Here’s the completed worksheet with correct answers and explanations:
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1. A nucleotide is made of three parts: a _phosphate_ group, a five carbon _sugar (deoxyribose)_, and a nitrogen containing _base_.
✔ Explanation:
A nucleotide — the building block of DNA — consists of:
- A
phosphate group (PO₄³⁻) — provides negative charge and links nucleotides.
- A
five-carbon sugar — in DNA, this is
deoxyribose.
- A
nitrogenous base — adenine (A), thymine (T), cytosine (C), or guanine (G).
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2. The sides of the DNA ladder consist of alternating _sugars_ and phosphates.
✔ Explanation:
The “backbone” of the DNA double helix is formed by alternating
sugar (deoxyribose) and
phosphate groups, linked by phosphodiester bonds. The rungs are the nitrogenous base pairs.
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3. The 5’ end of a single DNA strand contains a free _phosphate group_, while the 3’ end contains a free _hydroxyl group (–OH)_.
✔ Explanation:
DNA strands have directionality:
- The
5' end has a phosphate group attached to the 5th carbon of the sugar.
- The
3' end has a hydroxyl (–OH) group on the 3rd carbon of the sugar.
This polarity is essential for DNA replication and transcription.
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4. Chromosomes consist of DNA wrapped around _histones_.
✔ Explanation:
In eukaryotic cells, DNA is tightly coiled around proteins called
histones to form
nucleosomes, which further compact into chromatin and eventually chromosomes during cell division.
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5. DNA was not thought to be the genetic blueprint originally; instead many scientists believed _proteins_ contained the genetic code and blueprint of life.
✔ Explanation:
Before the 1950s, many scientists thought
proteins were the genetic material because they are more chemically diverse than DNA. This misconception was overturned by experiments like Griffith’s (1928), Avery-MacLeod-McCarty (1944), and Hershey-Chase (1952).
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6. Purines have _two_ rings, and pyrimidines have _one_ ring.
✔ Explanation:
Nitrogenous bases are classified by their ring structure:
-
Purines (adenine and guanine) =
double-ring structures.
-
Pyrimidines (cytosine, thymine, uracil) =
single-ring structures.
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7. Check each of the following statements that are true with regard to Griffith’s experiment:
> a.
✔ Griffith discovered transformation in bacteria.
> b.
✔ Mice injected with smooth bacteria quickly became sick and died.
> c.
✘ Encapsulated bacteria can become “naked” bacteria. *(False — it’s the reverse)*
✔ Explanation of Griffith’s Experiment (1928):
-
Smooth (S) bacteria → encapsulated → virulent → killed mice.
-
Rough (R) bacteria → non-encapsulated → non-virulent → mice survived.
-
Heat-killed S + live R → mice died → live S bacteria recovered →
transformation occurred.
- Griffith concluded that some “transforming principle” from dead S bacteria converted R bacteria into virulent S bacteria.
→ So:
-
a is TRUE — Griffith discovered bacterial transformation.
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b is TRUE — Smooth (encapsulated) bacteria are deadly.
-
c is FALSE — It’s
naked (R) bacteria that become encapsulated (S) after taking up DNA from dead S bacteria — not the other way around.
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✔ Final Answers Summary:
1. phosphate, sugar (deoxyribose), base
2. sugars
3. phosphate group, hydroxyl group (–OH)
4. histones
5. proteins
6. two, one
7. a.
✔, b.
✔, c.
✘
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Parent Tip: Review the logic above to help your child master the concept of dna and genes worksheet answers.
