Nucleic Acid Structure, DNA Replication, and Chromosome Structure

What is the primary role of genetic material?

It acts as a blueprint for building living organisms.

How does genetic material help organisms survive?

It contains the instructions that allow organisms to function and adapt to their environments.

What four criteria must genetic material meet?

Information, replication, transmission, and variation.

What does “information” mean as a requirement for genetic material?

It must contain the instructions needed to form and maintain an organism.

What does “replication” mean as a requirement for genetic material?

It must be able to make accurate copies of itself.

What does “transmission” mean as a requirement for genetic material?

It must be passed from parent to offspring (or from cell to cell).

What does “variation” mean as a requirement for genetic material?

It must have the capacity to change, creating genetic diversity.

What did scientists in the late 1800s propose about inheritance?

They proposed that a chemical substance inside cells was responsible for transmitting traits.

Why did researchers become convinced that chromosomes were the hereditary material?

Because chromosomes were observed to double and divide during cell division, matching the behaviour expected of hereditary material.

What two major components were found in chromosomes?

DNA and protein.

In the 1920s–1940s, which molecule did scientists expect to be the genetic material, and why?

Protein, because proteins were known to be structurally complex and varied compared to DNA, which seemed too simple at the time.

What happened when Griffith injected living Type S bacteria into a mouse?

The mouse died because Type S cells are virulent.

What happened when Griffith injected living Type R bacteria into a mouse?

The mouse survived because Type R cells are non-virulent (benign).

What happened when Griffith injected heat-killed Type S bacteria into a mouse?

The mouse survived because heat-killed Type S cells are harmless.

What happened when Griffith injected a mixture of living Type R bacteria and heat-killed Type S bacteria into a mouse?

The mouse died because something from the heat-killed Type S cells transformed the living R cells into virulent S cells.

What key discovery came from Griffith’s experiment?

A “transforming principle” exists that can transfer hereditary traits from dead cells to living ones.

What did the transformed bacteria acquire from the heat-killed Type S cells?

Information needed to make a capsule.

Why did the mouse die after transformation occurred?

The transformed bacteria produced a capsule, multiplied, and became virulent.

How did the transformation support the idea of genetic material?

The capsule-producing trait was replicated and transmitted to daughter cells, showing heredity.

What happened to the genetic material from the dead Type S cells?

It was taken up by the living Type R cells, giving them a new heritable trait—the ability to form a capsule.

What major limitation existed in Griffith’s experiment?

Griffith did not know the biochemical identity of the “transforming principle.”

What did Avery, MacLeod, and McCarty aim to determine in the 1940s?

Which substance from dead Type S bacteria is responsible for transforming live Type R bacteria.

Which purified molecules did they test from Type S bacteria?

DNA, RNA, and protein

Which purified molecule was able to convert Type R bacteria into Type S bacteria?

Only DNA.

What happened to transformation when the DNA extract was treated with RNase or protease?

Transformation still occurred, meaning RNA and protein were not responsible.

What happened when the DNA extract was treated with DNase?

Transformation did not occur, showing that DNA was essential for transformation.

What conclusion did Avery, MacLeod, and McCarty reach?

DNA is the genetic material.

What does DNase do?

Destroys DNA.

What does RNase do?

Destroys RNA.

What does protease do?

Destroys protein.

What is transformation in bacteria?

A process where bacteria take up DNA from the environment and incorporate it into their own genome.

Can bacteria naturally perform transformation?

Yes, many species—including Streptococcus pneumoniae—can naturally take up environmental DNA.

Can transformation also be done in the lab?

Yes, transformation can be artificially induced under specific laboratory conditions.

What enzyme inactivated the “transformation principle”?

DNase, which destroys DNA.

What do nucleotides form when linked together?

A single strand of DNA.

What do two DNA strands form when paired?

A double helix.

What is DNA associated with in living cells?

Proteins, forming chromatin.

How is chromatin packaged in the cell?

It is organized into chromosomes.

What is a genome?

The complete set of an organism’s genetic material.

What are the three main components of a DNA nucleotide?

Phosphate group, Pentose sugar (Deoxyribose), Nitrogenous base

What type of sugar is found in DNA nucleotides?

Deoxyribose

What are the two categories of nitrogenous bases in DNA?

Purines and Pyrimidines

Which nitrogenous bases are purines? (Hint: Pure As Gold)

Adenine (A) and Guanine (G)

Which nitrogenous bases are pyrimidines? (Hint: Cut The Pie)

Cytosine (C) and Thymine (T)

What are the three main components of an RNA nucleotide?

Phosphate group, Pentose sugar (Ribose), Nitrogenous base

What type of sugar is found in RNA nucleotides?

Ribose

What makes RNA less stable than DNA?

The extra OH group on ribose is reactive, making RNA less stable

Which nitrogenous bases in RNA are purines?

Adenine (A) and Guanine (G)

Which nitrogenous bases in RNA are pyrimidines?

Cytosine (C) and Uracil (U)

How are the carbon atoms in the sugar ring of nucleotides numbered?

From 1’ to 5’

Where is the 1’ carbon located in the sugar ring?

To the right of the ring oxygen; it attaches to the nitrogenous base

Which carbon does the phosphate group attach to?

