind Franklin’s Experiments

Extracted DNA from cells and crystallized it.
Employed X-ray diffraction techniques to analyze DNA crystals.
Produced an X-pattern on X-ray film, indicating a helical structure of DNA.

Watson and Crick’s DNA Model

Constructed a DNA model using Franklin’s X-ray data along with findings from other scientists.
Proposed DNA as a double helix with a sugar-phosphate backbone and base pairs located on the inside.
Spacing between bases is approximately 0.34 nm, and one complete turn of the helix measures 3.4 nm.
Strands of DNA run antiparallel, oriented from 5’-3’ and 3’-5’.
5' end features a phosphate group on the fifth carbon of the sugar; 3' end has a hydroxyl group on the third carbon.
Followed Chargaff’s rule: adenine (A) pairs with thymine (T) and guanine (G) with cytosine (C), connected by hydrogen bonds.
Base pairing follows complementary pairing rules; purines bond with pyrimidines.

Key Discoveries in DNA Structure

1950s discoveries confirmed DNA as the genetic material across all organisms.
Variation in percentages of A, T, G, and C observed among different species, affirming the helical nature of DNA via X-ray diffraction.
Chargaff’s rule highlights base composition differences across species.

Frederick Griffith's Experiment (1928)

Explored virulence through experiments with Streptococcus pneumoniae, identifying S (virulent) and R (nonvirulent) strains.
Mice infected with live S strain died, while R strain remained harmless.
Heat-killed S strain also did not kill mice, but the combination of heat-killed S strain and live R strain resulted in death.

Griffith's Conclusion

Surmised that virulence information from dead S strain cells transformed live R strain cells.
Introduced the concept of "transformation" for the transfer of genetic information.

Avery, MacLeod, & McCarty's Findings (1944)

Repeated Griffith’s experiments using purified cell extracts to identify genetic material.
Demonstrated that removing proteins from the transforming material did not inhibit transformation of R strain cells.
Concluded that DNA-digesting enzymes eliminated the transforming ability, confirming that DNA is the genetic material.

Implications of This Research

Established that DNA is responsible for carrying genetic information, shifting the scientific focus from proteins to DNA as the key molecule in heredity.
Laid the groundwork for future research on DNA, genetic engineering, and molecular biology.

Understanding Vaccines

Defined as biological substances that enhance immunity against specific diseases.
Consist of non-harmful forms of viruses, bacteria, or their components, and stimulate the immune system to produce antibodies.
Resulting immunity protects individuals from subsequent infections by the pathogen.

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Description

Explore the groundbreaking experiments of Rosalind Franklin on DNA crystals in this informative lecture. Delve into the X-ray diffraction technique that unveiled the double helix structure, contributing to Watson and Crick's DNA model. Understand the pivotal roles of various scientists in shaping our knowledge of genetic material.