DNA Structure and Function

DNA Structure and Function

High School

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Learning Objectives

• Describe the structure of a nucleotide and the double helix shape of DNA.

• Explain Chargaff's rules for base pairing in a DNA molecule.

• Outline the primary function of DNA and the central dogma of molecular biology.

• Summarize the main steps and enzymes in semi-conservative DNA replication.

• Identify the scientists who made major contributions to discovering DNA’s structure.

Key Vocabulary

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Nucleotide

The monomer of nucleic acids, consisting of a phosphate, a sugar, and a nitrogenous base.

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Double Helix

The twisted-ladder spiral shape of the two-stranded DNA molecule, as described by Watson and Crick.

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Chargaff's Rule

States that in DNA, Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G).

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Gene

A specific sequence of nucleotides in DNA that contains the instructions to make a single protein.

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DNA Replication

The biological process of producing two identical replicas of a DNA molecule from one original molecule.

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The Building Blocks of DNA

• DNA and RNA are polymers of a biomolecule called nucleic acid.

• ​DNA exists in all living things, inside the nucleus of eukaryotic cells.

• The building block, or monomer, of the DNA polymer is the nucleotide.

• Nucleotides have three parts: a phosphate group, deoxyribose sugar, and a nitrogenous base.

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Solved Example 1
In a DNA sample, 22% of the bases are guanine (G). What percentage of the bases would be adenine (A)?

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Step 1: Analyze and Sketch the Problem

• Goal: Determine the percentage of adenine (A) in a DNA sample.

• Knowns: The sample contains 22% guanine (G).

• Unknown: The percentage of adenine (A) = ?

• Formula: Chargaff's Rule states that A = T and G = C. The total of all bases is 100%.

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Solved Example 1
In a DNA sample, 22% of the bases are guanine (G). What percentage of the bases would be adenine (A)?

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Step 2: Solve for the Unknown

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Solved Example 1
In a DNA sample, 22% of the bases are guanine (G). What percentage of the bases would be adenine (A)?

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Step 3: Evaluate the Answer

• Check the total percentage: A(28%) + T(28%) + G(22%) + C(22%) = 100%.

• The percentages follow Chargaff's rule (A=T and G=C), so the answer is reasonable.

Multiple Choice

What are the three fundamental components that join together to form a single DNA nucleotide?

• A nitrogenous base, a lipid, and a phosphate group

• An amino acid, a deoxyribose sugar, and a phosphate group

• A phosphate group, a deoxyribose sugar, and a nitrogenous base

• A phosphate group, a ribose sugar, and a uracil base

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DNA's Double Helix Structure

• DNA's structure is a double helix, which looks like a twisted ladder.

• Its sides are the sugar-phosphate backbone; rungs are bases held by hydrogen bonds.

• The four bases are purines (A, G) and pyrimidines (C, T).

• Following Chargaff's Rule, Adenine (A) pairs with Thymine (T) and Cytosine (C) with Guanine (G).

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Multiple Choice

According to Chargaff's base-pairing rules, which of the following accurately represents a correct pairing of nitrogenous bases in a DNA double helix?

• Adenine bonding with Guanine

• A purine bonding with another purine

• Adenine bonding with Thymine

• Cytosine bonding with Thymine

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The Function of DNA and the Central Dogma

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Multiple Choice

What is the correct flow of genetic information as described by the Central Dogma of molecular biology?

• Protein → RNA → DNA

• DNA → RNA → Protein

• DNA → Protein → RNA

• RNA → DNA → Protein

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How DNA is Organized

• DNA is highly organized to fit into the nucleus of a cell.

• ​In non-dividing cells, DNA is in a less condensed form called chromatin.

• For cell division, DNA condenses into chromosomes, each with two identical chromatids.

• The order of condensation is DNA, which coils into chromatin, then chromosomes.

Multiple Choice

What is the term for the uncoiled, accessible form of DNA found in a non-dividing cell?

• Chromosome

• Centromere

• Chromatin

• Chromatid

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The Process of DNA Replication

• DNA replication is how a cell makes an identical copy of its DNA.

• The process is semi-conservative, using one old and one new DNA strand.

• Topoisomerase untangles the DNA, and then DNA Helicase 'unzips' the double helix.

• DNA Polymerase builds a new strand by adding complementary nucleotides to the original strands.

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Multiple Choice

In the semi-conservative model of DNA replication, what is the composition of the two new DNA molecules?

• Both molecules are made of fragmented old and new strands.

• One molecule consists of two old strands, and one consists of two new strands.

• Both molecules are made of entirely new strands.

• Each molecule consists of one old strand and one new strand.

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Key Scientists Behind DNA's Structure

• Discovering DNA's structure was a team effort involving several key scientists.

• James Watson and Francis Crick created the first accurate double-helix model of DNA.

• Rosalind Franklin’s X-ray images, like 'Photo 51', revealed DNA’s helical shape.

• Maurice Wilkins shared Franklin’s crucial X-ray diffraction data with Watson and Crick.

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Multiple Choice

Which scientist's X-ray diffraction image, known as 'Photo 51', was the critical piece of evidence that revealed the helical structure of DNA?

• Rosalind Franklin

• James Watson

• Francis Crick

• Maurice Wilkins

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Common Misconceptions About DNA

Misconception	Correction
DNA is always condensed into visible chromosomes.	It's usually uncoiled as chromatin, only condensing during cell division.
Watson and Crick discovered DNA by themselves.	They discovered DNA's structure using Rosalind Franklin's X-ray data.
Adenine pairs with Guanine.	Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G).

Multiple Choice

Why is the process of DNA replication referred to as 'semi-conservative'?

• Because only the enzymes DNA Polymerase and Helicase are conserved during the process.

• Because it conserves energy by not copying the entire chromosome.

• Because the process only copies half of the DNA molecule at a time.

• Because each new DNA molecule is composed of one original strand and one newly synthesized strand.

Multiple Choice

A DNA sample is found to contain 35% Thymine (T). According to Chargaff's rule, what percentage of Cytosine (C) should be present in the same sample?

• 30%

• 70%

• 35%

• 15%

Multiple Choice

If a mutation causes the enzyme DNA Helicase to become non-functional, what would be the immediate consequence for the process of DNA replication?

• The cell would be unable to produce the necessary nucleotides.

• The new DNA strands would be built with many errors.

• The DNA double helix would be unable to unwind or 'unzip', preventing replication from starting.

• The DNA would be unable to coil into a chromosome.

Multiple Choice

Analyze how a gene, the central dogma, and a physical trait are interconnected.

• A gene is a protein that follows the central dogma to create a trait made of DNA.

• The central dogma explains how a protein becomes a gene, which then creates a physical trait.

• A gene's DNA sequence is transcribed into RNA and then translated into a protein, which is expressed as a trait.

• A trait is directly coded by RNA, which is then reverse-transcribed into a gene as part of the central dogma.

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Summary

• DNA has a double-helix structure and is made of building blocks called nucleotides.

• The genetic code is based on the sequence of four bases: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).

• Chargaff's rule states that A always pairs with T, and C always pairs with G, holding the strands together.

• The main function of DNA is to store the instructions for making proteins.

• The Central Dogma explains the flow of genetic information: DNA → RNA → Protein.

• DNA replication is a semi-conservative process that creates two identical copies.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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