The 5’ carbon

Why is the 3’ –OH group important?

It forms the linkage between nucleotides (creates the phosphodiester bond)

What type of bond links nucleotides together in a DNA strand?

Covalent phosphodiester bonds

What does a chain of linked nucleotides form?

A DNA strand (a nucleotide polymer)

Do DNA strands have directionality?

Yes, each strand runs from a 5’ end to a 3’ end

Give an example of how a DNA sequence is written with directionality.

5’ – TACG – 3’

What happens when X-rays hit atoms in a substance

The atoms scatter the X-rays.

What does a repeating structure in a molecule produce when exposed to X-rays?

A diffraction pattern that reflects the arrangement of atoms.

What kind of diffraction pattern indicates a helical structure?

An X-shaped diffraction pattern.

What did Franklin’s X-ray diffraction image show about DNA?

X-rays were diffracted by wet DNA fibres onto a photographic plate, producing an X-shaped pattern characteristic of a helix.

Who analyzed DNA base composition across many species?

Erwin Chargaff

What pattern did Chargaff consistently observe?

Amount of adenine ≈ thymine, and amount of cytosine ≈ guanine

What is Chargaff’s rule?

• A = T

• C = G

What additional relationship did Chargaff identify?

A + G = C + T

How did Watson & Crick approach discovering DNA’s structure?

They pulled together existing knowledge and experimental findings from colleagues.

Did Watson & Crick find the structure immediately?

No, they tested several models before finding the one consistent with all available data.

When was the structure of DNA published and when did they receive the Nobel Prize?

Structure published in 1953; Nobel Prize awarded in 1962.

What forms the outer part of the DNA double helix?

The sugar-phosphate backbone.

Where are the nitrogenous bases located in the double helix?

On the inside of the helix.

What does it mean that the two DNA strands are antiparallel?

They run in opposite directions (5’→3’ and 3’→5’).

What stabilizes the helical structure?

Hydrogen bonds between nitrogenous bases (base pairing).

How many hydrogen bonds connect A–T and G–C?

• A–T: 2 hydrogen bonds

• G–C: 3 hydrogen bonds

What does complementary base pairing mean?

Each base pairs only with its complement: A with T, and G with C.

What is the complementary strand to 5’-GCGGATTT-3’?

3’-CGCCTAAA-5’

Complementary strand to 5’–ATATGGGC–3’?

3’-TATACCCG-5’

What is the overall shape of DNA?

A right-handed double helix composed of two antiparallel strands.

What forms the backbone of DNA?

A sugar–phosphate backbone consisting of alternating deoxyribose and phosphate groups.

What orientation do DNA strands run in?

Antiparallel: one runs 5' → 3' and the other 3' → 5'.

What type of bonds connect complementary bases across the two strands?

Hydrogen bonds.

Which bases pair together in DNA?

Adenine–Thymine (A–T) and Guanine–Cytosine (G–C).

How many hydrogen bonds are in each base pair?

A–T has 2 hydrogen bonds; G–C has 3 hydrogen bonds.

What effect does extra hydrogen bonding in G–C pairs have on DNA?

Increases DNA stability and melting temperature (Tm).

What is the length of one complete turn of the DNA helix?

10 bases (10 base pairs).

What does one nucleotide consist of?

A phosphate group, sugar, and nitrogenous base.

What are the two grooves of DNA called?

The major groove and the minor groove.

Which groove is the most accessible for sequence-specific binding?

The major groove.

Why is the major groove more accessible?

It exposes more chemical features of the base pairs, allowing proteins to “read” the DNA sequence more easily.

What is the function of the minor groove in DNA?

A narrow groove that mainly helps proteins recognize DNA shape, and serves as a binding site for some small molecules.

What is the conservative model of DNA replication?

The original (parental) strands stay together, and the newly synthesized strands form a completely new double helix—no mixing of old and new DNA.

What is the dispersive model of DNA replication?

DNA strands are made of mixed segments—old and new DNA are randomly distributed throughout each strand.

What is the semiconservative model of DNA replication?

Each new DNA molecule has one parental strand and one newly made daughter strand.

Who conducted the experiment that distinguished between the three DNA replication models?

Matthew Meselson and Franklin Stahl (1958)

What was the main goal of the Meselson–Stahl experiment?

o determine which replication mechanism—conservative, dispersive, or semiconservative—was correct.

What isotopes of nitrogen were used in the experiment?

• 15N = heavy isotope

• 14N = light, common isotope

What was the experimental procedure?

• Grow E. coli in 15N media so DNA becomes heavy.

• Transfer cells to 14N media.

• Allow cells to divide and collect samples after each generation.

• Separate DNA by density gradient centrifugation.

What happens to DNA labeled with 15N vs. 14N in the density gradient?

• 15N DNA → sinks lower (heavier)

• 14N DNA → stays higher (lighter)

What was observed after one generation in 14N?

A single intermediate (half-heavy) band — not consistent with conservative replication.

Why was the dispersive model ruled out?

After multiple generations, dispersive replication would never produce completely light DNA, but the experiment eventually showed a light DNA band.

What did generation 3 results show?

• Intermediate DNA

• Light DNA

→ consistent with the semiconservative model.